Wind is a form of solar power. As the earth rotates and the sun heats the atmosphere, the hot air rises above cooler air, creating a current. The wind turns the blades of a turbine, which spins an internal shaft connected to a generator and makes mechanical power or electricity used to power homes and other purposes. Power is extracted when wind makes contact with wings that are set at a slight angle to its direction. These wings are typically configured as blades attached to the hub of a revolving rotor. The wind impact forces the blades to move in a circular motion around the rotor's axle. The rotor axle turns the generator and creates electricity. The rotor, its axle and the generator assembly are typically set high on a platform or tower to allow clearance for the blade rotation and to maximize the impact of local winds. This basically simple description is made more complicated in practice by the physics of wind movement.

The mass flow of any liquid or gas increases linearly with increased velocity (Q = M x V, with Q equaling mass flow, M equaling the mass of the moving liquid or gas, and V equaling the flow velocity or wind speed). The kinetic energy available to any liquid or gas flow (or any moving object for that matter) is a function of the square of its velocity (K = ½ x M x V2 with K equaling the kinetic energy of the flowing gas). However, since the amount of mass (M) in contact with the blades increases with velocity, the power generated by wind is a function of the wind velocity cubed. This means a 20-mph wind has a power potential 64 times greater than that of a 5-mph wind, not 4 times greater. From this it can be easily seen that even minor fluctuations in wind velocity can result in extreme changes in power output. Most wind turbines are equipped with sets of slip rings and yaw mechanisms to reduce the extremes of power output. These yaw systems are important to prevent the rotors from overheating at high wind speeds.

Most of the blowing wind does not have direct impact on the blades. It has been determined that a wind turbine can extract at most 59% of the energy from available wind, the rest being lost to diversion and turbulence. No matter what the rotor blade design, this represents the theoretical upper limit of wind power extraction for any turbine. Practical efficiency limits are no higher than 35%.

In addition to the cube velocity law described above, wind turbines are also subject to the 1/7th power law. This results from the fact that wind blows faster at higher altitudes due to the reduction of drag along the Earth's surface and the reduced viscosity of the air at higher altitudes. According to the 1/7th power law, wind speed increases proportionately to the seventh root of altitude. For example, tripling the altitude of a wind turbine increases the available wind velocity by 17% (3 1/7 = 1.17) and its available power by 60% (1.17 3 = 1.60). As a general rule of thumb, wind power is economical when average wind speed is greater than 12.5 mph.

Distributed Wind Power Advantages and Disadvantages
As wind power becomes increasingly cost competitive, its advantages and disadvantages come into sharper focus.  Its primary disadvantage is its intermittence. Wind doesn't blow consistently or even all the time. As such, it can't be used a sole source of electricity and must be used to augment power supplied by existing transmission grids. However, small-scale and individual wind power users can utilize electrical batteries or fuel cells to store excess electricity generated by wind turbines during periods of high wind velocity for later use during periods of doldrums.

Another drawback is the fact that the best available wind power isn't always where it's most needed. The most abundant winds blow in the Midwest and the Great Plains, with North Dakota being the windiest state, followed by Texas, Kansas, South Dakota and Montana. Offshore winds that blow more or less continuously make coastal areas prime locations for wind power generation. However, recent technological advances make it possible to use wind power economically in any state.

Aesthetics and potential damage to bird populations are also concerns, especially when wind turbines are constructed in popular tourist areas, such as Cape Cod, Long Island, and the New Jersey shore. Protesters (who ironically would normally be in favor of such non-polluting renewable sources of energy) have opposed wind farm construction on the grounds that these facilities cause noise, are eyesores, and kill birds (including those that are on the federally protected endangered species list). Concern for species protection has resulted in limits to proposed wind turbine construction in Maryland. Aesthetic concerns and fears for the potential negative impacts on tourism have resulted in a 15-month moratorium on wind turbine construction off the New Jersey shore. Critics charge that a proposed wind farm in Vermont will ruin the rural countryside and hurt tourism. Kansas is proposing guidelines for the siting and construction of wind turbines to protect the last of the state's endangered prairie chicken populations.  Proponents of wind energy claim that bird kill fears are overstated (studies show that the number of birds killed by wind turbines is negligible compared to other causes—such as traffic, hunting, power lines, pets, and collisions with glass windows)—and the visual impact of wind farms is far less than that of oil-drilling operations and other energy generation facilities such as coal-fired electrical plants.

Wind power's abundant potential is perhaps its greatest advantage. Though not constant, wind is always blowing. Given its near universal accessibility, wind power is not subject to price shocks, supply shortages and international politics.  For every kWh generated by American wind turbines, one less kWh is needed from foreign sources in the form of oil shipments.  Continued technological advances can only continue to reduce the cost per kWh delivered for wind power facilities.

Wind power's environmental advantages are obvious. Compared to other renewable sources of energy, wind power has a relatively small “ecological footprint.”  With the common practice of farmers leasing already cleared land for the construction of wind turbines, very little change to the landscape (such as clearing of trees) is required and the land can still be used for farming and grazing. Wind turbines have no long-term environmental effects, such as hazardous waste disposal or noxious emissions.  For every 100 acres designated for use by a wind farm, only 1 acre is required for turbine tower foundations and access roads.  The other 99 acres can still be used by wildlife and farming activities.  Nationwide, the overall potential footprint for wind power is also relatively small. The National Wind Technology Center (NWTC) estimates that good wind areas, which include 6% of the contiguous United States, have the potential to generate more than one and a half times the electricity consumption of the US.

Financially, wind power has certain advantages. The payback period for wind turbine and wind farm construction is very short compared to other sources of energy. The total capital costs for production, installation, and decommission as well as the operating costs of a wind turbine are usually earned back within the first three months of operation. Though there are available tax credits for wind power installation, wind power does not require the massive government subsidies given to coal and nuclear power plants. Given the low potential for damage to human health and the environment, disaster insurance premiums required for wind power generators are exceptionally low for the power generating industry.
Another advantage is wind power's scalability. The size of a wind-power facility can range from a single small turbine to a massive wind farm with hundreds of large wind rotors. This provides site specific energy delivery and planning flexibility not found in typical fossil fuel or nuclear power plants that require a minimum customer base to justify the costs of construction.

Distributed Wind Power Economics
How much does wind power cost and how competitive is it with traditional fossil fuels and nuclear power?  Alone among renewable energy resources, wind power is actually competitive with traditional energy sources for many applications.  It already competes with new coal- and gas-fired generator plants as its cost per kWh continues to decline.  This decline is due to ongoing research and technological advances and has resulted in an 85% drop in the cost of creating energy from wind in the last 20 years. Its current cost per kWh is currently 4 to 6 cents. Depending on the region of the country, natural gas costs 4 to 4.5 cents per kWh, coal costs 4.8 to 5.5 cents per kWh, hydro costs 5.1 to 11.3 cents per kWh and nuclear costs 11 to 14.5 cents per kWh. These figures don't include many hidden costs (processing costs, environmental impacts, carbon dioxide emissions, ozone build up, hazardous waste disposal and acid rain). Wind power has none of these hidden costs and produces pollution-free electricity. 

However, like all renewable energy sources, it has a relatively low energy production density (as measured in kWh per acre of generation facility). Wind power is intermittent, inconsistent and highly variable from day to day and season to season. Wind power is also diffuse and requires multiple wind turbines over many dozens or hundreds of acres to produce the same amount of electricity as a coal or nuclear plant occupying only a few acres. The cost of capitalizing the land adds to the customer's overall monthly billing and the need for spread-out facilities severely limits wind power usage in densely populated areas. However, in certain areas, such as moderately populated rural areas where land prices are moderate or low, direct use of wind-generated electricity is economically feasible and desirable. In many cases, excess electricity generated by wind turbines can be sold back to a main power grid serving a large urban area.

What are the financial and tax advantages for switching to wind power?  Because of a recently renewed tax credit, the US has the potential to enter the top tier of wind power generation. The tax credit can provide a substantial return on initial capital costs. For example, an installation of 1 MW of wind turbine capacity costs approximately $1.5 million. Over the 10 years following installation, the credit can be equivalent to $500,000, or one third of the installation cost. Though in the top five nations in terms of overall wind power utilization, the US has a relatively low per capita level of wind energy use, and its wind generators tend to be isolated in secondary markets. American investors and investors from Europe stand ready to expand this market. However, the currently strong euro makes it difficult for European investors to directly compete in this country. It's hard to keep wind power generation technology secret or even proprietary, not to mention the technologies used by European companies like Vestas and Gamesa and the main American wind turbine manufacturer, General Electric. Though Gamesa is planning a blade factory in Pennsylvania, other European firms are keeping their production facilities in Europe.

So how much wind power is being generated and where is it being produced?  As of 2005, there are 47,317 MW of installed wind power capacity throughout the world and this amount is growing at a rate of 20% per year. This makes wind power the world's fastest growing energy source. That growth rate results in a doubling of wind-power capacity every four years. Five countries account for 80% of the world's wind-power capacity. These countries are: Germany, Spain, US, Denmark and India. Together, they have a total capacity of nearly 38,000 MW. Regional differences are also pronounced, with Europe (5,774 MW) having the highest wind-power usage rate and Africa (47 MW) having the lowest. 

Here in the US, wind power continues to grow apace. In 2004, wind power made up less than 1% of the America's electricity, but is projected to be at 6% by 2020. In 2005, wind generated over 17 billion kWh annually, enough to power about 1.6 million homes. By the year 2020, wind could supply power to 25 million homes, or about 6% of total electricity used in the US. 

Recent Distributed Wind Power Technical Advances
As mentioned above, technical advances both big and small (along with economics of scale as large-scale wind turbine production begins in earnest) have radically reduced the cost of wind-generated electricity over the last two decades.  Incremental improvements in design, operating mechanisms, manufacturing and construction have accumulated over the years to account for the bulk of the cost reduction.  New designs and operating concepts have the potential for greater cost savings.  Some of the more innovative new designs are listed below:

Push-Pull Vertical Axis Windmills
A new type of vertical axis wind turbine employs hydraulics to create both push on the front of the blades and pull on their back sides.  Studies show that this design may exceed previous limits on practical efficiency, extracting up to 40% of the wind's potential power.  The pull force exerted on the back of the blades is caused by a lift effect similar to that used by airplane wings.  This back pressure vortex causes the blades to move slightly faster than the wind speed.  Projected costs vary from 2.5 to 3.5 cents per kWh.  Though its optimal operating wind speed is about 30 mph, this design can also generate significant amounts of electricity from wind speeds as high as 70 mph without serious losses in efficiency. Its rugged design can handle up to 150-mph winds without structural damage.

The design resembles an empty “beer can” with curved paddle blades rotating around a vertical axis located in the center of the canister.  At a maximum height of 96 feet, the turbines can be placed in urban and residential areas where taller propeller turbines are not allowed. Its more compact shape and slower operating speeds (compared to traditional blade rotors) reduce potential aesthetic problems while greatly reducing the potential harm to birds.

Floating Turbines
In addition to oil, Norway has vast wind-power resources that can be extracted from the North Sea. The problem was how to build a stable platform for turbines to harvest the wind. The Norwegian company Norsk Hydro has solved this problem. Its concept is called Hywind, and it is basically a floating concrete platform (similar to structures developed for the North Sea oil industry) supporting large wind turbines. Hywind is planned to supplement, not replace, wind parks constructed on land. Its most economical use will be in energy-deficient areas without accessible land (coastal areas and steep fjords).

A demonstration pilot project planned for 2007 will evaluate the potential for wind parks far offshore in sea depths up to 1,000 feet. The location of such facilities eliminates one of the chief objections to wind-power development: that of aesthetics (this would be a classic case of “out of sight, out of mind”). Wind-speed data for the North Sea (based on weather data gathered over 30 years) indicate that Hywind will be exceptionally efficient. The pilot project will use wind turbines with a generating capacity of 3 MW installed on towers with heights of 260 feet with rotor diameters of 145 feet to provide clearance for storm waves as high as 115 feet. Future models will have generating capacities of 5 MW and rotor diameters of 400 feet. They will be part of floating wind parks with 200 turbines capable of producing 4 terawatt hours (TWh) of electricity per year, enough for 200,000 households.

Flying Windmills
One of the more exotic wind power proposals is, in effect, a “flying carpet” with power-generating turbines attached. The proposed facility is a helicopter-like craft which flies to an altitude of 15,000 feet and stays there, held aloft by wings that generate lift from the wind. It is held in placed by guy lines and cables attached to a ground anchor structure.  The cables also allow the transmission of electricity from the flying turbines to customers on the ground. Initially, the craft would achieve altitude by feeding electricity to the rotors, which then act like electric motors that turn the windmill blades like airplane propellers. The flying wind turbine craft will be deployed in the first layer of the atmosphere, called the troposphere. Estimates project that the craft will produce 200 kilowatts per hour of electricity. Once aloft, since winds usually blow in a horizontal direction, the turbine blades would catch the wind, providing both lift and generating electricity. To prevent loss of altitude if the wind speed declines, the craft can be augmented with dirigible balloons.
The operational efficiency of a flying wind turbine, depending on its location, can be as high as 90%. This is three times higher than a typical ground-based wind turbine. Wind-tunnel data suggests that a flying wind farm of 600 generators could produce as much energy as three nuclear power plants. As odd as this may sound, it solves two problems with wind power simultaneously: Noise and aesthetics are no longer concerns, and the constant wind speed in the upper atmosphere eliminates the intermittent nature of wind power. By definition, a flying wind turbine has zero environmental footprint.

Wind Power Projects
So where is wind power being used most effectively, and where are recent advances being applied? The following is a short list of some prominent wind power projects:

Charting Wind Power Potential with Wind Power Maps
Not exactly a wind farm construction project, the accurate mapping of the world's (and North America's) wind power potential makes the efficient location of wind farms possible. Scientists at Stanford University have produced a world map that plots wind power potential. This map is based on wind speed data readings taken from 7,500 surface stations and 500 balloon-launch stations. The balloons were floated to a height of 300 feet, the typical height of a wind turbine tower. Though all regions of the globe have wind power potential, the map shows that North America had the greatest potential for wind energy (with areas of greatest potential being the Great Lakes region and along both the New England and Pacific Northwest coastlines)

Iowa Project
A consortium of Iowa utilities plans to create an Iowa wind farm whose main crop is electricity. With $2.8 million dollars in seed money provided by the Department of Energy and the Electric Power Research Institute, the project has been led by the Cedar Falls Utilities. Cedar Falls acts as the agent for a consortium of seven municipal utilities (Cedar Falls Utilities, Algona Municipal Utilities, City of Estherville, City of Westfield, Ellsworth Municipal Electric, Fonda Municipal Electric, and Montezuma Municipal Light and Power) participating in the project.  Collectively, this consortium is known as the Iowa Distributed Wind Generation Project (IWDGP). Located near Algona, IA, where the wind speeds average 16.5 miles per hour, the IWDGP has constructed 750 kW wind turbines on three towers, each 165-feet high, for a total energy capacity of 2,250 kW. The turbines use the latest variable speed and variable pitch technologies to maximize energy efficiencies. Existing electrical transmission lines will carry the electricity generated by the turbines to regional customers.

The Judith Gap Wind Farm, Montana
This year, construction will be completed on a wind farm consisting of 90 turbines located just south of Judith Gap in Montana. With a reported construction cost of $150 million, the facility will provide enough electricity for 300,000 customers. Each tower will be 260 feet high, capped with rotor blades each measuring 387 feet in diameter. It is designed to generate 135-150 MW (equivalent to about 8% of the sponsoring utility's current capacity). When completed, the wind farm will cover an area of about 40 square miles, eight miles north to south and five miles east to west. Power transmission lines will connect the turbines to a substation that will feed the electricity to the local power grid.   Running parallel with these power lines is a system of fiber optic cables that allow the turbines to communicate with each other.

The Desert Sky Project in West Texas
Completed in 2001, the Desert Sky Wind Farm is a 160.5-MW facility located in Pecos County. The wind farm consists of 107 turbines rated at 1.5 MW each and covers an area of about 15 square miles. Each tower is approximately 200 feet in height and supports rotor blades with 231-foot diameters. Each turbine is designed to operate optimally at wind speeds between eight and 56 mph. Power generated by the wind turbines is carried by a 128-kV line to the Texas power grid, providing enough electricity for 40,000 homes.

DANIEL P. DUFFEY, P.E., is an environmental engineer in Cincinnati, OH.

DE - March/April 2006