CONCLUSIONS AND RECOMMENDATIONS
Wind turbine technology has demonstrated the potential for contributing to the energy needs of the United States. If the sites with acceptable wind characteristics were fully utilized, they could contribute up to about 10 percent of the nation's electrical energy needs. The limitation is based on utility system stability issues rather than available site locations. As in all energy investment decisions, the ultimate penetration level will be driven by the cost of energy that is produced. In turn, this is decided by the initial cost of the wind energy plant and the annual cost for maintenance and operation.
Since a number of U.S. electric power utilities are continuing to add capacity, there will be an opportunity to introduce a new, longer-lasting design for a wind turbine system. Moreover, renewed interest by the public in environmental issues associated with power generation gives a special advantage to wind power. A new wind turbine system probably will take advantage of advances in semiconductor power electronics to improve energy production as well as provide reactive power control, which will make wind-generated electric power more amenable for use by the electric utilities. New speed control schemes will be introduced, but the major advance must come through the design of less expensive, longer-lived, and higher-efficiency rotors. A guiding principle in creating this design should be that knowledge of aerodynamic forces must be carefully integrated with the structural response of the material, all balanced by the practicalities of field experience and tempered by the need to manufacture a consistently high-quality product at reasonable cost.
This committee has examined the experience base accumulated by wind turbines, and the accompanying R&D programs sponsored by the Department of Energy and has concluded that a wind energy system such as described above is within the capability of engineering practice. However, certain gaps in knowledge exist. The achievement of this goal without costly and inefficient trial and error requires certain critical research and development. Because of the fragile nature of the wind power equipment producers in the United States, this will require an R&D investment by the Department of Energy.
The committee cannot conclude without commenting on the status of the wind power equipment industry. Because of the decrease in the number of machines installed in the past 5 years, since the tax incentives expired, there currently is only one major integrated manufacturer in the United States. In addition, only a few companies are actively producing blades. Moreover, in recent years, a major Japanese manufacturer has entered the world market to join the European manufacturers who have been participants for some time. As a result, the U.S. industry is not in a financial position to engage in the R&D necessary to gain worldwide technological leadership for what the committee sees as a future growing worldwide market for wind power. The committee believes that the United States is facing a future major reduction in fossil fuel sources of energy. When this is coupled with a resurgence of public concern for environmental issues in energy production, the need to develop wind power energy to the fullest extent possible seems compelling.
In the recommendations below, specific research tasks are listed that need to be carried out. Within each category of research—materials, manufacturing, structural response, etc.—these research tasks are listed in approximate order of priority. However, we wishes to emphasize that the overall goal of an R&D program should be to develop a system to produce longer-lived, less expensive, and more efficient wind turbine rotors. Increased knowledge of the fatigue properties and fatigue failure mechanisms of blades should take precedence, but this cannot be separated from the search for better manufacturing processes or from design innovations that will either minimize the likelihood of failure or ease the aerodynamic constraints of blade shape that impede process innovation. The committee wish to emphasize that the four factors of fatigue, manufacturing, advanced materials, and design are closely interrelated in the quest to produce a more cost-effective blade.
Goal 1: To improve the material properties and design capability so that the structure will withstand higher stresses, or the same level of stress for a much longer period of time.
Long-term fatigue data should be developed for the most common glass-reinforced plastics (GRP) laminates and critical elements under appropriate environmental conditions. The data should be carried to 108 to 109 cycles if possible, at stress ratios of R = 0.1 (tension and compression) and R = -1 (fully reversed). An extensive search of all fatigue data on GRP composites should be conducted and published in a source convenient to blade designers. This should evolve into a databank of wind turbine blade materials.
The extensive compendium of mechanical data, including fatigue data, on wood/epoxy laminates should be published and made available to domestic blade designers. Very high cycle (108 to 109) fatigue tests should be conducted on this material to compare its fatigue response with that of GRP materials.
The potential benefits for significant weight reduction in blades (about 50 to 70 percent) while maintaining required stiffness through the use of hybrid composites (in which carbon or aramid fibers are placed in critical blade locations) should be explored through design studies and limited blade testing. Critical use of cost models must be a requirement in this work because of the strong industry reluctance to utilize materials that are more expensive than E-glass/vinyl ester or wood/epoxy.
Goal 2: To lower the operating stress levels by altering the structural/configuration design.
Simple cross-sectional analyses and computer codes need to be developed for determination of sectional elastic constants in composite blades with elastic couplings. These design tools should consider blade parameters such as curvature, twist, taper, and, above all, completely general material/geometry.
An aeroelastic design code should be developed for wind turbine blades. This would permit the investigation of aeroelastic tailoring as a passive control mechanism.
An investigation of new active control techniques for wind turbine blades should be initiated. This should be aimed at a new generation design in which gust loads are essentially reduced, thereby minimizing over-design of blades.
Goal 3: To improve the blade manufacturing process so that quality variations and cost are minimized.
The resin transfer molding process has demonstrated the capability of producing quality fan blades up to 40 feet in diameter. Prototype studies to make GRP blades by this process should be undertaken. The study must include trade-off studies of manufacturing cost and quality versus losses in aerodynamic efficiency to enhance producibility.
The pultrusion process has a demonstrated capability to produce low-cost GRP blades but with a relatively inefficient constant cross section. Aeroelastic tailoring may partially compensate for the lack of twist and tapered planform compared to usual wind turbine blade geometries. A feasibility study should be conducted.
The introduction of new manufacturing processes must be accompanied by fatigue testing of full-size blades. Only in this way can the design details and the material quality be validated for the manufacturing process. Baseline studies on blades produced by current practice are needed.
Goal 4: To reduce the cost of blades enough so that periodic replacement becomes cost-effective.
Because fatigue crack propagation is slow in GRP composites, and is believed to be readily visible on annual tower-top inspections, the strategy described by goal 4 may be feasible. This would eliminate the need for extensive and expensive high-cycle fatigue testing. Eliminating the uncertainties in accounting for long-life service in design calculations would result in considerable weight savings. A detailed feasibility study with a realistic cost model should be undertaken if the manufacturing studies show promise of significant reduction in blade life-cycle cost.