Seite - 146 - in Water, Energy, and Environment - A Primer

8.7.3 Tidalenergy A related form of ocean energy is tidal energy, in which the potential energy in a body of water trapped at high tide is converted to kinetic energy when this water is released. Generally, strong tidal flows exist where the water depth is relatively shallow and there is a broad vertical range beneath high and low tides. This occurs in only a few locations. These tides are created by the gravitational interactions of the Earth, Moon and Sun, and are also impacted by the Earth’s rotation and regional ocean temperature differences. These periodic gravitational forces create a bulge in the oceanwater, creating a temporary increase in sea level. Tides resultwhen such a bulge, impacted by the Earth’s rotation, meets shallowwater adjacent to coastlines. The consistent nature of these gravitational attractionsmakes tidalpowerahighlypredictable energysource. An interestingaspectof tidalpower is that energy is lost in the Earth–Moon system due to friction mechanisms in the oceans. This results in a slowing of the Earth’s rotation, and it is estimated that over the past 620million years the Earth has lost 17%of its rotational energy and the length of an Earth day has increased from21.9 to 24hours (56). 8.7.3.1 Barrage Oneformof tidalenergy(barrage)captureswaterathigh tideand releases it at low tide, a form of hydroelectricity. Essentially, barrages are dams across the full width of a tidal estuary. Several other tidal power configurations are also being investigated – for example, at the European Marine Energy Centre inOrkney, Scotland. Tidalpower is still inanearly stageofdevelopment relative to other formsof renewableenergy,havingsufferedfromhighcosts and limited availability of suitable sites. Nevertheless, recent technological developments suggest that tidal power’s potential maybemuchhigher thanpreviouslyanticipated, and that its costs Water,Energy, andEnvironment–APrimer146