U.S. Department of Energy Wind Energy Research Program for Low Wind Speed Technology of the Future

The United States is facing many challenges as it prepares to meet its energy needs during the twenty-first century. Electricity supply crises in California, fluctuating natural gas and gasoline prices, heightened concerns about the security of the domestic energy infrastructure and of foreign sources of supply, and uncertainties about the benefits of restructuring are all elements of the energy policy challenge.

In May 2001, the President’s National Energy Policy Development Group released a set of recommendations that have become the cornerstone of U.S. Energy Policy under the George W. Bush Administration 1. Pursuant to the release of the National Energy Policy recommendations, Secretary of Energy, Spencer Abraham, described the three priorities of the Department of Energy as:

• • Ensuring energy security by strengthening the energy production and delivery infrastructure

• • Focusing on programs that increase the supply of domestically produced energy, and that revolutionize how the country approaches conservation and energy efficiency

• • Directing research and development (R&D) budgets on ideas and innovations that are relatively immature and ensuring the greater application of mature technologies

The Assistant Secretary of Energy Efficiency and Renewable Energy, David Garman, stated that implementation of the Secretary’s priorities will involve nine elements 2, including:

• • Reducing dependence on foreign oil

• • Reducing the burden of energy prices on the disadvantaged

• • Increasing the efficiency of buildings and appliances

• • Reducing the energy intensity of industry

• • Creating a domestic biomass industry

Most relevant to the Wind Energy Research Program is the priority to:

“Increase the viability and deployment of renewable energy, by developing a diverse portfolio of renewable energy technologies that reduce the average cost of renewable energy production by 20% by 2010 and achieving cost-competitive parity with the average cost of energy by 2020.”

Competitive cost of energy (COE) levels have been achieved by focusing development on Class 6 wind resource sites (average wind speeds of 6.4 m/s @10-m height) and by taking advantage of the production tax credit (1.7 ¢/kWh in 2002 $). With favorable financial terms, Class 6 sites can market electricity at prices of 4 ¢/kWh or less before the subsidy. However, as more sites are developed, easily accessible prime Class 6 sites are disappearing. In addition, many Class 6 sites are located in remote areas that do not have easy access to transmission lines. The full development of accessible Class 6 sites may cause wind energy growth to plateau in the near future unless improvements in technology can make lower wind speed sites more cost effective.

Class 4 wind sites (5.8 m/s @10 m) cover vast areas of the Great Plains from central and northern Texas to the Canadian border. Class 4 winds sweep across the majority of North and South Dakota. Class 4 sites are also found along many coastal areas and along the shores of the Great Lakes. While the average distance of Class 6 sites from major load centers is 500 miles, Class 4 sites are significantly closer, with an average distance of 100 miles from load centers. Thus, utility access to Class 4 sites is more attractive and less costly. More importantly, Class 4 sites represent almost 20 times more developable wind resource than Class 6 sites. Figure 1 shows regions with Class 4 and greater resources.

Currently wind energy at Class 4 sites can be marketed at prices in the range of 5–6 ¢/kWh. Advanced low wind speed technology will be required for wind technology to be cost-competitive at Class 4 sites.

The program has defined goals for its technology development activities that will position wind as an attractive advanced technology option for the twenty-first century. The low wind speed technology (LWST) goal is to reduce cost of energy from large wind systems to 3 ¢/kWh in Class 6 wind resources by 2004, and to 3 ¢/kWh in Class 4 wind resources by 2010 (compared to a 2002 baseline of 4 ¢/kWh in Class 6 and 5.5 ¢/kWh in Class 4).

DOE researchers have developed an estimate of the additional capacity that could be installed in the U.S. as a result of the development of LWST. Their projections were developed using the National Energy Modeling System (NEMS), DOE’s primary electricity sector modeling tool. Assuming a turbine capable of producing electricity for 3 ¢/kWh in Class 4 sites became available in 2010, the NEMS projected wind capacity in 2020 would be about 50 GW or 40 GW above the baseline expectation, which represents a significant benefit to the nation. This projection assumes that there is no long-term production tax credit — a key objective of the low wind speed technology program is to eliminate the need for on-going subsidies.

The additional 40 GW of capacity would require approximately $30 billion in private sector capital investment. This would provide significant economic benefits to rural landowners and enhance energy security and environmental protection efforts. The low wind speed technology project is, therefore, closely aligned with the National Energy Policy goals described earlier in this paper.

In addition to activities to develop large-scale wind systems, DOE’s Wind Energy Program also supports the development of distributed (<100 kW) wind system technologies to achieve the same cost-effectiveness in Class 3 (5.3 m/s @10m) wind sites by 2007 as they currently have in Class 5 (6.2 m/s @10m) resources. This will open up new energy options for consumers in a wide range of applications: homes, farms, remote villages, water pumping, and battery charging.

In response to National Energy Policy priorities, DOE’s Wind Energy Program recently began charting new directions for its efforts. These directions are being organized around two thrusts described by Assistant Secretary Garman:

• 1 Increasing the viability of wind energy by developing new cost-effective technology for deployment in less-energetic, Class 4 wind regimes (see Fig. 2); developing cost-effective distributed, small-scale wind technology; and laying the groundwork for future work to tailor wind turbine technology to the production of hydrogen.

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  Class 4 wind regimes

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  Class 4 wind regimes

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• 2 Increasing the deployment of wind energy by providing supporting research in power systems integration, resource information, market acceptance, and industry support.

The development of low wind speed technology is a priority strategy being pursued to increase the viability of wind energy. At the heart of this new research plan is the development of wind turbine technology (machines larger than 100 kW) that is capable of producing electricity for 3 ¢/kWh at Class 4 sites by 2010. This objective will be achieved by bringing to bear all the resources of the Department of Energy, its laboratories, and a wide array of industry partners. The development of low wind speed technology, planned as a cooperative effort with industry, will be guided by several principles:

• • Program experience and stakeholder expertise will provide strategic input

• • Program evaluations performed regularly using performance-based management techniques will provide a strong analytical basis for performance criteria, periodic review, and adjustment

• • Public/private partnerships will be developed to support continuing innovation; they will be flexible and adaptive, support multiple pathways, and offer repeated opportunities for new players to enter the program

The LWST project is structured around three elements: concept and scaling studies, component development efforts, and full turbine system developments. In addition, it is supported by on-going program activities in wind systems integration and supporting research and testing. The program currently envisions that the LWST project will represent an increasingly large portion of total program funds over the remainder of this decade.

Since the late 1980s, the program has emphasized a public/private sector partnership in which new turbine designs are cost-shared and led by industry, while the Federal laboratories — the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (SNL) — provide the theoretical support through applied research and feedback from performance testing. Industry partners make commercial decisions, and the laboratories, via the National Wind Technology Center (NWTC), transfer improved technology to industry while providing unique testing services not otherwise available to them. The level of cost-sharing required depends on the size of the procurement and the accompanying technical risk, and is consistent with guidance provided in the Energy Policy Act of 1992.

The low wind speed technology project began with a request for proposals (RFP) issued October 19, 2001. The RFP offered bidders an opportunity to participate in one of three technical areas: 1) concept and scaling studies, 2) component development, and 3) low wind speed turbine prototype development. This RFP offered industry partners several different approaches to the problem of synthesizing new systems designs. Because different project teams may be at different stages of systems development, additional RFPs will be issued within the next 1–2 years. These new opportunities to participate will allow partners to take the results from one technical area and move into another. An example would be completing a systems design study under the first RFP and proposing a component development effort in a later RFP. These later RFPs also represent additional points of entry into the program for industry partners who are not currently ready to submit a proposal. Under the initial RFP, proposals have been evaluated and negotiations have begun with six prospective awardees in the three technical areas.

The low wind speed technology project is tightly focused on hardware improvements. Myriad design approaches for decreasing COE have been proposed and many explored at some level. COE reductions over the past two decades have resulted from the combination of many improvements in a wide range of areas, including manufacturing, engineering, and business practices. Although turbine size has increased significantly, wind turbines today look much like they did 10 years ago. Design changes are visible only to the most knowledgeable observer. Will the wind turbine of tomorrow look much different? DOE laboratories and industry have been conducting studies to try and answer this question and better understand the course of future technology 3,4 5.

Some of the more important findings of these studies include:

• • No single technology improvement or innovation will achieve the LWST goals

• • Taller towers are important for improved energy capture at low wind speed sites

• • Advanced blade designs and materials are necessary

• • All rotor configurations can be optimized equally to help improve COE

• • Transportation and crane erection limitations must be overcome

• • Advanced controls are extremely important for loads alleviation.

COE reductions will be the result of combining a number of technical improvements. Some potential examples are:

• • Advanced rotors and controls, −15% ± 7% (Flexible, low-solidity, higher speed, hybrid carbon-glass, and innovative designs)

• • Advanced drive train concepts,−10% ± 7% (Hybrid drive trains with low-speed permanent magnet generators and other innovative designs, including reduced cost power electronics)

• • New tower concepts, −2% ± 5% (Taller, modular, field assembled, and load feedback control)

• • Improved availability and reduced losses, −5% ± 3% (Better controls, siting, and improved availability)

• • Manufacturing improvements, −7% ± 3% (New manufacturing methods, volume production, and learning effects)

• • Region and site tailored designs, −5% ± 2% (Tailoring the designs of turbines for unique sites of larger [100 MW] wind farms).

It is unlikely that the machine producing electricity for 3 ¢/kWh in 2010 in a Great Plains site will look drastically different from a machine in a California pass in 2000. But beneath the shell, improved technologies will be evident from blade tip to tower base and beyond.

The machine of the future is likely to consist of an innovative blade made from composites of advanced carbon fabrics integrated with more conventional fiber-glass. The blade may use adaptive designs that cause the blade to twist in such a way as to reduce the loads. Blades will be longer and perhaps thinner, operating at higher tip speeds and taking advantage of an improved understanding of aerodynamic loads and aero-acoustics. Rotors will mitigate loads through innovative independent blade pitch control algorithms based on feedback from blades, drive train, and towers. Drive trains may consist of single-stage drives linked to lightweight, high-power density permanent magnet generators. Or perhaps the drive train will have no gearing at all. Machines will be variable speed, processing their varying frequency power through an expanding range of power converters using new circuit designs and cheaper electronic components. Towers will soar to 150 m and will erect themselves quickly. They will also be able to lower a machine to the ground in a matter of hours to allow major maintenance on the ground without the deployment of large cranes. Components and structures throughout will be designed with an increased knowledge of turbulence and inflow in new environments and the use of more accurate codes that allow designers to reduce design factors based on improved certainty.

Wind turbines will still look like wind turbines. But the power produced by these machines of the future will compete head to head on a cost of energy basis with coal and natural gas.