After 10 years’ intensive R&D and accumulated knowledge of the sector, we consider the following as pre-requisites for the commercial exploitation of wave energy:

Survival: Especially challenging for the North Atlantic. We design for the 100-year extremes, the greatest hazard being a freak ‘wall-of-water’ presently estimated to be ~24metres.  Certification by Det Norske Veritas or similar is necessary for marine insurance.  Fail-safe modes are essential during extreme events and breakdowns (eg failure within the device or of grid connection).

Deep water: Ocean waves lose energy and become steeper as the water shoals; losses become significant as the depth becomes less than half a wave-length.  The North Atlantic energy ‘hot spot’ West of Ireland is centred on ~178 metre wavelengths, ie longer wavelengths are important. The equivalent off West Coast USA is over 300 metres.  As might be expected, the bathymetry is ideal in the regions mentioned above, deep water is available within a few kilometres of the shore.

25+ year life on site: The main hull structures should be capable of remaining on site for at least 25 years, and be readily decommissioned thereafter.  The costs of recovering and re-deploying a device at any intermediate stage should be avoided completely.

Self-reacting point absorbers: Oscillating systems capable of resonant energy absorption have been the subject of a great deal of attention since the 1970’s.  The theory is now well established but, until recently, a number of technical challenges limited the prospects of commercial success.   Self-reacting point absorbers have two advantages, - independence from the sea-bed (other than slack moorings and grid connection) thus minimising installation and maintenance costs and, secondly, if axi-symmetric, can respond to waves from any direction.

Arrays:  The energy density of ocean waves is considerably greater than wind and consequently closer spacing is possible.  Theoretically defined by each unit’s absorption or capture width, in practice an array layout will be dictated by moorings (slack, for self-reacting devices), the prevailing wave direction, and foreshore consents.  

Tuning and control: Ocean waves are typically a mix of wind-waves and swell. Most of the time the wave climate is far from regular, and varies very significantly.  North Atlantic wave periods and wave heights can more than double within 24 hours as depressions pass over.  It is essential that any commercial device will have autonomous control (on-board ‘intelligence’) allowing it to tune to changing conditions and to maximise useful power output.  An ability to vary bandwidth is desirable.

Significant installed capacity:  Installed capacities should be greater than 1MW, otherwise per unit costs of moorings, grid connection, operations and maintenance become excessive.  

Power capacity: The amount of wave energy that an oscillating system can in theory absorb is a function of the prevailing wavelength and the oscillating mode(s).  For a North Atlantic site the theoretical limits are well above 1MW, averaged across the expected distribution, ie there are many occasions when the theoretical limit is much higher. A good point absorber, if ‘run backwards’, becomes a good wave generator.  To do so requires that a suitably large volume of water is displaced each wave period, and that is a function of water-plane area and stroke length.

Fabrication:  Low cost / long life / low maintenance materials such as concrete are to be preferred over steel or polymers, other things being equal.  Similarly, any need for large dry docks, deep water harbours, jack-up barges, etc., will add to costs and limit the number of suitable facilities for the construction and deployment stages.

Cost / kWh: This is a matter of minimising costs (capital, opex) and maximising useful electrical power delivered to the grid.   

Health and safety:  Although not expected to carry permanent crew or volatile hydrocarbons, access for routine and un-planned on-board basic maintenance requires clear procedures.  The installed equipment should be safely housed and accessible above the water-line.  Boarding and dis-embarking via a rib, small service craft or helicopter should be well within acceptable standards up to at least Force 5.  The device must be capable of being switched remotely to a non-operational safe mode, and of failing safe.