Analysis of Force-Capping for Large Wind Turbine Rotors

A standard design procedure is to optimise wind turbine rotors for maximum power coefficient within a specified range of wind speeds, up to the rated wind speed. The downwind thrust force upon the rotor is greatest at this design condition. Reacting large forces using cantilever beams necessitates high material costs for the blades, tower and foundations. This paper shows that limiting the maximum allowable downwind thrust can lower the capital expenditure (CAPEX) of a wind farm per MW installed. This saving is made at the expense of sacrificing power coefficient over the small range of wind speeds at which the thrust limit is enforced. We describe this concept as ‘force-capping’. For small reductions in the maximum allowable force, the effect on annual energy production (AEP) is minor. This paper outlines a parametric analysis procedure to evaluate how reducing the material content of a wind turbine affects the energy production. Dynamic forces and fatigue life are considered. For small reductions in turbine material, we show that the change in CAPEX is first order, whereas the loss of AEP is second order. We therefore conclude that force-capping must be beneficial in every case to at least some extent. Our analysis shows that the cost of energy can reduce by approximately 0.2% through imposing a force-cap of ~90% upon the maximum thrust force for a 5MW machine. The case for force-capping strengthens further as the cost of borrowing and cost of material rises.