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Assessment of Research Needs for Wind Turbine Rotor Materials Technology Executive Summary The committee's charge was to define a research and development agenda for the Department of Energy's wind energy program focusing on materials aspects of the wind turbine rotor technology. In particular, the committee interpreted its mandate to include an assessment of the potential for new design methods, better manufacturing processes, and advanced control methods in addition to advanced materials for improvement in turbine rotor performance and lifetime. In addition, the committee took as its responsibility the development, in broad outline, of a research and development program that would place U.S. technology in rotors for wind power in a preeminent world position. In carrying out its charge the committee examined an extensive literature on wind machines and wind turbine rotor materials and was briefed by many experts on these subjects, both in the wind industry and in related industries. Wind-driven power systems represent a renewable energy technology that is still in the early stages of development. Arrays of interconnected wind turbines convert the power carried by the wind into electricity for users of the utility power grid. Major concentrations of this technology exist in California, Denmark, and Hawaii. At the end of 1989, the wind power plants in California comprised a power-generating capacity of 1335 MW, equivalent to a medium-sized utility power plant. Not only do these wind power plants produce no gaseous emissions, particulates, or radioactive by-products, but they can be installed rather quickly as modular units, each providing capacity from a few tens of kilowatts to hundreds of megawatts. They can be readily integrated into existing utility generation-transmission-distribution systems. While economic utilization of wind energy depends critically on location and siting, many available sites exist in the United States. While land intensive, wind power plants can coexist with other uses of the land on which they are situated. Wind power plants installed in the early 1980s suffered structural failures chiefly because of incomplete understanding of the wind forces (especially the turbulence component) acting on these large structures and in some cases because of poor quality in manufacture. Failure of the rotor blades was one of the principal and most serious structural failures. Failures from these causes are now somewhat better understood. An associated additional limitation to achievement of the full economic potential of wind energy is uncertainty about the long-term response of wind turbine rotor materials to the turbulent stochastic loadings to which they are subjected. Over their projected operational lifetimes (typically 20 to 30 years), these structures are subjected to as many as a billion stress cycles. Since a number of U.S. electric power utilities are continuing to add capacity, there will be an opportunity to introduce new, longer-lasting designs. Moreover, renewed public interest in environmental issues associated with power generation gives a renewed impetus to wind power. A new wind turbine system will probably take advantage of advances in semiconductor power electronics to produce changes in the system configuration that will make wind-generated electric power more amenable for use by electric utilities. New speed control schemes will be introduced, but a major advance must come
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Assessment of Research Needs for Wind Turbine Rotor Materials Technology through the design of less expensive, longer-lived, and higher-efficiency rotor blades. 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. We have concluded that a wind energy system such as described above is within the capability of engineering practice. But certain gaps in knowledge exist, so achieving the 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 from 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 rate of installation of machines in the last 5 years (since the tax incentives expired), there currently is only one major integrated manufacturer in the United States; only a few companies are actively producing blades. In recent years a major Japanese manufacturer has entered the world market, joining 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 reduction in most fossil fuel sources of energy. When this is coupled with a resurgence of public concern over environmental issues in energy production, the need to develop wind power energy to the fullest extent possible seems compelling. RESEARCH RECOMMENDATIONS The committee has identified four goals to guide the research needs in wind turbine rotor technology: Goal 1 To improve the material properties and design capability so that the structure will either withstand higher stresses or the same level of stress for a much longer period of time. Goal 2: To lower the operating stress levels by altering the structural/configuration design. Gual 3: To improve the blade manufacturing process so that quality variations and cost are minimized. Goal 4: To reduce the cost of blades enough so that periodic replacement becomes cost-effective. The details of specific research recommendations are given in Chapter 7 (Conclusions and Recommendations) and, in greater depth, at the ends of Chapters 2, 3, 4, 5, and 6. The main research tasks are summarized below. MATERIALS Initiate a program to generate long-term (10-year-equivalent), high-cycle fatigue (108-109) data for candidate structural materials: glass-reinforced plastic, wood/epoxy, and advanced high-modulus composite materials under appropriate environmental conditions. The program would contribute an element in a needed databank for wind turbine blade materials. DESIGN The wind turbine industry needs design tools that are beyond the capability for development by the private sector. For example, it is necessary to be able to compute laminate stresses in three dimensions and to
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Assessment of Research Needs for Wind Turbine Rotor Materials Technology relate these stresses by means of a suitable failure theory. It is also necessary to rigorously relate blade material and geometry to beam-like stiffnesses. In turn, the stiffness and mass must be combined with suitable aerodynamic models to determine structural dynamic response in the rotating field of the rotor. This type of analysis will help to assess the benefits of active control as well as passive (such as by elastic tailoring) load relief systems. Most of these tools either exist or are being developed in the aerospace industry. They are largely developed under government systems procurements and should be extracted and adapted to the special needs of the wind turbine rotor. Emerging technologies in active and passive control of both the rotor and the generator need to be studied on an overall system basis considering the probable gains in structural efficiency and reduction in blade life-cycle cost. MANUFACTURING Two emerging manufacturing processes, resin transfer molding, and pultrusion, offer significant opportunities for cost reduction but with attendant limitations on blade design freedom. A feasibility study should be conducted to evaluate each of these processes in a real application and at a reasonable scale, allowing realistic design and cost trade studies to be accomplished. For example, the cost of a geometrically simple pultruded blade can be little more than the cost of the materials, thus making it appreciably less costly than the curved and shaped wind turbine blades currently employed. However, while less costly, such blades are also less efficient aerodynamically. Thus, from a life-cycle economics perspective, it is not clear that the attendant reduction in the initial cost of the wind turbine can compensate for the associated decrease in energy production over the wind turbine lifetime.
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