Presentations

Abstract

The efficiency of wind farms, especially offshore, is driven by both local and non-local losses. Local losses include inter-turbine wakes. Non-local losses include effects such as global blockage and wind-farm-induced gravity waves which are manifestations of the temperature stratification of the atmosphere and the height of the atmospheric boundary layer (ABL). A typical ABL is capped by a very stable inversion layer displaying a rapid increase in potential temperature with a less strong stable layer aloft in the free atmosphere. The height of this inversion layer, its strength and the stratification of the free atmosphere have been theoretically shown through numerical modelling to have a strong impact on wind farm efficiency. Low level jets (LLJs) which are low level maxima in wind speed deviating from a standard monotonically increasing profile with height have also been the subject of research to assess their impact on performance. It has been suggested that jets above a wind farm might aid wake recovery and jets below hub height might hinder wake recovery [1]. There had been little work done to date to assess the impact of ABL stratification and LLJs on operational offshore wind farms. Using SCADA data from several operational wind farms in the North Sea and meteorological data from the ERA5 reanalysis we show that the effect of temperature stratification beyond the surface layer has a tangible impact on wind farm efficiency and wind farm yield. ERA5 reanalysis temperature data at multiple vertical levels are used to classify ABL height, inversion layer strength and free atmospheric lapse rate. The efficiency of six operational wind farms in the North Sea was calculated under the different temperature stratification classes. Wind farm efficiency, calculated as the ratio of farm power output to that of the equivalent number of isolated turbines based on the local ERA5 wind speed, can vary by a factor of up to 2.5 with the biggest reduction in efficiency seen for very low boundary layer heights and strong inversion strengths. The results show a good correlation with previous LES simulations of a hypothetical regular wind farm array, particularly when considering the non-local efficiency [2]. LLJs were classified using wind speed data from ERA5 in terms of their magnitude and height between 140m and 290m. In all cases, wind farm efficiency was shown to be significantly reduced compared to the absence of a low-level jet. However, jet height was found to be strongly correlated with ABL height. The lower ABL height is likely to be the dominant effect, though this needs further research. Considering that offshore ABLs are frequently relatively low (<500m), these results show that modelling their impact on efficiency is important as otherwise energy yield can be over-estimated. References [1] Gadde SN, Stevens RJAM. Effect of low-level jet height on wind farm performance. J. Renewable Sustainable Energy 1 January 2021; 13 (1): 013305. [2] Lanzilao L, Meyers J. A parametric large-eddy simulation study of wind-farm blockage and gravity waves in conventionally neutral boundary layers. Journal of Fluid Mechanics. 2024;979:A54. doi:10.1017/jfm.2023.1088