Posters

Abstract

The wind conditions prevailing at a wind farm site inform the selection of suitable wind turbine technology. This includes consideration of the class of wind turbine that is appropriate, with a rotor swept area that keeps the mechanical fatigue loads that are propagated through the drive train within design limits. The rotor area also determines how much energy is extracted from the wind for a given wind speed, with larger rotors deployed on lower wind sites to achieve rated power at lower wind speeds. Some sites exhibit a high degree of seasonal variation in wind speed, such that a smaller rotor may be more suitable during a windy season, while a larger rotor would be appropriate during a calm season. However, blade length, hence rotor diameter, is typically a fixed quantity, and turbine technology must be selected on the basis of annual average characteristics, with seasonal, diurnal and directional variations in wind conditions sometimes accommodated by operational strategies. Variations in blade length have been considered in other contexts. For example, studies have been undertaken previously to explore the viability of blade extensions when the observed conditions at a site suggest a larger rotor area can be tolerated by the system. Detachable blade tips have also been used to address logistical challenges when transporting blades to site in difficult terrain. This presentation considers the potential benefits of a blade length that can be varied on a seasonal basis to match the rotor diameter to the conditions prevailing during a particular season, using techniques such as those mentioned in relation to logistics. The potential benefits are explored by applying wind climates with a high degree of seasonality to different power curves, representing IEC Class I to III, to show what degree of seasonality represents a threshold beyond which there is a benefit to switching between classes by the use of seasonal blades. A simple seasonality metric is introduced in order to support this analysis. This established seasonality by fitting a circle to a polar plot of wind speed against time of year, where time is the polar angle, reflecting the cyclical nature of seasonality. Seasonality is then the ratio of the distance of the centre of the circle from the origin to the radius of the circle, taking a value between zero (no seasonal variation) and unity (maximum seasonal variation from flat calm to peak wind speed). Different circles are fitted to different periods to represent complex seasonality, such as monsoon climates. The impact of seasonality on production with and without seasonal blades changing the class of the turbine to match the season is investigated using ERA5 data from a number of sites around the world to determine when the strategy is financially feasible.