Kenneth Thomsen

Fatigue loads for wind turbines operating in wakes

Abstract Loads for a wind turbine operating in the wake of an upstream wind turbine was investigated with the objective of identifying the load generating changes in the wind field parameters compared to a free flow situation. The investigation was based on the measurements from the offshore wind farm Vindeby in Denmark. The following wind field parameters were included in the wind field analysis: Wind speed deficit, turbulence intensity, horizontal shear and turbulence spectrum length scale. The identified wind field changes were implemented in the aeroelastic code HawC, which was used to predict the wind turbine loads. For both free flow and wake cases, good agreement with measured loads was obtained. The most important load generating parameters for the wind farm in Vindeby were the increased turbulence intensity and the reduced turbulence length scale. The increased fatigue loading in the wind farm compared to free flow was found to be between 5% and 15%, depending on the wind farm layout. The load increase caused by wake effects was found to be the same for an offshore and a land site.

Site-Specific Design Optimization of 1.5–2.0 MW Wind Turbines

A method is presented for site-specific design of wind turbines where cost of energy is minimized. A numerical optimization algorithm was used together with an aeroelastic load prediction code and a cost model. The wind climate was modeled in detail including simulated turbulence. Response time series were calculated for relevant load cases, and lifetime equivalent fatigue loads were derived. For the fatigue loads, an intelligent sensitivity analysis was used to reduce computational costs. Extreme loads were derived from statistical response calculations of the Davenport type. A comparison of a 1.5 MW stall regulated wind turbine in normal onshore flat terrain and in an offshore wind farm showed a potential increase in energy production of 28% for the offshore wind farm, but also significant increases in most fatigue loads and in cost of energy. Overall design variables were optimized for both sites. Compared to an onshore optimization, the offshore optimization increased swept area and rated power whereas hub height was reduced. Cost of energy from manufacture and installation for the offshore site was reduced by 10.6% to 4.6 O. This reduction makes offshore wind power competitive compared with todays onshore wind turbines. The presented study was made for one wind turbine concept only, and many of the involved sub models were based on simplified assumptions. Thus there is a need for further studies of these models.