Harnessing the wind to do work is not new. In 3000 B.C.E. wind propelled the sail boats of ancient peoples living along the coasts of the Mediterranean Sea. The Swiss and French used wind-powered pumps in 600, which was shortly followed by windmills used to make flour from grain. By 1086, there were 5,624 water mills south of the Trent and the Severn rivers in England. Holland alone once had over 9,000. As late as the 1930s in the United States, windmills were the primary source of electricity for rural farms all across the Midwest. Although the advent of fossil fuel technologies shifted energy production from animal, wind, and water, dependence upon these finite fossil energy sources and concern over atmospheric pollutants, including carbon dioxide, are causing a resurgence of interest in early energy sources such as wind.
Wind turbines in use at the turn of the twenty-first century primarily produce electricity. In developed countries where electric grid systems connect cities, town, and rural areas, wind farms consisting of numerous wind turbines produce electricity directly to the electric grid. In developing countries, remote villages are not connected to the electric grid. Smaller wind turbines, singly or in groups, have tremendous potential to bring electricity to these remote locations without requiring the significant investment in transmission lines that would be required for grid connection. Turbines range in size from less than one meter in diameter to more than 50 meters in diameter, with power ratings from 300 watts to more than one megawatt. This wide variation enables village systems to be sized to meet specific electrical demand in the kilowatt range, while large, grid-connected utility applications produce many megawatts of electricity with a few large wind turbines.
HOW WIND TURBINES WORK
Wind is the motion of the atmosphere, which is a fluid. As the wind approaches an airfoil-shaped object, the velocity changes as the fluid passes the object, creating a pressure gradient from one side of the object to the other. This pressure gradient creates a net force on one side of the object, causing it to move in the fluid. When wind hits the airfoil-shaped blade of a turbine, the lift force that is created causes the blade to rotate about the main shaft. The main shaft is connected to an electric generator. When the rotor spins due to forces from the wind, the generator creates electricity that can be fed directly into the electric grid or into a system of batteries.
Aerodynamic drag has also been used to capture energy from the wind. Drag mechanisms consist of flat or cup-shaped devices that turn the rotor. The wind simply pushes the device around the main shaft. Anemometers used to measure wind speed are often drag devices, as are traditional farm windmills.
Airplane propeller analysis relies upon the "axial momentum" theory, which is based on energy, momentum, and mass conservation laws. This theory has been applied to wind turbines as well. The power (Pw) of a fluid passing across an area perpendicular to the flow is where ρ is the air density, A is the disk area perpendicular to the wind, and Vw> is the wind speed passing through the disk area. For instance, if the wind speed is 10 m/s and the rotor area is 1,200 m2, the available power is 600 kW. When the wind speed doubles to 20 m/s, the available power increases to 4,800 kW. This value represents the total power available in the wind, but the turbine cannot extract all of that power. If the turbine were able to extract all the available power, the wind speed would drop to zero downwind of the rotor.
A simple, ideal model of fluid flow through a rotor was used by both F. W. Lanchester and A. Betz (Lanchester, 1915; Betz, 1920) to study the limitation of power extracted from the wind. This "actuator disk" model is shown in Figure 1. It assumes that a perfectly frictionless fluid passes through an actuator disk, which represents the wind turbine. The fluid approaching the actuator disk slows, creating a pressure greater than atmospheric pressure. On the downwind side of the disk, the pressure drops below atmospheric pressure due to extraction of energy from the impinging fluid. As the fluid moves further downstream of the turbine, atmospheric pressure is recovered. Using axial momentum theory, Betz and Lanchester independently showed that the maximum fraction of the wind power that can be extracted is 16/27 or about 59 percent. This is known as the Lanchester/Betz limit, or, more commonly, the Betz limit, and is assumed to be an upper limit to any device that extracts kinetic energy from a fluid stream. Real wind turbines capture from 25 percent to more than 40 percent of the energy in the wind. More refined engineering analyses account for fluid friction, or viscosity, and wake rotation (Eggleston and Stoddard, 1987; Hansen and Butterfield, 1993).
There are two major types of wind turbines: horizontal-axis and vertical-axis. A wind turbine that rotates about an axis parallel to the wind is a horizontal-axis wind turbine (HAWT). Although HAWTs have not been proven clearly superior to Vertical-Axis Wind Turbines (VAWTs), they have dominated recent installations. Of all the utility-scale turbines installed today, 97 percent are HAWTs. A variety of configurations exist. HAWT rotors differ in orientation (upwind or downwind of the tower), flexibility (rigid or teetered), and number of blades (usually two or three).
Horizontal-Axis Wind Turbines (HAWTs)
An upwind turbine rotates upwind of the tower. In order to maintain the rotor's upwind position, a yaw mechanism is required to position the rotor as the wind direction shifts. In this configuration, the wind flowing toward the rotor is unobstructed over the entire rotor. Conversely, the wind flowing toward a downwind rotor is obstructed by the turbine tower over part of the rotor area. This causes fluctuating loads on each blade as it passes behind the tower, which can decrease the fatigue life of the rotor. The downwind turbine, however, aligns itself with the prevailing wind passively, eliminating the need for additional yaw drive components.
Flexibility at the rotor hub has been used to alter the load conditions on the blades. A rigid hub turbine generally has two or three blades attached to the hub. A teetered rotor, however, consists of two blades attached to the hub, which forms a hinge with the main shaft of the turbine. When the wind speed above the main shaft is higher than that below the main shaft, the rotor moves slightly downwind over the top half of the rotational cycle to reduce the loads on the blades. Although the cyclic loads are reduced by teetering the rotor, when the wind causes the rotor to exceed its maximum flexibility range hitting the teeter stops, large, transient loads can be introduced.
Although multiple-blade turbines are effective, two- and three-blade rotors are most cost-effective. Two-blade rotors commonly use teetering hinges, and all three-blade rotors are mounted on rigid hubs. Rotors up to 15 m in diameter are economically feasible and simplified with three blades. For large rotors exceeding 30 m in diameter, the blade weight is directly related to turbine costs. Thus, reducing the number of blades from three to two results in lower system costs with respect to the potential power available in the wind. For mid-range turbines, 15 m to 30 m in diameter, the trade off between power production and reduced cost as a result of reduced blade weight is more difficult to determine.
Most turbines are designed to rotate at a constant speed over a specific range of wind speed conditions. The generators in these turbines produce electricity compatible with the established grid system into which electricity is fed. Operating the turbine at variable rotor speeds increases the range of wind speeds over which the turbine operates. The amount of energy produced annually is increased as well. However, sophisticated power electronics is required to convert the electricity to the grid standard frequency.
Vertical-Axis Wind Turbines
A vertical-axis wind turbine (VAWT) rotates about an axis perpendicular to the wind. The design resembling an eggbeater was patented by D. G. M. Darrieus, a French inventor, in the 1920s. Because the axis of rotation is perpendicular to the ground, components such as the generator and gearbox are closer to the ground, making servicing these turbines fairly easy. Also, the turbine is not dependent upon its position relative to the wind direction. Since the blades cannot be pitched, these turbines are not self-starting. The greatest disadvantage of VAWTs is the short machine lifetime. The curved blades are susceptible to a variety of vibration problems, that lead to fatigue damage.
A modern VAWT that relies upon aerodynamic drag is known as a Savonius wind turbine. Sigurd Savonius, a Finnish inventor, developed this design in 1924. Two S-shaped panels are arranged to cup the wind and direct it between the two blades. This recirculation improves the performance of this drag device, but at best only 30 percent of the power available in the wind can be extracted.
Utility-scale wind turbines range in size from 25 m to more than 50 m in diameter, with power ratings from 250 kW to 750 kW. Modern, electricity-producing wind turbines are often placed in large areas called wind farms or wind power plants. Wind farms consist of numerous, sometimes hundreds, of turbines regularly spaced on ridges or other unobstructed areas. The spacing varies depending upon the size of the turbine. Until recently California had the largest wind farms in the world. In the late 1990s, installations with capacities of 100–200 MW in the midwestern United States. Figure 2 shows the producing wind power plants in the U.S. resulting in a total capacity of 2.471 MW in 2000. This represents a small fraction of the 750 GW generating capacity in the U.S. Globally, wind power exceeded 10 GW of installed capacity in 1999. Much of the worldwide growth in wind energy use is in Europe, particularly Denmark, Germany, and Spain. To reduce pollution, many European countries subsidize wind generated electricity.
Future wind turbines will exploit the accomplishments of many years of U.S. government-supported research and development, primarily at the national laboratories, over the past twenty-five years. During the 1980s, a series of airfoils specifically designed for wind turbine applications were shown to increase annual power production by 30 percent. Detailed studies of the aerodynamics of wind turbines operating in the three-dimensional environment have led to improved models that are used for turbine design. These modeling capabilities allow turbine designers to test new concepts on paper before building prototype machines.
Several avenues for improving power production and cutting costs are being pursued by turbine designers. Building taller towers places the turbine in higher wind regimes, which increases the potential power production. In addition to taller towers, larger rotor diameters will improve power production. Increasing blade flexibility will reduce loads that reduce the fatigue life of blades, but sophisticated dynamic models are required to make such designs. Sophisticated control algorithms will monitor the turbines in order to accommodate extreme load conditions.
Because one-third of the world's population does not have access to electricity, many countries lacking grid systems in remote, rural areas are exploring various methods of providing citizens with access to electricity without costly grid extensions. Wind turbines are an intermittent source of electricity because the wind resource is not constant. For this reason, some form of energy storage or additional energy source is required to produce electricity on demand. Systems designed to supply entire villages or single homes are used throughout the world.
Battery systems are the most common form of energy storage. In some developing countries, battery-charging stations have been built using wind turbines. These sites simply charge batteries that people rent. The discharged batteries are exchanged for charged batteries at regular intervals. Other systems are designed for storage batteries to provide electricity during times when the wind is not blowing.
Wind turbine systems are often combined with other energy sources such as photovoltaic panels or diesel generators. Many remote areas currently rely upon diesel generators for electricity. Transportation costs limit the amount of diesel fuel that can be supplied, and diesel fuel storage poses environmental risks. By combining the generators with wind turbines, diesel fuel use is reduced. Wind and solar resources often complement each other. It is common in many areas for wind resources to be strongest in seasons when the solar resource is diminished, and vice versa. Systems that combine energy sources are called hybrid systems.
Although electricity production is the primary use of wind turbines, other applications still exist. Water pumping, desalination, and ice-making are applications that wind turbines serve. Wind turbine rotors for small-scale applications generally range in size from less than 1 m diameter to 15 m diameter, with power ratings from 300 W to 50 kW.
SITING WIND TURBINES
To obtain the best productivity from a wind turbine, it must be sited adequately. Whether establishing a wind farm with a hundred turbines or a single turbine for a home, documenting the wind conditions at a given site is a necessary step. Several databases of wind conditions have been established over the years (Elliot et al., 1987). Improvements in geographic information systems have enabled mapping wind conditions for very specific regions. Figure 3 shows the wind resource throughout the United States. Class 1 is the least energetic, while Class 7 is the most energetic. Wind turbines are generally placed in regions of Classes 3-7. This map indicates that a large part of the country has wind conditions that are conducive to wind turbine applications. These advanced mapping techniques can be used to compare one ridge to another in order to place the turbines in the most productive regions.
In addition to obtaining adequate wind resources, site selection sites for wind turbines must also consider avian populations. Several studies have been performed to determine the impact that turbines have on bird populations, with inconclusive results (Sinclair and Morrison, 1997). However, siting turbines to avoid nesting and migration patterns appears to reduce the impact that turbines have on bird mortality.
Other considerations in siting wind turbines are visual impact and noise, particularly in densely populated areas (National Wind Coordinating Committee Siting Subcommittee, 1998). Due to Europe's high population density, European wind turbine manufacturers are actively examining the potential of placing wind turbines in offshore wind farms.
Deregulation of the electric utility industry presents many uncertainties for future generation installations. Although the outcome of deregulation is unknown, ownership of the generation facilities and transmission services will most likely be distributed among distinct companies. In the face of such uncertainties, it is difficult to predict which issues regarding renewable energy will dominate. However, the benefits of integrating the utility mix with wind energy and the determination of the cost of wind energy, are issues that will be relevant to most deregulation scenarios.
Wind energy economics focuses on the fuel-saving aspects of this renewable resource, but capacity benefits and pollution reduction are important considerations as well. The capital costs are significant, but there is no annual fuel cost as is associated with fossil fuel technologies. Thus, wind energy has been used to displace fossil fuel consumption through load-matching and peak-shaving techniques. In other words, when a utility requires additional energy at peak times or peak weather conditions, wind energy is used to meet those specific needs. In addition to fuel savings, wind energy has been shown to provide capacity benefits (Billinton and Chen 1997). Studies have shown that although wind is an intermittent source, wind power plants actually produce consistent, reliable power that can be predicted. This capacity can be 15 percent to 40 percent below the installed capacity. Last, the emission-free nature of wind turbines could be exploited in a carbon-rading scenario addressing global climate change.
Two methods of determining the cost of wind energy are the Fixed Charge Rate (FCR) and the Levelized Cost of Energy (LCOE). An FCR is the rate at which revenue must be collected annually from customers in order to pay the capital cost of investment. While this incorporates the actual cost of the wind turbines, this method is not useful for comparing wind energy to other generation sources. The LCOE is used for comparison of a variety of generation technologies that may vary significantly in size and operating and investment time periods. This metric incorporates the total life cycle cost, the present value of operation and maintenance costs, and the present value of depreciation on an annual basis.
Subsidies, in the form of financing sources, and tax structures significantly impact the levelized cost of energy. The Renewable Energy Production Incentive (REPI), enacted in 1992, at $0.015/kilowatt-hour (kWh), applies only to public utilities. This tax incentive is renewed annually by Congress, making its longevity uncertain. Private owners of wind power plants are eligible for the federal Production Tax Credit (PTC), which is also $0.015/kWh. A project generally qualifies for this tax credit during its first ten years of operation. This credit is also subject to Congressional approval. Ownership by investor-owned utilities (IOUs), and internal versus external project financing, also affect the LCOE. Cost differences that vary with ownership, financing options, and tax structures can be as great as $0.02/kWh (Wiser and Kahn, 1996).
The annual LCOE for wind turbines has decreased dramatically since 1980. At that time, the LCOE was $0.35/kWh. In 1998, wind power plant projects were bid from $0.03kWh to $0.06/kWh. These numbers still exceed similar figures of merit for fossil fuel generation facilities in general. However, wind energy can become competitive when new generation capacity is required and fossil fuel costs are high, when other incentives encourage the use of clean energy sources. For example, a wind power plant installed in Minnesota was mandated in order to offset the need for storing nuclear plant wastes.
Incentives that currently encourage the use of clean energy sources may or may not survive the deregulation process. Green pricing has become a popular method for utilities to add clean energy sources, with consumers volunteering to pay the extra cost associated with renewable energy technologies. Some proposed restructuring scenarios include a Renewable Portfolio Standard (RPS), which mandates that a percentage of any utility's energy mix be comprised of renewable energy sources. Last, distributed generation systems may receive favorable status in deregulation. Small generation systems currently are not financially competitive in many areas, but through deregulation individuals or cooperatives may be able to install small systems economically.
Wind energy was the fastest growing energy technology from 1995 to 1999. This translates to an annual market value of over $1.5 billion. Supportive policies in some European countries, improved technology, and the dramatic drop in the cost of wind energy, have contributed to the growth of wind energy.
M. Maureen Hand
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