Salinity Gradients

Seawater is approximately 200 times more saline than fresh river water, derived from rain, snowmelt or groundwater and is delivered to the coast by major rivers. Global salinity differences arise from submarine and surface current movements (Fig. 26.6). The relatively high level of salinity in seawater thus establishes a pressure potential with sweet river water, which can be used to generate electricity or derive fresh (drinking) water from the seawater. This 'osmotic' pressure differential—equivalent to a hydraulic head over 120 m—can be harnessed and used to drive a conventional Pelton Wheel turbine to generate electricity. It can also be used as a chemical potential to generate electricity directly.

100mW 0* 100'E

Fig. 26.6 Average global sea surface salinity. (NASA 2009)

100mW 0* 100'E

Fig. 26.6 Average global sea surface salinity. (NASA 2009)

Whilst freshwater salt content is unlikely to be seasonably variable, the same is not true for ocean salinity. There is strong evidence of seasonal variability of salinity, which may also be related to decadal variations, such as the El Nino Southern Oscillation (Alves et al. 2010). However, the impact of these seasonal and decadal fluctuations may be relatively small on potential power production. In any event the technologies are not sufficiently developed to confirm that salinity variation will lead to seasonal variations in energy production.

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