The Economics of Multi-Axis Point Absorber Wave Energy Converters

Abstract

This paper examines the economic advantages and disadvantages of multi-axis point absorber wave energy converters in comparison to conventional heave-only point absorbers. A multi-axis point absorber wave energy converter (MA-PAWEC) is classified as a point absorber device that has a power take off (PTO) system extracting energy from more than one mode of motion (e.g. heave and surge). The majority of existing point absorber devices operate in heave mode alone. Therefore the forces exerted along other axes must be resisted by the mooring system, any reciprocal component of which constitutes a wasted opportunity to extract energy. The economics of PAWECs are governed by the available resource, energy generated by the device, capital cost and operational cost. These factors are examined for MA-PAWECs and compared to a generic heave-PAWEC. For a performance comparison, a simple generic body PAWEC is examined under heave mode operation and multi-axis operation in a representative spectrum. The modelling is based on linear potential theory. The potential advantages of MA-PAWECS are identified as greater energy absorption, fewer installed devices for a given capacity, and greater array control. Disadvantages include higher capex, higher maintenance costs and sensitivity to PTO costs. The performance and costs are assigned an estimated economic scaling factor and are applied to a generic heave-PAWEC for an economic comparison of the two devices. This indicates that a multi-axis approach to point absorbers could offer a 21% lower cost of electricity than the incumbent heave-response devices.