Types of marine power

There are three main ways to harness the energy stored in the sea and make electricity – wave power, tidal power and ocean thermal power. Several designs exist that can generate power from these resources.

Getting energy from the waves can be done with devices like Pelamis, CETO, Oyster and Oscillating Water Columns (OWCs). With tidal power, tidal barrages or offshore turbines do the trick. Ocean Thermal Energy Conversion (OTEC) power involves using the heat energy from warm water in tropical seas to make electricity.

How does it work?

Ocean thermal power

With OTEC, heat energy is converted to electrical energy. The warm seawater is put into a low-pressure environment so that the water changes into steam. The steam then turns a turbine and generates electricity. With other marine power devices, the kinetic energy stored in the water is converted to electrical energy.

Wave power

There are several different designs that all perform the energy conversion. In Pelamis the waves move hydraulic machinery in the hinges of the device which then drives an electrical generator. With Oyster, the waves move a big hinge on the sea floor which pumps water to a turbine.

Oscillating Water Columns are often built on rocks on the coast or float out at sea. They use the waves to push air through a pipe to make turbines move and power generators.

The following links to videos can show you how some of these devices work.

Tidal power

As the tide rises and falls, the movement of the water can be used to turn turbines and generate electricity. This is usually done in two ways, with tidal current turbines and tidal barrages. Tidal turbines work in a similar way to wind turbines except they are underwater and the water flow turns the blades rather than the wind.Tidal barrages are similar to hydroelectric dams. They are built across big estuaries. Once the tide has peaked, the barrage closes its gates so that the water is kept back. When the gates are opened again, the water drives turbines within the barrage to generate electricity. Some barrages generate electricity both as the tide is rising and when it is falling.

Cost and efficiency

Wave power

Most current devices are full-scale prototypes so they are more expensive than commercial products are eventually expected to be. Oyster currently costs about £9 million.

Tidal power

Barrages are enormously expensive to build and cost billions of pounds, but could produce a lot of power. Tidal turbines are still mainly in the prototype stage. OTEC is 2.5% efficient.

Typical size

Wave power

The size of the machine depends on the design. Pelamis is 120m long. OWCs tend to be a lot smaller.

Tidal power

Tidal turbines are big but tidal barrages are huge and cut off estuaries.

Locations around the UK

La Rance tidal barrage in northern France was the first to be built. Many wave machine prototypes are being tested in Scotland at a place called the European Marine Energy Centre (EMEC). Companies rent a space at the centre to test their new device. There is a tidal current turbine being tested in Strangford Lough in Ireland. There is talk of building a barrage across the Severn estuary between England and Wales.

Good news

Both waves and tides are reliable and generally predictable sources of energy. The machinery, once built, does not generate carbon dioxide or waste. They also have the potential to produce lots of electricity.

Bad news

Wave devices and tidal turbines must be able to cope with bad weather and seawater corrosion. They also need big electricity cables on the sea bed to connect them up to the grid. Tidal barrages are expensive and there are not many suitable sites for them to be built.

Energy storage

The energy is not easily stored, but tides in particular have regular patterns making the energy reliable and predictable so there is less need to store it. Tidal barrages do have some ability to store the energy in the water by closing the gates and opening them later when more electricity is needed.

Environmental impact

Wave and tidal turbine installations do not affect the environment very much. There are strict guidelines of where they can be located. Sometimes they can actually help as they can create nursery zones for fish, particularly as no shipping or fishing boats would be allowed in the area.

Tidal barrages have an impact on the environment. They obstruct the natural rising and falling of the tides and permanently floods some of the areas behind the barrage. This can affect all wildlife in a similar way to hydroelectric dams.

Human impact

Some wave devices may be noisy or ugly although many of them are located far out to sea and cannot be seen.

Is it a viable power source for the UK?

We could not get all our electricity from marine power as it would take up too much space to do so, but we do have lots of good locations for wave and tidal turbines. Tidal barrages produce much more power but we may not have enough sites for these to be built on in the UK to cover our energy requirements, and the environmental costs may prohibit some of these being used.

Current research

Scientists are currently working on various areas of marine energy technology. There are researchers looking at the environmental and human impact of proposed tidal barrages to investigate whether they should be built. Others use computers to investigate what could go wrong in the wave machines to try and solve the problems before the real thing is built. Many more are improving the performance of these technologies and coming up with new devices for making electricity from the sea.

More information