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by Micah Garlich-Miller
"The answer, my friend, is blowing in the wind" -- Bob Dylan
It is unlikely that Bob Dylan knew how prophetic his words were to become. A few years after he penned "Blowing in the Wind," the oil embargo of 1973 created an energy crisis that rocked the US. Federal funding for wind energy research soared, from $2 million in 1974 to $60 million by 1980 (unadjusted dollars). The modern day wind energy movement was born.
Twenty years later wind energy is poised to lose its label as an "alternative" energy. Wind energy has come of age as a reliable, cost-competitive energy source. From virtual non-existence in the early 1980s, the amount of wind-generated electricity has jumped above 7.5 terawatt-hours (TWh) annually and is expected to exceed 20 TWh by the year 2000. According to the Worldwatch Institute, wind energy is the fastest-growing energy source in the world, increasing at a rate of 20 percent annually. Wind energy capacity has grown by 150 percent since 1990 and this growth shows little signs of abating. European environmental concerns and skyrocketing energy demands in developing countries are driving development. Manufacturers anticipate nearly $2 billion (US) in 1996 sales.
And then there's the bad news. Despite the tremendous worldwide growth, the once powerful US wind energy market is at a standstill. Of the 1300 additional megawatts (MW) of wind power installed worldwide in 1995, less than 50 MW were installed in the US.
With recent advances in the design of wind turbines, the key elements for the successful development of wind energy have become more political than technological. Although wind turbine design has not been optimized, well-designed machines achieve vastly different levels of penetration in different political climates. Here in the US, the restructuring of the electric utility industry is having disastrous effects on the domestic wind energy market. Utility power planners, unsure of the outcome of the restructuring process, are hesitant to install any new capacity from wind or other sources. Wind energy, with its higher up-front capital costs, has been especially hard hit. Although wind energy is competitive with conventional energy sources based on a life-cycle analysis (it has no long-term fuel costs), uncertainty in the market leads to a prevalence of lower capital cost technologies.
Political legislation has also had positive effects on the wind industry. In 1978 Congress abolished the monopoly that utilities had over the production of electricity, thus making independent power production possible. This, coupled with California's favorable tax incentives for renewable energy, resulted in an investment of over $1.5 billion in wind electricity in that state. With a market to support it, the cost of domestic wind energy fell by 80 percent between 1982 and 1992. This was also a period of significant technological advancement. In the early 1980s, poor reliability meant that wind turbines in California were available for generation on average only 60 percent of the time. Today, if a wind turbine is to be competitive in the market, it must be available for operation 97 to 99 percent of the time. The productivity of the turbines, the amount of energy gathered per meter squared of swept rotor, has also increased. In California, the productivity of a turbine has increased on average from 500 kilowatt-hours per meter squared (kWh/m2) per year in the early 1980s to 800 kWh/m2 per year in the mid-1990s. Today, well-designed, well-maintained wind turbines may produce substantially more energy.
Another positive development was the Energy Policy Act of 1992 which provided wind energy with a $0.016 per kWh production incentive. The goal of this production credit was to help level the playing field between wind and other conventional energy sources, such as coal, which have received considerable public subsidy. With this incentive, wind power is now cost-competitive with new power plants in high wind regimes. In the past year, Congress attempted to repeal this production credit. Although the measure was defeated, energy planners who must plan for the long term are justifiably nervous.
It is important to note that wind energy is not a new idea. Its existence as a provider of useful mechanical power dates back at least 1,000 years, and only 100 years ago was it surpassed by steam as the principal prime mover. In the United States of the late 19th century, 77 firms were manufacturing farm windmills of one form or another. Today more than one million American farm windmills are still in use delivering more than 250 MW in pumping power to rural people around the globe. To this day, they are a common sight along the rural landscape of this country.
The fuel which powered these water pumps has not been exhausted. The high-quality wind resources of North Dakota alone could produce 36 percent of the electric consumption of the lower 48 states. Excellent wind regimes are scattered from Texas up into Canada. If exploited, these wind resources could easily provide more than the total power consumption of the US. However, unless load patterns significantly change, the intermittent nature of wind energy makes it unlikely wind will ever provide more than 20 percent of this country's electricity. With today's wind turbine technology, achieving this 20 percent mark would require the development of about 16,000 square miles of good wind resource area (about the size of four Montana counties). Of this land, less than 5 percent would be occupied by wind turbines and support infrastructure. The remainder of the land would be available for farming and ranching as it is now.
In the electric power business, the technology option is often decided by small economic differences. Since the power available from the wind is a function of the cube of the wind speed, good wind resource assessment and siting is critical to the economics of wind energy. Excluding other factors, a site with 6 meters/second (m/s) winds can produce 70 percent more power than a site with 5 m/s winds. In general the cost-effective application of small grid-connected wind machines requires winds exceeding 5 m/s (11 mph), while utility wind farms require speeds of 5 m/s (13 mph).
In the last decade the typical configuration of the modern utility scale wind turbine has emerged. The most successful design has been a three-bladed, upwind, horizontal-axis wind turbine (HAWT). Another well-known design is the vertical-axis wind turbine (VAWT) whose appearance has led people to dub it an "egg beater." Although the VAWT has the advantage of placing the generator close to the ground, the design has not had great market success. Two-bladed and downwind designs have also been explored, but they tend to have greater cyclical loads that adversely affect the life span of the turbine. Upwind designs eliminate the load caused by moving the blade through the wind shadow of the tower, and the three-bladed (or more) design is dynamically symmetrical (the same mass moment of inertia about its hub). All modern utility scale wind turbines use aerodynamic lift rather than drag to drive their blades. Although drag devices have the advantage of being able to produce useful mechanical power at very low wind speeds, lift devices, which use the pressure difference created by flow over a wing, may produce up to 50 times more power per unit of blade area.
In a HAWT, all moving parts are located at the top of the tower. Low maintenance is an important design requirement. A HAWT power train typically consists of a rotor, a turbine shaft, a speed-increasing gearbox, a rotor brake, and an electrical generator. These components must be designed for continuous use over a life span of approximately 30 years. The turbine shaft assembly, which serves both structural and mechanical roles, is one of the most critical design components in a HAWT. Wind loading, rotor support, and applied torque combine to contribute to the fatigue loading of this component.
The gearbox typically has a step up ratio that ranges from 1.0 (direct drive power train) to 100 (large scale HAWT). Parallel axis gearboxes, despite having greater space and rigidity requirements, are more often used than epicyclical or planetary gears due to their availability and lower cost. Parallel axis gears also allow cables and piping to be connected to the rotor through the main shaft.
Many blade designs have been used. Glass-fiber composites (both molded and filament wound), laminated wood composites, steel spars with non-structural composite fairings, and welded steel airfoils have all had success. The choice of blade materials is an engineering decision involving considerations of size, strength, stiffness, weight design, manufacturing expertise, maintenance, and cost.
Another important design consideration is the overspeed control of the turbine. Modern wind turbines use redundant designs that combine mechanical braking, generator braking, and aerodynamic speed reduction strategies. Medium and large scale turbines typically have adjustable blade pitch thereby providing a means of controlling starting and stopping torque along with peak power. However, pitch control can be costly and maintenance intensive. For this reason, small and medium scale turbines often have fixed pitch stall controlled blades that rely on aerodynamic stall to limit peak power. Tip brakes have also been used. These brakes use centrifugal (or control) forces to place the blade tip at right angles to the tip motion. Aerodynamic drag limits the speed, often in a noisy fashion. And finally, when damaging wind conditions have been detected, modern wind turbines can use their yaw motors to furl the rotor out of the wind.
Other technical design issues relate to the integration of wind power plants with utility operating systems. According to a new paper by Robert Putnam of the consulting firm Electrotek Concepts, integration of wind power into the utility grid "has not been a problem." Putnam's work is based on interviews with system operators and dispatchers from Pacific Gas & Electric Co. (PG&E) and Southern California Edison Co. (SCE), two utilities which have had extensive experience with the integration of wind energy on their systems since the early 1980s. Putnam divides integration issues into interface engineering issues that deal with the quality of the wind energy produced and operational issues that affect the manner in which the utility chooses to fulfill its energy requirements while handling both expected and unexpected load fluctuations.
Interface issues include harmonics, reactive power supply, voltage regulation, and frequency control of the electricity produced. Though these issues were problematic in early wind turbine designs, the advanced converter systems available today produce output which meets IEEE Power Quality standards.
Operational issues include operating reserve (the capacity and ability to meet load fluctuations which occur in a timely fashion), unit commitment (availability scheduling of power plants), system stability (ensuring quality of power on a system wide level), and transmission and distribution system impacts (sudden changes of power flow). These are problems which utilities are forced to confront whether wind is included in the energy mix or not. Wind does, however, introduce some uncertainty into this process, leading Putnam to suggest that further research into wind prediction on an hourly basis would be useful. Despite this, Putnam concludes, "Any issues that have developed, such as intermittency and voltage regulation, can be addressed by accepted power system procedures and practices." Putnam adds that in the case of SCE and PG&E, the short-term variations in wind plant output are small relative to normal load fluctuations and therefore do not significantly impact the system regulating capacity.
Wind energy has other attractive qualities. Decommissioning a wind farm is straightforward and may even provide a source of revenue to the utility via the turbine salvage value. Wind farms are also highly modular in nature, ideal for planners. If electricity demands grow at a site, it is generally a simple matter to add to the number of turbines in use.
In 1990, Scientific American described how the cost of wind energy would fall to $0.04 per kWh by the year 2000. Other predictions lowered this estimate to a ridiculously low $0.02 per kWh. One industry analyst went so far as to suggest that wind energy would become "too cheap to meter." Paul Gipe, in his book Wind Energy Comes of Age, points out that such rampant predictions end up hurting the industry. Since the capital costs of a wind plant are primarily up front (there are no fuel costs), developers delay installation while the prices decrease. This has the effect of stalling the development of the wind industry. Nonetheless, wind energy is undeniably becoming more cost-competitive.
The cost of wind energy in California fell from nearly $0.45 per kWh in the early 1980s to less than $0.10 per kWh in the early 1990s. Here in Minnesota, Northern States Power (NSP) estimates a cost of $0.047 per kWh for electricity from a 25 MW wind farm it installed in 1994. Utility scale projects have been bid at even lower costs.
These values, competitive with other new installations of conventional energy sources, are still much higher than the average price that utilities pay for electricity. Coal and other power plants that may be 30 years old have completely amortized their costs, and the price of electricity reflects the currently low fuel prices. Still, greater fuel diversity leads to less dependence on fossil fuels, which are often subject to rapid price fluctuations and supply problems. With many countries rushing to install low capital gas-fired electric generating capacity, the number and degree of gas supply fluctuations are likely to increase.
The money invested in a wind farm primarily serves to create jobs. According to the New York State Energy Office, wind creates more high-wage jobs than other energy sources, and as Carl Weinberg, a former PG&E executive, notes, "Wind power pays for people, not fuel."
Minnesota is one of the few places in the nation where wind energy development is occurring. Favorable energy policies here and in Iowa have led to several wind farm installations in recent years, a trend that is likely to continue. In a paid advertisement Northern States Power Co. proclaims: "We at NSP believe that having a variety of energy resources is the best way to ensure clean, reliable, low cost electricity now and in the future. Wind power is part of that energy supply." NSP plans to install 425 MW of wind power by the year 2002.
Most of this development will occur atop Buffalo Ridge in southwestern Minnesota, one of the windiest spots in the nation. In 1994, NSP installed 25 MW of wind capacity on the ridge near Lake Benton, Minnesota. The wind farm is comprised of 73 Kenetech Model 33M-VS wind turbines that are rated between 300 and 405 kW. Based on 1993 system average emission rates from NSP sources in Minnesota, this wind farm is projected to reduce carbon dioxide, sulfur dioxide, and nitrogen oxide emissions by 49,000, 125,000, and 146,000 tons per year respectively. Kenetech, however, has not faired well economically and filed for Chapter 11 bankruptcy last summer.
The contract for the next 100 MW development has been awarded to Zond Systems, Inc., based out of California. Worldwide the company has installed and operates over 2,400 wind turbines comprising 250 MW of generating capacity. One reason Zond was selected is the high system availability (97 percent) shown on its existing turbine fleet (reliability was one of the factors that ultimately doomed Kenetech). Turbine construction on this project is anticipated to begin in 1997. One hundred forty-three of Zond's Z46 (46 m rotor diameter) turbines will be needed to provide this amount of power. The power contract which Zond signed with NSP is reportedly down around $0.03 per kWh with the $0.016 per kWh federal tax incentive. Many of the components for Zond's 700-kW wind turbines will be manufactured in the Midwest and both the blades and the towers may come from Minnesota-based manufacturers. Zond has begun efforts to site an assembly facility in the southwest corner of the state. The assembly facility should employ 15 to 20 full-time employees, and about 10 employ
Other utility scale wind energy development is taking place in Minnesota. Minneapolis-based Northern Alter-native Energy (NAE) owns and operates a five-turbine wind farm in Marshall, Minn. The wind farm, commissioned in 1992, has produced over two million kWhs for the Marshall Municipal Utility. NAE has also teamed up with Micon Wind Turbines (US) Inc. to install Micon's first 600 kW wind energy turbine produced in the United States at a wind farm owned by NAE in Sibley, Iowa. This same partnership has also applied for permission to build a 11.25 MW wind project in southwestern Minnesota. The project would use 15 Micon 750 kW turbines on land acquired by NAE and NSP. If successful, these small cluster-like wind farms may provide a model for future wind development in this country.
The future for wind energy around the world is obviously bright. At home and abroad, consumers are beginning to demand renewable energy. Burgeoning international markets are opening, and the vast indigenous wind resources of this country suggest that a strong domestic market may one day develop here.
