4.3.3.8 Ocean energy
The potential marine-energy resource of wind-driven waves, gravitational tidal ranges, thermal gradients between warm surface water and colder water at depths of >1000 m, salinity gradients, and marine currents is huge (Renewables, 2004), but what is exploitable as the economic potential is low. All the related technologies (with the exception of three tidal-range barrages amounting to 260 MW, including La Rance that has generated 600 GWh/yr since 1967) are at an early stage of development with the only two commercial wave-power projects totalling 750 kW. To combat the harsh environment, installed costs are usually high. The marine-energy industry is now in a similar stage of development to the wind industry in the 1980s (Carbon Trust, 2005). Since oceans are used by a range of stakeholders, siting devices will involve considerable consultation.
The best wave-energy climates (Figure 4.16) have deep-water power densities of 60–70 kW/m but fall to about 20 kW/m at the foreshore. Around 2% of the world’s 800,000 km of coastline exceeds 30 kW/m, giving a technical potential of around 500 GW assuming offshore wave-energy devices have 40% efficiency. The total economic potential is estimated to be well below this (WEC, 2004d) with generating cost estimates around 80–110 US$/MWh highly uncertain, since no truly commercial scale plant exists (IEA, 2006d).
Errata
Extracting electrical energy from marine currents could yield in excess of 10 TWh/yr (0.4 EJ/yr) if major estuaries with large tidal fluctuations could be tapped, but cost estimates range from 450–1350 US$/MWh (IEA, 2006a). A 1 km-stretch of permanent turbines built in the Agulhas current off the coast of South Africa, for example, could give 100 MW of power (Nel, 2003). However, environmental effects on tidal mud flats, wading birds, invertebrates etc. would need careful analysis. In order for these new technologies to enter the market, sustained government and public support is needed.
Ocean thermal and saline gradient energy-conversion systems remain in the research stage and it is still too early to estimate their technical potential. Initial applications have been for building air conditioning (www.makai.com/p-pipelines/) for desalination in open- and hybrid-cycle plants using surface condensers and in future could benefit tropical island nations where power is presently provided by expensive diesel generators.