Tides flow in and out of estuaries and rivers on a cycle of approximately 12.4 hours–2 tides every 24 hours and 50 minutes. The tides are caused primarily by the gravitational pull of the moon, although the sun has a secondary effect, leading to tides varying in height throughout the year. The tides are 100% predictable and therefore offer a completely reliable energy source.
Energy is taken from the tidal water movement by building a dam across a river with sluices and turbines; the revolution of the turbines is used to generate electricity. There are a variety of turbine designs and they can be designed to operate in both directions–ebb and flow.
There are 2 basic options for Eco-THEP (tidal hydro energy plant) design and operation.
Ebb generation allows water to fill the basin to the full normal level, before closing the sluices at full tide. At this point, with the water level on both sides, the turbines can be used as pumps to raise the water level higher in the basin. The water is then held back for 1-2 hours while the sea level drops, the turbines are opened and the water flows through the turbines for 4-5 hours, producing electricity through to low tide. The energy returned by originally pumping the extra water over a very small head is much greater given the large head when it is released.
Two-way generation uses the turbines to generate on both flood and ebb tide. All the water flows through the turbines, with the water not being held back at full tide. The energy produced is usually less than ebb generation but electricity is produced over a longer period of around 8 hours per tide.