Load and power averaging in multirotor wind turbine systems

Abstract

Multirotor wind turbine systems (MRS) are an innovative alternative to conventional single rotor (SR) wind turbines. For this paper, the focus is on several selected and optimized 20 MW MRS designs based on previous simplified concept analyses. Based on aerodynamic simulations for turbulent wind fields, the so-called load averaging effect is investigated. Due to the individual ability of each rotor of the MRS to control speed and pitch, therefore reducing the local thrust forces, an averaging of loads, extreme and fatigue loads, is shown. This averaging also affects the MRS power curves. In a turbulent wind field slightly below rated wind speed at hub height, the upper located rotors are experiencing higher wind speeds due to wind shear and therefore go into full load operation, thereby limiting their power output and thrust. An SR in the same wind field would endure locally increased loads on the rotor, but cannot locally limit its power output. The SR can therefore generate a higher power output when approaching rated wind speed. In partial load operation mode, the MRS power curve outperforms the SR. The MRS reacts better on a spatial turbulent wind field by its multitude of rotors and their individual speed control.