The decline in specific power reduces wind power density and system efficiency

US Wind power generation has grown significantly over the last decades, both in number and average size of operating turbines. A lower specific power, i.e. larger rotor blades relative to wind turbine capacities, allows to increase capacity factors and to reduce cost. However, this development also reduces system efficiency, i.e. the share of power in the wind flowing through rotor swept areas which is converted to electricity. At the same time, also output power density, the amount of electric energy generated per unit of rotor swept area, may decrease due to the decline of specific power. The precise outcome depends, however, on the interplay of wind resources and wind turbine models. In this study, we present a decomposition of historical US wind power generation data for the period 2001-2021 to study to which extent the decrease in specific power affected system efficiency and output power density. We show that as a result of a decrease in specific power, system efficiency fell and therefore, output power density was reduced during the last decade. Furthermore, we show that the wind available to turbines has increased substantially due to increases in the average hub height of turbines since 2001. However, site quality has slightly decreased during the last 20 years.