Home / Energy Types /

Wave Energy

Porthleven (D Davies, 2007)Wave energy occurs due to movements of water near the surface of the sea. Waves are formed by winds blowing over the water surface, which make the water particles adopt circular motions. This motion carries kinetic energy. The amount of energy in waves depends on their height and period (the time between successive peaks). The annual average power per unit length of wave crest (e.g. 40 kW/m) is a first indicator of how energetic a particular site is.

Waves occur only in the volume of water closest to the water surface, in waves, the water acts as a carrier for energy, moving it in some direction, but does not undergo a net movement itself.

Wave Energy can be:

  • Shoreline
  • Nearshore
  • Offshore

Marine renewable energy can provide a significant contribution to our energy needs in the future.

The Resources

Praa Sands - D Davies 2008The energy contained in ocean waves is a huge potential source of renewable energy and the UK has wave power levels that are amongst the highest in the world. However, the technology, infrastructure and support need to be put in place to make best use of this clean inexhaustible energy supply.

By harnessing this resource efficiently, it is believed that we will have a sustainable and environmentally friendly source of power for generations to come.

Wave energy is a concentrated form of wind energy. Wave energy occurs in the movement of water near the surface of the sea. The stronger the wind and the longer the distance over which it blows, the larger the waves and the more energy they carry. Waves travel great distances without significant losses and so act as an efficient energy transport mechanism across thousands of miles.

For this reason, waves on the west coast of the UK tend to contain more energy than those on the east coast as the wind can blow all the way across the Atlantic. As waves reach the shallower water near our coasts, they begin to lose energy through friction with the seabed and eventually break on the shore.

This means that the greatest amount of energy is available in deeper well-exposed waters offshore.

The UK’s electricity consumption is around 350,000,000,000 kilowatt hours (kWh) per year. Given the available resource, marine renewable energy could perhaps produce between 15 and 20% of this.

Technology

The Carbon Trust (www.carbontrust.co.uk) designed the Marine Energy Challenge. This was a £3.0m, 18-month programme of targeted engineering support, intended to improve understanding of wave and tidal stream generation technologies by helping technology developers advance their designs.

Energy from the waves can be captured by various devices, which produce enough movement either of air or water to drive generators that converts the energy into electricity.

(The following information on the technologies used in wave energy is mainly taken from The British Wind Energy Association website (www.bwea.com))

Placement - The decision of where to place a wave energy device is often related to design requirements for water depth (this generally increasing with distance from shore), the energy content of waves (this being greater offshore), and access for deployment, retrieval, operation and maintenance.

Fixing - Near-shore and offshore devices may be either bottom-mounted or floating.

Reaction - Wave energy devices need a system of reacting forces in order to extract energy and this is one of the biggest design challenges. To create such a system, two or more bodies need to move relative to each other, while at least one body interacts with the waves. One approach is to allow one body to move freely with the waves, while another is held static (as in the case of a floating buoy reacting against the seabed). Alternatively, all of the bodies may be dynamic.

End stop - Within the reaction system, a common requirement is to avoid situations where the relative motion is so large that destructively high forces occur between the bodies. Some of the most well-known device concepts are introduced below.

Oscillating Water Column (OWC) - This comprises a partly submerged structure (‘collector’) which is open to the sea below the water surface so that it contains a column of water. Air is trapped above the surface of the water column. As waves enter and exit the collector, the water column moves up and down and acts like a piston on the air, pushing it back and forth. The air is channelled towards a turbine and forces it to turn. The turbine is coupled to a generator to produce electricity.

Overtopping - This consists of a structure over which the waves topple, a reservoir to collect the water and hydro turbines installed at the bottom of the reservoir. The head of collected water turns the turbines as it flows back out to sea and the turbines are coupled to generators to produce electricity.

Point absorber - This is a floating structure that absorbs energy in all directions by virtue of its movements at or near the water surface. This feature is useful to maximise the amount of power that is available for capture. The power take-off system may take a number of forms, depending on the configuration of displacers/reactors.

Terminator - This is also a floating structure that moves at or near the water surface, but it absorbs energy in only a single direction. The device extends in the direction normal to the predominant wave direction, so that as waves arrive, the device restrains them.

Attenuator - This device is a long floating structure like the terminator, but is orientated parallel to the waves rather than normal to them. It rides the waves like a ship and movements of the device at its bow and along its length can be restrained so as to extract energy.

Cost

Marine renewable energy has the potential to become competitive with other generation forms in future. At present, it is likely to be more expensive than other renewables and conventional generation until at least hundreds of megawatts capacity are installed. This capacity is equivalent to several offshore wind farms at the scale currently being constructed.

Some estimates on costs of Energy from wave energy farms have been between 12p/kWh and 44p/kWh, with central estimates for offshore wave farms in the sub-range 22p/kWh to 25p/kWh.

SperboySPERBOY™ was devised by Embley Energy Ltd to produce electrical energy from ocean waves. Embley Energy Ltd took part in the Carbon Trust's Marine Energy Challenge where SPERBOY™ was studied and a best-case, cost-of-energy generation of 5p/kWh was calculated for a commercial wave farm. The planned design will use a concrete structure, low maintenance requirements and a 40-50 year life expectancy.

For every kWh of power generated from waves and tidal streams, 430 grams of CO2 emissions would be avoided.

Current uses and Future Development

Wave Hub

Wavehub Right: An artist's impression of the Wave Hub.

Wave Hub is a groundbreaking renewable energy project in the South West of England that aims to create the UK's first offshore facility for the demonstration and proving of the operation of arrays of wave energy generation devices.

www.wavehub.co.uk

Potential

Skills - We already have many of the skills needed to develop marine renewable energy systems. The technologies used are very similar to those in other industries such as in shipbuilding, exploring and drilling for oil in the North Sea. We have developed complex underwater robots, installed sub-sea cable systems, designed new floating offshore platforms and exported this technology all over the world. We can learn from this experience and create a world-wide market for these new systems.

As an island, the UK is well placed to use its numerous coastal harbour and dock facilities to build these marine renewable energy systems for domestic and global supply. This will create huge regional opportunities as well as enabling some of our once thriving shipbuilding areas to regenerate and be at the forefront of industry once again.

Links to some wave energy device developers: