WIND ENERGY Wind Energ. 2002; 5:261–279 (DOI: 10.1002/we.61) Research Article Site-specific Design Optimization of Wind Turbines P. Fuglsang * and C. Bak, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark J. G. Schepers and B. Bulder, Netherlands Energy Research Foundation, Westerduinweg 3, NL- 1755 ZG Petten, The Netherlands T. T. Cockerill and P. Claiden, Renewable Energy Centre, School of Sciences, The University of Sunderland, Ryhope Road, Sunderland SR2 7EE, UK A. Olesen, Bonus Energy A/S, Fabriksvej 4, DK-7330 Brande, Denmark R. van Rossen, Lagerwey Windturbine BV, PO Box 279, NL-3770 AG Barneveld, The Netherlands Key words: wind turbines; aeroelasticity; optimization; cost modelling; cost of energy This article reports results from a European project, where site characteristics were incorporated into the design process of wind turbines, to enable site-specific design. Two wind turbines of different concept were investigated at six different sites comprising normal flat terrain, offshore and complex terrain wind farms. Design tools based on numerical optimization and aeroelastic calculations were combined with a cost model to allow optimization for minimum cost of energy. Different scenarios were optimized ranging from modifications of selected individual components to the complete design of a new wind turbine. Both annual energy yield and design-determining loads depended on site characteristics, and this represented a potential for site-specific design. The maximum variation in annual energy yield was 37% and the maximum variation in blade root fatigue loads was 62%. Optimized site-specific designs showed reductions in cost of energy by up to 15% achieved from an increase in annual energy yield and a reduction in manufacturing costs. The greatest benefits were found at sites with low mean wind speed and low turbulence. Site-specific design was not able to offset the intrinsic economic advantage of high-wind-speed sites. It was not possible to design a single wind turbine for all wind climates investigated, since the differences in the design loads were too large. Multiple-site wind turbines should be designed for generic wind conditions, which cover wind parameters encountered at flat terrain sites with a high mean wind speed. Site-specific wind turbines should be designed for low-mean-wind-speed sites and complex terrain. Copyright  2002 John Wiley & Sons, Ltd. Introduction The wind climate in mountainous complex terrain and offshore can be substantially different from that of normal flat terrain. This will affect turbine design assumptions and represents a potential for cost reduction by site-specific design. Such design can be achieved by optimization of the wind turbine for the specific site, by taking into account terrain and wind conditions as shown in Reference 1. Throughout the wind turbine industry, aeroelastic codes are now routinely used for the calculation of dynamic loads, annual energy yield and power quality. Proper modelling of the atmospheric wind climate, with turbulence as well as other site characteristics, is essential for reliable estimates of loads. Such methodology has been developed to a high level of complexity and is now to a large extent being used in the development of new large multi-megawatt prototypes. Aeroelastic calculations are complicated and the derivation of extreme and fatigue loads involves L Correspondence to: P. Fuglsang, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark. E-mail: peter.fuglsang@risoe.dk Contract/grant sponsor: European Commission; Contract/grant number: JOR3-CT98-0273. Published online 23 August 2002 Received 1 October 2001 Copyright  2002 John Wiley & Sons, Ltd. Revised 6 February 2002 Accepted 7 March2002