The Influence of Specific Rating on the Cost of Wind Energy

Abstract

The optimization of rotor and drive train design is an important task in making the cost of wind energy competitive with other energy sources and the ratio of rated power to swept area, or specific rating, is a key parameter. This report presents the results of an extension to DOE/NREL’s WindPACT program by studying the effect of specific rating on the cost of energy from certain onshore wind turbine designs in a range of wind regimes. The results show that the optimum specific rating is sensitive to several variables including the mean wind speed, the Weibull shape factor, the design tip speed ratio, and the balance of station costs. INTRODUCTION Much effort has been spent over the past decade to reduce the cost of wind energy by improving the efficiency of the rotors and minimizing the cost of all components. One aspect of this task is to determine the optimum ratio of the rated power of the turbine and the swept area, or the “specific rating”. In 2000 the National Renewable Energy Laboratory (NREL) launched the Wind Partnership for Advanced Component Technologies (WindPACT) program to examine ways in which the cost of wind energy could be reduced a target of ¢3.0/kWh. The WindPACT rotor study [1, 2] was based on a rotor with a specific rating of 0.39 kW/m which was typical of some commercial machines (see Table 1) and the study has been extended to determine how the earlier results might be affected by changes in the specific rating. Full details of that study are available in [5] which can be accessed from the NREL website at www.nrel.gov/publications/. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The question has become more relevant since several manufacturers now offer more than one specific rating for a certain machine (usually by changing the diameter while the rating is unaltered). Typically the less energetic the wind regime, the lower the specific rating. At the same time, some researchers and authors have proposed that the specific rating should be increased to lower the cost of energy. The present state of the industry is summarized in Table 1 which shows the specific ratings of some selected current wind turbines. Table 1. Specific Rating of Selected Current Wind Turbines Manufacturer Model Diameter (m) Rated Power (kW) Specific Rating (kW/m) Vestas V47 47 660 0.38 Tacke TW1.5 65 1500 0.44 Enercon E66 66 1500 0.45 GE Wind TZ1.5 70.5 1500 0.38 Vestas V80 8