Wave power devices extract energy directly from the surface motion of ocean waves or from pressure fluctuations below the surface.Through time several concepts to extraction of energy from waves have been invented and developed. More than 100 concepts are globally in varying levels of development and several are in demonstration phases.
There are four overall types of wave energy extractors:
- Oscillating water column - These devices contain a chamber where the water level changes as waves go in and out. As the water level changes so does the pressure, which causes air to flow through a turbine.
- Attenuator or linear absorber – The linear absorber structure is oriented roughly parallel to the direction of wave propagation and is composed of multiple sections that rotate in pitch and yaw relative to each other.
- Overtopping terminator - A terminator is a device that reflects or absorbs all the energy in a wave. Water is forced through a turbine by the power of the wave.
- Point absorber - These devices capture energy from the "up and down" motion of the waves. They may be fully or partially submerged. They are small compared to a typical wavelength and can absorb energy from all directions.
Wave energy technologies are designed to extract energy from the energy contained in the movement of waves. Systems to convert wave energy into electricity can be located at deep water sites (offshore), at shallow water sites (near shore) or eventually built into the coast line (shoreline). They vary in size from very large systems installed on their own, to smaller ones grouped into farms or arrays.
Major challenges for wave energy devices
The single most important challenge for wave energy technology is to develop survivability in full scale ocean environments. This has to be possible at a reasonable cost of investment to indicate a future economy in the cost of energy production. For a wave energy device to survive in the chosen wave environment several issues have to be solved, including anchoring or position control to allow a stable grid connection, storm protection and in general a strength of construction to withstand the three dimensional forces of the ocean.
Another challenge is to find funding for the needed large-scale demonstration projects.
Favourable feed-in tarifs for wave energy that is fed into the transmission grid; values from 20-25 c€/kWh may be necessary in the demonstration phase and are provided in some countries, such as Ireland, Portugal, and the UK. Other countries provide values from more than 9 c€/kWh, such as Germany, France, and Spain; the feed-in tariff concept is a form of revenue support, in order to enable renewable energy technologies to overcome the initial phase in that they cannot compete with traditional generation technologies. They consist of a premium paid by the grid operators in addition to the regular price per kWh of electricity produced.
The cost of energy production during the lifetime of the installation is also a challenge. The device needs to have a mass production potential and a forecast of materials costs, installation and maintenance expenses. For now, a good projection will be sufficient to allow a project to go forward, but this projection has to include the cost of grid connection and bringing the electricity onshore.
The short history of wave devices in demonstration
One of the first shore-based, grid-connected wave power unit to be installed was an oscillating water column system built into the coastline of the Island of Islay in Scotland in 2000. The 500-kW device was developed by Wavegen in cooperation with Queen’s University Belfast.
In 2003, Wave Dragon installed the world's first floating grid-connected wave power unit. The 57-meter-wide prototype was an exact replica (at a scale of 1:4.5) of a 260-meter-wide 4-MW machine with a capacity of 20 kW. Because of its subscale nature, this unit was deployed in a protected bay in Nissum Bredning in Denmark. Experimental data from more than 20,000 hours of operation har been collected.
In July 2004, Pelamis was the first company to deploy a full-scale grid-connected wave power unit, in open seas at the European Marine Energy Center (EMEC) in Orkney, Scotland. Based on successful testing of this 750-kW unit at EMEC, Pelamis announced the first commercial sale of three devices wave power plant in Portugal. The 2.25-MW pilot plant, the 30-MW plant was deployed off the coast of Portugal in 2008 but is no longer operating primaruly due to financial problems.
An Oscillating Water Column (OWC) onshore device of 400 kW in Pico, Portugal operates from time to time.
The first section out of ten of a 500 kW near shore device, Wave Star has been in operation since 2009 at Hanstholm in Denmark. It has an installed power of 110 kW.
Demonstration projects are also ongoing and planned in Australia, Canada, China, Ireland, Japan, Portugal, Spain, the United Kingdom, Denmark, and the U.S.