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Grid-Connected Systems
Grid-connected systems are so named because they connect directly to the electrical grid. They are also referred to as batteryless grid-connected or batteryless utility-tied systems because they do not employ batteries to store surplus electricity1.
In batteryless grid-tie systems, the electrical grid accepts surplus electricity — electricity produced by the turbine in excess of
1. Electric wire from turbine carries wild AC electricity to house
2. DC control panel
3. DC disconnect
4. Inverter
5. AC disconnect
6. Breaker box
7. Electrical outlets
8. Electrical meter
9. Electric wire carries AC electricity to and from hous
Fig. 3.3: Grid-Connected Wind System. The grid-connected wind system is the simplest of all systems. Wild AC electricity produced by the turbine is first fed into the controller. The inverter produces grid-compatible AC electricity to power household loads. Surpluses are backfed onto the electrical grid.
demand. When a wind system is inactive, the grid supplies electricity to the home or business. The grid therefore serves as the storage medium.
As shown in Figure 3.3, a batteryless grid-connected system consists of six main components: (a) a wind generator specifically designed for grid connection, (b) a tower, (c) an inverter/power conditioner, (d) a main service panel (e) meters, and (f ) safety disconnects.
In most batteryless grid-connected systems, the wind generator produces “wild AC” electricity. Wild AC is alternating current electricity the frequency and voltage of which vary with wind speed. Frequency is the number of times electrons switch direction every second and, as noted earlier, is measured in hertz, or cycles per second.
The flow of electrons through an electrical wire is created by an electromotive force that scientists call voltage. Voltage is electrical pressure, the driving force that causes electrons to move. The unit of measurement for voltage is volts. In most small wind turbines, the faster the blades spin, the higher the voltage and the greater the frequency.
Wild AC, produced by wind turbines, is not directly usable. Appliances and electronic devices require a tamer version of electricity — alternating current with a fairly constant frequency and voltage, like that available from the grid. In a grid-connected system, the wild voltage must first be “tamed.” That is, its frequency and voltage must be converted to standard values. This occurs in two additional components, the controller and inverter (Figure 3.3). The inverter converts the electricity to grid-compatible AC — 60 cycle per second, 120-volt (or 240-volt) electricity. Because the
inverter produces electricity in sync with the grid, it’s often referred to as a synchronous inverter.
While grid-compatible wind generators typically produce wild AC, another type of wind generator is also found in the small wind market.
The 120-volt or 240-volt AC produced by the inverter (or directly by an induction generator) flows to the breaker box, which is where the circuit breakers are found. From here, the electricity flows along wires in a house or business to electrical devices drawing power. If the wind machine is producing more electricity than is needed, the excess is fed onto the grid.
Surplus electricity backfed onto the grid travels from the main service panel through the utility’s electric meter, typically mounted on the outside of the building. It then flows through the wires that connect to the grid. The surplus electricity then travels along the power lines where it flows into neighboring homes or businesses.
A utility electric meter monitors the electricity fed onto the grid so the utility can credit the producer for its contribution. The meter also keeps track of electricity the power company supplies to homes and businesses when their wind systems are not generating. To learn how the electric company measures what you are putting onto the grid and how they “pay” for it, check out the accompanying box, “Net Metering in Grid-Connected Systems.”
In addition to the electric meter — or meters (some utilities require two or more meters) — that monitor the flow of electricity onto and off the local utility grid, grid-connected wind energy systems often contain safety disconnects. These are manually operated switches that enable service personnel to disconnect at a couple of key points in the system to prevent electrical shock if service is required. As shown in Figure 3.3, an AC disconnect is located between the inverter and breaker box. When switched off, it disconnects and isolates the wind energy system from the household circuits and the grid. The AC disconnect must be mounted outside so that it is accessible to utility company personnel so they can isolate the wind system from the grid when working on the electric lines in your area without fear of shock — for instance, if a line goes down in an ice storm. The AC disconnect must also contain a switch that can be locked in the off position by the utility worker so that the homeowner or a family member doesn’t accidentally
turn the system back on prior to the completion of repairs.
Lockable AC disconnects are required by most utilities. However, many experienced electric companies like the large
California utilities and Colorado’s Exel, which collectively have thousands of solar- and wind-electric systems connected to their systems, have dropped the requirement for utility company-accessible, lockable disconnects. The California utilities have found that they can’t use them because there are too many to disconnect at once. More importantly, these companies have come to realize that they’re simply not needed. That’s because grid-compatible inverters automatically shut off when the utility power goes down. As a result, no electricity can flow onto the grid. A properly installed grid-connected wind-electric system will not backfeed a dead grid. Period.
The Pros and Cons of Grid-Connected Systems
Batteryless grid-connected systems represent the majority of all new wind systems in the United States. Their pluses and minuses are summarized in Table 3.1.
On the positive side, batteryless grid-connected systems are relatively simple and typically the least expensive option — often 25% cheaper than battery-based systems. They also require less maintenance than battery-based systems.
Another substantial advantage of these systems is that they can store an unlimited amount of electricity on the grid (so long as the grid is operational). Although grid-connected systems don’t physically store excess electricity on site like a battery-based system for later use, they “store” surplus electricity on the grid in the form of a credit on your utility bill. When winds fail to blow — or a wind turbine isn’t producing enough electricity to meet demand — electricity is drawn from the grid, using up the credit. Unlike a battery bank, you can never “fill up” the grid. It will accept as much lectricity
as you can feed it.
By crediting a producer for electricity fed onto the grid, a utility says, “You’ve supplied us with x kilowatt-hours of electricity. When you need electricity, we’ll supply you with an equal amount at no cost. If at the end of the month you’ve supplied more than you consume, we’ll either pay you for it or carry the surplus over to the next month.”
Another advantage of grid storage is utility storage of electricity is not subject to losses that occur when electricity is stored in a battery. when electricity is stored in a battery, it is converted to chemical energy. When electricity is
needed, the chemical energy is converted back to electrical energy. As much as 20 to 30 percent of the electrical energy fed into a battery bank is lost due to conversion inefficiencies and other factors. In sharp contrast, electricity stored on the grid comes back in full. If you deliver 100 kilowatt-hours of electricity, you can draw off 100 kilowatt-hours. (The grid has losses too, however, net metered customers get 100 percent return on their stored electricity.)
Another advantage of grid-tie systems is that they are greener than battery-based systems. Although utilities aren’t the greenest entities in the world, they are arguably greener than battery-based systems. Battery production requires an enormous amount of energy and raw materials. Batteries also contain highly toxic sulfuric acid. Although old lead-acid batteries are recycled, they’re often recycled under abysmally poor conditions in less developed countries, exposing
employees (often young children) and the environment to toxic chemicals.
Grid-tie systems, when net metered, can provide some income. In windy sites, they may produce surpluses month after month. If the local utility pays for surpluses at retail rates, the surpluses can generate income that helps reduce the cost of the system and the annual cost of producing electricity.
On the downside, grid-connected systems may require extensive negotiations with local utilities. This, though, may become a thing
of the past. Although some utilities may throw up roadblocks, more and more are becoming cooperative as they become more comfortable with these systems.
Another downside of these systems is that when the grid goes down, so does a batteryless grid-connected wind system. Even when winds are blowing, batteryless grid-tied wind energy systems shut down if an electric line comes crashing down in an ice storm or lightning strikes a nearby transformer, both of which result in a power outage. Even though the winds are blowing, you’ll get no power from your system.
If power outages are a recurring problem and outages occur for long periods, you may want to consider installing a standby gas or diesel generator that switches on automatically when the grid goes down. Because a backup generator takes many seconds to start up and come on line, you may want to consider installing an uninterruptible power supply (UPS) on critical equipment such as computers. A UPS contains a battery and a small inverter. If the utility power goes out, it supplies power instantly until its battery runs low. Another alternative is to install a grid-connected system with battery backup, discussed next. In these systems batteries provide backup power to a home or business when the grid goes down.
Grid-Connected Systems with Battery Backup
Grid-connected systems with battery backup are also known as battery- based utility-tied systems. These systems ensure a continuous supply of electricity, even when freezing rain wipes out the electrical supply to your home or business. Figure 3.4 shows the components of these systems: (1) a wind turbine on a tower, (2) a charge controller,
(3) an inverter, (4) safety disconnects, (5) breaker box or main service panel, and (6) meters to keep track of electricity delivered to and drawn from the grid.
1. Carries wild AC from turbine
2. Controller
3. Battery bank
4. Transfer switch
5. Inverter
6. Subpanel (for critical loads)
7. Main breaker box (all household loads)
8. Utility meter
9. Service line
Fig. 3.4 : Grid-Connected Wind System with Battery Backup.
Although grid-connected systems with battery backup are similar to batteryless grid-connected systems, they differ in several ways. The most obvious difference is that battery-based grid-connected systems contain a bank of batteries. They also require a different type of inverter. These systems also contain a meter that monitors the flow of electricity into and out of the battery bank and a device known as a charge controller.
Batteries for grid-connected systems with battery backup are either flooded lead-acid batteries or sealed lead-acid batteries. Battery banks in grid-connected systems are typically smaller than those in off-grid systems because they are usually sized to provide sufficient storage to run a handful of critical loads for a day or two until the utility company restores electrical service. Critical loads might include a refrigerator and freezer, a few lights, a well pump, and the blower of a furnace or the boiler and pump in a radiant heating system.
Keeping batteries fully charged is a high priority in these systems. Battery banks are maintained at full charge day in and day out to ensure a ready supply of electricity should the grid go down. It’s only when the batteries are topped off and a household’s demands are being met that excess electricity is backfed onto the grid.
Batteries are called into duty only when the grid goes down. They’re a backup power source. They’re not there to supply additional power to run loads that exceed the wind system’s output. When demand exceeds supply, electricity is supplied by the electrical grid, not the batteries. When the winds are dead, the grid, not the battery bank, becomes the power source.
Maintaining a fully charged battery bank requires a fair amount of electricity. That’s because batteries self-discharge when sitting idly by. Thus, a good portion of the surplus electricity a wind system generates may be devoted to keeping batteries full. Keeping batteries topped off consumes 5 to 10 percent of a system’s daily output. (In systems with a low-efficiency and technologically unsophisticated inverter and a large or older battery bank, consumption can be as high as 25 to 50 percent.)
Battery banks in grid-connected systems don’t require careful monitoring like those in off-grid systems, but it is a very good idea to keep a close eye on them — just to be sure they’ll be functional when the grid goes down. Owners can monitor batteries through a meter that indicates the total amount of electricity stored in the battery bank at any one time. These meters give readings in amphours or kilowatt-hours. What do these terms mean?
As most readers know, electricity is the flow of electrons through a wire. Like water flowing through a hose, electricity flows through conductors at varying rates. The rate of flow depends on the voltage.
The flow of electrons through a conductor is measured in amperes or amps for short. An amp is 6.24 x 1018 electrons passing by a point on a conductor per second. The greater the amperage, the faster the electrons are flowing.
One amp of electricity flowing through a wire for an hour is one amp-hour. This term is also frequently used to define a battery’s storage capacity. A flooded lead-acid battery, for example, might store 420 amp-hours of electricity. Amp-hours can also be converted to kilowatt-hours, as explained shortly.
Charge controllers also monitor battery voltage. They use this information to protect batteries from being overcharged — having too much electricity driven into them. Overcharging can permanently damage the lead plates in batteries, dramatically reducing battery life.
When the charge controller sees that the batteries are fully charged, it terminates the flow of electricity to them. Surplus
Fig. 3.5: Dump Load. Resistive heaters like this one are used as dumps for surplus electricity from off-grid wind energy systems.
electricity is then fed onto the grid, or if the grid is not operational, to a diversion or dump load. Diversion loads are typically resistance- type devices that convert surplus electricity into heat. They are installed in water heaters or as separate space heaters in the basement or a nearby utility room and help put to use the surplus electricity (Figure 3.5).
Pros and Cons of Grid-Connected Systems with Battery Backup
Grid-connected systems with backup power protect against utility failures, although typically only a handful of critical loads can be run from a battery bank. These systems allow homeowners to heat their homes, keep food cold, power a radio, and run emergency medical equipment. In businesses, they protect computers and other vital equipment required to continue operations.
Grid-connected systems with battery backup do have some drawbacks. They cost more to install and operate than batteryless
grid-connect systems. Flooded lead-acid batteries used in these systems require periodic maintenance and replacement every five to ten years, whether they’re used or not. Keeping batteries topped off can also consume a fair amount of a system’s daily electrical output.
When contemplating a battery-based grid-tie system, ask yourself three questions: (1) How frequently does the grid fail in your utility’s service area? (2) What are your critical loads and how important is it to keep them running? (3) How do you react when the grid fails?
If the local grid is extremely reliable, you don’t have medical support equipment to run or need computers for critical financial transactions, and you don’t mind using candles on the rare occasions when the grid goes down, why buy, maintain, and replace costly batteries? See Table 3.2 for a quick summary of the pros and cons of battery-based grid-connected systems.
Off-Grid (Stand-Alone) Systems
Those who want to or must supply all of their needs through wind energy or a combination of wind and solar and don’t want to be connected to the grid install off-grid systems. As shown in Figure 3.6, this system bears a remarkable resemblance to a grid-connected system with battery backup.
The main source of electricity in an off-grid system is a batterycharging wind turbine. These turbines produce wild AC electricity that is converted (rectified) to DC electricity by rectifiers located in the charge controller.
The controller delivers DC electricity to the battery bank. When electricity is needed, it is drawn from the battery bank via the inverter. The inverter converts the DC electricity from the battery bank, typically 24 or 48 volts, to higher-voltage AC, either 120 or 240 volts, required by households and businesses. The AC then flows to active circuits in the house via the breaker box.
Although off-grid systems resemble grid-connected systems with battery banks, there are some noticeable differences. The first and most obvious is that there are no power lines running from the house or business to the grid. In these systems, then, the wind turbine produces all of the electricity required to meet the owner’s needs. Surplus generated during windy periods is stored in batteries for use during low- or no-wind periods. If the batteries are full, the surplus is typically sent to the diversion load.
Off-grid systems are also typically equipped with another source of electricity, often a PV array or a gasoline or diesel generator (gen-set). They help make up for shortfalls.
Off-grid systems also require safety disconnects to permit servicing. A DC disconnect is located between the charge controller and inverter. These systems also contain charge controllers to protect the batteries from overcharging and a low-voltage disconnect to prevent deep discharge of the battery bank.
Off-grid wind energy systems are the most complex of all options. Some systems contain DC circuits. These circuits are fed directly from the battery bank, bypassing the inverter, to power DC lights or refrigerators. Bypassing the inverter saves energy, because inverters are not 100 percent efficient. It takes a little energy to convert DC to AC — usually about 5 to 10 percent.
DC appliances are generally small, difficult to find, expensive, and not always that reliable or as fully equipped as AC appliances. DC refrigerators, for example, do not come with the features that
1. Electric wire carries wild AC to controller
2. Controller
3. Battery bank
4. DC disconnect
5. Inverter
6. AC disconnect
7. Main breaker box
8. AC circuits
Fig. 3.6: Most wind turbines in off-grid wind systems produce AC electricity that’s converted to DC electricity by the controller. The inverter draws electricity from the batteries, converting it into AC electricity for household use.
many individuals expect, such as automatic defrost or ice makers. DC circuits also require larger, more costly wires and special receptacles. Moreover, the energy lost as low-voltage DC electricity flows through wires is about the same as the losses in an inverter.
To simplify installation of battery-based systems, you may want to consider installing a power center (Figure 3.7). Power centers contain many of the essential components of a renewable energy system, including one or more inverters, the meters needed to monitor
Fig. 3.7: Power Center. Power centers like this one contain all of the components needed for a successful installation, all mounted on one panel. They’re easy to wire and pass inspection with ease.
system performance, safety disconnects, and the charge controller. Power centers provide connection points to which the wires to the battery bank, the inverter and the wind generator connect. Although power centers may cost a bit more than buying all the components separately, they are easier and cheaper to install.
Pros and Cons of Off-Grid Systems
Off-grid systems provide freedom from power outages, energy independence, and total emancipation from the electric utility (Table 3.3). If designed and operated correctly, they will provide sufficient energy to meet your needs for many years.
Although, they do free you from utilities, you will still very likely need to buy a generator (gen-set) and fuel to power it. Gen-sets produce pollution and cost money to maintain and operate. Offgrid systems are also the most expensive of all systems because of the need for batteries and backup power (via PV systems and/or gen-sets), which add substantially to the cost. They also require more wiring and additional space to house battery banks and generators.
They require more maintenance, too, thanks to the batteries and generators. Batteries require replacement every five to ten years, depending on the quality of batteries you buy and how well you maintain them. Battery production and recycling also exact a toll on the environment.
Although cost is a major downside, there are times when offgrid systems cost the same or less than grid-connected systems — for example, if a home or business is located more than a few tenths of a mile from the utility lines. Under such circumstances, it can cost more to run electric lines to a home than to install an off-grid wind system.
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