On the difficulty of inventing an economical sea wave energy converter: A personal view

Abstract

Sea waves are a promising potential source of renewable energy, but the technology has not yet settled down to one or two basic forms as has happened with most major inventions. New ideas continue to arise, and no general agreement about how to proceed seems to be emerging. Moreover, although practicable wave energy converters (WECs) have been developed, they are not as economical as might have been hoped. This paper explores a particular aspect of the cost problem. Broadly speaking, the costs of WECs are high because they deal with large forces moving slowly. It is reasonable to expect that where a working surface can be identified, the bigger its area is relative to the overall surface area and the faster the surface moves, the more economical it is likely to be. This suggests two criteria for an economical WEC: a large working area relative to its size and a high ratio of the speed of that surface to the particle speed of the wave. The second criterion indicates a resonant system, and this paper is confined to WECs that have quasi-resonant working surfaces (QR WECs). The quasi- is because the mechanics involved is not quite that of classical resonance. Promising types are listed in four groups according to the source of reaction, because this is generally the most difficult function to provide in QR WECs. Other types of WEC are left out, most often because they do not meet the criteria of a strongly coupled working surface and resonance, and usually require large amounts of material. While size is not uniquely related to cost, WECs using much material are likely to be expensive.