Limiting reactive power flow peaks in wave energy systems

Abstract

Optimal control of wave energy converters has been shown to require an injection of power into the WEC system to maintain an optimal velocity profile. While this consumption of power results in an overall increase in energy capture, it also brings more stringent requirements on the power take-off (PTO) system. Specifically, the PTO must cater for bi-directional power flow and a source available for the provision of this reactive power, either via a storage device, or the electrical grid itself. However, one aspect which has received relatively little attention is the magnitude of the reactive power peaks, which may have implications for the required overall power rating of the system. In particular, though reactive power flow may only be required for a small fraction of the wave period, reactive power peaks well in excess of active power levels bring a potentially significant capital cost in terms of system power rating, along with a unfavourable capacity factor rating. This paper examines the circumstances under which reactive power flow peaks exceed active power levels and proposes a solution which puts a finite (nonzero) limit on reactive power flow, consistent with active power levels. The problem is solved as a nonlinear constrained optimisation problem, while the consequences of imposing such a limit on energy capture are also examined.