A Re-examination of Power Coefficient as a Measure of Performance for Horizontal Axis Wind Turbines

Abstract

Linear momentum theory as applied to horizontal axis wind turbines (HAWTs) provides perhaps the most useful basis for understanding their operation. In particular, the theoretically derived expression for power coefficient represents a convenient measure of performance, as well as provides insight into optimal operating conditions. The typical interpretation of power factor as an energy conversion efficiency, however, especially in the context of converting the “power in the wind” to a power output, has several conceptual difficulties. In this paper it is argued that the energy efficiency interpretation of power coefficient can be misleading, potentially leading to misinterpretation of performance of different wind turbine designs. Instead, an interpretation of power coefficient as the “relative capture area” of a wind turbine is suggested, analogous to the relative capture width parameter for ocean wave energy conversion devices. Such an interpretation gives a more physically coherent picture of wind turbine performance and provides a more pragmatic measure of performance, one that can also be applied to other wind machine designs.