Energy-yield-based optimization of an H-Darrieus wind turbine

Small-size H-Darrieus wind turbines are gaining more popularity in the wind energy market, thanks to some particular benefits (simplicity, reliability and low noise emissions) and to the efforts of industrial manufacturers to propose new exterior solutions coupled with tempting rated-power offers. The actual operating conditions of a rotor over a year can be, however, very different from the nominal one and strictly dependent on the features of the installation site. Based on these considerations, a turbine optimization oriented to maximize the annual energy yield, instead of the maximum power, is thought to represent a more interesting solution. With this goal in mind, 5400 test cases of H-Darrieus rotors were compared on the basis of their energy-yield capabilities for different annual wind distributions in terms of average speed. To this purpose, the wind distributions were combined with the predicted performance maps of the rotors obtained with a specifically developed numerical code based on a Blade Element Momentum (BEM) approach. The limits related to the wind turbine start-up behavior and to the structural loads (i.e. maximum rotational speed and maximum wind velocity) were also taken into account. The analysis, developed in terms of dimensionless parameters, highlighted the configurations which ensure the largest annual energy yield for each wind distribution. In addition, the differences between the results of a design process oriented to the maximum power output or to the maximum annual-energy-yield were analyzed; in particular, the comparison showed that a design oriented to the maximum energy-yield is assumed to provide a notable increase in the extracted energy (up to around 20%, with the selected design assumptions) whenever wind distributions with low average wind speeds are considered.