Marine Renewables

Tidal Energy

Ocean tides are periodic rises and falls in the level of the sea, and are formed by the gravitational attraction of the Moon and the Sun on the water in the ocean. Tides are one of the most predictable forces of nature. Water level rises during "flood tides", and falls during "ebb tides". Each day, there are two flood tides and two ebb tides - or more precisely two cycles for every 24.84 hours, which is the period of the rotation of the moon around Earth. The difference in water levels between consecutive flood and ebb tides is called the "tidal range".

Since the earth and the moon rotate around the sun, the relative positions of the moon and the sun also affect the tides. When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called "spring tides". When the sun and moon are not aligned, the tides are not as dramatically high and low. These are called "neap tides".

Tidal energy is stored by means of an impounded basin and later released through turbines. The larger the impounded basin is, the more energy can be stored due to tidal level changes. For a constant cross-sectional area impounded water basin with a constant water level difference of H metres, the potential energy available from the water basin per unit area of the basin will be 4.91 H2 kJ/m2. This being the theoretical maximum for the tidal energy resource potential.

Working Principles of Tidal Power Devices

Tidal power can be captured by means of a barrage or dam built across an estuary to form an impounded basin. During flood tides, water will fill the impounded basin via a series of sluices which are then closed when the water starts to ebb. When the water level at the seaward side lowers further, and the "water head" is increased, and electricity can be generated by releasing the water stored in the basin through a series of conventional turbines, which drive the electric generators.

Above: Tidal barrage for ebb generation

Power can be generated from a barrage either by passing the incoming tide through the turbines mounted in the barrage (flood generation); or by allowing the flow to pass through sluices without generating power and then trapping the high tide behind the barrage by closing the sluices. The head of the water is then passed back through the turbines on the outgoing ebb tide (ebb generation). In simple ebb or flood generation, large installed capacity is only used for a short period of time (3-6 hours) in each tidal cycle, producing short bursts of power.

Another method, two-way generation, extracts energy from both ebb and flood tides. However, as the output will be more evenly distributed in time, there will be a net decrease in power.