Survey of Energy Resources 2007
Tidal - the Resource
Wave energy can be considered as a concentrated form of solar energy, where winds generated by the differential heating of the earth pass over open bodies of water, transferring some of their energy to form waves. The amount of energy transferred and hence the size of the resulting waves, depends on the wind speed, the length of time for which the wind blows and the distance over which it blows (the 'fetch'). In this way, the original solar power levels of typically ~ 100 W/m2 can be transformed into waves with power levels of over 1 000 kW per metre of wave crest length.
Waves lying within or close to the areas where they are generated (storm waves) produce a complex, irregular sea. These waves will continue to travel in the direction of their formation even after the wind dies down. In deep water, waves lose energy only slowly, so they can travel out of the storm areas with minimal loss of energy as regular, smooth waves or 'swell' and this can persist at great distances from the point of origin. Therefore, coasts with exposure to the prevailing wind direction and long fetches tend to have the most energetic wave climates, such as the western coasts of the Americas, Europe, Southern Africa and Australia/New Zealand, as shown in Fig. 14-1 .
The global wave power resource in deep water (i.e. 100 m or more) is estimated to be ~ 1-10 TW (Panicker, 1976). As the waves move to shallower waters they lose energy, but detailed variation of sea-bed topography can lead to the focusing of wave energy in concentrated regions near the shoreline, called 'hot spots'. The economically exploitable resource varies from 140-750 TWh/yr for current designs of devices when fully mature (Wavenet, 2003) and could rise as high as 2 000 TWh/yr (Thorpe, 1999), if the potential improvements to existing devices are realised.