Batteries work hard for those of us who live off-grid. They need to be properly installed and kept at the right temperature. Enclosures for flooded lead-acid batteries need to be vented. Batteries also need to be periodically filled with distilled water. You must also monitor their state of charge and recharge them quickly after deep discharging.
Generators in such systems need attention, too. If you install one in your system, you will need to periodically change oil and air filters. If you install a manually operated generator, you’ll need to fire it up from time to time to raise the charge level or to equalize your batteries. You may also have to haul your generator in for an occasional repair.
In grid-connected systems with battery backup, you’ll have much less to worry about. If you install sealed batteries, for example, you’ll never need to check the fluid levels or fill batteries.
Batteries may seem complicated and difficult to get along with, but if you understand them, you can get lots of years of service from them. Break the rules and, well, you’re going to pay for your carelessness.
Thursday, May 31, 2012
Battery Maintenance and Safety
Battery care and maintenance are vital to the long-term success of battery-based renewable energy systems. Proper maintenance increases the service life of a battery. Because batteries are expensive, longer service life results in lower operating costs over the long haul. The longer your batteries last, the cheaper your electricity will be.
Keep Them Warm
Lead-acid batteries like to be kept warm. For optimal function, batteries should be kept between 75 to 80° F. In this range, they’ll accept and deliver tons more electricity. Guaranteed! Cold temperatures slow down the chemical reactions in batteries, reducing the amount of electricity a battery can store.
Although batteries can’t be housed in cold rooms, care must be taken to avoid exposure to high temperatures as well. High temperatures increase the release of explosive hydrogen gas, known as outgassing.They also increase water loss, which reduces battery fluid levels. Higher temperatures also lead to higher rates of self-discharge in batteries. (As a rule, older batteries lose charge faster than new batteries.)
If you can’t maintain batteries in a 75 to 80° F range, at least ensure they’re housed in a room where the temperature ranges between 50 and 80° F. Rarely should batteries fall below 40° F or exceed 100° F. Whatever you do, don’t store batteries in a cold garage, barn or shed. Besides delivering less electricity, they won’t last long. They could even freeze under certain conditions, causing their cases to crack, spilling acid and creating a dangerous mess.
Batteries should not be stored on concrete floors. Cold floors cool them down and reduce their rate of chemical reaction and their capacity. Always raise batteries off the floor.
Ideally, batteries should be housed in a separate, conditioned (heated and cooled) battery room or in a battery box inside a conditioned space to maintain proper temperature. Battery boxes are typically built from plywood. An acid-resistant liner is required to contain possible acid spills. Lids should be hinged and sloped to discourage people from storing items on them. As a side note, batteries should be located as close to the inverter and other power conditioning equipment as possible. Doing so minimizes power losses.
Ventilate Your Batteries
Batteries release potentially explosive hydrogen gas when being charged, so battery boxes and battery rooms containing flooded lead-acid batteries should be well ventilated (Figure 7.3). This allows hydrogen to escape. Never place batteries in a room with a gas-burning appliance or an electrical device, such as a water heater, even if the enclosure is vented. A tiny spark could ignite the hydrogen gas, causing an explosion. Note that this applies to the inverter
Fig. 7.3: Battery Vent System. (a) An outdoor battery room should be well insulated and possibly heated and cooled to maintain temperatures in the optimum range. A passive vent system is needed to allow hydrogen gas to escape. (b) Indoor battery rooms need not be insulated, but require venting.
and all other conditioning equipment. Although you want them near the batteries, they should not be in the same space.
Keep Kids Out
Battery rooms and battery boxes should be inaccessible and locked if young children are present. This will prevent children from coming in contact with the batteries, risking electrical or acid burns. Although electrocution is not a hazard at 12, 24 or 48 volts, dropping a tool or other metal object on the battery terminals could result in an electrical arc that can cause burns, or could result in an explosion of the battery, resulting in acid burns.
Avoiding Deep Discharge to Ensure Longer Battery Life
Keeping flooded lead-acid batteries warm and topped off with distilled water to replace water lost during charging ensures a long life span. Longevity can also be ensured by keeping batteries as fully charged as possible. Like many technologies, lead-acid batteries last longer the less you use them. That is to say, the fewer times a battery is deeply discharged, the longer it will last.
This topic (like so many others) is complicated. While deep discharging reduces the lifespan of a battery, what renewable energy users want from batteries is not simply for them to last a long time but to cycle a lot of energy. Theoretically, you’ll get the most bang for your buck by cycling in the 40 to 60 percent deep-discharge range.
It’s also important to recharge batteries as quickly as possible after deep cycling. For long life, you should also never leave batteries at a low state of charge for a long time. This results in the formation of large lead sulfate crystals, described earlier. Unfortunately, achieving these goals is easier said than done. If your system is small and you don’t pay much attention to electrical use, you’ll very likely overshoot the 40 to 60 percent mark time and time again.
One way of reducing deep discharge is to conserve and use electricity efficiently. Conserving energy means not leaving lights and electronic devices running when they’re not in use. It also means getting rid of phantom loads. Energy efficiency means installing energy-efficient lighting, appliances, electronics, etc. You can also adjust electrical use according to the state of charge of your batteries— in other words, cut back on electrical usage when batteries are more deeply discharged and shift demand for electricity to times when the batteries are more fully charged. You may, for instance, run your washing machine and microwave when the wind’s blowing and your batteries are full, but hold off when batteries are running low.
To track battery state of charge you can install a digital amphour or watt-hour meter. These meters keep track of the amount of electricity stored in a battery bank each day. They also indicate the amount of electricity drawn from the batteries. In addition, they keep track of the total amount stored in a battery bank at any one time — how full the batteries are. This information is used to adjust consumption. If batteries are approaching the 40 to 60 percent
discharge mark, you may hold off on activities that consume lots of electricity. Or, you may run your backup generator to charge the batteries.
Watering and Cleaning Batteries
To maintain batteries you must also periodically add distilled water. This replaces water lost during charging. Water loss occurs by electrolysis, the splitting of water molecules in the electrolyte when electricity flows into a battery. Electricity splits water molecules into hydrogen and oxygen. (Electrolysis is the source of the potentially explosive mixture of hydrogen and oxygen gas that makes battery room venting necessary.)
Hydrogen and oxygen produced during electrolysis are both gases. These gases can escape through the vents in the battery caps in flooded lead-acid batteries, lowering water levels. Water can also evaporate through the vents in a flooded lead-acid battery at any time, and a mist of sulfuric acid can escape through the vents during charging, depleting fluid levels.
All of these sources of water loss add up over time and can run a battery dry. When the plates are exposed to air, they quickly begin to corrode. When this happens, a battery’s life is pretty well over.
To prevent batteries from running dry, check battery fluid levels regularly. Many experts recommend checking batteries monthly. Others recommend checking batteries every two to three months. When replacing fluid, be sure to only add distilled or deionized water, and do not overfill batteries. Never use tap water. It may contain minerals or chemicals that will contaminate the battery fluid, reducing a battery’s life span.
The tops and terminals of batteries may also need to be cleaned with distilled water and paper towels or a clean rag. When cleaning batteries, be sure to wear gloves, protective eyewear and a long-sleeved shirt you don’t care about. If you get acid on your skin, wash it off immediately with soap and water.
When filling batteries, be sure to take off watches, rings and other jewelry, especially loose-fitting jewelry. Metal jewelry will conduct electricity if it contacts both terminals of a battery. Such an event will leave your jewelry in a puddle of metal — along with some of your flesh. One 6- or 12-volt cell can produce more than 8,000 amps if the positive and negative terminals of a battery are connected. In addition, sparks could ignite hydrogen and oxygen gas in the vicinity, causing an explosion. Shorting out a battery can also crack the case, releasing battery acid.
Also be careful with tools when working on batteries — for example, tightening cable connections. A metal tool that makes a connection between oppositely charged terminals on a battery may be instantaneously welded in place. The tool will become red hot and could also ignite hydrogen gas, causing an explosion. Wrap hand tools used for battery maintenance in electrical tape so that only one inch of metal is exposed on the working end; that way it can’t make an
electrical connection. Or buy insulated tools to prevent this from happening. Try to have a set of these insulated tools dedicated just to battery maintenance. That way you won’t grab the insulated wrench for another project and then use whatever non-insulated wrench is at hand when it comes time for battery maintenance.
You may also have to clean the battery posts every year or two. To clean the posts, use a small wire brush, perhaps in conjunction with a spray-on battery cleaner purchased at a hardware store. To reduce maintenance, coat battery posts with Vaseline or a battery protector/sealer, available at hardware and auto supply stores. This protects the posts and the nuts that secure the battery cables on the posts.
Equalization
To get the most out of batteries, you need to periodically equalize them. Equalization is a controlled overcharge of batteries.
Why Equalize?
Periodic equalization is performed for three reasons. The first is to drive lead sulfate crystals off the lead plates, preventing the formation of larger crystals that reduce battery capacity. In addition to coating the plates, the large crystals can also flake off, removing lots of lead from the plates.
Batteries must also be periodically equalized to stir the electrolyte. Sulfuric acid tends to settle near the bottom of the cells in flooded lead-acid batteries. During equalization, hydrogen and oxygen gases released by the breakdown of water (electrolysis) create bubbles. They mix the fluid so that the concentration of acid is equalized throughout each cell of each battery, ensuring better function.
Equalization also helps bring all of the cells in a battery bank to the same voltage. That’s important because some cells sulfate more than others. As a result, their voltage may be lower. A single lowvoltage cell in one battery reduces the voltage of the entire string. In many ways, then, a battery bank is like a camel train. It travels at the speed of the slowest camel.
Although equalization removes lead sulfate from plates, which restores function, some lead flakes off the plates during equalization and settles to the bottom of the batteries. As a result, even properly equalized batteries lose lead over time and never regain their full capacity.
How to Equalize
Equalizing batteries is a simple process. In those systems with a gen-set for backup, the owner simply sets the inverter to the equalization mode and then cranks up the generator. The inverter controls the process from that point onward. In wind/PV hybrid systems, the operator can also set the controller to the equalize setting during a storm or period of high wind. The controller takes over from there.
How often batteries should be equalized depends on whom you talk to and how hard you work your batteries. Some installers recommend equalization every three months. If your batteries are frequently deep discharged, however, you may want to equalize more frequently. If batteries are rarely deep discharged, they’ll need less frequent equalization. For example, batteries that are rarely discharged below 50 percent may only need to be equalized every six months.
Rather than second guess your batteries’ needs for equalization, it is wise to check the voltage of each battery, using a digital volt meter (multimeter), every month or two. If you notice that the
Fig. 7.4: Hydrometer. Hydrometers measure specific gravity. Low specific gravity indicates that the battery needs recharging, perhaps even equalization.
voltage of one or two batteries is substantially lower than others, it’s time to equalize.
Another way to test batteries is to measure the specific gravity of the battery acid using a hydrometer (Figure 7.4). Specific gravity is a measure of the density of battery acid. Density is related to the concentration of battery acid — the higher the concentration, the higher the specific gravity. If significant differences in the specific gravity of the battery acid are detected in the cells of a battery bank, it is time to equalize.
If at all possible, use your wind turbine to equalize batteries in off-grid systems. You’d be amazed at how well this works. As a final note on the topic, be sure only to equalize flooded lead-acid batteries. A sealed battery, either gel cell batteries or absorbed glass matt sealed batteries, cannot be equalized! If you try to, you’ll ruin the battery.
Reducing Battery Maintenance
Battery maintenance should take no more than 30 minutes a month. To reduce maintenance time, you can install sealed batteries. Another way to reduce time spent babying batteries is to replace factory battery caps with Hydrocaps (Figure 7.5). Hydrocaps capture much of the hydrogen and oxygen gases released by batteries when charging under normal operation. The gases are recombined in a small chamber in the cap filled with tiny beads coated with a platinum catalyst. Water formed in this reaction drips back into the batteries, reducing water losses by about 90 percent.
Another option is Water Miser caps. They capture moisture and acid mist escaping from batteries’ fluid, reducing water loss by about 30 to 75 percent.
Yet another way to reduce maintenance is to install an automatic or semiautomatic battery filling system (Figure 7.6). Dan uses a manually operated Qwik-Fill battery watering system manufactured by Flow-Rite Controls in Grand Rapids, Michigan (sold
Fig. 7.5: Hydrocaps. These simple devices help reduce battery watering by reducing water losses.
Fig. 7.6: Battery Filling System. Distilled water can be fed automatically to battery cells or manually pumped into them through plastic tubing. Both approaches save a lot of time and energy and help to keep battery fluid levels topped off to ensure battery longevity.
Fig. 7.7: Battery Filler Bottle. If you can access your batteries relatively easily, this filler bottle is one of the easiest and most economic means of adding distilled or deionized water to them.
online through Jan Watercraft Products). He’s found that this system works extremely well even after many years of service and has turned battery maintenance from a chore to a pleasure. Although they’re a bit pricey, the systems quickly pay for themselves in reduced maintenance time and ease of operation. The convenience of quick battery watering overcomes the procrastination that leads to costly battery damage. A cheaper alternative is a half-gallon battery filler bottle (Figure 7.7).
Keep Them Warm
Lead-acid batteries like to be kept warm. For optimal function, batteries should be kept between 75 to 80° F. In this range, they’ll accept and deliver tons more electricity. Guaranteed! Cold temperatures slow down the chemical reactions in batteries, reducing the amount of electricity a battery can store.
Although batteries can’t be housed in cold rooms, care must be taken to avoid exposure to high temperatures as well. High temperatures increase the release of explosive hydrogen gas, known as outgassing.They also increase water loss, which reduces battery fluid levels. Higher temperatures also lead to higher rates of self-discharge in batteries. (As a rule, older batteries lose charge faster than new batteries.)
If you can’t maintain batteries in a 75 to 80° F range, at least ensure they’re housed in a room where the temperature ranges between 50 and 80° F. Rarely should batteries fall below 40° F or exceed 100° F. Whatever you do, don’t store batteries in a cold garage, barn or shed. Besides delivering less electricity, they won’t last long. They could even freeze under certain conditions, causing their cases to crack, spilling acid and creating a dangerous mess.
Batteries should not be stored on concrete floors. Cold floors cool them down and reduce their rate of chemical reaction and their capacity. Always raise batteries off the floor.
Ideally, batteries should be housed in a separate, conditioned (heated and cooled) battery room or in a battery box inside a conditioned space to maintain proper temperature. Battery boxes are typically built from plywood. An acid-resistant liner is required to contain possible acid spills. Lids should be hinged and sloped to discourage people from storing items on them. As a side note, batteries should be located as close to the inverter and other power conditioning equipment as possible. Doing so minimizes power losses.
Ventilate Your Batteries
Batteries release potentially explosive hydrogen gas when being charged, so battery boxes and battery rooms containing flooded lead-acid batteries should be well ventilated (Figure 7.3). This allows hydrogen to escape. Never place batteries in a room with a gas-burning appliance or an electrical device, such as a water heater, even if the enclosure is vented. A tiny spark could ignite the hydrogen gas, causing an explosion. Note that this applies to the inverter
Fig. 7.3: Battery Vent System. (a) An outdoor battery room should be well insulated and possibly heated and cooled to maintain temperatures in the optimum range. A passive vent system is needed to allow hydrogen gas to escape. (b) Indoor battery rooms need not be insulated, but require venting.
and all other conditioning equipment. Although you want them near the batteries, they should not be in the same space.
Keep Kids Out
Battery rooms and battery boxes should be inaccessible and locked if young children are present. This will prevent children from coming in contact with the batteries, risking electrical or acid burns. Although electrocution is not a hazard at 12, 24 or 48 volts, dropping a tool or other metal object on the battery terminals could result in an electrical arc that can cause burns, or could result in an explosion of the battery, resulting in acid burns.
Avoiding Deep Discharge to Ensure Longer Battery Life
Keeping flooded lead-acid batteries warm and topped off with distilled water to replace water lost during charging ensures a long life span. Longevity can also be ensured by keeping batteries as fully charged as possible. Like many technologies, lead-acid batteries last longer the less you use them. That is to say, the fewer times a battery is deeply discharged, the longer it will last.
This topic (like so many others) is complicated. While deep discharging reduces the lifespan of a battery, what renewable energy users want from batteries is not simply for them to last a long time but to cycle a lot of energy. Theoretically, you’ll get the most bang for your buck by cycling in the 40 to 60 percent deep-discharge range.
It’s also important to recharge batteries as quickly as possible after deep cycling. For long life, you should also never leave batteries at a low state of charge for a long time. This results in the formation of large lead sulfate crystals, described earlier. Unfortunately, achieving these goals is easier said than done. If your system is small and you don’t pay much attention to electrical use, you’ll very likely overshoot the 40 to 60 percent mark time and time again.
One way of reducing deep discharge is to conserve and use electricity efficiently. Conserving energy means not leaving lights and electronic devices running when they’re not in use. It also means getting rid of phantom loads. Energy efficiency means installing energy-efficient lighting, appliances, electronics, etc. You can also adjust electrical use according to the state of charge of your batteries— in other words, cut back on electrical usage when batteries are more deeply discharged and shift demand for electricity to times when the batteries are more fully charged. You may, for instance, run your washing machine and microwave when the wind’s blowing and your batteries are full, but hold off when batteries are running low.
To track battery state of charge you can install a digital amphour or watt-hour meter. These meters keep track of the amount of electricity stored in a battery bank each day. They also indicate the amount of electricity drawn from the batteries. In addition, they keep track of the total amount stored in a battery bank at any one time — how full the batteries are. This information is used to adjust consumption. If batteries are approaching the 40 to 60 percent
discharge mark, you may hold off on activities that consume lots of electricity. Or, you may run your backup generator to charge the batteries.
Watering and Cleaning Batteries
To maintain batteries you must also periodically add distilled water. This replaces water lost during charging. Water loss occurs by electrolysis, the splitting of water molecules in the electrolyte when electricity flows into a battery. Electricity splits water molecules into hydrogen and oxygen. (Electrolysis is the source of the potentially explosive mixture of hydrogen and oxygen gas that makes battery room venting necessary.)
Hydrogen and oxygen produced during electrolysis are both gases. These gases can escape through the vents in the battery caps in flooded lead-acid batteries, lowering water levels. Water can also evaporate through the vents in a flooded lead-acid battery at any time, and a mist of sulfuric acid can escape through the vents during charging, depleting fluid levels.
All of these sources of water loss add up over time and can run a battery dry. When the plates are exposed to air, they quickly begin to corrode. When this happens, a battery’s life is pretty well over.
To prevent batteries from running dry, check battery fluid levels regularly. Many experts recommend checking batteries monthly. Others recommend checking batteries every two to three months. When replacing fluid, be sure to only add distilled or deionized water, and do not overfill batteries. Never use tap water. It may contain minerals or chemicals that will contaminate the battery fluid, reducing a battery’s life span.
The tops and terminals of batteries may also need to be cleaned with distilled water and paper towels or a clean rag. When cleaning batteries, be sure to wear gloves, protective eyewear and a long-sleeved shirt you don’t care about. If you get acid on your skin, wash it off immediately with soap and water.
When filling batteries, be sure to take off watches, rings and other jewelry, especially loose-fitting jewelry. Metal jewelry will conduct electricity if it contacts both terminals of a battery. Such an event will leave your jewelry in a puddle of metal — along with some of your flesh. One 6- or 12-volt cell can produce more than 8,000 amps if the positive and negative terminals of a battery are connected. In addition, sparks could ignite hydrogen and oxygen gas in the vicinity, causing an explosion. Shorting out a battery can also crack the case, releasing battery acid.
Also be careful with tools when working on batteries — for example, tightening cable connections. A metal tool that makes a connection between oppositely charged terminals on a battery may be instantaneously welded in place. The tool will become red hot and could also ignite hydrogen gas, causing an explosion. Wrap hand tools used for battery maintenance in electrical tape so that only one inch of metal is exposed on the working end; that way it can’t make an
electrical connection. Or buy insulated tools to prevent this from happening. Try to have a set of these insulated tools dedicated just to battery maintenance. That way you won’t grab the insulated wrench for another project and then use whatever non-insulated wrench is at hand when it comes time for battery maintenance.
You may also have to clean the battery posts every year or two. To clean the posts, use a small wire brush, perhaps in conjunction with a spray-on battery cleaner purchased at a hardware store. To reduce maintenance, coat battery posts with Vaseline or a battery protector/sealer, available at hardware and auto supply stores. This protects the posts and the nuts that secure the battery cables on the posts.
Equalization
To get the most out of batteries, you need to periodically equalize them. Equalization is a controlled overcharge of batteries.
Why Equalize?
Periodic equalization is performed for three reasons. The first is to drive lead sulfate crystals off the lead plates, preventing the formation of larger crystals that reduce battery capacity. In addition to coating the plates, the large crystals can also flake off, removing lots of lead from the plates.
Batteries must also be periodically equalized to stir the electrolyte. Sulfuric acid tends to settle near the bottom of the cells in flooded lead-acid batteries. During equalization, hydrogen and oxygen gases released by the breakdown of water (electrolysis) create bubbles. They mix the fluid so that the concentration of acid is equalized throughout each cell of each battery, ensuring better function.
Equalization also helps bring all of the cells in a battery bank to the same voltage. That’s important because some cells sulfate more than others. As a result, their voltage may be lower. A single lowvoltage cell in one battery reduces the voltage of the entire string. In many ways, then, a battery bank is like a camel train. It travels at the speed of the slowest camel.
Although equalization removes lead sulfate from plates, which restores function, some lead flakes off the plates during equalization and settles to the bottom of the batteries. As a result, even properly equalized batteries lose lead over time and never regain their full capacity.
How to Equalize
Equalizing batteries is a simple process. In those systems with a gen-set for backup, the owner simply sets the inverter to the equalization mode and then cranks up the generator. The inverter controls the process from that point onward. In wind/PV hybrid systems, the operator can also set the controller to the equalize setting during a storm or period of high wind. The controller takes over from there.
How often batteries should be equalized depends on whom you talk to and how hard you work your batteries. Some installers recommend equalization every three months. If your batteries are frequently deep discharged, however, you may want to equalize more frequently. If batteries are rarely deep discharged, they’ll need less frequent equalization. For example, batteries that are rarely discharged below 50 percent may only need to be equalized every six months.
Rather than second guess your batteries’ needs for equalization, it is wise to check the voltage of each battery, using a digital volt meter (multimeter), every month or two. If you notice that the
Fig. 7.4: Hydrometer. Hydrometers measure specific gravity. Low specific gravity indicates that the battery needs recharging, perhaps even equalization.
voltage of one or two batteries is substantially lower than others, it’s time to equalize.
Another way to test batteries is to measure the specific gravity of the battery acid using a hydrometer (Figure 7.4). Specific gravity is a measure of the density of battery acid. Density is related to the concentration of battery acid — the higher the concentration, the higher the specific gravity. If significant differences in the specific gravity of the battery acid are detected in the cells of a battery bank, it is time to equalize.
If at all possible, use your wind turbine to equalize batteries in off-grid systems. You’d be amazed at how well this works. As a final note on the topic, be sure only to equalize flooded lead-acid batteries. A sealed battery, either gel cell batteries or absorbed glass matt sealed batteries, cannot be equalized! If you try to, you’ll ruin the battery.
Reducing Battery Maintenance
Battery maintenance should take no more than 30 minutes a month. To reduce maintenance time, you can install sealed batteries. Another way to reduce time spent babying batteries is to replace factory battery caps with Hydrocaps (Figure 7.5). Hydrocaps capture much of the hydrogen and oxygen gases released by batteries when charging under normal operation. The gases are recombined in a small chamber in the cap filled with tiny beads coated with a platinum catalyst. Water formed in this reaction drips back into the batteries, reducing water losses by about 90 percent.
Another option is Water Miser caps. They capture moisture and acid mist escaping from batteries’ fluid, reducing water loss by about 30 to 75 percent.
Yet another way to reduce maintenance is to install an automatic or semiautomatic battery filling system (Figure 7.6). Dan uses a manually operated Qwik-Fill battery watering system manufactured by Flow-Rite Controls in Grand Rapids, Michigan (sold
Fig. 7.5: Hydrocaps. These simple devices help reduce battery watering by reducing water losses.
Fig. 7.6: Battery Filling System. Distilled water can be fed automatically to battery cells or manually pumped into them through plastic tubing. Both approaches save a lot of time and energy and help to keep battery fluid levels topped off to ensure battery longevity.
Fig. 7.7: Battery Filler Bottle. If you can access your batteries relatively easily, this filler bottle is one of the easiest and most economic means of adding distilled or deionized water to them.
online through Jan Watercraft Products). He’s found that this system works extremely well even after many years of service and has turned battery maintenance from a chore to a pleasure. Although they’re a bit pricey, the systems quickly pay for themselves in reduced maintenance time and ease of operation. The convenience of quick battery watering overcomes the procrastination that leads to costly battery damage. A cheaper alternative is a half-gallon battery filler bottle (Figure 7.7).
Sizing a Battery Bank
Properly sizing a battery bank is key to designing a reliable off-grid system. The principal goal when sizing a battery bank is to install a sufficient number of batteries to carry your household or business through periods when the wind or wind and sun (in hybrid systems) are not available.
Battery banks are typically sized to meet the need for electricity for three days. Longer reserve periods — five days or more — may be required for some areas. As noted in Chapter 3, backup fossil- fuel generators are often included in off-grid systems. Backup generators can reduce the size of the battery bank and are used to equalize the batteries. For more details on wiring and sizing battery banks for off-grid systems, you may want to check out Dan’s book, Power from the Wind. Because batteries are expensive, it’s a good idea to make your home as efficient as possible. This will reduce the size of your wind system and battery bank.
Battery banks are typically sized to meet the need for electricity for three days. Longer reserve periods — five days or more — may be required for some areas. As noted in Chapter 3, backup fossil- fuel generators are often included in off-grid systems. Backup generators can reduce the size of the battery bank and are used to equalize the batteries. For more details on wiring and sizing battery banks for off-grid systems, you may want to check out Dan’s book, Power from the Wind. Because batteries are expensive, it’s a good idea to make your home as efficient as possible. This will reduce the size of your wind system and battery bank.
Wiring a Battery Bank
Batteries are wired by installers to produce a specific voltage and amp-hour storage capacity. Small renewable energy systems — for example, those used to power RVs, boats and cabins — are typically wired to produce 12-volt electricity. The electronics in these applications run entirely off 12-volt DC electricity. Systems in off-grid homes and businesses are typically wired to produce 24- or 48-volt DC electricity. The low-voltage DC electricity, however, is converted to AC electricity by the inverter. It also boosts the voltage to 120- and 240-volts, commonly used in homes and businesses.
Will Any Lead-Acid Battery Work?
Lead-acid batteries come in many varieties, each one designed for a specific application. Car batteries, for example, are designed and manufactured for use in cars, light trucks and vans; deep-cycle marine batteries are designed for boats; golf cart batteries are for golf carts; and forklift batteries for forklifts.
For off-grid systems, you have three options: (1) deep-cycle flooded lead-acid battery like those made by Trojan, Rolls and Deka (Figure 7.2), (2) forklift batteries, and (3) golf cart batteries. Car batteries won’t work. Their thin lead plates are not designed for the deep discharges that commonly occur in renewable energy systems. Although the lead sulfate crystals that form on the plates of a battery during deep discharge are removed when batteries are recharged, some crystals fall off before recharge occurs. The thin plates of a car battery are whittled away to nothing very quickly.
After twenty or so deep discharges, the batteries would be ruined — no longer able to accept a charge.
Battery-based wind and solar systems require deep-discharge lead-acid batteries with thick lead plates. It’s the thickness of the plates that allows them to withstand multiple deep discharges. Even though the plates lose a little lead over time, they are so thick that the small losses are insignificant. Consequently, deep-cycle batteries can be deeply discharged hundreds, sometimes a few thousand times, over their lifetime.
Fig. 7.2: Deep-Cycle Lead-Acid Batteries. These batteries contain thick lead plates and are used in many batterybased renewable energy systems. The thick plates permit deep cycling so long as the batteries are recharged soon after each deep discharge.
For optimum long-term performance, deep-cycle batteries still need to be recharged promptly after deep discharging. Don’t forget this! With proper care, these batteries could last for seven to ten years, perhaps longer.
Forklift batteries are high-capacity, deep-discharge batteries designed for a fairly long life and operate under fairly demanding conditions. They can withstand 1,000 to 2,000 deep discharges — more than many other deep-cycle batteries used in renewable energy systems — and thus work well. They are, however, rather heavy, bulky and expensive. If you can acquire them new at a decent price, you may want to use them.
Golf cart batteries may also work. Like forklift batteries, golf cart batteries are designed for deep discharge. However, they typically cost a lot less than other heavier duty deep-cycle batteries. While the lower cost may be appealing, golf cart batteries don’t store as much electricity and don’t last as long as the alternatives. They may last only five to seven years, if well cared for. Shorter lifespan means more frequent replacement. More frequent replacement means higher long-term costs and more hassle.
What about Used Batteries?
Another option is used batteries. Although they can often be purchased inexpensively, they’re rarely worth it. Used batteries are often being sold because they’ve failed or have experienced a serious decline in function. As a buyer, you also have no idea how well — or how poorly — they’ve been treated. Have they been deeply discharged many times? Have they been left in a state of deep discharge for long periods? Have they been filled with tap water rather than distilled water? Although there are exceptions, most people we know who’ve purchased used flooded lead-acid batteries have been disappointed.
When shopping for batteries for a renewable energy system, look for high-quality deep-cycle batteries. Although you might be able to save some money by purchasing cheaper alternatives, including used batteries, frequent replacement is time consuming.
Batteries are heavy and it takes quite a lot of time and effort to disconnect old batteries and rewire new ones. Bottom line: the longer a battery will last — because it’s the right battery for the job and it’s well made and well cared for — the better!
Sealed Batteries
Grid-connected systems with battery backup often incorporate another type of lead-acid battery, known as sealed lead-acid batteries or captive-electrolyte batteries. Sealed batteries are filled with electrolyte at the factory, charged, and then permanently sealed. This makes them easy to handle. They can be shipped without fear of leaking. They won’t leak even if the battery casing is cracked, and they can be installed in any orientation — even on their sides. But
most important, they never need to be watered.
Two types of sealed batteries are available: absorbed glass mat (AGM) batteries and gel cell batteries. In absorbed glass mat batteries, thin absorbent fiberglass mats are placed between the lead plates. The mat consists of a network of tiny pores that immobilize the battery acid. These tiny pockets also capture hydrogen and oxygen gases given off by the battery when it is charging. Unlike a flooded lead-acid battery, the gases can’t escape. Instead, they recombine in the pockets, reforming water. That’s why sealed AGM batteries never need watering.
In gel batteries, the sulfuric acid electrolyte is converted to a substance much like hardened Jell-O by the addition of a small amount of silica gel. The gel-like substance fills the spaces between the lead plates.
Sealed batteries are also known as “maintenance-free” batteries because fluid levels never need to be checked and because the batteries never need to be filled with water. They also never need to be (and should not be!) equalized, a process discussed shortly. Eliminating routine maintenance saves a lot of time and energy. It makes sealed batteries a good choice for grid-connected systems with battery backup. In these systems, batteries are rarely used and maintained like those in a grid-connected system. Sealed batteries are also ideal off-grid systems in remote locations where routine maintenance is problematic — for example, rarely occupied backwoods cabins.
Sealed batteries offer several additional advantages over flooded lead-acid batteries. They charge faster and do not release explosive gases, so there’s no need to vent battery rooms or battery boxes where they’re stored. In addition, sealed batteries are much more tolerant of low temperatures. They can even handle occasional freezing, although this is never recommended. Sealed batteries selfdischarge more slowly than flooded lead-acid batteries when not in use. (All batteries self-discharge when not in use.)
Unfortunately, sealed batteries are much more expensive, store less electricity, and have a shorter lifespan than flooded lead-acid batteries. They also can’t be rejuvenated (equalized) if left in a state of deep discharge for an extended period. During such times, lead sulfate crystals on the plates begin to grow. Large crystals reduce a
battery’s ability to store electricity. Batteries then take progressively less charge and have less to give back. Over time, entire cells may die, substantially reducing a battery’s storage capacity.
Large crystals on the plates of flooded lead-acid batteries can be removed by a controlled overcharge, a procedure known as equalization. Although equalization is safe in unsealed flooded lead-acid batteries, it results in pressure buildup inside a sealed battery. Pressure is vented through the pressure release valve on the sealed battery, which releases electrolyte and could destroy or seriously decrease the storage capacity of the sealed battery. So, while maintenance- free batteries may seem like a good idea, they are not suitable for many applications.
For off-grid systems, you have three options: (1) deep-cycle flooded lead-acid battery like those made by Trojan, Rolls and Deka (Figure 7.2), (2) forklift batteries, and (3) golf cart batteries. Car batteries won’t work. Their thin lead plates are not designed for the deep discharges that commonly occur in renewable energy systems. Although the lead sulfate crystals that form on the plates of a battery during deep discharge are removed when batteries are recharged, some crystals fall off before recharge occurs. The thin plates of a car battery are whittled away to nothing very quickly.
After twenty or so deep discharges, the batteries would be ruined — no longer able to accept a charge.
Battery-based wind and solar systems require deep-discharge lead-acid batteries with thick lead plates. It’s the thickness of the plates that allows them to withstand multiple deep discharges. Even though the plates lose a little lead over time, they are so thick that the small losses are insignificant. Consequently, deep-cycle batteries can be deeply discharged hundreds, sometimes a few thousand times, over their lifetime.
Fig. 7.2: Deep-Cycle Lead-Acid Batteries. These batteries contain thick lead plates and are used in many batterybased renewable energy systems. The thick plates permit deep cycling so long as the batteries are recharged soon after each deep discharge.
For optimum long-term performance, deep-cycle batteries still need to be recharged promptly after deep discharging. Don’t forget this! With proper care, these batteries could last for seven to ten years, perhaps longer.
Forklift batteries are high-capacity, deep-discharge batteries designed for a fairly long life and operate under fairly demanding conditions. They can withstand 1,000 to 2,000 deep discharges — more than many other deep-cycle batteries used in renewable energy systems — and thus work well. They are, however, rather heavy, bulky and expensive. If you can acquire them new at a decent price, you may want to use them.
Golf cart batteries may also work. Like forklift batteries, golf cart batteries are designed for deep discharge. However, they typically cost a lot less than other heavier duty deep-cycle batteries. While the lower cost may be appealing, golf cart batteries don’t store as much electricity and don’t last as long as the alternatives. They may last only five to seven years, if well cared for. Shorter lifespan means more frequent replacement. More frequent replacement means higher long-term costs and more hassle.
What about Used Batteries?
Another option is used batteries. Although they can often be purchased inexpensively, they’re rarely worth it. Used batteries are often being sold because they’ve failed or have experienced a serious decline in function. As a buyer, you also have no idea how well — or how poorly — they’ve been treated. Have they been deeply discharged many times? Have they been left in a state of deep discharge for long periods? Have they been filled with tap water rather than distilled water? Although there are exceptions, most people we know who’ve purchased used flooded lead-acid batteries have been disappointed.
When shopping for batteries for a renewable energy system, look for high-quality deep-cycle batteries. Although you might be able to save some money by purchasing cheaper alternatives, including used batteries, frequent replacement is time consuming.
Batteries are heavy and it takes quite a lot of time and effort to disconnect old batteries and rewire new ones. Bottom line: the longer a battery will last — because it’s the right battery for the job and it’s well made and well cared for — the better!
Sealed Batteries
Grid-connected systems with battery backup often incorporate another type of lead-acid battery, known as sealed lead-acid batteries or captive-electrolyte batteries. Sealed batteries are filled with electrolyte at the factory, charged, and then permanently sealed. This makes them easy to handle. They can be shipped without fear of leaking. They won’t leak even if the battery casing is cracked, and they can be installed in any orientation — even on their sides. But
most important, they never need to be watered.
Two types of sealed batteries are available: absorbed glass mat (AGM) batteries and gel cell batteries. In absorbed glass mat batteries, thin absorbent fiberglass mats are placed between the lead plates. The mat consists of a network of tiny pores that immobilize the battery acid. These tiny pockets also capture hydrogen and oxygen gases given off by the battery when it is charging. Unlike a flooded lead-acid battery, the gases can’t escape. Instead, they recombine in the pockets, reforming water. That’s why sealed AGM batteries never need watering.
In gel batteries, the sulfuric acid electrolyte is converted to a substance much like hardened Jell-O by the addition of a small amount of silica gel. The gel-like substance fills the spaces between the lead plates.
Sealed batteries are also known as “maintenance-free” batteries because fluid levels never need to be checked and because the batteries never need to be filled with water. They also never need to be (and should not be!) equalized, a process discussed shortly. Eliminating routine maintenance saves a lot of time and energy. It makes sealed batteries a good choice for grid-connected systems with battery backup. In these systems, batteries are rarely used and maintained like those in a grid-connected system. Sealed batteries are also ideal off-grid systems in remote locations where routine maintenance is problematic — for example, rarely occupied backwoods cabins.
Sealed batteries offer several additional advantages over flooded lead-acid batteries. They charge faster and do not release explosive gases, so there’s no need to vent battery rooms or battery boxes where they’re stored. In addition, sealed batteries are much more tolerant of low temperatures. They can even handle occasional freezing, although this is never recommended. Sealed batteries selfdischarge more slowly than flooded lead-acid batteries when not in use. (All batteries self-discharge when not in use.)
Unfortunately, sealed batteries are much more expensive, store less electricity, and have a shorter lifespan than flooded lead-acid batteries. They also can’t be rejuvenated (equalized) if left in a state of deep discharge for an extended period. During such times, lead sulfate crystals on the plates begin to grow. Large crystals reduce a
battery’s ability to store electricity. Batteries then take progressively less charge and have less to give back. Over time, entire cells may die, substantially reducing a battery’s storage capacity.
Large crystals on the plates of flooded lead-acid batteries can be removed by a controlled overcharge, a procedure known as equalization. Although equalization is safe in unsealed flooded lead-acid batteries, it results in pressure buildup inside a sealed battery. Pressure is vented through the pressure release valve on the sealed battery, which releases electrolyte and could destroy or seriously decrease the storage capacity of the sealed battery. So, while maintenance- free batteries may seem like a good idea, they are not suitable for many applications.
Which Types of Batteries Work Best?
Batteries used in most off-grid renewable energy systems are deepcycle, flooded lead-acid batteries. These batteries can be charged and discharged (cycled) hundreds of times before they wear out.
Lead-acid batteries contain three separate 2-volt compartments, known as cells. Inside each cell is a series of thick, parallel lead plates (Figure 7.1). The cells are connected internally (wired in series) so that they produce 6-volt electricity. The cells are filled with sulfuric acid (hence the term “flooded”). A partition wall separates each cell so that fluid cannot flow from one cell to the next. The cells are encased in a heavy-duty plastic case.
As illustrated in Figure 7.1, lead-acid batteries contain two types of plates: positive and negative. The positive plates connect to a positivemetal post or terminal; the negative plates connect to a negative post. The posts allow electricity to flow into and out of batteries.
The positive plates of lead-acid batteries are made from lead dioxide (PbO2). The negative plates are made from pure lead. The
Fig. 7.1: Anatomy of a Flooded Lead-Acid Battery.
sulfuric acid that fills the spaces between the plates is referred to as the electrolyte.
How Lead-Acid Batteries Work
Like all other types of batteries, lead-acid batteries convert electrical energy into chemical energy when they are charged. When discharging, that is, giving off electricity, chemical energy is converted back into electricity. Electricity, of course, consists of electrons, tiny negatively charged particles, that flow through conductors. The electrons flow out of the battery at the positive post, creating an electrical current. During the chemical reactions that take place during discharge, lead on the surface of the negative plates reacts with sulfuric acid in the battery, creating tiny lead sulfate
crystals on the surface of the plates.
Although the chemistry of lead-acid batteries is a bit complicated, it is important to remember that this system works because electrons can be stored in the chemicals within the battery when a battery is charged. The stored electrons can be drawn out by reversing the chemical reactions. Through this reversible chemical reaction, the battery is acting as a “charge pump,” moving electrical charges through a circuit on demand.
Lead-acid batteries contain three separate 2-volt compartments, known as cells. Inside each cell is a series of thick, parallel lead plates (Figure 7.1). The cells are connected internally (wired in series) so that they produce 6-volt electricity. The cells are filled with sulfuric acid (hence the term “flooded”). A partition wall separates each cell so that fluid cannot flow from one cell to the next. The cells are encased in a heavy-duty plastic case.
As illustrated in Figure 7.1, lead-acid batteries contain two types of plates: positive and negative. The positive plates connect to a positivemetal post or terminal; the negative plates connect to a negative post. The posts allow electricity to flow into and out of batteries.
The positive plates of lead-acid batteries are made from lead dioxide (PbO2). The negative plates are made from pure lead. The
Fig. 7.1: Anatomy of a Flooded Lead-Acid Battery.
sulfuric acid that fills the spaces between the plates is referred to as the electrolyte.
How Lead-Acid Batteries Work
Like all other types of batteries, lead-acid batteries convert electrical energy into chemical energy when they are charged. When discharging, that is, giving off electricity, chemical energy is converted back into electricity. Electricity, of course, consists of electrons, tiny negatively charged particles, that flow through conductors. The electrons flow out of the battery at the positive post, creating an electrical current. During the chemical reactions that take place during discharge, lead on the surface of the negative plates reacts with sulfuric acid in the battery, creating tiny lead sulfate
crystals on the surface of the plates.
Although the chemistry of lead-acid batteries is a bit complicated, it is important to remember that this system works because electrons can be stored in the chemicals within the battery when a battery is charged. The stored electrons can be drawn out by reversing the chemical reactions. Through this reversible chemical reaction, the battery is acting as a “charge pump,” moving electrical charges through a circuit on demand.
Wednesday, May 30, 2012
Protecting Against Lightning
Although lightning is not attracted to tall metal objects, such as a wind generator tower, as is commonly thought, it is important to install lightning protection. Grounding rods attached to the tower bleed off the static charge created as air masses move across the Earth’s surface. They’ll reduce the likelihood of a direct strike. Ground rods are eight-foot long copper-coated rods. They are driven into the ground at the base of the wind turbine tower. For details, contact a local installer or your turbine/tower manufacturer. (For more on the subject, see Mick Sagrillo’s article “Residential Wind Turbines and Lightning,” available online at renewwisconsin.org/wind/Toolbox-Fact%20Sheets/Lightning.pdf.)
Grounding a tower minimizes lightning strikes, but does not guarantee that lightning will not strike your tower. Backup is
needed in the form of lightning arrestors. Lightning arrestors “bleed off ” electricity in case of a direct or nearby strike, protecting sensitive equipment. A professional installer will provide recommendations.
Surge arrestors should also be installed on the electrical wire running down the tower and the utility wiring for grid-tied systems. They protect against surges of electricity induced in the wire by lightning strikes. The surge protectors on the utility side of the system protect against lightning strikes in utility lines, which are much more frequent than on properly grounded wind turbine towers.
While protecting against direct lightning strikes is important, nearby strikes pose the gravest danger to a wind system. When lightning strikes the ground near a home, it creates an electrical current in the atmosphere and/or the ground. This current creates a voltage wave that resembles ripples in a quiet pond after a pebble is dropped into it. If one of these waves crosses a conductor like a wind generator tower or buried wires, an electrical current will be created (induced) in the conductor. This current can fry sensitive electronic components of a wind system. Surge protectors in a wind system will help protect against this phenomenon.
Grounding a tower minimizes lightning strikes, but does not guarantee that lightning will not strike your tower. Backup is
needed in the form of lightning arrestors. Lightning arrestors “bleed off ” electricity in case of a direct or nearby strike, protecting sensitive equipment. A professional installer will provide recommendations.
Surge arrestors should also be installed on the electrical wire running down the tower and the utility wiring for grid-tied systems. They protect against surges of electricity induced in the wire by lightning strikes. The surge protectors on the utility side of the system protect against lightning strikes in utility lines, which are much more frequent than on properly grounded wind turbine towers.
While protecting against direct lightning strikes is important, nearby strikes pose the gravest danger to a wind system. When lightning strikes the ground near a home, it creates an electrical current in the atmosphere and/or the ground. This current creates a voltage wave that resembles ripples in a quiet pond after a pebble is dropped into it. If one of these waves crosses a conductor like a wind generator tower or buried wires, an electrical current will be created (induced) in the conductor. This current can fry sensitive electronic components of a wind system. Surge protectors in a wind system will help protect against this phenomenon.
Aircraft Safety and the FAA
Another factor to consider when installing a wind turbine is its impact on aviation. If the tower exceeds a certain height and is within a certain distance from an airport, you will need to file for a permit from the Federal Aviation Administration (FAA). As for height, an FAA permit is required if the tower is over 200 feet, which is extremely rare for a small wind turbine.
The second condition that requires an FAA review and permit is if the wind generator is approximately two to four miles from a “public use” or military airport. Whether you need a permit depends on the length of the runway. In such instances, the FAA will determine the height of the tower you can install. They may also require top-of-tower warning lights. Note that permits are not required when siting a turbine near private landing strips with no public access, airfields not shown on FAA maps, or landing strips that are not in use. If you’re hiring a professional installer, he or she can advise you on this matter.
The second condition that requires an FAA review and permit is if the wind generator is approximately two to four miles from a “public use” or military airport. Whether you need a permit depends on the length of the runway. In such instances, the FAA will determine the height of the tower you can install. They may also require top-of-tower warning lights. Note that permits are not required when siting a turbine near private landing strips with no public access, airfields not shown on FAA maps, or landing strips that are not in use. If you’re hiring a professional installer, he or she can advise you on this matter.
Tower Height Considerations
Once you’ve identified the best site, you must determine optimum tower height. To produce as much electrical energy as possible at a site, the rule of thumb is that the entire rotor should be at least 30 feet above the tallest obstacle within a radius of 500 feet.
When calculating minimum tower height, don’t forget to take tree growth into account. If the trees on your property will grow 20 feet in the next 20 to 30 years, the life expectancy of a wind system, add that to the tower height for the best long-term performance. (To learn two ways to estimate the height of trees and buildings, see the accompanying box.)
If your site is within a quarter of a mile from a forest or goodsized wooded lot, the top of the nearby tree line is the height you want to exceed. Mount the wind turbine using the tree line as the height you must exceed. Don’t forget to factor in tree growth.
If you are installing a wind turbine in an area with more than 50 percent deciduous tree cover, the effective ground level is two thirds of the tree height. If trees are 60-feet high, for instance, the
effective ground level is 40 feet. A 100-foot equivalent tower would, therefore, need to be 140 feet high to take into account the trees.
Bear in mind that the height recommendation is the minimum acceptable tower height. Savvy wind energy installers exceed the rule and see increased performance because of it. It usually costs very little to increase tower height by another 20 to 40 feet and the return on this small investment is quite impressive. We don’t know anyone who has installed a wind turbine who says, “I wish I’d bought a shorter tower.” However, we know lots of people who wish they had purchased a taller one.
Tall Tower Economics: Overcoming the Small-Turbines-on-Short-Tower Myth
When you talk to professional wind system installers, you may hear statements to the effect that it doesn’t make sense to mount a smaller turbine, for example, one with a seven- or eight-foot diameter rotor, on a tall tower. This is flawed reasoning. Tower height should be determined by the height of obstructions in the area, not the size of the wind turbine or the towers a manufacturer or dealer sells. A 50-foot tower slightly downwind from a 65-foot-high tree
line isn’t going to produce much electricity. Moreover, the turbine will produce even less electricity as the trees grow over the 20- to 30-year life of the wind system. Remember: energy output and the economics of the wind system are both proportional to V3 (the cube of the wind speed).
Although it is sometimes hard to justify a tall tower for a small turbine, that doesn’t mean that the right decision is a short tower. The right decision is to invest enough in your tower to make the most of your turbine’s potential — or choose another renewable energy system.
If you are thinking about installing a smaller wind generator, but are nervous about the cost of a taller tower, we recommend that you calculate how much more the tower will cost and how much more electricity the turbine will produce on a taller tower. In our experience, installing a taller tower always results in the production of substantially more electricity. Even though it will always cost more money, the important question to ask is whether the increased tower height is justified economically by the increase in electrical production. In most cases, it is.
When calculating minimum tower height, don’t forget to take tree growth into account. If the trees on your property will grow 20 feet in the next 20 to 30 years, the life expectancy of a wind system, add that to the tower height for the best long-term performance. (To learn two ways to estimate the height of trees and buildings, see the accompanying box.)
How High?
Determining the height of a tower seems pretty straightforward until you have to do it. The first challenge you’ll
face is determining the actual height of nearby objects, such as trees or barns. How do wind site assessors determine
the height of ground clutter?
One way, shown in Figure 6.15, is to place a stake (a metal fence post, for instance) next to the object you want to measure. On a sunny day, measure the height of the stake and then measure its shadow. Then measure the shadow of the object under question.
The height of the object can be determined by ratios using the equation: _H__1 H2 = S__L_1 SL2. H1 is the unknown height and H2 is the height of the fence post. SL1 is the length of the shadow of the object you are trying to measure. SL2 is the length of the shadow of the fence post. To solve for H1, you just need to rearrange the equation: H1 = (_S__L_1_)_(_H_2_) SL2. Note that the ground around both the tree and the fence post must be level for this method to be accurate.
Consider an example. Let’s assume that the fence post is four feet high and the shadow it casts is two feet long.
The shadow cast by the tree or building is 14 feet. How high is the tree? As illustrated, in Figure 6.15a, you begin
by setting up ratios: __x 4 = _1_4_ 2 , then solve for x: x = (4 x _1_4_ 2 ,) = 28 feet. Another simple method is explained in Figure 6.15b.
If your site is within a quarter of a mile from a forest or goodsized wooded lot, the top of the nearby tree line is the height you want to exceed. Mount the wind turbine using the tree line as the height you must exceed. Don’t forget to factor in tree growth.
If you are installing a wind turbine in an area with more than 50 percent deciduous tree cover, the effective ground level is two thirds of the tree height. If trees are 60-feet high, for instance, the
Fig. 6.15a and 6.15b: Measuring Height. (a) Driving a fence post or some other object of known length into the ground next to an object of unknown height and comparing the length of the shadows allows one to calculate the height of an object. (b) Another method for determining height is shown here. In this method, you’ll be solving for C, the height of the tree. Measure the distance from the tree (D). Measure the distance from your eye to the ruler in your hand. This is B. Measure the height of the object in inches on the ruler. This is A. Then set up a ratio equivalence as follows: __AB= __C___ B + D . The rest of the math is shown in the figure.
effective ground level is 40 feet. A 100-foot equivalent tower would, therefore, need to be 140 feet high to take into account the trees.
Bear in mind that the height recommendation is the minimum acceptable tower height. Savvy wind energy installers exceed the rule and see increased performance because of it. It usually costs very little to increase tower height by another 20 to 40 feet and the return on this small investment is quite impressive. We don’t know anyone who has installed a wind turbine who says, “I wish I’d bought a shorter tower.” However, we know lots of people who wish they had purchased a taller one.
Tall Tower Economics: Overcoming the Small-Turbines-on-Short-Tower Myth
When you talk to professional wind system installers, you may hear statements to the effect that it doesn’t make sense to mount a smaller turbine, for example, one with a seven- or eight-foot diameter rotor, on a tall tower. This is flawed reasoning. Tower height should be determined by the height of obstructions in the area, not the size of the wind turbine or the towers a manufacturer or dealer sells. A 50-foot tower slightly downwind from a 65-foot-high tree
line isn’t going to produce much electricity. Moreover, the turbine will produce even less electricity as the trees grow over the 20- to 30-year life of the wind system. Remember: energy output and the economics of the wind system are both proportional to V3 (the cube of the wind speed).
Although it is sometimes hard to justify a tall tower for a small turbine, that doesn’t mean that the right decision is a short tower. The right decision is to invest enough in your tower to make the most of your turbine’s potential — or choose another renewable energy system.
If you are thinking about installing a smaller wind generator, but are nervous about the cost of a taller tower, we recommend that you calculate how much more the tower will cost and how much more electricity the turbine will produce on a taller tower. In our experience, installing a taller tower always results in the production of substantially more electricity. Even though it will always cost more money, the important question to ask is whether the increased tower height is justified economically by the increase in electrical production. In most cases, it is.
Proper Siting of a Wind Machine
A wind turbine must be mounted in a good wind site, well above ground clutter in the strongest, smoothest winds. Wind site assessors begin the process of siting a wind turbine by determining the prevailing wind direction at a site. Although winds blow in different directions at different times of the year, or even within the same day, they arrive from one or two directions predominantly over the course of the year. In many places in North America, winds come predominantly from the southwest — thanks to the Coriolis effect. They often blow from the northwest in the winter.
To determine the predominant wind flow, ask the advice of farmers, who work outdoors and hence are familiar with wind patterns, or contact a local airport. They may be able to provide you with a wind rose, a graphical representation of wind direction (Figure 6.14). In a wind rose, the length of the spokes around the circle is an indication of how frequently the wind blows from a particular direction. The longer the line, the greater the frequency. In the wind rose in Figure 6.14, the winds blow predominantly from the southwest. A wind rose also indicates the percentage of total wind energy from each direction, which is very helpful.
In an open site, with little ground clutter, a wind turbine can be located almost anywhere — so long as the entire rotor is mounted 30 feet above the tallest obstacle within a 500-foot radius and you’ve taken into account future tree growth, if trees are the tallest objects.
Fig. 6.14: Wind Rose. This unique graph shows how often winds blow from various directions and the percent energy of the wind for various directions. The wider white bars represent the percent of total energy from different directions and the narrower, shaded bars illustrate the percent of total time from each of the sixteen different direction sectors.
Unfortunately, very few of us live on ideal sites. There’s almost always some major obstacles.
To site a wind turbine, first determine the prevailing wind direction, then look for a location for the tower that’s upwind of major obstacles. Although winds will shift so that upwind temporarily becomes downwind, situating your wind turbine and tower this way will ensure that it can take advantage of the strongest prevailing winds.
When siting a wind machine, it is also a good idea to minimize wire runs from the turbine to the controller and inverter to reduce line loss. As a rule, the higher the wind turbine’s voltage, the farther it can be sited from the point of use. When installing a turbine, contact the manufacturer or an experienced installer for recommendations.
To determine the predominant wind flow, ask the advice of farmers, who work outdoors and hence are familiar with wind patterns, or contact a local airport. They may be able to provide you with a wind rose, a graphical representation of wind direction (Figure 6.14). In a wind rose, the length of the spokes around the circle is an indication of how frequently the wind blows from a particular direction. The longer the line, the greater the frequency. In the wind rose in Figure 6.14, the winds blow predominantly from the southwest. A wind rose also indicates the percentage of total wind energy from each direction, which is very helpful.
In an open site, with little ground clutter, a wind turbine can be located almost anywhere — so long as the entire rotor is mounted 30 feet above the tallest obstacle within a 500-foot radius and you’ve taken into account future tree growth, if trees are the tallest objects.
Fig. 6.14: Wind Rose. This unique graph shows how often winds blow from various directions and the percent energy of the wind for various directions. The wider white bars represent the percent of total energy from different directions and the narrower, shaded bars illustrate the percent of total time from each of the sixteen different direction sectors.
Unfortunately, very few of us live on ideal sites. There’s almost always some major obstacles.
To site a wind turbine, first determine the prevailing wind direction, then look for a location for the tower that’s upwind of major obstacles. Although winds will shift so that upwind temporarily becomes downwind, situating your wind turbine and tower this way will ensure that it can take advantage of the strongest prevailing winds.
When siting a wind machine, it is also a good idea to minimize wire runs from the turbine to the controller and inverter to reduce line loss. As a rule, the higher the wind turbine’s voltage, the farther it can be sited from the point of use. When installing a turbine, contact the manufacturer or an experienced installer for recommendations.
Tower Anchors and Bases
When installing a tower, you’ll need to install a strong base. If the tower is guyed, you’ll also need to install anchors for the guy cables. All major manufacturers provide well-engineered plans for suitable foundations. Follow the plans very carefully. Don’t cut corners to save time or money. Take a wind workshop or two before installing your wind turbine or hire a professional.
Tower Base
Virtually all towers are mounted on concrete pads reinforced with rebar (Figure 6.13). The depth required for a foundation, known as the critical depth, is the depth that prevents a foundation (and anchors) from being pried out of the ground by the force of the wind or uprooted by freeze-thaw cycles. The critical depth of a foundation depends on many factors, such as the type and height of the tower, wind speed, depth of the frost line and soil characteristics.
Fig. 6.13: Concrete Base. Most wind turbine towers rest on a solid concrete base. This photo shows the base for each leg of a freestanding lattice tower
Freestanding towers require deeper and more robust foundations than guyed towers. The taller the tower, the stronger the foundation. The stronger the winds, the deeper and more robust the foundation. The deeper the frost line, the deeper the foundation. Some soils hold tower foundations in place better than others. For instance, clay-rich and heavier soils have more holding power than sandy soils. For advice on critical tower depth, contact the turbine and tower manufacturer.
Engineered plans are typically provided in a turbine’s installation manual. Even so, be sure to consult local building codes, soil engineers or local excavators to be sure your tower foundation is sufficiently strong.
For maximum strength, concrete bases should cure at least 28 days prior to installation of the tower. Do not place a tower on a concrete pad before that! Inadequately cured concrete weakens the strength of the concrete. Your tower and wind machine rely on a sturdy, wellcured foundation. Remember, too, that cold weather slows the curing process. More time may be required in such conditions.
Anchors
In addition to a concrete base, fixed guyed towers and guyed tilt-up towers also require anchors to attach the guy cables. Anchor options for wind generator towers are many and varied. For most wind turbines, concrete anchors are the best choice. They are created by digging a deep hole, installing rebar according to the engineered drawing, and then pouring concrete into the hole. A screw-in auger, anchor, or angle steel is then embedded into the hole and angled
towards the tower as per manufacturer instructions. Once the concrete has cured, the hole is filled in. Be sure to pour anchors well below the frost line.
Tower Base
Virtually all towers are mounted on concrete pads reinforced with rebar (Figure 6.13). The depth required for a foundation, known as the critical depth, is the depth that prevents a foundation (and anchors) from being pried out of the ground by the force of the wind or uprooted by freeze-thaw cycles. The critical depth of a foundation depends on many factors, such as the type and height of the tower, wind speed, depth of the frost line and soil characteristics.
Fig. 6.13: Concrete Base. Most wind turbine towers rest on a solid concrete base. This photo shows the base for each leg of a freestanding lattice tower
Freestanding towers require deeper and more robust foundations than guyed towers. The taller the tower, the stronger the foundation. The stronger the winds, the deeper and more robust the foundation. The deeper the frost line, the deeper the foundation. Some soils hold tower foundations in place better than others. For instance, clay-rich and heavier soils have more holding power than sandy soils. For advice on critical tower depth, contact the turbine and tower manufacturer.
Engineered plans are typically provided in a turbine’s installation manual. Even so, be sure to consult local building codes, soil engineers or local excavators to be sure your tower foundation is sufficiently strong.
For maximum strength, concrete bases should cure at least 28 days prior to installation of the tower. Do not place a tower on a concrete pad before that! Inadequately cured concrete weakens the strength of the concrete. Your tower and wind machine rely on a sturdy, wellcured foundation. Remember, too, that cold weather slows the curing process. More time may be required in such conditions.
Anchors
In addition to a concrete base, fixed guyed towers and guyed tilt-up towers also require anchors to attach the guy cables. Anchor options for wind generator towers are many and varied. For most wind turbines, concrete anchors are the best choice. They are created by digging a deep hole, installing rebar according to the engineered drawing, and then pouring concrete into the hole. A screw-in auger, anchor, or angle steel is then embedded into the hole and angled
towards the tower as per manufacturer instructions. Once the concrete has cured, the hole is filled in. Be sure to pour anchors well below the frost line.
Tuesday, May 29, 2012
Tower Kits
Several turbine manufacturers sell tilt-up tower kits designed and engineered for their wind machines. Because steel pipe is heavy and expensive to ship long distances, most kits include all of the materials you need except the pipe. You purchase pipe locally.
Wind turbines often require adaptors, known as stub towers, to fit onto commercially available towers. Stub towers consist of a short piece of pipe, with a flange that bolts onto the base of the turbine. Be sure to obtain an adaptor for your turbine. For best results, buy a hot-dipped galvanized adaptor. They last much longer than ungalvanized steel sub-towers.
When buying a tower, be sure you purchase a model that’s approved by the wind turbine manufacturer. When shopping for a wind generator tower kit, also be aware that some kits may not include anchors, because the type of anchor needed for a tower varies from one location to the next, depending on the soil type. In most cases, anchor foundations are constructed on site using concrete and rebar.
When mounting a wind turbine, be sure to avoid roof-top towers. Such installations are an unequivocally bad idea for several reasons. First, roof-mounted turbines in most locations are too close to the ground, where average annual wind speeds are much lower. Lower average annual wind speeds mean much lower output. A 2007 study of roof-mounted wind turbines by Encraft Ltd, a British consulting firm, showed that wind turbines mounted on homes and apartment buildings produced much less electricity than predicted by computer models. On residential structures, annual wind speeds were only about 5 to 7 miles per hour, well below the models’ predicted wind speeds of 10 to 12 miles per hour — and well below the speed at which the wind turbines produce electricity. (The models obviously didn’t account for ground drag and turbulence).
Meters were installed to monitor the output of the micro-turbines and small turbines in this study. The researchers compared energy generated by the urban turbines to the manufacturers’ performance predictions. The researchers found that on average, wind turbines exported less than 0.5 kilowatt-hours a day — that’s 5 cents worth of electricity per day. Some of the turbines generated less power than the inverters consumed and were negative energy producers at their low wind speed sites. That is, these systems consumed more electricity than they generated. Overall, the turbines produced only 50 to 60 kWh of electricity per year — that’s $5 to $6 per year!
Rooftop installations are not a good idea because they’re also exposed to turbulence created by ground clutter, trees and buildings. As noted in previous chapters, turbulence reduces the quality and quantity of wind. This reduces energy production and increases wear and tear on the turbine, resulting in shorter turbine life.
Buildings are also rarely designed and engineered to support the load and handle the vibrations produced by a wind turbine. These vibrations can cause structural damage to buildings and vibrations are conducted into the building, which can be annoying to occupants.
Wind turbines often require adaptors, known as stub towers, to fit onto commercially available towers. Stub towers consist of a short piece of pipe, with a flange that bolts onto the base of the turbine. Be sure to obtain an adaptor for your turbine. For best results, buy a hot-dipped galvanized adaptor. They last much longer than ungalvanized steel sub-towers.
When buying a tower, be sure you purchase a model that’s approved by the wind turbine manufacturer. When shopping for a wind generator tower kit, also be aware that some kits may not include anchors, because the type of anchor needed for a tower varies from one location to the next, depending on the soil type. In most cases, anchor foundations are constructed on site using concrete and rebar.
When mounting a wind turbine, be sure to avoid roof-top towers. Such installations are an unequivocally bad idea for several reasons. First, roof-mounted turbines in most locations are too close to the ground, where average annual wind speeds are much lower. Lower average annual wind speeds mean much lower output. A 2007 study of roof-mounted wind turbines by Encraft Ltd, a British consulting firm, showed that wind turbines mounted on homes and apartment buildings produced much less electricity than predicted by computer models. On residential structures, annual wind speeds were only about 5 to 7 miles per hour, well below the models’ predicted wind speeds of 10 to 12 miles per hour — and well below the speed at which the wind turbines produce electricity. (The models obviously didn’t account for ground drag and turbulence).
Meters were installed to monitor the output of the micro-turbines and small turbines in this study. The researchers compared energy generated by the urban turbines to the manufacturers’ performance predictions. The researchers found that on average, wind turbines exported less than 0.5 kilowatt-hours a day — that’s 5 cents worth of electricity per day. Some of the turbines generated less power than the inverters consumed and were negative energy producers at their low wind speed sites. That is, these systems consumed more electricity than they generated. Overall, the turbines produced only 50 to 60 kWh of electricity per year — that’s $5 to $6 per year!
Rooftop installations are not a good idea because they’re also exposed to turbulence created by ground clutter, trees and buildings. As noted in previous chapters, turbulence reduces the quality and quantity of wind. This reduces energy production and increases wear and tear on the turbine, resulting in shorter turbine life.
Buildings are also rarely designed and engineered to support the load and handle the vibrations produced by a wind turbine. These vibrations can cause structural damage to buildings and vibrations are conducted into the building, which can be annoying to occupants.
Tower Options
Towers for small wind machines come in three basic varieties: (1) freestanding, (2) fixed guyed, and (3) tilt-up towers (Figure 6.1). Each type has some variations, listed in Table 6.1.
Fig. 6.1: Wind Tower Options. (a) Freestanding, (b) fixed guyed, and (c) tilt-up. Freestanding towers can be both lattice (shown here) and monopoles. Fixed guyed towers are typically lattice towers. Tilt-up towers can be either lattice or tubular.
Freestanding Towers
Freestanding wind generator towers are self-supporting structures. They stand on their own, like flag poles or street lights or the Eiffel Tower. Freestanding towers are made of steel and are firmly anchored to the ground via well-reinforced concrete foundations. The combination of heavy-duty steel tower construction and a secure anchorage ensures that the tower can withstand powerful winds that could pry the foundation loose and topple the tower and your expensive turbine. They also ensure that the tower can support the turbine.
The most common type of freestanding tower is a lattice or truss tower, like those shown in Figure 6.2. The Eiffel Tower in Paris is a good example of a lattice structure.
Is Wind Right for You?
Periodic inspection and maintenance and occasional repair of wind turbines are essential to the long-term success of a wind energy system. The towers on which they stand present a formidable barrier to these activities. Many wind system owners fail to perform these tasks because they don’t want to lower or climb their towers once a year. If you are a “put it up and forget about it” kind of person and can’t afford to hire someone to perform an annual inspection and maintenance, we recommend that you consider installing a PV system instead. PV systems are as close to maintenance-free technology as you can get (provided there are no batteries in the system). If you install a wind system, you will either need to climb the tower or lower it to the ground once or, preferably, twice a year to inspect the turbine, wires, connections, and perform maintenance, as required.
Fig. 6.2a and 6.2b: Lattice Tower. Freestanding lattice towers are made of (a) heavy-duty angle iron, as in this tower erected at the Midwest Renewable Energy Association’s headquarters, or (b) tubular steel. Horizontal and vertical bracing made of steel angle iron that runs between the tubular steel legs.
Lattice towers are made from tubular steel or angle iron with horizontal and diagonal cross bracing bolted to the vertical steel legs. Ladders or step bolts are incorporated so the towers can be climbed for inspection, maintenance and repair. Freestanding lattice towers are sometimes fitted with a small platform near the top, which provides a secure place to work.
Another, more expensive option for freestanding towers is the monopole (Figure 6.3). They consist of a single, sturdy pole made from round tubular steel. Rungs or foot pegs are attached for climbing.
Freestanding towers are secured to massive steel-reinforced concrete foundations, as shown in Figure 6.4. The taller the tower, the heftier (and more expensive) the foundation.
Fig. 6.3: Monopole Tower. Monopole towers are sturdy, well-anchored by a solid foundation, but extremely costly for reasons explained in the text.
Fig. 6.4: Concrete Piers and Base of Tower. This massive, deep foundation supports a 120-foot freestanding lattice tower. Each leg of the tower will attach to a steel leg embedded in each of the vertical piers. The piers and base of the foundation are made of concrete reinforced with rebar.
Assembling and Installing Freestanding Towers
Freestanding lattice towers are typically assembled on the ground in sections, 20 feet at a time. The legs and bracing are bolted together on the ground.
After a lattice tower is assembled, the turbine is often attached. The tower and turbine are lifted with a crane. The tower is bolted to steel anchors embedded in the concrete foundation. To facilitate tower construction, some lattice towers are hinged at the base. That way, the tower can be assembled on the ground, and then tilted up into position with a crane (Figure 6.5). In some instances, the tower is erected and raised without a turbine. The turbine is then hoisted onto the top of the tower. A reasonably level area is needed to assemble a freestanding tower and to lift it with the crane.
Although freestanding lattice towers are typically assembled on the ground and lifted with a crane, it is possible to construct towers vertically one section at a time using a vertical gin pole. This technique is time-consuming and requires extreme caution and is only used in crane-inaccessible sites.
Fig. 6.5: Crane Lifting Tower and Turbine. This 80-ton crane lifts a massive turbine and tower into place. The hinges at the base of the tower allow the crane to tilt the tower into position.
Like lattice towers, monopole towers come in sections. They are fitted together on the ground. When completed, the tower is hoisted into place with a crane and the tower is secured to the foundation.
Freestanding monopole towers are typically the most expensive of all options, because they require the most steel and the most robust foundations (Table 6.2).
Pros and Cons of Freestanding Towers
Freestanding towers offer advantages over other types. One of the most important is that they require much less space (Figure 6.6). Their smaller footprint makes a freestanding wind generator tower ideal for locations with extensive tree cover.
Freestanding towers are more aesthetically appealing to many people than guyed towers. A freestanding tower is also one of the safest towers to install. Almost all the work can be done on the ground, and a single crane lift can erect the tower, turbine, wiring, etc.
Embodied energy is the energy that it takes to make a product — from the extraction of the raw materials to the completion of the finished product, including shipping to retail outlets where it is sold. Because they require so much concrete and steel and because these materials require huge amounts of energy to produce, freestanding towers have a much higher embodied energy than other options. If your primary motivation is to decrease your environmental footprint by using renewable energy, a freestanding tower is not your best choice.
Fig. 6.6: Tower footprints: (a) Tilt-up, (b) Fixed Guyed, and (c) Freestanding.
Freestanding towers also require periodic ascent to perform routine inspection, maintenance, and repair, which can be a plus or minus, depending on your point of view. To prevent catastrophic falls, a safety harness or safety work belt must be worn while climbing and working on a tower (Figure 6.7). Safety harnesses are equipped with several D-rings (three-D-ring models should be used for tower work). The D rings are used to secure you to the tower via lanyards to prevent falls when working on a tower. A “positioning” or “restraint” lanyard holds a worker in place to allow him or her to work hands-free. A “shock absorbing” lanyard is used to
arrest a fall, that is, gradually slow a worker who has fallen to prevent a harmful jerk.
Towers should be equipped with a safety cable that runs the length of the tower along the climbing rungs or ladder (Figure 6.8). Workers attach their safety harness to the cable when climbing by an anti-fall device, such as a Lad-Saf. This sliding “climbing car” follows you as you ascend but locks onto the cable to arrest a fall if you lose your footing and fall.
Once you are atop the tower, belt in with lanyards and disconnect from the anti-fall cable. You must always be “attached” to the
Fig. 6.7: Safety Harness. Mick demonstrates proper use of a safety harness in one of his workshops.
Fig. 6.8a and 6.8b: Safety Cable and Lad-Saf. (a) Worker prepares to climb tower. Note safety harness and Lad-Saf attached to safety cable. This prevents the worker from falling. (b) Close up of connection to Lad-Saf and safety cable.
tower. When climbing a tower without a safety cable, “Always climb using two lanyards in an alternating pattern so that one of them is clipped onto the tower at all times,” advises small wind expert Jim Green.
If you are not willing or able to climb a tower, you must be willing to hire someone to do it. If not, consider installing a tilt-up tower or a PV system.
Fixed Guyed Towers
The second type of tower is the fixed guyed tower (Figure 6.1b). Most are lattice towers. The legs of fixed guyed lattice towers are made of steel tube or pipe, or sometimes solid steel rods. The three legs of the lattice tower are usually 18 inches apart and are secured by horizontal and diagonal steel cross braces (Figure 6.9b).
Guyed towers are bolted to a concrete foundation and are supported by guy cables. Guy cables consist of high-strength stranded-steel cable or aircraft cable. They extend from attachments on the tower to steel-reinforced concrete anchors embedded in the
Fig. 6.9a and 6.9b: Fixed Guyed Lattice Tower. (a) This lattice tower is anchored by guy cables and is one of the most popular and least expensive tower options. (b) Close-up showing details.
ground. Guy cables are strung out in three directions 120 degrees apart. The guy radius, that is, the distance from the base of the tower to the anchors, ranges from 50 to 80 percent of the tower height, depending on the construction of the tower. Usually it is about 75 percent. For a 100-foot tower, then, the anchors would be 120 degrees apart and 50 to 80 feet from the base.
Fixed guyed towers are also made from pipe or tubular steel that comes in 20-foot sections. Like guyed lattice towers, tubular towers are supported by guy cables.
Assembling and Installing Fixed Guyed Towers
Guyed towers are usually assembled on the ground. Lattice towers are bolted together, one section at a time. After the tower is assembled, the wind turbine and electrical wire are attached. The tower and turbine are then erected by a crane. If the tower is 80 feet or taller, it may be necessary to lift a lattice tower or tower made from steel tubing in sections. The wind turbine is lifted onto the tower after the last section is in place.
Fixed guyed towers can also be assembled vertically, one section at a time, using a vertical gin pole — an inexpensive, temporary vertical “crane” that’s bolted onto the tower. Installers use it to raise one section of a tower at a time. After a section is in place, the gin pole is moved up so the next section can be installed, and so on. Vertical gin pole assembly is time-consuming and tedious, and it can be a bit dangerous. Those who’ve tried it do not recommend it. If no crane
is available or the crane cannot access the site, however, a vertical gin pole may be your only option.
Fixed guyed towers rest on concrete pads, though the towers are generally not bolted to them. Guy cables are attached to the tower during assembly. After the tower is upright and plumbed,
workers tension the cables. If the turbine was not previously attached, it is then lifted by the crane and fastened to the top.
Pros and Cons of Fixed Guyed Towers
Fixed guyed towers cost much less than freestanding towers because they require much less steel and their foundations require a lot less concrete. Lattice towers used by installers are also mass produced for the telecommunications industry, making them less expensive and widely available. Fixed guyed towers require more space than freestanding towers, but less than tilt-up towers, discussed next.
Fixed guyed towers must also be climbed for routine maintenance and repair, like freestanding towers. Some people consider the guy cables to be an eyesore, although guy wires disappear into
the background from most vantage points, except up close. Guy wires may also present a hazard to birds, although we’ve never heard of a bird killed by them.
Tilt-Up Towers
The third type of tower option is a guyed tilt-up tower. Unlike freestanding and fixed guyed towers, a guyed tilt-up tower can be raised and lowered for inspection, maintenance and repair. Guyed tilt-up towers may be made from steel pipe or lattice sections.
Guyed tilt-up towers require four sets of guy cables at each level. Cables are located 90 degrees apart. The fourth cable is required for stability when raising or lowering a tower. That is, it allows workers to safely raise and lower the tower. Without them, the tower would topple during these operations.
As illustrated in Figure 6.10, a tilt-up tower is raised and lowered with the aid of a gin pole. Unlike the vertical gin pole discussed earlier, this pole is permanently attached to the base of the tower at a 90° angle to the mast. It is a lever arm that allows the tower to be tilted up and down.
Tilting a tower also requires a hinge between the mast and the concrete base (Figure 6.11). When the tower is down — that is, lying on the ground and ready to be raised — the gin pole sticks straight up. When the tower is vertical, the gin pole lies near and parallel to the ground. As illustrated in Figure 6.10, a steel cable connects the free end of the gin pole to a lifting device such as a tractor.
Fig. 6.10: Guyed Tilt-Up Tower. Guyed tilt-up towers are raised and lowered using a truck, tractor, electric winch or grip hoist.
Guy cables hold a tilt-up tower upright and resist the force of the wind. The guy radius is 35 to 80 percent of tower height, depending on the type of tower. For a 100-foot tilt-up tower, the anchors should be located 35 to 80 feet from the base of the tower.
Assembling and Raising a Tilt-Up Guyed Tower
Steel pipe or tubing and lattice towers are both used for guyed tiltup towers. They come in 20-foot lengths. The individual lengths
Fig. 6.11: Hinged Base of a Guyed Tilt-Up Tower. The hinge at the base of this tilt-up tower allows it to be tilted up and down to maintain and service the wind turbine.
Fig. 6.12: Gin Pole. The gin pole is attached to the anchor. Notice the electric winch to the left of the attachment.
of pipe are secured by bolts or joined by slip-fit couplings on the ground. While the tower is on the ground, guy cables are attached to the tower and the concrete anchors.
Once assembled, the tower is tilted into position (Figure 6.10). This is accomplished with the assistance of a tractor, a pickup truck, a heavy-duty electric winch or a manually operated device known as a grip hoist.
When installing a tall tower for the first time, some installers raise one or two sections of the tower at a time. After each section is raised, the tower is plumbed and the guy cables are tensioned. This tower is then lowered and an additional piece is added. It is then raised, plumbed, and cables tensioned. This continues until the entire tower is assembled, plumbed and properly tensioned.
Experienced installers also recommend lifting (and plumbing) the entire tower before attaching the wind turbine to be sure that everything is correct. Also, make sure to train workers so they all know what they are doing by the time the turbine and tower are lifted.
Pros and Cons of a Guyed Tilt-Up Tower
The main benefit of tilt-up towers is that they never have to be climbed. They can be raised and lowered fairly quickly and all inspections and work can be performed on terra firma.
Although they’re ideal for those who cringe at the idea of climbing a tall tower, tilt-up guyed towers have the largest footprint of all (Figure 6.6). You’ll need to ensure that there’s a clear path for the lifting vehicle and a lay-down zone as long as the tower.
Raising and lowering a tower requires a few helpers to ensure that everything runs smoothly — for example, that the cables don’t get tangled. And, of course, you’ll need a truck, tractor or some
other lifting device. Be careful when using a tow vehicle because they can slip. Accidents can also occur if the anchors are not correctly positioned or the guy cables get too tight while lowering or
raising the tower. A strong wind could come along and blow the tower over when it is being raised or lowered, ruining the turbine.
Fig. 6.1: Wind Tower Options. (a) Freestanding, (b) fixed guyed, and (c) tilt-up. Freestanding towers can be both lattice (shown here) and monopoles. Fixed guyed towers are typically lattice towers. Tilt-up towers can be either lattice or tubular.Freestanding Towers
Freestanding wind generator towers are self-supporting structures. They stand on their own, like flag poles or street lights or the Eiffel Tower. Freestanding towers are made of steel and are firmly anchored to the ground via well-reinforced concrete foundations. The combination of heavy-duty steel tower construction and a secure anchorage ensures that the tower can withstand powerful winds that could pry the foundation loose and topple the tower and your expensive turbine. They also ensure that the tower can support the turbine.
The most common type of freestanding tower is a lattice or truss tower, like those shown in Figure 6.2. The Eiffel Tower in Paris is a good example of a lattice structure.
Is Wind Right for You?
Periodic inspection and maintenance and occasional repair of wind turbines are essential to the long-term success of a wind energy system. The towers on which they stand present a formidable barrier to these activities. Many wind system owners fail to perform these tasks because they don’t want to lower or climb their towers once a year. If you are a “put it up and forget about it” kind of person and can’t afford to hire someone to perform an annual inspection and maintenance, we recommend that you consider installing a PV system instead. PV systems are as close to maintenance-free technology as you can get (provided there are no batteries in the system). If you install a wind system, you will either need to climb the tower or lower it to the ground once or, preferably, twice a year to inspect the turbine, wires, connections, and perform maintenance, as required.
Fig. 6.2a and 6.2b: Lattice Tower. Freestanding lattice towers are made of (a) heavy-duty angle iron, as in this tower erected at the Midwest Renewable Energy Association’s headquarters, or (b) tubular steel. Horizontal and vertical bracing made of steel angle iron that runs between the tubular steel legs.Lattice towers are made from tubular steel or angle iron with horizontal and diagonal cross bracing bolted to the vertical steel legs. Ladders or step bolts are incorporated so the towers can be climbed for inspection, maintenance and repair. Freestanding lattice towers are sometimes fitted with a small platform near the top, which provides a secure place to work.
Another, more expensive option for freestanding towers is the monopole (Figure 6.3). They consist of a single, sturdy pole made from round tubular steel. Rungs or foot pegs are attached for climbing.
Freestanding towers are secured to massive steel-reinforced concrete foundations, as shown in Figure 6.4. The taller the tower, the heftier (and more expensive) the foundation.
Fig. 6.3: Monopole Tower. Monopole towers are sturdy, well-anchored by a solid foundation, but extremely costly for reasons explained in the text. 
Fig. 6.4: Concrete Piers and Base of Tower. This massive, deep foundation supports a 120-foot freestanding lattice tower. Each leg of the tower will attach to a steel leg embedded in each of the vertical piers. The piers and base of the foundation are made of concrete reinforced with rebar.Assembling and Installing Freestanding Towers
Freestanding lattice towers are typically assembled on the ground in sections, 20 feet at a time. The legs and bracing are bolted together on the ground.
After a lattice tower is assembled, the turbine is often attached. The tower and turbine are lifted with a crane. The tower is bolted to steel anchors embedded in the concrete foundation. To facilitate tower construction, some lattice towers are hinged at the base. That way, the tower can be assembled on the ground, and then tilted up into position with a crane (Figure 6.5). In some instances, the tower is erected and raised without a turbine. The turbine is then hoisted onto the top of the tower. A reasonably level area is needed to assemble a freestanding tower and to lift it with the crane.
Although freestanding lattice towers are typically assembled on the ground and lifted with a crane, it is possible to construct towers vertically one section at a time using a vertical gin pole. This technique is time-consuming and requires extreme caution and is only used in crane-inaccessible sites.
Fig. 6.5: Crane Lifting Tower and Turbine. This 80-ton crane lifts a massive turbine and tower into place. The hinges at the base of the tower allow the crane to tilt the tower into position.
Like lattice towers, monopole towers come in sections. They are fitted together on the ground. When completed, the tower is hoisted into place with a crane and the tower is secured to the foundation.Freestanding monopole towers are typically the most expensive of all options, because they require the most steel and the most robust foundations (Table 6.2).
Pros and Cons of Freestanding Towers
Freestanding towers offer advantages over other types. One of the most important is that they require much less space (Figure 6.6). Their smaller footprint makes a freestanding wind generator tower ideal for locations with extensive tree cover.
Freestanding towers are more aesthetically appealing to many people than guyed towers. A freestanding tower is also one of the safest towers to install. Almost all the work can be done on the ground, and a single crane lift can erect the tower, turbine, wiring, etc.
Embodied energy is the energy that it takes to make a product — from the extraction of the raw materials to the completion of the finished product, including shipping to retail outlets where it is sold. Because they require so much concrete and steel and because these materials require huge amounts of energy to produce, freestanding towers have a much higher embodied energy than other options. If your primary motivation is to decrease your environmental footprint by using renewable energy, a freestanding tower is not your best choice.
Fig. 6.6: Tower footprints: (a) Tilt-up, (b) Fixed Guyed, and (c) Freestanding.
Freestanding towers also require periodic ascent to perform routine inspection, maintenance, and repair, which can be a plus or minus, depending on your point of view. To prevent catastrophic falls, a safety harness or safety work belt must be worn while climbing and working on a tower (Figure 6.7). Safety harnesses are equipped with several D-rings (three-D-ring models should be used for tower work). The D rings are used to secure you to the tower via lanyards to prevent falls when working on a tower. A “positioning” or “restraint” lanyard holds a worker in place to allow him or her to work hands-free. A “shock absorbing” lanyard is used to
arrest a fall, that is, gradually slow a worker who has fallen to prevent a harmful jerk.
Towers should be equipped with a safety cable that runs the length of the tower along the climbing rungs or ladder (Figure 6.8). Workers attach their safety harness to the cable when climbing by an anti-fall device, such as a Lad-Saf. This sliding “climbing car” follows you as you ascend but locks onto the cable to arrest a fall if you lose your footing and fall.
Once you are atop the tower, belt in with lanyards and disconnect from the anti-fall cable. You must always be “attached” to the
Fig. 6.7: Safety Harness. Mick demonstrates proper use of a safety harness in one of his workshops.
Fig. 6.8a and 6.8b: Safety Cable and Lad-Saf. (a) Worker prepares to climb tower. Note safety harness and Lad-Saf attached to safety cable. This prevents the worker from falling. (b) Close up of connection to Lad-Saf and safety cable.tower. When climbing a tower without a safety cable, “Always climb using two lanyards in an alternating pattern so that one of them is clipped onto the tower at all times,” advises small wind expert Jim Green.
If you are not willing or able to climb a tower, you must be willing to hire someone to do it. If not, consider installing a tilt-up tower or a PV system.
Fixed Guyed Towers
The second type of tower is the fixed guyed tower (Figure 6.1b). Most are lattice towers. The legs of fixed guyed lattice towers are made of steel tube or pipe, or sometimes solid steel rods. The three legs of the lattice tower are usually 18 inches apart and are secured by horizontal and diagonal steel cross braces (Figure 6.9b).
Guyed towers are bolted to a concrete foundation and are supported by guy cables. Guy cables consist of high-strength stranded-steel cable or aircraft cable. They extend from attachments on the tower to steel-reinforced concrete anchors embedded in the
Fig. 6.9a and 6.9b: Fixed Guyed Lattice Tower. (a) This lattice tower is anchored by guy cables and is one of the most popular and least expensive tower options. (b) Close-up showing details.ground. Guy cables are strung out in three directions 120 degrees apart. The guy radius, that is, the distance from the base of the tower to the anchors, ranges from 50 to 80 percent of the tower height, depending on the construction of the tower. Usually it is about 75 percent. For a 100-foot tower, then, the anchors would be 120 degrees apart and 50 to 80 feet from the base.
Fixed guyed towers are also made from pipe or tubular steel that comes in 20-foot sections. Like guyed lattice towers, tubular towers are supported by guy cables.
Assembling and Installing Fixed Guyed Towers
Guyed towers are usually assembled on the ground. Lattice towers are bolted together, one section at a time. After the tower is assembled, the wind turbine and electrical wire are attached. The tower and turbine are then erected by a crane. If the tower is 80 feet or taller, it may be necessary to lift a lattice tower or tower made from steel tubing in sections. The wind turbine is lifted onto the tower after the last section is in place.
Fixed guyed towers can also be assembled vertically, one section at a time, using a vertical gin pole — an inexpensive, temporary vertical “crane” that’s bolted onto the tower. Installers use it to raise one section of a tower at a time. After a section is in place, the gin pole is moved up so the next section can be installed, and so on. Vertical gin pole assembly is time-consuming and tedious, and it can be a bit dangerous. Those who’ve tried it do not recommend it. If no crane
is available or the crane cannot access the site, however, a vertical gin pole may be your only option.
Fixed guyed towers rest on concrete pads, though the towers are generally not bolted to them. Guy cables are attached to the tower during assembly. After the tower is upright and plumbed,
workers tension the cables. If the turbine was not previously attached, it is then lifted by the crane and fastened to the top.
Pros and Cons of Fixed Guyed Towers
Fixed guyed towers cost much less than freestanding towers because they require much less steel and their foundations require a lot less concrete. Lattice towers used by installers are also mass produced for the telecommunications industry, making them less expensive and widely available. Fixed guyed towers require more space than freestanding towers, but less than tilt-up towers, discussed next.
Fixed guyed towers must also be climbed for routine maintenance and repair, like freestanding towers. Some people consider the guy cables to be an eyesore, although guy wires disappear into
the background from most vantage points, except up close. Guy wires may also present a hazard to birds, although we’ve never heard of a bird killed by them.
Tilt-Up Towers
The third type of tower option is a guyed tilt-up tower. Unlike freestanding and fixed guyed towers, a guyed tilt-up tower can be raised and lowered for inspection, maintenance and repair. Guyed tilt-up towers may be made from steel pipe or lattice sections.
Guyed tilt-up towers require four sets of guy cables at each level. Cables are located 90 degrees apart. The fourth cable is required for stability when raising or lowering a tower. That is, it allows workers to safely raise and lower the tower. Without them, the tower would topple during these operations.
As illustrated in Figure 6.10, a tilt-up tower is raised and lowered with the aid of a gin pole. Unlike the vertical gin pole discussed earlier, this pole is permanently attached to the base of the tower at a 90° angle to the mast. It is a lever arm that allows the tower to be tilted up and down.
Tilting a tower also requires a hinge between the mast and the concrete base (Figure 6.11). When the tower is down — that is, lying on the ground and ready to be raised — the gin pole sticks straight up. When the tower is vertical, the gin pole lies near and parallel to the ground. As illustrated in Figure 6.10, a steel cable connects the free end of the gin pole to a lifting device such as a tractor.
Fig. 6.10: Guyed Tilt-Up Tower. Guyed tilt-up towers are raised and lowered using a truck, tractor, electric winch or grip hoist.Guy cables hold a tilt-up tower upright and resist the force of the wind. The guy radius is 35 to 80 percent of tower height, depending on the type of tower. For a 100-foot tilt-up tower, the anchors should be located 35 to 80 feet from the base of the tower.
Assembling and Raising a Tilt-Up Guyed Tower
Steel pipe or tubing and lattice towers are both used for guyed tiltup towers. They come in 20-foot lengths. The individual lengths
Fig. 6.11: Hinged Base of a Guyed Tilt-Up Tower. The hinge at the base of this tilt-up tower allows it to be tilted up and down to maintain and service the wind turbine.
Fig. 6.12: Gin Pole. The gin pole is attached to the anchor. Notice the electric winch to the left of the attachment.of pipe are secured by bolts or joined by slip-fit couplings on the ground. While the tower is on the ground, guy cables are attached to the tower and the concrete anchors.
Once assembled, the tower is tilted into position (Figure 6.10). This is accomplished with the assistance of a tractor, a pickup truck, a heavy-duty electric winch or a manually operated device known as a grip hoist.
When installing a tall tower for the first time, some installers raise one or two sections of the tower at a time. After each section is raised, the tower is plumbed and the guy cables are tensioned. This tower is then lowered and an additional piece is added. It is then raised, plumbed, and cables tensioned. This continues until the entire tower is assembled, plumbed and properly tensioned.
Experienced installers also recommend lifting (and plumbing) the entire tower before attaching the wind turbine to be sure that everything is correct. Also, make sure to train workers so they all know what they are doing by the time the turbine and tower are lifted.
Pros and Cons of a Guyed Tilt-Up Tower
The main benefit of tilt-up towers is that they never have to be climbed. They can be raised and lowered fairly quickly and all inspections and work can be performed on terra firma.
Although they’re ideal for those who cringe at the idea of climbing a tall tower, tilt-up guyed towers have the largest footprint of all (Figure 6.6). You’ll need to ensure that there’s a clear path for the lifting vehicle and a lay-down zone as long as the tower.
Raising and lowering a tower requires a few helpers to ensure that everything runs smoothly — for example, that the cables don’t get tangled. And, of course, you’ll need a truck, tractor or some
other lifting device. Be careful when using a tow vehicle because they can slip. Accidents can also occur if the anchors are not correctly positioned or the guy cables get too tight while lowering or
raising the tower. A strong wind could come along and blow the tower over when it is being raised or lowered, ruining the turbine.
Towers and Tower Installation
The tower on which a wind turbine “flies” plays a huge role in a successful wind system. It raises a wind turbine to a nonturbulent height at which it can harvest lots of energy from the wind. A tower also withstands nature’s fury.
Tower installation involves a significant investment in money, too. In larger systems, the cost of a tower may account for nearly a quarter of the price tag. For smaller wind turbines, an appropriately sized tower plus installation may cost two to three, sometimes five times, more than the turbine itself.
Tower installations also require a significant amount of time. Most students at wind energy workshops are surprised to learn that most of their time is spent not working on the turbine, but assembling and raising the tower.
In this chapter, we’ll explore three tower options, tower assembly and installation. We’ll discuss proper siting and the economic benefits of installing a turbine on a tall tower. We’ll end with a description of safety concerns and give you some advice on buying a tower.
Tower installation involves a significant investment in money, too. In larger systems, the cost of a tower may account for nearly a quarter of the price tag. For smaller wind turbines, an appropriately sized tower plus installation may cost two to three, sometimes five times, more than the turbine itself.
Tower installations also require a significant amount of time. Most students at wind energy workshops are surprised to learn that most of their time is spent not working on the turbine, but assembling and raising the tower.
In this chapter, we’ll explore three tower options, tower assembly and installation. We’ll discuss proper siting and the economic benefits of installing a turbine on a tall tower. We’ll end with a description of safety concerns and give you some advice on buying a tower.
Building Your Own Wind Machine
If don’t have the cash to buy a wind turbine or want the challenge of making your own turbine, you may want to consider building your own wind turbine (Figure 5.11). Building your own wind generator provides invaluable experience.
Directions for how to build a wind machine are available online. At sites like Hugh Piggott’s (scoraigwind.com) or Dan Bartmann’s
Fig. 5.11: This amazingly quiet wind turbine designed and built by the folks from otherpower.com is remarkably reliable and efficient. Other Power sells kits that require some assembly for independent and budget-constrained buyers.
(otherpower.com), you’ll find a listing of other valuable websites and resources.
Homebrew wind turbine master Hugh Piggott’s website sells step-by-step plans for building axial-flux alternator turbines.
There are even a couple of books on the subject: Windpower Workshop by Hugh Piggott and a more recent title, Homebrew Wind Power: A Hands-On Guide to Harnessing the Wind by Other Power’s Dan Bartmann and Dan Fink, which is an extremely well-written and thorough treatise on the subject. Rather than provide a full description of this subject, which would take a chapter or two, maybe even an entire book, we recommend you consult these resources. We also recommend that you sign up for a workshop, for example, through The Evergreen Institute or otherpower.com.
Building a wind generator is not that difficult, if you are mechanically inclined and persistent. You will need a shop and some common hand and power tools. “The hardest part of designing a small
windmill for electricity production is to find a suitable generator,” writes Piggott in Windpower Workshop. “For best performance you’ll need a reliable, low-speed generator that’s pretty efficient in light winds,” he adds. Piggott strongly recommends permanent magnet alternators. “Unfortunately, permanent magnetic alternators are difficult to find.” Piggott lists some sources, among them motorcycle alternators and welders. Unfortunately, each one has a significant downside. See his website and book for more on this and other options.
Car alternators are popular among do-it-yourselfers; however, they require high rpm and are not always very efficient. They also require a lot of electricity to power the electromagnets that create the magnetic field. You’ll also need to rewind the stator coils so they produce electricity at the lower speeds common in wind turbines. This is a big job for those without experience. Most people who’ve tried this route give up in failure.
When building a wind turbine, you’ll also have to design and build blades and install controls. Blades and rotors are particularly tricky. They need to be carefully matched to the generator. Most homemade wind turbines use wooden blades, which must be hand carved.
You will also need to incorporate overspeed controls and a shutdown mechanism. And don’t forget you will need an inverter matched to the turbine. Be wary of cheaply made wind turbines like those with blades fashioned from PVC pipe. Remember, too, that you need to install the turbine on a tower. Tower designs can also be found online. Be careful, however; homemade towers often leave much to be desired. Towers are far more complicated to design than wind
turbines. Your best and safest bet is to buy a new or used tower.
Directions for how to build a wind machine are available online. At sites like Hugh Piggott’s (scoraigwind.com) or Dan Bartmann’s
Fig. 5.11: This amazingly quiet wind turbine designed and built by the folks from otherpower.com is remarkably reliable and efficient. Other Power sells kits that require some assembly for independent and budget-constrained buyers.(otherpower.com), you’ll find a listing of other valuable websites and resources.
Homebrew wind turbine master Hugh Piggott’s website sells step-by-step plans for building axial-flux alternator turbines.
There are even a couple of books on the subject: Windpower Workshop by Hugh Piggott and a more recent title, Homebrew Wind Power: A Hands-On Guide to Harnessing the Wind by Other Power’s Dan Bartmann and Dan Fink, which is an extremely well-written and thorough treatise on the subject. Rather than provide a full description of this subject, which would take a chapter or two, maybe even an entire book, we recommend you consult these resources. We also recommend that you sign up for a workshop, for example, through The Evergreen Institute or otherpower.com.
Building a wind generator is not that difficult, if you are mechanically inclined and persistent. You will need a shop and some common hand and power tools. “The hardest part of designing a small
windmill for electricity production is to find a suitable generator,” writes Piggott in Windpower Workshop. “For best performance you’ll need a reliable, low-speed generator that’s pretty efficient in light winds,” he adds. Piggott strongly recommends permanent magnet alternators. “Unfortunately, permanent magnetic alternators are difficult to find.” Piggott lists some sources, among them motorcycle alternators and welders. Unfortunately, each one has a significant downside. See his website and book for more on this and other options.
Car alternators are popular among do-it-yourselfers; however, they require high rpm and are not always very efficient. They also require a lot of electricity to power the electromagnets that create the magnetic field. You’ll also need to rewind the stator coils so they produce electricity at the lower speeds common in wind turbines. This is a big job for those without experience. Most people who’ve tried this route give up in failure.
When building a wind turbine, you’ll also have to design and build blades and install controls. Blades and rotors are particularly tricky. They need to be carefully matched to the generator. Most homemade wind turbines use wooden blades, which must be hand carved.
You will also need to incorporate overspeed controls and a shutdown mechanism. And don’t forget you will need an inverter matched to the turbine. Be wary of cheaply made wind turbines like those with blades fashioned from PVC pipe. Remember, too, that you need to install the turbine on a tower. Tower designs can also be found online. Be careful, however; homemade towers often leave much to be desired. Towers are far more complicated to design than wind
turbines. Your best and safest bet is to buy a new or used tower.
Other Considerations
Although swept area, weight, annual energy output, governing mechanisms, shut-down mechanisms and sound levels are the most important factors to consider when buying a wind machine, there are other details that manufacturers provide. We think readers should be familiar with them, but not let them overly influence their judgment.
Cut-In Speed
One factor that is of little relevance is the cut-in speed — the wind speed at which a wind generator starts producing electricity. Most turbines don’t produce appreciable amounts of electricity until wind speeds reach 10 miles per hour. They produce full power at speeds from about 23 to 30 miles per hour.
Some manufacturers recommend installing a turbine with the lowest cut-in speed possible in areas of low wind to make better use of the wind resource. However, because low wind speeds hold very little energy, low speed cut-in isn’t important.
Power Curves
Another useless bit of information provided by wind turbine manufacturers is the power curve like the ones shown in Figure 5.8. Power curves are graphical representation of the power production of a wind turbine (in watts) at different wind speeds.
While power curves make for good visuals, they aren’t always accurate. They’re not always obtained under the most ideal test conditions. Moreover, they are a bit misleading to the uninitiated. That’s because most wind turbines operate at relatively low wind speeds — around 10 to 20 miles per hour — and very rarely operate at the peak of the power curve. As you can see, wind turbines produce much less electricity at lower wind speeds. When considering a wind turbine, what’s important is how much energy it will produce at your site on your tower at your average wind speed — that is, the AEO.
Rated Power
Last but not least is a measurement known as rated power, one we’ve avoided using as much as possible in this book. Rated power is the output in watts at rated wind speed. Much like the output of solar electric modules, rated power of wind turbines was devised to give buyers a way to compare products. Buyers, for instance, typically compare one kilowatt or ten kilowatt turbines as they would compare solar modules.
Rated power is of limited usefulness in large part because there are currently no standards in the wind industry (as there are in the solar industry) for determining rated power. Manufacturers determine rated power in different ways and at different wind speeds — the rated wind speed. This situation will change because the small wind industry is developing a standard technique of measuring performance.
To see why rated power is less than ideal, consider three wind turbines with identical rated power of 1,000 watts or 1 kilowatt: (1) Bergey’s XL.1, (2) Southwest Windpower’s Whisper 200, and (3) Eveready’s Kestrel 1000. At first blush, you might assume that all three wind turbines are identical. After all, they’re one kilowatt turbines. However, Southwest Windpower’s Whisper 200 achieves its 1,000-watt rated output at 26 miles per hour. Bergey’s XL.1 produces its 1,000-watt rated power at 24.6 miles per hour, and Eveready’s Kestrel cranks out 1,000 watts at 23.5 miles per hour.
Although it might seem that the wind turbine that produces 1,000 watts at the lowest wind speed would be the best buy, that’s not necessarily true. There’s much more to consider, including swept area, tower top weight and AEO. As noted learned earlier, in most cases the wind turbine with the largest swept area produces the most electricity, but don’t make that assumption without looking at the annual energy output. At an average wind speed of 8 miles
per hour, the Kestrel 1000 will produce about 900 kilowatt-hours of electricity annually. The Whisper 200 will produce about 720 kilowatt-hours annually and the Bergey will produce approximately 660. (These numbers are based on the manufacturers’ estimates.)
It is also important to note that rated power is one point on the power curve and is the output at a wind speed that is much higher than those typically encountered at most sites. Thus, the rated wind speed only represents a small fraction of the wind resource at a site. We’re interested in output in the typical range of wind speeds at our site, which is well below the rated power. Our advice on rated power: forget it.
Final Factors
When shopping for a wind turbine, be sure to check into the company’s customer service record — how well it supports its dealers and customers. Some companies like Bergey Windpower and
Abundant Renewable Energy have stellar service records. Be sure that the company you buy from offers technical advice should you have trouble and that their technical support staff speaks your native tongue.
Another factor to consider is how long the company has been in business. Doing business with a company that been in business for a while, say ten or more years, is a good idea. A related criterion to take into consideration is how long a wind turbine has been on the market. The longer a wind turbine has been on the market, the better. It has been tried and tested and improved upon.
It’s also important to check on the availability of parts. Parts shipped from foreign countries, such as China, Europe, South Africa or Mexico may take months to arrive, if they’re available at all. Meanwhile, your $30,000 wind energy system sits idle. When shopping, check to see if the importer of the wind turbine you are interested in stocks spare parts.
It’s a good idea to consider the number of moving parts and wear points in a turbine. Some designs, like the Jacobs turbines manufactured by Wind Turbine Industries in Minnesota, contain a lot of moving parts, which results in numerous wear points — up to 300. These machines may require a lot of tinkering at 100 feet.
Be sure to check out the warranty. Warranties typically run five years. The longer the better. In June 2009, Bergey Windpower boosted its warranty to 10 years; others have pledged to follow suit. Also be sure to determine what warranties cover. Is it materials and workmanship or parts only? Does it include shipping? Unfortunately, most warranties do not cover the cost of labor.
We recommend buying from a local dealer/installer, too. That way, you’ll have local support, even if you opt to do the installation yourself. Be sure to ask for references and interview them.
If you buy from an online wholesaler, be aware that most of these companies do not offer technical support, installation advice or assistance. Nor do they offer replacement parts or repair services. If you buy cheap, you are on your own when you need help.
We believe that it is a good idea to stick with name brand wind turbines and avoid newcomers and foreign imports, especially Chinese-made machines, at least at this time. Buying a used or
reconditioned wind turbine may be an economical option, but it’s not a very good idea. The problem with used wind turbines is that you don’t often know what you’re getting.
If you buy a remanufactured wind turbine, you want to be sure that the machine has been fully reconditioned, not just painted and with new blades installed. Make sure that the equipment supplied with the turbine, like the controller or inverter are code compliant. You may also want to obtain an extended warranty. Also be sure you can find a tower for the turbine before you purchase it. Suitable tall towers for the larger small-scale wind machines are hard to find.
Cut-In Speed
One factor that is of little relevance is the cut-in speed — the wind speed at which a wind generator starts producing electricity. Most turbines don’t produce appreciable amounts of electricity until wind speeds reach 10 miles per hour. They produce full power at speeds from about 23 to 30 miles per hour.
Some manufacturers recommend installing a turbine with the lowest cut-in speed possible in areas of low wind to make better use of the wind resource. However, because low wind speeds hold very little energy, low speed cut-in isn’t important.
Power Curves
Another useless bit of information provided by wind turbine manufacturers is the power curve like the ones shown in Figure 5.8. Power curves are graphical representation of the power production of a wind turbine (in watts) at different wind speeds.
While power curves make for good visuals, they aren’t always accurate. They’re not always obtained under the most ideal test conditions. Moreover, they are a bit misleading to the uninitiated. That’s because most wind turbines operate at relatively low wind speeds — around 10 to 20 miles per hour — and very rarely operate at the peak of the power curve. As you can see, wind turbines produce much less electricity at lower wind speeds. When considering a wind turbine, what’s important is how much energy it will produce at your site on your tower at your average wind speed — that is, the AEO.
Rated Power
Last but not least is a measurement known as rated power, one we’ve avoided using as much as possible in this book. Rated power is the output in watts at rated wind speed. Much like the output of solar electric modules, rated power of wind turbines was devised to give buyers a way to compare products. Buyers, for instance, typically compare one kilowatt or ten kilowatt turbines as they would compare solar modules.
Rated power is of limited usefulness in large part because there are currently no standards in the wind industry (as there are in the solar industry) for determining rated power. Manufacturers determine rated power in different ways and at different wind speeds — the rated wind speed. This situation will change because the small wind industry is developing a standard technique of measuring performance.
To see why rated power is less than ideal, consider three wind turbines with identical rated power of 1,000 watts or 1 kilowatt: (1) Bergey’s XL.1, (2) Southwest Windpower’s Whisper 200, and (3) Eveready’s Kestrel 1000. At first blush, you might assume that all three wind turbines are identical. After all, they’re one kilowatt turbines. However, Southwest Windpower’s Whisper 200 achieves its 1,000-watt rated output at 26 miles per hour. Bergey’s XL.1 produces its 1,000-watt rated power at 24.6 miles per hour, and Eveready’s Kestrel cranks out 1,000 watts at 23.5 miles per hour.
Although it might seem that the wind turbine that produces 1,000 watts at the lowest wind speed would be the best buy, that’s not necessarily true. There’s much more to consider, including swept area, tower top weight and AEO. As noted learned earlier, in most cases the wind turbine with the largest swept area produces the most electricity, but don’t make that assumption without looking at the annual energy output. At an average wind speed of 8 miles
per hour, the Kestrel 1000 will produce about 900 kilowatt-hours of electricity annually. The Whisper 200 will produce about 720 kilowatt-hours annually and the Bergey will produce approximately 660. (These numbers are based on the manufacturers’ estimates.)
It is also important to note that rated power is one point on the power curve and is the output at a wind speed that is much higher than those typically encountered at most sites. Thus, the rated wind speed only represents a small fraction of the wind resource at a site. We’re interested in output in the typical range of wind speeds at our site, which is well below the rated power. Our advice on rated power: forget it.
Final Factors
When shopping for a wind turbine, be sure to check into the company’s customer service record — how well it supports its dealers and customers. Some companies like Bergey Windpower and
Abundant Renewable Energy have stellar service records. Be sure that the company you buy from offers technical advice should you have trouble and that their technical support staff speaks your native tongue.
Another factor to consider is how long the company has been in business. Doing business with a company that been in business for a while, say ten or more years, is a good idea. A related criterion to take into consideration is how long a wind turbine has been on the market. The longer a wind turbine has been on the market, the better. It has been tried and tested and improved upon.
It’s also important to check on the availability of parts. Parts shipped from foreign countries, such as China, Europe, South Africa or Mexico may take months to arrive, if they’re available at all. Meanwhile, your $30,000 wind energy system sits idle. When shopping, check to see if the importer of the wind turbine you are interested in stocks spare parts.
It’s a good idea to consider the number of moving parts and wear points in a turbine. Some designs, like the Jacobs turbines manufactured by Wind Turbine Industries in Minnesota, contain a lot of moving parts, which results in numerous wear points — up to 300. These machines may require a lot of tinkering at 100 feet.
Be sure to check out the warranty. Warranties typically run five years. The longer the better. In June 2009, Bergey Windpower boosted its warranty to 10 years; others have pledged to follow suit. Also be sure to determine what warranties cover. Is it materials and workmanship or parts only? Does it include shipping? Unfortunately, most warranties do not cover the cost of labor.
We recommend buying from a local dealer/installer, too. That way, you’ll have local support, even if you opt to do the installation yourself. Be sure to ask for references and interview them.
If you buy from an online wholesaler, be aware that most of these companies do not offer technical support, installation advice or assistance. Nor do they offer replacement parts or repair services. If you buy cheap, you are on your own when you need help.
We believe that it is a good idea to stick with name brand wind turbines and avoid newcomers and foreign imports, especially Chinese-made machines, at least at this time. Buying a used or
reconditioned wind turbine may be an economical option, but it’s not a very good idea. The problem with used wind turbines is that you don’t often know what you’re getting.
If you buy a remanufactured wind turbine, you want to be sure that the machine has been fully reconditioned, not just painted and with new blades installed. Make sure that the equipment supplied with the turbine, like the controller or inverter are code compliant. You may also want to obtain an extended warranty. Also be sure you can find a tower for the turbine before you purchase it. Suitable tall towers for the larger small-scale wind machines are hard to find.
Monday, May 28, 2012
What to Look for When Buying a Wind Machine
While there are many turbines on the market, careful load and site analysis will narrow the field considerably. Once you have determined your average monthly electrical load and the average wind speed on your site, you can select a wind turbine that will produce enough electricity to meet your demands.
Manufacturers provide a plethora of technical data on their wind machines that can be used to make comparisons. Unfortunately, most of it is useless. Further complicating matters, “There can be a big difference in reliability, ruggedness, and life expectancy from one brand to the next,” according to Mike Bergey, president of Bergey Windpower.
So how do you go about selecting a wind machine?
Although wind turbines can be compared using many criteria, there are only a handful that really matter: (1) swept area, (2) durability, (3) annual energy output, (4) governing mechanism, (5)
shut-down mechanism, and (6) sound.
Swept Area
Swept area is the area of the circle described by the spinning blades of a turbine. Because the blades of a wind turbine convert wind energy into electrical energy, the swept area is the collector area of the turbine. The greater the swept area, the greater the collector
area. The bigger the swept area, the more energy you’ll be able to capture from the wind. To get the most out of a wind turbine — to produce the most electricity at the lowest cost — select a wind turbine with the greatest swept area. Swept area allows for easy comparison of different models.
Swept area is determined by rotor diameter. The rotor diameter is the distance from one side of the circle created by the spinning blades to a point on the opposite side or about twice the length of the blades. When comparing wind turbines, then, the rotor diameter is a pretty good measure of how much electricity a turbine will generate. Although other features such as the efficiency of the generator and the design of the blades influence energy production, for most turbines they pale in comparison to the influence of rotor diameter and, hence, swept area.
Manufacturers list the rotor diameter in feet or meters — often both. The greater the blade length, the greater the rotor diameter and the greater the swept area.
Most manufacturers also list the swept area of the rotor. Swept area is presented in square feet or square meters — sometimes both.
Annual Energy Output
Another, even more useful, measure is the annual energy output (AEO) or annual energy production (AEP) at various wind speeds. The AEO of a given wind turbine is presented as kilowatt-hours of electricity produced at various average wind speeds. Like the US EPA’s estimated gas mileage for vehicles, AEO gives buyers a convenient way to compare models. As in the estimated gas mileage rating, however, AEOs won’t tell you exactly how much electricity
a wind machine will produce at a site. Performance varies depending on a number of factors such as turbulence and the density of the air.
Durability: Tower Top Weight
Another extremely important criterion is durability. The most important measure of durability is tower top weight — how much a wind turbine weighs. Four turbines that produce about the same amount of electricity are for example, the Proven WT2500 (419 pounds), the ARE110 (315 pounds), the Skystream 3.7 (170 pounds) and the Whisper 500 (155 pounds). The weight differences are in some cases substantial.
In our experience, heavyweight wind turbines tend to survive the longest — sometimes many years longer than medium or lightweight turbines. Weight is usually reflected in the price. Remember, however, that you get what you pay for. Producing electricity on a precarious
perch 80 to 165 feet above the ground isn’t a job you want to relegate to the lowest bidder, which is invariably the lightest turbine.
Balance of System Cost
Before you buy a machine, consider the total system cost. You’ll need to purchase a tower and pay for installation, unless, of course, you install the tower yourself. Even then, you’ll need to pay for concrete, rebar and equipment to excavate the foundation and anchors. You’ll also need to run electrical wire from the turbine to the house and purchase an inverter (although they’re included in most batteryless grid-tie wind turbines). If you’re going off-grid or want battery backup for your grid-connected system, you’ll also need to buy batteries. All of this will add to the cost. The cost of the turbine itself may range from 10 to 40 percent of the total system cost.
Governing Systems
Found in all wind generators worth buying, governing, or overspeed control, systems are designed to prevent a wind generator from burning out or breaking apart in high winds. They do this by slowing down the rotor when the wind reaches a certain speed, known as the governing wind speed. Why is this necessary?
As wind speed increases, the rotor of a wind turbine spins more rapidly. The increase in the revolutions per minute (rpm) increases electrical output. Although electrical output is a desirable goal, if it exceeds the machine’s rated output, the generator could overheat and burn out. In addition, centrifugal forces in high wind speeds exert incredible forces on wind turbines that can tear them apart if the rotor speed is not governed.
A governing system is essential because it allows the turbine to shed extra energy when the winds are really strong. Not all wind turbines come with governing mechanisms, however. Many of the smallest wind turbines, the micro-turbines, with rated outputs of around
Fig. 5.5: Microturbines. Many microturbines like the Marlec (shown here) have no governing mechanism to slow the rotor in high winds. They rely on the relatively low rotor speed and rugged construction to endure high winds.
400 watts, for example, have no governing mechanisms (Figure 5.5). (These turbines are too small to produce a significant amount of electricity for most applications.) Larger wind turbines, those with swept areas over 38 square feet, however, come with overspeed controls. Two types are commonly found: furling and blade pitch.
Furling
Most manufacturers protect their wind turbines by furling. Furling is accomplished in one of two ways, both of which shift the position of the rotor (hub and blades) relative to the wind. This turns the blades out of the wind, decreasing the amount of rotor swept area that intercepts the wind. Reducing the swept area reduces the speed at which the rotor turns and the energy collected. Slowing the rotor will protect the wind turbine from damage.
Manufacturers employ two main types of furling: horizontal and vertical. In horizontal furling, the rotor turns out of the wind by turning sideways. For this reason, horizontal furling is also
known as side furling. In vertical furling, the rotor rotates upward with the same effect. “Angle furling” is a combination of the two.
Horizontal or side furling is achieved, in part, by hinging the tail. In side-furling turbines, a hinge is located between the tail boom and the body of the turbine. As you can see from Figure 5.6,
the turbine is also slightly offset from the yaw axis — that is, the yaw bearing is attached to the side of the turbine body, not its center so the turbine is not directly over the tower. Because the turbine is offset from the yaw axis, the force of the wind on the blades tends
Fig. 5.6: Side Furling. This wind turbine is not broken, it is side furling in high winds, which slows the rotor and protects the machine from damage.
Fig. 5.7: Vertical Furling
to rotate the machine around the yaw axis. However, the tail resists this rotation and keeps the rotor facing into the wind.
In light winds, the forces on the rotor and tail are small and the wind holds the tail in its normal position — straight behind the turbine. However, in strong winds, the increasing forces on the rotor overcome the force of the wind on the tail. Since the tail creates more force than the offset rotor, the tail stays mostly aligned with the wind and the turbine turns away from the wind. As a result, the turbine folds on itself. This slows the rotor.
Vertical furling is achieved by moving the hinge in front of the yaw axis and rotating it slightly. In high winds, the force of the wind tilts the rotor up, while the tail stays oriented downwind. As in side furling, this reduces its speed (Figure 5.7).
When fully furled, the rotor of a vertical furling turbine resembles a helicopter rotor. When wind speed declines, however, the rotor returns to its normal operating position. Shock absorbers are
often used to ease the rotor back into position.
Furling reduces the amount of energy collected by the rotor. Although electrical output typically continues, it usually occurs at a lower rate, as shown in the power curve of the ARE442 in Figure 5.8.
Changing Blade Pitch
Fig. 5.8: Power Curves. The electrical production of the ARE442, like that of many other turbines, declines significantly in high wind speeds as a result of overspeed controls that protect the wind turbine from damage. The power production of the ARE110 plateaus, so the machine continues to produce a significant amount of energy in high winds.
Fig. 5.9: Pitch Control — Blade-Actuated Governor. Numerous ingenious methods of blade pitch control have been devised. In this turbine, a Jacobs 31-20, the springs are part of a complex and effective blade pitch control mechanism.
pitch changes automatically in these turbines as wind speed increases over a certain level. The greater the wind speed above the operating range of the machine, the more the blades rotate (pitch). Changing the angle of the blade reduces rotor speed.
Pitch control typically requires springs, gears and weights ingeniously
arranged to produce the desired effect (Figure 5.9). Some machines, like the Jacobs, use the weight of the blade itself to change the pitch.
Blade pitch functions admirably, but is not as widely used as horizontal and vertical furling mechanisms. Of the two, blade pitch control is more expensive, but provides better control of blade speed and is more reliable. Bottom line: although furling mechanisms are cheaper, cheap is not necessarily better when it comes to a wind machine. The goal in buying a wind machine is to purchase the most reliable and most durable turbine. That said, you may only have a few choices among the turbines that produce the amount of electricity you need and most of home-scale wind turbines use furling.
Shut-Down Mechanisms
Small wind turbines should include a reliable shut-down mechanism. They allow a turbine to be turned off so operators can maintain and repair a wind turbine without fear of injury. They also provide a means of shutting a wind machine down when extremely violent storms, especially thunderstorms, are approaching. Maintenance personnel engage the shut-down mechanisms when they need to work on a turbine, but they also typically secure the blades with rope — just in case the wind comes up while they’re servicing a turbine.
Wind turbines contain two types of shut-down mechanisms: mechanical and electrical. Mechanical systems include disc brakes and folding tails. Both are manually activated. They’re attached to a cable that runs down the tower. Tightening the cable activates the brake or folds the tail (side furling the machine), stopping the rotor (Figure 5.10).
Fig. 5.10: Cable Winch on Tower. Clay Sterling, MREA’s Education Director, shuts down a Jacobs wind turbine by tightening the cable attached to the tail of the turbine.
Although disc brakes may seem like a good idea, they are not fail-safe. If the cable breaks in violent storm, for example, an operator would be helpless to stop the turbine. There’s no way to apply the brakes!
Although folding the tail protects the rotor from overspeeding, it doesn’t stop it from rotating. This presents a potential risk to service personnel working on the tower, unless another means of stopping the rotor, such as a disc brake, is available. Furthermore, if the cable breaks in high winds, when the machine is shut down, the tail will swing back into the wind and the wind turbine will start back up. If the winds are strong enough, this could seriously damage the turbine.
Some wind turbines come with electrical brakes, a.k.a. dynamic brakes. Dynamic braking is the least expensive option and is found in many small-scale wind turbines.
Dynamic braking is a fairly simple approach that is found in turbines equipped with permanent magnet alternators. It consists of a switch inside the house or at the base of the tower. When the
brake switch is closed, it short-circuits the wind machine, rapidly slowing the rotor.
In dynamic braking, the braking force is proportional to the rotor speed. As the rotor slows down, the braking force diminishes. As the rotor speed approaches zero, so does the braking force. In low to moderate winds, dynamic braking should either stop the rotor or slow it down considerably. However, dynamic braking may not completely stop the rotor in high winds. In winds blowing over 20 miles per hour, for instance, dynamic brakes can’t be counted on. If a wind machine is shut down prior to a storm’s arrival, strong winds may overpower the brakes, causing the rotors to start turning. In high wind speeds that force the blades to start spinning
slowly, energy is dissipated in the windings of the alternator, which could cause it to burn up. Not all dynamic brakes are created equal. Those found in Southwest Windpower’s Skystream 3.7 and turbines made by Proven and Abundant Renewable Energy are 100% reliable, as far as we can tell.
Shut-down mechanisms of a wind turbine should be high on the list of considerations, right up there with swept area and tower top weight. If the turbine is to be serviced on the tower, the shut-down mechanism should be capable of completely stopping the rotor. Don’t buy a turbine without a shut-down mechanism. Inexpensive wind turbine designs without a reliable shut-down mechanism are a short-sighted gamble, at best.
Sound Levels
The sound a turbine produces is another important factor to consider, both for your own peace of mind and your neighbors’. All residential wind machines produce sound. Sounds emanate from
the blades as they spin. They produce a swooshing sound. Sound is also produced when a turbine furls in high winds. Rotation of the rotor in the alternator also produces sound, as do gears in geardriven wind turbines. (Sound test reports can be found at the National Renewable Energy Laboratory’s website.)
Sound levels increase as wind speed increases. However, sound from a wind turbine is often difficult to detect and is rarely a nuisance. Remember, too, that mounting a turbine high off the ground — typically 80 to 120 feet — to reach the smoothest, most powerful winds significantly reduces sound levels at ground level.
Even so, it is important to consider sound levels. One way is to observe turbines you are considering in operation under a variety of wind speeds. If you can’t, you may want to ask homeowners or business owners who have installed the turbines you are considering for their experiences.
Another method is to check out the rpm of the turbines at their rated outputs. Rated output is the output in watts at a certain wind speed, known as rated speed. Knowing this gives an idea of how much sound they’ll produce — the higher the rpm, the more sound. The rpm of a wind turbine also give an indication of quality. Generally, less expensive and less durable turbines spin at a higher rpm. They rely on less expensive generators that operate at high speeds to produce energy. In addition, higher rpm machines are subject to more wear and tear and tend not to last as long.
Manufacturers provide a plethora of technical data on their wind machines that can be used to make comparisons. Unfortunately, most of it is useless. Further complicating matters, “There can be a big difference in reliability, ruggedness, and life expectancy from one brand to the next,” according to Mike Bergey, president of Bergey Windpower.
So how do you go about selecting a wind machine?
Although wind turbines can be compared using many criteria, there are only a handful that really matter: (1) swept area, (2) durability, (3) annual energy output, (4) governing mechanism, (5)
shut-down mechanism, and (6) sound.
Swept Area
Swept area is the area of the circle described by the spinning blades of a turbine. Because the blades of a wind turbine convert wind energy into electrical energy, the swept area is the collector area of the turbine. The greater the swept area, the greater the collector
area. The bigger the swept area, the more energy you’ll be able to capture from the wind. To get the most out of a wind turbine — to produce the most electricity at the lowest cost — select a wind turbine with the greatest swept area. Swept area allows for easy comparison of different models.
Swept area is determined by rotor diameter. The rotor diameter is the distance from one side of the circle created by the spinning blades to a point on the opposite side or about twice the length of the blades. When comparing wind turbines, then, the rotor diameter is a pretty good measure of how much electricity a turbine will generate. Although other features such as the efficiency of the generator and the design of the blades influence energy production, for most turbines they pale in comparison to the influence of rotor diameter and, hence, swept area.
Manufacturers list the rotor diameter in feet or meters — often both. The greater the blade length, the greater the rotor diameter and the greater the swept area.
Most manufacturers also list the swept area of the rotor. Swept area is presented in square feet or square meters — sometimes both.
Annual Energy Output
Another, even more useful, measure is the annual energy output (AEO) or annual energy production (AEP) at various wind speeds. The AEO of a given wind turbine is presented as kilowatt-hours of electricity produced at various average wind speeds. Like the US EPA’s estimated gas mileage for vehicles, AEO gives buyers a convenient way to compare models. As in the estimated gas mileage rating, however, AEOs won’t tell you exactly how much electricity
a wind machine will produce at a site. Performance varies depending on a number of factors such as turbulence and the density of the air.
Durability: Tower Top Weight
Another extremely important criterion is durability. The most important measure of durability is tower top weight — how much a wind turbine weighs. Four turbines that produce about the same amount of electricity are for example, the Proven WT2500 (419 pounds), the ARE110 (315 pounds), the Skystream 3.7 (170 pounds) and the Whisper 500 (155 pounds). The weight differences are in some cases substantial.
In our experience, heavyweight wind turbines tend to survive the longest — sometimes many years longer than medium or lightweight turbines. Weight is usually reflected in the price. Remember, however, that you get what you pay for. Producing electricity on a precarious
perch 80 to 165 feet above the ground isn’t a job you want to relegate to the lowest bidder, which is invariably the lightest turbine.
Balance of System Cost
Before you buy a machine, consider the total system cost. You’ll need to purchase a tower and pay for installation, unless, of course, you install the tower yourself. Even then, you’ll need to pay for concrete, rebar and equipment to excavate the foundation and anchors. You’ll also need to run electrical wire from the turbine to the house and purchase an inverter (although they’re included in most batteryless grid-tie wind turbines). If you’re going off-grid or want battery backup for your grid-connected system, you’ll also need to buy batteries. All of this will add to the cost. The cost of the turbine itself may range from 10 to 40 percent of the total system cost.
Governing Systems
Found in all wind generators worth buying, governing, or overspeed control, systems are designed to prevent a wind generator from burning out or breaking apart in high winds. They do this by slowing down the rotor when the wind reaches a certain speed, known as the governing wind speed. Why is this necessary?
As wind speed increases, the rotor of a wind turbine spins more rapidly. The increase in the revolutions per minute (rpm) increases electrical output. Although electrical output is a desirable goal, if it exceeds the machine’s rated output, the generator could overheat and burn out. In addition, centrifugal forces in high wind speeds exert incredible forces on wind turbines that can tear them apart if the rotor speed is not governed.
A governing system is essential because it allows the turbine to shed extra energy when the winds are really strong. Not all wind turbines come with governing mechanisms, however. Many of the smallest wind turbines, the micro-turbines, with rated outputs of around
Fig. 5.5: Microturbines. Many microturbines like the Marlec (shown here) have no governing mechanism to slow the rotor in high winds. They rely on the relatively low rotor speed and rugged construction to endure high winds.400 watts, for example, have no governing mechanisms (Figure 5.5). (These turbines are too small to produce a significant amount of electricity for most applications.) Larger wind turbines, those with swept areas over 38 square feet, however, come with overspeed controls. Two types are commonly found: furling and blade pitch.
Furling
Most manufacturers protect their wind turbines by furling. Furling is accomplished in one of two ways, both of which shift the position of the rotor (hub and blades) relative to the wind. This turns the blades out of the wind, decreasing the amount of rotor swept area that intercepts the wind. Reducing the swept area reduces the speed at which the rotor turns and the energy collected. Slowing the rotor will protect the wind turbine from damage.
Manufacturers employ two main types of furling: horizontal and vertical. In horizontal furling, the rotor turns out of the wind by turning sideways. For this reason, horizontal furling is also
known as side furling. In vertical furling, the rotor rotates upward with the same effect. “Angle furling” is a combination of the two.
Horizontal or side furling is achieved, in part, by hinging the tail. In side-furling turbines, a hinge is located between the tail boom and the body of the turbine. As you can see from Figure 5.6,
the turbine is also slightly offset from the yaw axis — that is, the yaw bearing is attached to the side of the turbine body, not its center so the turbine is not directly over the tower. Because the turbine is offset from the yaw axis, the force of the wind on the blades tends
Fig. 5.6: Side Furling. This wind turbine is not broken, it is side furling in high winds, which slows the rotor and protects the machine from damage.
Fig. 5.7: Vertical Furlingto rotate the machine around the yaw axis. However, the tail resists this rotation and keeps the rotor facing into the wind.
In light winds, the forces on the rotor and tail are small and the wind holds the tail in its normal position — straight behind the turbine. However, in strong winds, the increasing forces on the rotor overcome the force of the wind on the tail. Since the tail creates more force than the offset rotor, the tail stays mostly aligned with the wind and the turbine turns away from the wind. As a result, the turbine folds on itself. This slows the rotor.
Vertical furling is achieved by moving the hinge in front of the yaw axis and rotating it slightly. In high winds, the force of the wind tilts the rotor up, while the tail stays oriented downwind. As in side furling, this reduces its speed (Figure 5.7).
When fully furled, the rotor of a vertical furling turbine resembles a helicopter rotor. When wind speed declines, however, the rotor returns to its normal operating position. Shock absorbers are
often used to ease the rotor back into position.
Furling reduces the amount of energy collected by the rotor. Although electrical output typically continues, it usually occurs at a lower rate, as shown in the power curve of the ARE442 in Figure 5.8.
Changing Blade Pitch
Fig. 5.8: Power Curves. The electrical production of the ARE442, like that of many other turbines, declines significantly in high wind speeds as a result of overspeed controls that protect the wind turbine from damage. The power production of the ARE110 plateaus, so the machine continues to produce a significant amount of energy in high winds. 
Fig. 5.9: Pitch Control — Blade-Actuated Governor. Numerous ingenious methods of blade pitch control have been devised. In this turbine, a Jacobs 31-20, the springs are part of a complex and effective blade pitch control mechanism.pitch changes automatically in these turbines as wind speed increases over a certain level. The greater the wind speed above the operating range of the machine, the more the blades rotate (pitch). Changing the angle of the blade reduces rotor speed.
Pitch control typically requires springs, gears and weights ingeniously
arranged to produce the desired effect (Figure 5.9). Some machines, like the Jacobs, use the weight of the blade itself to change the pitch.
Blade pitch functions admirably, but is not as widely used as horizontal and vertical furling mechanisms. Of the two, blade pitch control is more expensive, but provides better control of blade speed and is more reliable. Bottom line: although furling mechanisms are cheaper, cheap is not necessarily better when it comes to a wind machine. The goal in buying a wind machine is to purchase the most reliable and most durable turbine. That said, you may only have a few choices among the turbines that produce the amount of electricity you need and most of home-scale wind turbines use furling.
Shut-Down Mechanisms
Small wind turbines should include a reliable shut-down mechanism. They allow a turbine to be turned off so operators can maintain and repair a wind turbine without fear of injury. They also provide a means of shutting a wind machine down when extremely violent storms, especially thunderstorms, are approaching. Maintenance personnel engage the shut-down mechanisms when they need to work on a turbine, but they also typically secure the blades with rope — just in case the wind comes up while they’re servicing a turbine.
Wind turbines contain two types of shut-down mechanisms: mechanical and electrical. Mechanical systems include disc brakes and folding tails. Both are manually activated. They’re attached to a cable that runs down the tower. Tightening the cable activates the brake or folds the tail (side furling the machine), stopping the rotor (Figure 5.10).
Fig. 5.10: Cable Winch on Tower. Clay Sterling, MREA’s Education Director, shuts down a Jacobs wind turbine by tightening the cable attached to the tail of the turbine.Although disc brakes may seem like a good idea, they are not fail-safe. If the cable breaks in violent storm, for example, an operator would be helpless to stop the turbine. There’s no way to apply the brakes!
Although folding the tail protects the rotor from overspeeding, it doesn’t stop it from rotating. This presents a potential risk to service personnel working on the tower, unless another means of stopping the rotor, such as a disc brake, is available. Furthermore, if the cable breaks in high winds, when the machine is shut down, the tail will swing back into the wind and the wind turbine will start back up. If the winds are strong enough, this could seriously damage the turbine.
Some wind turbines come with electrical brakes, a.k.a. dynamic brakes. Dynamic braking is the least expensive option and is found in many small-scale wind turbines.
Dynamic braking is a fairly simple approach that is found in turbines equipped with permanent magnet alternators. It consists of a switch inside the house or at the base of the tower. When the
brake switch is closed, it short-circuits the wind machine, rapidly slowing the rotor.
In dynamic braking, the braking force is proportional to the rotor speed. As the rotor slows down, the braking force diminishes. As the rotor speed approaches zero, so does the braking force. In low to moderate winds, dynamic braking should either stop the rotor or slow it down considerably. However, dynamic braking may not completely stop the rotor in high winds. In winds blowing over 20 miles per hour, for instance, dynamic brakes can’t be counted on. If a wind machine is shut down prior to a storm’s arrival, strong winds may overpower the brakes, causing the rotors to start turning. In high wind speeds that force the blades to start spinning
slowly, energy is dissipated in the windings of the alternator, which could cause it to burn up. Not all dynamic brakes are created equal. Those found in Southwest Windpower’s Skystream 3.7 and turbines made by Proven and Abundant Renewable Energy are 100% reliable, as far as we can tell.
Shut-down mechanisms of a wind turbine should be high on the list of considerations, right up there with swept area and tower top weight. If the turbine is to be serviced on the tower, the shut-down mechanism should be capable of completely stopping the rotor. Don’t buy a turbine without a shut-down mechanism. Inexpensive wind turbine designs without a reliable shut-down mechanism are a short-sighted gamble, at best.
Sound Levels
The sound a turbine produces is another important factor to consider, both for your own peace of mind and your neighbors’. All residential wind machines produce sound. Sounds emanate from
the blades as they spin. They produce a swooshing sound. Sound is also produced when a turbine furls in high winds. Rotation of the rotor in the alternator also produces sound, as do gears in geardriven wind turbines. (Sound test reports can be found at the National Renewable Energy Laboratory’s website.)
Sound levels increase as wind speed increases. However, sound from a wind turbine is often difficult to detect and is rarely a nuisance. Remember, too, that mounting a turbine high off the ground — typically 80 to 120 feet — to reach the smoothest, most powerful winds significantly reduces sound levels at ground level.
Even so, it is important to consider sound levels. One way is to observe turbines you are considering in operation under a variety of wind speeds. If you can’t, you may want to ask homeowners or business owners who have installed the turbines you are considering for their experiences.
Another method is to check out the rpm of the turbines at their rated outputs. Rated output is the output in watts at a certain wind speed, known as rated speed. Knowing this gives an idea of how much sound they’ll produce — the higher the rpm, the more sound. The rpm of a wind turbine also give an indication of quality. Generally, less expensive and less durable turbines spin at a higher rpm. They rely on less expensive generators that operate at high speeds to produce energy. In addition, higher rpm machines are subject to more wear and tear and tend not to last as long.
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