Tidal Energy

Unlike wind and sun, tides are always moving. Harnessing tidal energy is one of the greatest potential sources for renewable energy. They’re reliable, we can calculate the tides years into the future and they contain a huge amount of energy.

Harnessing tidal energy is not new. In France, tidal mills have existed since the 12th Century, used for grinding grain. These mills were built in shallow creeks by the coast. Gates were constructed that swung open inwards. As the tide came in, the waves pushed the gates open. When the tide turned, the gates were forced shut, forcing the retreating water to flow seaward through the watercourse of the mill.

The tides are 'powered' by the moon. The gravitational pull of the moon pulls the water in Earth’s oceans, creating a bulge on the side facing the moon. A second bulge on the opposite side of the Earth is created due to the centrifugal force of the planet rotating. These two bulges create our high tides. Tidal power captures the energy from the rising and lowering sea levels. There are two main types:

Tidal Flow Turbines

Tidal flow turbines are located in areas of strong currents and tidal flows, such as around a headland or close to estuaries or islands. They typically resemble wind turbines on the seabed, with propellers shaped to minimise the impact on sea life and to ensure the turbines do not get caught up with debris and underwater plant matter.

A number of manufacturers are now producing tidal flow turbines and they are being trialled in a number of places around the world. ScottishPower Renewables is currently installing a tidal array off the coast of Islay in Scotland. Consisting of ten 1MW Andritz Hydro Hammerfest HS1000 Tidal Turbines, the system will be fully submerged on the seabed and is expected to generate about 30GWh of electricity annually: sufficient to power the local communities living on the islands of Jura and Islay where the array is to be situated.

Tidal Barrages

A dam is constructed across the mouth of an estuary. When the tide comes in, water is held back by the dam until there is sufficient head to generate power. The gates are then opened and water flows into the dam, flowing through hydro-turbines and generating electricity. As the high tide peaks, the gates are closed, holding water behind the barrage. As the tide recedes, the trapped water is once again released through the hydro-turbines, generating additional electricity.

Tidal barrages are the most established source of electricity from our oceans. La Rance barrage near St. Malo on the north French coast has been in place and operational for 49 years. It generates electricity for around 20 hours each day and has never been closed except for routine maintenance.

La Rance tidal barrage, pictured below, generates an average of 64MW of power and has an annual generation capacity of 540GWh. The barrage is 145 metres long and the plant generates a little over 1% of all electricity use in France.

Tidal barrages have also been installed in South Korea and in Canada. A tidal lagoon that is similar in capacity to the La Rance plant is currently planned for Swansea bay in the United Kingdom.

Small scale tidal energy

As with wave energy, tidal energy is an emerging technology and at present there are no systems available for small scale power generation. They are covered at a high level within Introducing Renewable Energy as they are exciting technologies with huge potential as a source of reliable renewable energy and no book that claims to introduce renewable energy is complete without mentioning them.

Experimental small-scale tidal energy systems are being developed and tested. The most common design is a small propeller-based system that resembles an outboard motor from a boat. These are designed to be mounted on jetties or on stationary boats in harbour.

There are still a number of issues that need to be overcome before small scale tidal energy can become feasible. The first issue is robustness. Tidal energy generators are complex pieces of electrical equipment. They are submerged in salt water and installed in locations where there are powerful currents. They are constantly pounded in every direction, not just by the water, but by anything picked up by the currents – rocks, sand and general debris.

Getting the electricity out of the sea is also complicated and expensive. For submerged systems, maintenance and repair is also problematic: safe access to the generating equipment is difficult and expensive. The final issue is protecting marine life. Tidal energy systems are inevitably installed in fast moving, comparatively shallow water and many of them are near the shore. Installing power generation can have a significant disruptive impact on the local ecology and the designs need to minimise this wherever possible.