Wind and Wind Energy

As you learned in the last chapter, wind is a clean, abundant, and renewable energy resource that can be tapped to produce electricity. This chapter explores how wind is generated and introduces
you to two types of wind — local and global. We’ll also explore ways local topography affects wind, introducing you to two key concepts: ground drag and turbulence. This information provides the
practical knowledge you will need to select the best site for a wind turbine and the optimum tower height.

What is Wind?
Wind is air in horizontal motion across the Earth’s surface. All winds are produced by differences in air pressure between two regions. Differences in pressure result from differential heating of the surface of the Earth. Heating, of course, is caused by sunlight striking the Earth’s surface.

Like most other forms of energy in use today, even coal, oil and natural gas, wind is a product of sunlight — solar energy. Some wind advocates, refer to wind as “the other solar energy” or “secondhand solar energy.” Let’s begin by looking at two types of local winds: (1) offshore and onshore winds and (2) mountain-valley breezes.

Offshore and Onshore Winds

Offshore and onshore winds are generated along the shores of large lakes, such as the Great Lakes of North America, and along the coastlines of the world’s oceans. Offshore and onshore winds blow regularly, nearly every day of the year. They are produced by the differential heating of land and water, caused by solar energy.

Here’s how this happens: As shown in Figure 2.1a, sunlight shining on the Earth’s surface heats the land and water simultaneously. As the water and adjoining land begin to warm, they radiate
some of the heat (infrared radiation) into the atmosphere. This heat, in turn, warms the air above them. When air is heated it expands, and as it expands it becomes less dense and rises. The upward movement of air is called a thermal or updraft.

Although water and land both heat up when warmed by the sun, land masses warm more rapidly than neighboring bodies of water. Because air over land heats up more quickly than air over water, air pressure over land is lower than over neighboring surface waters. As warm air rises over land, cooler, high pressure air moves in to fill the void, resulting in a steady breeze known as onshore wind.

the void, resulting in a steady breeze known as onshore wind. At night, the winds blow in the opposite direction — from land to water — as illustrated in Figure 2.1b. These are known as offshore breezes or offshore winds.

Like onshore winds that occur during the day, offshore winds are created by differences in air pressure between the air over land and neighboring water bodies. Here’s what happens: after sunset, the land and the ocean both begin to cool. Land, however, cools more rapidly than water. Because the water cools more slowly, air above it is warmer. Warm air expands and rises. Cooler high pressure air

Fig. 2.1a and 2.1b: Onshore and offshore breezes. Onshore (a) and offshore (b) breezes occur along the coastlines of major lakes and oceans.

flows from the land to the water at night (Figure 2.1b). The result is an offshore breeze: steady winds that flow from land to water.

Offshore and onshore breezes operate day in and day out on sunny days, providing a steady supply of wind energy. Because offshore and onshore winds are fairly reliable, coastal regions of the world are often ideal locations for small (and large) wind turbines.

Coastal winds are more consistent than winds over the interior of continents and also tend to be more powerful because of the relatively smooth and unobstructed surface of open waters. That is to say, wind moves rapidly over water because lakes and coastal waters provide very little resistance to its flow, unlike forests or cities and suburbs, which dramatically lower surface wind speeds.

Mountain-Valley Breezes

Like coastal winds, mountain-valley breezes arise from the differential heating of the Earth’s surface. To understand how these winds are formed, let’s begin in the morning.

As the Sun rises on clear days, sunrays strike the valley floor and begin heating the ground, valley walls and mountains. As the ground and valley walls begin to warm, the air above them warms. It then expands and begins to flow upward. This process is known as convection. (Convection is the transfer of heat in a fluid or a gas that is caused by the movement of the heated air or fluid itself.) While some of this warm air rises vertically, mountain valleys also tend to channel the solar-heated air through the valley toward the mountains (Figure 2.2). As the warmed air moves up a valley, cooler air from surrounding areas flows in to replace it. This wind is known as a valley breeze.

Throughout the morning and well into the afternoon, breezes flow up-valley — from the valley floor into the mountains. These breezes tend to reach a crescendo in the afternoon. When the Sun sets, however, the winds reverse direction, flowing down valley.

Winds flow in reverse at night because the mountains cool more quickly than the valley floor. Cool, dense air (high-pressure air) from the mountains sinks and flows down through the valleys like

Fig. 2.2a and 2.2b: Mountain-Valley Breezes. Mountain-valley winds can provide a reliable source of wind power if conditions are just right. (a) Up-valley winds. (b) Down-valley winds.

the water in a mountain stream, creating steady and often predictable down-valley or mountain breezes.

Together, valley and mountain winds are known as mountainvalley breezes. As a rule, mountain breezes (down-flowing winds) tend to be stronger than daytime valley breezes.

Mountain-valley breezes typically occur in the summer, a time when solar radiation is greatest. They also typically occur on calm days when the prevailing winds (larger regional winds, which will be discussed shortly) are weak or nonexistent.

Mountain-valley winds also form in the presence of prevailing winds — for example, when a storm moves through an area. In such instances, mountain or valley winds may “piggy back” on the prevailing winds, creating even more powerful (and hence higher energy) winds. When consistently flowing in the same direction, such winds can provide a great deal of power that can be tapped to produce an abundance of electricity.

The Pros and Cons of Wind Energy

Wind is a seemingly ideal fuel source that could ease many of the world’s most pressing problems. Like all energy sources, small wind power has its advantages and disadvantages. Let’s look at the downsides of small wind systems first.

Disadvantages of Wind Energy

Small wind’s disadvantages are few and often grossly exaggerated or only perceived problems. They include wind’s variability, bird mortality, aesthetics, property values and unwanted sound. Some people are concerned about wind being more site specific than solar electricity. There’s even concern about ice falling from turbines after ice storms and interference with radio and televisions signals.

Variability and Reliability of the Wind

Perhaps the most significant “problem” with small wind is that the wind does not blow 100 percent of the time in most locations. Wind is a variable resource, to be sure. It’s not available 24 hours a day like coal or oil. In fact, a wind turbine may operate for four days in a row, producing a significant amount of electricity, then sit idle for two days — or a week.

Wind resources vary seasonally, too. In most locations, winds are typically strongest in the fall, winter and early spring, but decline during the summer. Wind even varies during the course of a day. Winds may blow in the morning, die down for a few hours, then pick up later in the afternoon and blow throughout the night.

Even though wind is a variable resource, it is not unreliable. Just like solar energy, you can count on a certain amount of wind at a given location during the year. With smart planning and careful
design, you can design a wind system to meet your electrical needs.

Wind’s variable nature can be managed to our benefit by installing batteries to store surplus electricity in off-grid systems. The stored electricity can power a home or office when the winds fail to blow.

Surplus electricity can also be stored on the electrical grid in many systems. Thus, when a wind-electric system is producing more power than a home or business is using, the excess is fed onto the grid. In times of shortfall, electricity is drawn from the grid.

Wind’s variable nature can also be offset by coupling small wind systems with other renewable energy sources, for example, solarelectric systems. Such systems are referred to as hybrid systems. Solar-electric systems (or photovoltaic [PV] systems) generate electricity when sunlight strikes solar cells in solar modules. Hybrid systems can be sized to provide a steady year-round supply of electricity. Residential wind-generated electricity can also be supplemented by small gas or diesel generators.

Bird Mortality

One perceived problem with wind power is bird mortality. Unfortunately, this issue has been blown way out of proportion. Although a bird may occasionally perish in the spinning blades of
a residential wind machine, this is an extremely rare occurrence. Ian is aware of only one instance of a bird kill, when a hawk flew into a small wind turbine. “Because of their relatively smaller blades and short tower heights, home-sized wind machines are considered too small and too dispersed to present a threat to birds,” notes Mick Sagrillo in his article, “Wind Turbines and Birds,” published by Focus on Energy, Wisconsin’s renewable energy program.

The only documented bird mortality of any significance occurs at large commercial-scale wind turbines — but even then, the number of deaths is relatively small. Commercial wind turbines kill an estimated 50,000 birds per year. While this may sound like a lot, this number pales in comparison to other lethal forces, among them domestic cats, automobiles, windows in buildings, and communication towers. All in all, cats are probably the most lethal “force” that birds encounter. Scientists estimate that our beloved cats kill about 270 million birds a year nationwide — though the number is very likely much higher.

Aesthetics

Although many people view small wind turbines as things of great beauty, others contend that they detract from natural beauty. Ironically, those who find wind turbines to be unsightly often ignore the great many forms of visual blight in the landscape, among them cell phone towers, water towers, electric transmission lines, radio towers and billboards. To be fair, there are differences between a wind tower and common sources of visual pollution. For one, a wind turbine’s spinning blades call attention to these machines. Another is that we’ve grown used to the ubiquitous electric lines and radio towers. As a result, people often fail to see them anymore.
Given the opportunity to oppose a structure in their “viewshed” (for example, at a public hearing that may be required for permission to install a residential wind system) neighbors will often speak up in opposition. If you need to apply for permission to install a turbine on a tall tower, you may encounter this problem.

Proximity to Homes and Property Values

Critics raise legitimate concerns when it comes to the placement of wind machines near their property. Although most of the issues over proximity have been raised by individuals and groups that oppose large commercial wind farms, residential systems can also cause a stir among neighbors. Some may be concerned about aesthetics. Others may worry about safety.

To avoid problems, we recommend installing machines in locations out of sight and hearing of neighbors. Safety concerns are typically related to tower collapse — an extremely rare event that is always the result of bad design and improper installation. Even though homeowner’s insurance should cover damage to individuals and property, it is best to place a wind turbine and tower well away from your neighbors’ property lines.

Unwanted Sound

Opponents of wind energy and apprehensive neighbors sometimes voice concerns about unwanted sound, a.k.a. noise, from residential wind machines. Small wind turbines do produce sound, and as the wind speed increases, sound output increases. Sound is produced primarily by the spinning blades and alternators. The faster a turbine spins, the more sound it produces.

You can reduce unwanted sound by selecting a quieter, low-rpm wind turbine rather than a louder, high-rpm wind turbine. If you are concerned about sound, make this a high priority as you shop for a turbine and let your neighbors know you are sensitive to this issue.

Wind turbines have governing mechanisms, systems that slow down the machines when winds get too strong to protect them from damage. Different governing systems result in different sound levels. When researching your options, we recommend that you listen to the turbines you’re considering buying in a variety of wind conditions, including those that require governing.

To reduce sound at ground level, be sure to mount your turbine on a tall tower. Suitable tower heights, which we’ll discuss later, are usually 80 to 120 feet. A residential wind turbine mounted high on a tower catches the smoother and stronger — and hence most productive — winds. This strategy also helps reduce sound levels on the ground because sound dissipates quickly over distance.

Residential (and commercial) wind machines are also much quieter than many people suspect because the sounds they make are partially drowned out by ambient sounds on windy days. Rustling leaves and wind blowing past one’s ears often drown out much of the sound produced by a residential wind turbine.

Sound is measured in two ways — by loudness and frequency. Loudness is measured in decibels (dB). Frequency is the pitch. A low note sounded on a guitar has a low frequency or pitch. A high
note has a high frequency. The average background noise in a house is about 50 dB. Nearby trees on a breezy day measure about 55 to 60 dB. Most of today’s residential wind turbines perform very near ambient levels over most of their operating range.

Even though the intensity of sound produced by a wind generator may be the same as ambient sound, the frequency may differ. As a result, wind turbine sounds may be distinguishable from ambient noises, even though they are not louder. You’ll hear a swooshing sound. In other words, while the sound of a wind turbine can be picked out of surrounding noise if a conscious effort is made to hear it, home-sized wind turbines are not the noisy contraptions that some people make them out to be.

Site Specific

Yet another criticism of small wind is that it is more site specific — or restricted — than solar energy.

To understand what this means, we begin by pointing out that there are good solar areas and good wind areas. In a good solar region, most people with a good southern exposure can access the same amount of sun. In a windy area, however, hills and valleys or stands of trees can dramatically reduce the amount of wind that blows across a piece of property. Therefore, even if you live in an area with sufficient winds, you may be unable to tap into the wind’s energy because of topography or nearby forests or stands of tall trees. That’s what critics mean when they say that wind energy is more site specific than solar.

That said, we should point out that solar resources also vary. If you live in a forest in a sunny location, for example, you’ll have a lot less solar energy than a nearby neighbor whose home is in a field. In addition, homeowners can access the wind at less-than-optimum sites by installing turbines on tall towers. Tall towers help you overcome topographical and other barriers.

Ice Throw

Like trees and power lines, wind turbines can ice up under certain conditions. Ice falling off the blades is known as ice throw, and is a concern that may arise during zoning hearings on residential wind turbines.

While ice builds up on blades and wind turbine towers during ice storms, it is typically deposited in very thin sheets. When the blades are warmed by sunlight, the ice tends to break up into small
pieces, not huge dangerous chunks, and drop to the ground.

Ice buildup on the blades of a wind turbine dramatically reduces the speed at which a turbine can spin. It’s a little like trying to drive a car with four flat tires. As a result, ice is not thrown from a turbine, it falls around the base of the tower — just as it does from trees and power lines.

Any prudent person would stay away from the tower base when ice is shed from the blades, as they would from trees or power lines covered with ice warming in the sun. Ice-laden trees are also considerably more dangerous, as branches can and often do break and fall to the ground, damaging power lines, cars and houses. Entire trees can topple as a result of ice buildup.

On the rare occasion that ice builds up on a wind turbine, experienced wind turbine operators shut down their machines until the Sun or warmer temperatures melt the ice since they cannot generate electricity spinning at such low revolutions per minute anyway.

Interference with Telecommunications

Some opponents of wind energy raise the issue of interference with telecommunications signals. This is simply not a problem. Turbines for homes and small businesses have small blades that do not interfere with such signals. Moreover, the blades of modern wind turbines are made out of materials that are “transparent” to telecommunications signals. As a result, small wind turbines are often installed to power remote telecommunications sites. Telecommunication equipment
wouldn’t be installed in such locations if there were a problem with interference.

The Advantages of Wind Energy

Although residential wind turbines and their energy source, the wind, have a few downsides, wind energy is an abundant and renewable resource. We won’t run out of wind for the foreseeable future, unlike oil and natural gas.

Small-scale wind energy could also help decrease our reliance on declining and costly supplies of oil — if electricity generated by wind is used to power electric or plug-in electric hybrid cars and
trucks, displacing gasoline, which is refined from oil.

Wind energy can also play a meaningful role in offsetting declining US natural gas supplies. In the United States, approximately 18 percent of all electricity is currently generated by natural
gas, according to the US Department of Energy. As supplies decline, wind could help ease the crunch, supplying a growing percentage of our nation’s electricity.

Wind could even eventually reduce our dependence on nuclear power as well. In the United States, nuclear power plants generate about 20 percent of the nation’s electricity. Although wind energy does have its impacts, it is a relatively benign technology compared to conventional sources of electricity. It could help all countries create cleaner and safer energy at a fraction of the environmental cost of conventional electrical energy production. Wind energy can help nations reduce global warming and devastating changes in our climate. Wind can also help homeowners and businesses do their part in solving other costly environmental problems such as acid rain.

Another benefit of wind energy is that, unlike oil, coal and nuclear energy, the wind is not owned by major energy companies or controlled by foreign nations. An increasing reliance on wind energy could therefore ease international political tension. Reducing our reliance on Middle Eastern oil could reduce costly military operations aimed, in part, at stabilizing a region where the largest oil reserves reside.

Wind is also a free resource. The cost of wind is not subject to price increases. A wind- and solar-powered future might be one subject to less inflation. This is not to say that wind energy will be
free of price increases. While the fuel itself (the wind) is free, the price of wind generators is likely to increase. That’s because it takes energy to extract and process minerals to make the steel and copper needed for wind turbines and towers. It also takes energy to make turbines and towers and ship and install them. As the price of conventional fuels and raw materials increases, the cost of wind energy also will go up.

Yet another advantage of wind-generated electricity is that it uses existing infrastructure, the electrical grid, and existing technologies. A transition to wind energy could occur fairly seamlessly.

Thanks to generous tax credits and other financial incentives, individuals in rural areas with good wind resources can meet all or part of their energy needs at rates that are often competitive with
conventional sources. In remote locations, wind or wind and solar electric hybrid systems can be cheaper than conventional power,

Fig. 1.5: Plug-In Hybrid. Electric cars and plug-in hybrids like the one shown here are the most promising automobile technologies on the horizon. They could be powered by electricity from the Sun and wind.

which requires the installation of costly electric lines that transport electricity from power plants to end users.

World Wind Energy Resources

Wind energy is clearly on the rise and could become a major source of electricity in years to come because wind is widely available and often abundant in many parts of the world. Significant resources are found on every continent. Tapping into the world’s windiest locations could theoretically provide 13 times more electricity than is currently produced worldwide, according to the Worldwatch Institute, a Washington, DC-based nonprofit organization.


Fig. 1.3: Small Wind Turbine on Tower. This ARE442 wind turbine installed at Mick’s house is
mounted on a guyed lattice tower. Maintenance is performed by climbing the tower.

Rated Power in Watts or Kilowatts

Wind turbines are commonly described in terms of rated power, also known as rated output or rated capacity. Rated power is the instantaneous output of the turbine (measured in watts) at a certain wind speed (called the rated speed) at a standard temperature and altitude. The rated power of small wind turbines falls in the range of 1,000 to 100,000 watts. One thousand watts is one kilowatt (kW). Large wind turbines include all of those turbines over 100 kilowatts. Most larger turbines, however, are rated at one megawatt or higher. A megawatt is a million watts or 1,000 kilowatts. It is important to note that wind turbines do not produce their rated power all of the time, only when they’re running at their rated wind speed.

In North America, wind is abundant much of the year in the Great Plains and in many northern states. It is also a year-round source of energy along the Pacific and Atlantic coasts and the shores of the Great Lakes. Tapping into the windiest locations in the United States, for example, in North and South Dakota could produce enough electricity to supply all of the nation’s electrical needs. Proponents of wind energy estimate that wind could eventually provide at least 20 to 30 percent of the electricity consumed in the United States and other countries.

Proponents of renewable energy envision a future powered by wind and a host of other clean, affordable renewable energy resources, among them solar energy, biomass, geothermal energy, tidal energy, wave energy and ocean currents (Figure 1.4).

Fig. 1.4: Solar Array. In a renewable energy future, large solar electric installations like this one will supplement electricity produced by other renewable resources, including wind, hydropower and biomass.