Horizontal Axis Wind Turbine Power Coefficient as Relative Capture Area

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 coefficient as an energy conversion efficiency, however, especially in the context of converting the “power in the wind” to a power output, comes with several conceptual difficulties. In this paper we argue that power coefficient is better interpreted as the “relative capture area” of a wind turbine, a parameter analogous to relative capture width for ocean wave energy conversion devices. Such an interpretation removes the ambiguities associated with the efficiency concept, gives a more physically coherent picture of wind turbine operation, and provides the most pragmatic measure of performance. In addition, the relative capture idea is universally valid, applicable not only to HAWTs but all other wind machine designs as well.