Solar Energy produces the wind and the wind makes ocean waves, which then transfer energy from storm centres to our coasts. Wave patterns are irregular in amplitude, frequency, phase and direction, and power levels vary markedly from wave to wave and from season to season. Cost effective energy capture is therefore a difficult technical task. Concerted research was triggered by the oil crisis in the 1970’s. The resource is abundant but diffuse and best off western seaboards at temperate latitudes. Funds to build prototype devices have been scarce so the burden of development has fallen on laboratory experiments and mathematical modelling.
The basic requirements are a system with mass, buyoancy and damping to harness huge wave forces at low velocity. The technical problem is their conversion to low force at high velocity. This objective has spawned much inventiveness and the evolution of devices based on a wide variety of ideas. Mathematically, their description has been satisfactorily accomplished with linear mass-spring-damper models. Linear hydrodynamic theory is well-established and can predict wave-body interaction well for small amplitude motion. Devices which rely on large amplitude non-linear response for energy capture will be expensive. However, such response must obviously be analysed to answer questions of design strength and survival.
The assumption of linearity in the describing equations means that we may invoke the principle of superposition and decompose the response analysis into components at various wave frequencies. Spectral analysis of sea surface movement at sites of interest will then define the wave energy resource and the monochromatic forces which excite the device system. Off the west coast of the .UK, Norway and Canada, the mean annual power flux crossing a north-south line is about 50 kW/m. A good representative sea-state for design purposes has an energy period (the mean wave period weighted by energy content) of 10 seconds and an RMS surface elevation of 1 metre. It is worth remembering that the annual average is achieved by the three months or so of near flat calm being balanced by brief winter storms bearing several hundred kW/m.