The advantage in using wave or wind power is that there is good seasonal matching between power supply and demand. The major drawback with wind and wave power is the lack of control engineers have over the power supplied. The use of water is also seen in hydroelectric power stations and tidal power stations. A typical hydroelectric scheme uses a dammed river or lake to provide a reservoir that supplies a steady flow of water to drive turbines. In areas where the demand for power fluctuates, pumped storage plants may be installed, where the surplus power created during off-peak times is used to pump water back into the same or maybe a different reservoir.
Tidal power uses reversible turbines that both manipulate the flood and ebb tides.
At high tide sluice gates are closed, creating a reservoir in the tidal basin. The water can then be released at will. The largest tidal scheme in La Rance, France can produce 240MW, enough to satisfy the needs of a city of several hundred thousand inhabitants. In many developing countries they are the only source of energy with emphasis now being placed on the planting of fast-growing trees in order to supply firewood. Biomass is also being converted into alcohol which is being used as fuel for internal combustion engines when mixed with petrol.