Performance analysis of wind turbines with leading-edge erosion and erosion-safe mode operation

Abstract

For offshore wind turbines, Leading-Edge Erosion (LEE) due to rain is posing a serious risk to structural integrity and can lead to a performance loss of the order of a few percent of the Annual Energy Production (AEP). A proposed mitigation strategy is the so-called Erosion-Safe Mode (ESM). In this work, the AEP losses caused by LEE or by operating in the ESM are compared for two reference turbines, i.e. the IEA 15MW and the NREL 5MW turbines. For both turbines, the performance is evaluated in uniform and sheared inflow conditions. The effects of erosion are modeled by creating clean and rough airfoil polars in XFOIL. It is assumed that erosion occurs once a critical blade element section speed is exceeded. Power curves for LEE and ESM are calculated by using the free-wake vortex method CACTUS. Results show that LEE negatively affects the power production below rated capacity, while operating in ESM predominantly sheds performance at rated power of the turbine. This study, therefore, shows that a break-even point for the ESM exists. The AEP loss due to erosion can be successfully mitigated with the ESM at sites with low mean wind speed, however, at sites with higher mean wind speed, operation with erosion leads to a lower AEP loss. The break-even point shows little sensitivity to the blade design and to mean shear variations, but strongly depends on the frequency ESM needs to be applied. The latter is driven by the predicted amount of damaging rain events. In conclusion, erosion-optimal operation is strongly governed by the site characteristics and much less by turbine design, and the viability of an ESM strategy can be significantly expanded by a better understanding of blade damage mechanisms and improved forecasting of the related weather events.