Wave and Tidal Power

Inevitably, it has been easier over the past few decades to test a PV panel or a wind turbine – companies and researchers have not needed access to an area of sea or part of the coast, so PV and Wind generation have been developed much more quickly and are now basically at the commodity stage – they work, people are familiar with them, their pay-back parameters have been calculated etc.

The main advantage of wave energy, if you live in a country with a significant length of coast, is that it is there all the time – the waves go up and down all day, every day, though systems to extract energy from them are usually relying on a regular up and down motion and do not cope well with storms. However, so far the systems which have been put in are fairly small-scale and very high cost. Designing devices which will continue to work for a long time in salt water and which will withstand all weather conditions, increases the manufacturing cost.

There are several systems so far proposed for converting wave energy to electricity. These basically depend on:

• Changes in pressure as waves go past a fixed installation being used to pump air or water past a turbine
• Floating devices flexing or rocking, and that motion being used to turn a turbine and create electricity.

It is not clear how efficient any of these systems will be, but some companies have put in relatively low power test installations.

Like tidal systems there are potential down-sides. Anything large at the surface of the sea might be unsightly, or in the way of recreational boating and leisure activities, if close to the shore, and could get in the way of shipping, if further out to sea. Anything submerged could have an environmental impact on local sea-life.

Pressure-driven devices:

In Oscillating Water Column devices, there is a partly-submerged structure with openings near the surface where water will flow in as a wave passes. When the water enters, it compresses air in the column above and the compressed air turns a turbine to produce electricity.

In a Submerged Pressure-Driven device, the pressure changes, as a wave rolls over above. This compresses air and turns a turbine. A company called M3Wave have designed and deployed this sort of device. As it is submerged, it does not suffer so much during storms.

Wave-driven water pumps and Overtopping Systems pump water into a reservoir which can then run out past turbines to generate electricity.

Floating flexing or rocking:

Attenuators have floating structures which flex as the waves pass, and the flexing motion turns a generator. One of these was built by a company called Pelamis Wave Power, which sadly went into administration in 2014. While they were still trading, they sold a successful wave energy convertor to Scottish Power to operate in the Orkneys. The device generated 750kW, measured 180m long, was 4 metres in diameter and weighed 1350 tonnes.

Inverted pendulums or buoys rock and the rocking motion can be used to turn a generator. This sort of device is produced by a company called Ocean Power Technologies, though their products all seem to be designed for providing local, relatively low power, to devices which need to be moored offshore – like communications or weather-monitoring devices, rather than anything which would contribute to an onshore power grid.