2 Harnessing tidal energy
The principle of tidal power generation is very similar to that employed in conventional hydropower schemes, except the working head of water is no greater than about 10 to 15 m. The critical term in Equation 5.6 is the water discharge rate. Tidal barrages across estuaries can extract energy from the incoming (flood) and the outgoing (ebb) tides using turbines located in sluices through the barrage. Estuarine barrages can enclose vast amounts of water, including river flow into the estuary, thereby increasing power capacity.
The most common type of barrage uses just the ebb tide. The incoming flood tide is allowed to pass though sluices with the generators idling, to trap the water behind the barrage at high tide. This head of water then drives the turbines to generate power on the ebb tide. Flood tide generation is simply the opposite but in either case, given that there are two tides a day, power is produced in two bursts every day. Two-way operation is possible, using both ebb and flow, although power output is not doubled. This is because the tidal basin would normally empty and fill through its entire natural outlet, and when constrained through sluices the full cycle would not be complete before the tide turned. The major disadvantage of tidal barrages is the same as for hydroelectricity dams, they silt up. Moreover, by constraining tidal flow through narrow sluices instead of the full width of an estuary the natural processes of tide-dominated sedimentation and erosion are changed over large areas.
There are alternatives to estuarine barrages, albeit with much lower power capacity. One is to create artificial lagoons in embayed coasts or to cut off parts of estuaries to minimize environmental change. In especially powerful tidal flows that pose insurmountable challenges to construction, such as those around western and northern Scotland, tidal energy could be 'harvested' using small, floating turbines anchored to the sea bed.
As with wind and wave energy, that available from tides is geographically highly varied, and obviously restricted to maritime countries. Some, like the UK are well endowed, but most have far less potential, particularly islands surrounded by major oceans and areas with simple coastlines.