US wind power generation has grown significantly over the last decades, in line with the number and average size of operating turbines. However, wind power density has declined, both measured in terms of wind power output per rotor swept area as well as per spacing area. To study this effect, we present a decomposition of US wind power generation data for the period 2001--2021 and examine how changes in input power density and system efficiency affected output power density. Here, input power density refers to the amount of wind available to turbines, system efficiency refers to the share of power in the wind flowing through rotor swept areas which is converted to electricity and output power density refers to the amount of wind power generated per rotor swept area. We show that, while power input available to turbines has increased in the period 2001--2021, system efficiency has decreased. In total, this has caused a decline in output power density in the last 10 years, explaining higher land-use requirements. The decrease in system efficiency is linked to the decrease in specific power, i.e. the ratio between the nameplate capacity of a turbine and its rotor swept area. 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 in this period.

翻译：近几十年来，美国风力发电量显著增长，这与运行中的风力涡轮机数量和平均尺寸的增加相一致。然而，无论是按转子扫掠面积还是按间距面积测量的风能输出功率密度均有所下降。为研究这一现象，我们对2001—2021年间美国风力发电数据进行分解，并考察输入功率密度和系统效率的变化如何影响输出功率密度。其中，输入功率密度指涡轮机可利用的风能；系统效率指流经转子扫掠面积的功率中转化为电能的比例；输出功率密度指单位转子扫掠面积产生的风能。我们发现，尽管2001—2021年间涡轮机可利用的功率输入有所增加，但系统效率却持续下降。总体而言，这导致过去十年中输出功率密度下降，从而解释了更高的土地使用需求。系统效率的降低与单位功率（即涡轮机额定容量与其转子扫掠面积的比值）的下降相关。此外，我们还发现自2001年以来，由于涡轮机平均轮毂高度的增加，可利用风能大幅提升，但同期场地质量略有下降。