Wind Energy 101

R. Gerald Nix
National Renewable Energy Laboratory

Wind energy is a commercially available renewable energy source, with state-of-the-art wind plants producing electricity at about $0.05 per kWh.
However, even at that production cost, wind-generated electricity is not yet fully cost-competitive with coal- or natural-gas-produced electricity for the bulk electricity market.
The wind is a proven energy source; it is not resource-limited in the United States, and there are no insolvable technical constraints.

This paper describes current and historical technology, characterizes existing trends, and describes the research and development required to reduce the cost of wind-generated electricity to full competitiveness with fossil-fuel-generated electricity for the bulk electricity market.

Potential markets are described.

The Resource

One way to characterize winds is to use seven classes according to power density:

• class 1 is the lowest and class 7 is the greatest.
• The wind power density is proportional to the wind velocity raised to the third power (velocity cubed).
• For utility applications, class 4 or higher energy classes are usually required.
• Class 4 winds have an average power density in the range of 320-400 W/m2, which corresponds to a moderate speed of about 5.8 m/s (13 mph measured at a height of 10 m)

Researchers estimate that there is enough wind potential in the United States to displace at least 45 quads of primary energy annually used to generate electricity [1].

• This is based on "class 4" winds or greater and the judicious use of land. For reference, the United States used about 30 quads of primary energy to generate electricity in 1993 [2].
• A quad is a quadrillion (1015) BTUs or about equivalent to the energy in 167,000,000 barrels of oil.
• Although almost all of the currently installed wind electric generation capacity is in California, the major wind energy resource is virtually untapped in the Great Plains region.
• About 90% of the wind energy resource in the contiguous United States is contained in 11 Great Plains states.
• This area ranges from Texas north to Canada, and east from Colorado into Iowa. Expansion of wind energy into this high resource area is just beginning, with promise of significant future implementation.
• For reference, about 40,000 MW of wind-generated electricity is required to displace 1 quad of primary energy consumption for fossil-fueled power generation.
• A good description of the wind resource is found in the article by Schwartz [3].

Conversion Techniques

Wind energy appears to be a conceptually simple technology: a set of turbine blades driven by the wind turns a mechanical shaft coupling to a generator which produces electricity. These include the rotor blades, gearbox, generator, nacelle and tower. It is the reduction of this simple concept to practice which results in significant engineering and materials challenges.

The general goals of wind energy engineering are to:

• Reduce the cost of the equipment
• Improve energy capture from the wind
• Reduce maintenance, increase system and component lifetimes, Increase reliability
• Addressing aesthetics and environmental effects.

Modern turbines are either horizontal-axis or vertical-axis machines,that make full use of lift-generating airfoils (older generation windmills relied primarily on drag forces rather than aerodynamic lift forces to turn the rotor).

Each type of turbine has advantages and disadvantages. Both types are commercially available although the horizontal-axis turbine is predominant. Horizontal-axis turbines are built with differing numbers of blades, typically two or three.

Turbines for utility applications are normally installed in clusters of 5 to 50 MW which are called windplants or wind farms.

• Modern wind turbines have efficiencies of about 40%, with availabilities typically exceeding 97%.
• Capacity factor (ratio of annual produced energy to annual nameplate energy) has typical field value of 20% to 25%.
• Capacity factor is very site specific because it reflects the fraction of the time that the wind blows.
• In areas of relatively constant winds, e.g., trade winds, capacity factor can be as great as 60% to 70%. A description of various types of wind turbines is found in Eldridge [4].
  History

More than six million windmills and wind turbines have been installed in the United States in the last 150 years. Most were windmills with a rating of less than 1 hp. The most common windmill application has been water pumping, especially on remote farms and ranches. Wind turbines, usually rated at 1 kW or less, were originally used to supply electricity to remote sites.

• Typical is the Jacobs turbine, tens of thousands of which were produced from 1930 to 1960.
• The first large wind turbine was the Smith-Putman unit, which was erected in southern Vermont during World War II. It was rated at 1.25 MW of alternating current (ac) electricity and used a two-bladed metal rotor 53.3 m (175 ft) in diameter.

By 1960, the production of wind turbines in the United States had essentially stopped as most of the rural United States had been electrified via a grid of wires carrying electricity from more cost-effective central fossil-fired generating stations. The fuel-oil uncertainties, fuel-price escalations, and heightened environmental awareness of the 1970s brought a flurry of activity to develop cost-effective wind turbines.

The U.S. Department of Energy (DOE) and the National Aeronautics and Space Administration (NASA) led the activity by developing large machines rated up to 4.5 MW.

These large research and development machines had mechanical and structural problems, and efforts were stopped before the technology reached maturation. Nevertheless, these machines provided valuable experience and proved the value of many technical innovations. None of these large turbines are currently operating in a utility system.

• Numerous other machines (rated at 50-300 kW) were developed by industry in the 1980s and installed to produce electricity that was fed into the utility grid.
• Smaller turbines (1-10 kW) were developed for remote applications. All of these turbines were significantly advanced beyond the technology of the older machines, although there were still opportunities for significant improvements.

Most of the utility-size turbines (100-300 kW) were installed in California under lucrative power purchase agreements and favorable investment tax credits.

The three primary locations are:

• Altamont Pass near San Francisco,
• Tehachapi near Bakersfield
• San Gorgonio near Palm Springs.

The turbines were of widely differing quality, as were the developers and operators of the wind plants. However, after a sorting-out period, well-managed and well-operated wind plants resulted.

Current Status

More than 16,000 wind turbines are currently installed in California with a total generating capacity approaching 1700 MW.

• The turbines in the wind plants are privately owned, with the electricity sold to the local utilities.
• These turbines generate more than 3 billion kWh of electricity per year-enough electricity to meet the residential requirements of a city of about 1 million people.
• This combined capacity is equivalent to a medium-sized nuclear plant.
• About 1% of the electricity used in California is generated from wind.
• For reference, about 40,000 MW of wind-generated electricity is required to displace 1 quad of primary energy consumption for fossil-fueled power generation.
  
• Most of the early wind farms in California used early 1980s technology to produce electricity at a cost of $0.07-$0.10 per kWh, depending on the location, design, and operating policy.
• State-of-the-art plants are being built to produce electricity at a selling price of less than $0.05 per kWh at class 4 or greater wind sites.
• Around the year 2000, when the innovative next-generation wind turbines begin operating, the cost of wind-generated electricity is estimated to drop to less than $0.04 per kWh [5] at these sites.
  

Potential Markets: There are 4 major potential markets:

• 1) domestic utility grids,
• 2) foreign utility grids,
• 3) village power systems in developing countries
• 4) domestic remote power systems.

These markets vary in size and have different characteristics.

• The domestic and foreign utility grid-connected applications typically require larger (300-500+ kW) turbines installed in clusters of 5-50+ MW.

The village power market is significant because a large number of people (> 1 billion) live without electricity, often in areas where a large grid construction or expansion is prohibitively costly. The village power market is available now, with an important driving force being the need to stem the flow of individuals from rural areas to already overburdened cities of the third world.

In many cases, supplying electricity to rural villages will allow development of a local industrial economy which results in jobs and a lessening of the incentive to migrate to a larger city.

Often the power plant of choice for village power applications is a hybrid system, with wind turbines coupled to a diesel engine and often including other renewable energy sources and battery storage.

The value of electricity for village power is much greater than that in large grid utilities.

Finally, the domestic remote power market is relatively small and specialized. An example is powering remote telecommunication stations.

There is significant competition for supplying turbines and turn-key power systems to these markets. The United States must compete with European companies, primarily Danish and German companies.

In many cases, a significant factor in choice of supplier will be the availability of a financing package, especially for third world applications.

Technical Challenges

• Advanced wind turbines must be more efficient, more robust, and less costly than current turbines.
• Significant additional wind resource measurements are needed, especially long-term measurements to enable a better understanding of annual variation in the wind energy resource.
• A better understanding of turbulence within the wind, and how local terrain and other structures generate turbulence, is needed.
• Turbulence within wind farms is greater than that in open terrain, resulting in structural and fatigue loads which limit turbine component lifetimes or dictate maintenance schedules for turbines and components like gearboxes [6].
• It appears that there is a coherent structure to some of the turbulent flows generated from upwind turbines and terrain. Research is underway to allow prediction and mitigation of turbulence induced loads [7].
• Wind forecasting is an important factor to allow the operators to better plan and control operations.
• Micrositing is important to maximizing wind plant output-proper siting can substantially enhance the income from a wind plant.

More efficient airfoils

• NREL has developed airfoils tailored to meet the specific demands of wind turbines [8]. This has resulted in greater efficiency of energy capture (10-30%) than was possible with the existing airfoils.
• Older airfoils, which were based on designs for helicopters, have major problems: a decrease in efficiency when the airfoil's leading edge becomes fouled, and generator burn-out because of excessive energy capture from wind gusts.
• The NREL airfoils are the first of a new generation of airfoils that will significantly improve performance and make wind energy more competitive in areas with wind power densities lower than class 4.
• Energy capture gains of up to 30% have been accomplished for stall regulated turbines using the NREL airfoils.

Better blade manufacturing

• Better composite materials, better designs, and more cost effective manufacturing techniques are needed for components such as blades.
• Blades are usually fiberglass composites or wood laminates, although some of the earlier large machines used aluminum blades. [Some newer baldes are made of Carbon fiber]

Dynamic stall is thought to be an important factor determining mechanical loads on a turbine, especially when the blades experience transients in which they go in and out of stall regimes. Objectives include understanding the basic phenomena, and defining and implementing simple mechanical modifications to minimize the resulting structural loads. The result will be better design methods and improved turbines.

• Most utility-scale turbines have been operated at constant speed, with typical rotor speeds from 40 to 60 rpm. This constant input shaft speed is increased through use of a gearbox to give a significantly higher generator speed which results in specified power quality, say 60 Hz.
• The power quality is closely controlled to ensure wind plant electricity meets utility specifications.

Wind energy is not considered a firm power source by utilities because of the variable nature of the resource.

The use of multiple wind plant sites within a region, especially where the correlation between windiness at sites is understood, can potentially result in a situation in which the output of one wind plant can increase when the output of another decreases because of wind fluctuations.

A recent investigation indicated that for utility applications, pumped hydro energy storage is most cost-effective [13].

Transmission access is important, especially in sparsely populated states with very substantial wind resources, such as Montana.

• If a number of large wind plants were constructed in a sparsely populated area, it would be necessary to transmit the electricity to the distant population centers.
• If existing transmission lines are available and if they have adequate capacity, the economics will be substantially better than if new lines must be constructed at a typical cost of about $1 million per mile.
• Wind plant access to transmission lines may actually be enhanced by building fossil-fueled

This is a summary - the Entire article is available at Palmsprings.com