Tidal Energy

Tidal energy is a form of hydropower that utilises large amounts of power within the ocean’s tides to generate electricity. Tidal energy is a renewable energy source, as the Earth uses the gravitational forces of both the moon and the sun everyday to move vast quantities of water around the oceans to produce tides.

There are different kinds of tidal power systems. A tidal barrage is a type of tidal power generation that involves the construction of a fairly low dam wall, known as a ‘barrage’, across the entrance of a tidal inlet or basin, creating a tidal reservoir. This dam has a number of underwater tunnels cut into its width allowing seawater to flow through them in a controllable way, using a sluice gate on the sea and reservoir side, which can slide down or up to release or retain water as desired. Fixed within the tunnels are propellers that are turned by the tidal flow and they in turn spin a turbine. The movement creates a magnetic field within the generator above, which is converted to electricity.

One disadvantage of tidal barrage electricity generation is that it can only generate electricity when the tide is actually flowing either in or out, as during high and low tide times the tidal water is stationary. However, as the tides are completely predictable, it is straightforward to plan how to compensate for low generation times with other providers in the energy mix. Supporters of tidal power also point out that other renewable energy resources, such as solar and wind farms, are much more unpredictable and intermittent. Other disadvantages of a tidal barrage system are the high construction costs and the environmental effects that a long concrete dam may have on the estuary it spans.

A tidal stream generation system reduces some of the environmental effects of tidal barrages by using turbine generators beneath the surface of the water. Major tidal flows and ocean currents, like the Gulf Stream, can be exploited to extract their tidal energy, using underwater rotors and turbines. Tidal stream generation is very similar in principal to wind power generation, except this time, water currents flow across a turbine’s rotor blades that rotate the turbine, much like how wind currents turn the blades for wind power turbines. In fact, tidal stream generation areas on the seabed can look just like underwater wind farms.

Unlike offshore wind power turbines, which can suffer from storms or heavy sea damage, tidal stream turbines operate just below the sea surface or are fixed to the seabed. Tidal streams are formed by the horizontal fast flowing volumes of water caused by the ebb and flow of the tide, as the profile of the seabed causes the water to speed up as it approaches the shoreline.

As water is much denser than air and has a much slower flow rate, tidal stream turbines have much smaller diameters and higher tip speed rates compared to an equivalent wind turbine.

One of the disadvantages of tidal stream generation is that, as the turbines are submerged under the surface of the water, they can create hazards to navigation and shipping.

A good example of a successful tidal power project is the La Rance power station in France.

This tidal barrage is still the largest tidal power station in the world, in terms of installed capacity, with a peak rating of 240 megawatts generated by its 24 turbines, and an annual output of approximately 600 gigawatts. The development costs were significant, even back in 1966 when it was opened, but these have now been recovered and electricity production costs are lower than that of nuclear power. The high cost of tidal barrages, however, is what has discouraged the further construction of similar projects.

The environment at La Rance has remained healthy, but there have been changes.

The barrage has caused limited silting of the Rance ecosystem, although this has been manageable. Sand eels and plaice have reduced in numbers, but sea bass and cuttlefish have returned to the river. The tidal flows are regulated in the estuary by the operators, who adjust them to minimise the biological impact.

The La Rance tidal plant produces a source of energy that is clean, renewable and sustainable. It has no impact on climate, because it does not emit any greenhouse gases.

The pattern of the tides is preserved, so that the impact on species living in the estuary is minimal. The operator monitors the tides and weather forecasts to program the barrage operations on a weekly basis.

Since the construction of the barrage, a new ecological equilibrium has been established in the Rance estuary and there is an abundance of fish, bird and other wild life. The mean water level in the lagoon is higher than it was before the construction, which has promoted an increase in boating and sailing activities. The facility attracts approximately 70,000 visitors per year and a canal lock in the west end of the dam permits the passage of 20,000 vessels each year between the English Channel and the Rance.

Tidal power offers society a clean and renewable source of energy. Although technology is still at a relatively immature stage, economic projections indicate that tidal energy could become cost-competitive over the long-term and governments should explore potential sites for taking advantage of these natural opportunities.