Wind Energy as a Significant
Source of Electricity
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
Winds arise because of the uneven heating of the earth's surface
by the sun. 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.
Figure 1 shows a wind resource map (annual average) for the contiguous
United States. 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. 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. Figure 2 is
a simplified schematic drawing of wind turbines, showing the major
components. 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,
and increase reliability while at the same time addressing aesthetics
and environmental effects. This requires significant efforts in
fundamental aerodynamics, materials engineering, structures, fatigue,
power electronics, controls, and manufacturing techniques.
Modern turbines are either horizontal-axis or vertical-axis
machines, Figure 2, 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, and San Gorgonio
near Palm Springs. Figure 3 shows a typical wind plant. 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. Figure
4 shows a production history for U.S. wind turbines, most of which
are located in California. 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.
DOE, through the National Renewable Energy Laboratory (NREL) and
Sandia National Laboratories, has research and development programs
to improve or define the turbines of today, tomorrow, and the next
century. The approach is to develop a technology base which will
enable the private sector to perform the final development necessary
to build a viable industry. Much of this research and development
is cost-shared, with the industry and utilities typically supplying
30% to 70% of the funding. Current market projections from DOE estimate
that 2% of the 2010 U.S. electricity supply will come from wind
energy.
Costs and Goals
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.
Still, there are obstacles to widespread commercialization of
wind energy. Wind energy technology has made substantial advances,
but the competing technologies have also improved and the competitive
situation has changed as the available supply of inexpensive natural
gas has significantly increased. In addition, there is a potential
change in the electric power industry to provide a structure which
will result in increased competition. This change will probably
enhance electricity generation by independent power producers, with
an optimizing criterion being minimum cost of electricity. External
costs, such as pollution avoidance and damage, are being discounted
or totally ignored. This tends to make it more difficult for wind-generated
electricity to effectively compete. Simply put, technical advances
will have to cut the cost of wind energy even further for the DOE
projections to become reality. The required technical advances appear
achievable with sustained research and development.
Potential Markets
There are 4 major potential markets: 1) domestic utility grids,
2) foreign utility grids, 3) village power systems in developing
countries, and 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. These are large
potential markets, with the foreign markets possibly developing
earlier than the domestic market because the electricity often has
greater value in the foreign markets. In addition, many of the potential
foreign markets are in areas where a significant air quality improvement
is required, which does not favor expansion of coal-fired generation
plants. 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. DOE, its national laboratories,
universities, and the wind industry are working together to accomplish
these improvements through various research and development programs.
Each program is aimed at specific goals ranging from improving the
current generation of turbines and components to defining, researching,
and testing the innovative turbines of the next century. The technical
challenges these programs will have to address include the following:
Better characterization of the resource
This involves taking better measurements of wind characteristics,
especially within wind plants, and developing better siting methods.
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. The current
technique for manufacturing fiberglass blades is hand lay-up. This
technique is labor intensive and quality is difficult to control.
Substantial gains can potentially be made by using automated techniques.
The life of a utility-quality turbine with good maintenance is about
30 years, with the blades having a projected life of about 15 years,
which necessitates a replacement of blades during the turbine life.
A goal is to achieve blade life equivalent to turbine unit life.
Better understanding of aerodynamics
Time-variant, three-dimensional aerodynamic phenomena are significantly
more complex than those observed in steady, two-dimensional wind-tunnel
tests. NREL researchers are generating better field data to provide
an enhanced understanding of the basic phenomena [9].
There are significant interactions with universities, industry,
and foreign researchers in the area of fundamental aerodynamics.
The approach is to perform both wind tunnel and field tests with
very sophisticated and rapid data collection systems to understand
boundary layer flow over the blade. 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.
Development of theoretical models and computer codes
Substantial effort is being devoted to developing computer characterizations
of every component in the integrated wind turbine [10]
. Objectives include understanding basic phenomena, load generation
and the load path from the tip of the blades to the turbine foundation,
and how to model dynamic loads for the integrated wind turbine.
As a result, improved designs of components and systems will give
rise to longer lifetimes and will allow cost reductions while meeting
the structural requirements of the components. A goal is "virtual
prototyping" in which validated computer models are used to understand
the performance, lifetime, and cost of each component in a proposed
design. Iterations to improve the designs will be done on the computer,
allowing the first physical prototypes which are constructed to
be significantly advanced beyond those which would result if conventional
design techniques were used.
Better understanding of fatigue and structures
Work is under way to better predict fatigue effects on components
[11]. The goal is more robust and innovative designs.
Research includes significant materials and structures testing,
in addition to the computer modeling described above. Fatigue is
the most important factor in turbine and component lifetime. Turbine
components are subject to fluctuating random loads, which are much
more difficult to characterize and design for than static loads.
Testing is an integral part of blade development.
Better turbine configurations
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. A more efficient
approach is to allow the rotor to run at varying speed as determined
by the wind. This will result in potential energy production gains
of about 15%, but necessitates the use of sophisticated power electronics
to change the output electricity from time varying characteristics
to the required time invariant characteristics. An even more efficient
approach is to eliminate the gearbox and to operate with a low-speed
generator operated at variable speed. These approaches require that
new power electronic control techniques and control equipment be
developed, and that new types of generators be designed and developed.
Better control techniques
This involves using power electronics to generate higher quality
electricity at a higher efficiency and the use of better control
techniques to enhance turbine operational efficiencies. These advances
are possible because of the improvements in computers. Some of the
techniques being considered include fuzzy logic, neural networks,
and other adaptive control schemes. Not only should efficiency be
enhanced, but it should be possible to reduce structural loads while
ensuring higher quality power.
Integation Issues
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.
Accurate forecasting can significantly enhance the value of wind
generated electricity-a recent investigation indicates that the
value increase can be as much as $0.01 to $0.02/kWh [12].
Energy storage is an important technical challenge that could
enhance the dispatchability of wind plants. Batteries, pumped hydro,
compressed air, and superconducting magnets are candidate storage
techniques. A recent investigation indicated that for utility applications,
pumped hydro energy storage is most cost-effective [13].
For smaller applications such as village power, battery storage
can be cost-effective. This is especially the case in hybrid systems
in which a diesel engine is included and the cost of diesel fuel
is very high. When storage is integrated with wind plants, the value
of wind-generated electricity will probably be much greater than
the current value, which for most utility applications in the United
States is presently considered equal to the avoided fuel cost.
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 plants nearby
to enable maximum utilization of the investment in the transmission
lines. Obviously, this is a very location specific situation, but
one which is important to the economics of building large wind plants.
Environmental Issues
Wind energy is environmentally positive. Annual wind generated
electricity production in California displaces the energy equivalent
of 5 million barrels of oil and avoids the release of 2.6 billion
pounds of greenhouse gases per year, in addition to avoiding other
emissions such as sulfur and nitrogen oxides which contribute to
smog and acid rain.
However, some environmental concerns must be addressed. The death
of birds by flying into operating turbines is a concern, especially
when the birds are raptors such as golden eagles. There are numerous
investigations under way to determine the significance of the concern,
and to define and validate mitigation techniques. A typical example
is the investigation being performed by researchers from the University
of California at Santa Cruz [14]. Researchers
are collecting data to understand the effect of wind turbines on
the population of golden eagles in one area of the Altamont Pass
wind resource area. The approach is to radio-tag and track a sufficient
number of eagles so that the population dynamics can be understood.
Other researchers are investigating mitigation techniques such as
eliminating tower members suitable for bird perching, using acoustic
warning devices, appropriately painting warning colors and patterns
on turbine blades, controlling vegetation around the towers to minimize
prey availability, and siting turbines more carefully. The avian
situation is an emotional issue, with arguments ranging from doomsday
to the other extreme that the population is actually increasing
because of the wind turbines. While avian problems are not thought
to be widespread, this is a significant issue which is being addressed
in a very serious and scientific manner.
Another concern is aesthetics. What is beautiful to an engineer
may simply be ugly to others. Therefore, wind plant siting and layout
are important. It appears that wind plants that have an orderly
layout in rows may be preferable to layouts which follow ridges
and flow patterns. In general, use of small wind turbine clusters
located at multiple sites may be preferred to one very large plant.
Aesthetics is a challenge that can be met by developing and using
better siting guidelines and by better educating the public about
the value of wind plants.
Conclusion
Wind energy will be one of the most important, widely applied
of the renewable energy forms during the next several decades. There
are substantial challenges to be met, but all appear solvable. Successful
research and development will potentially result in generation from
wind energy of about 10% of the electricity used in the United States.
A strong U.S. wind industry will be competitive to supply wind turbines
to the rest of the world, along with the significant environmental
and societal benefits of wind energy.
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