Wave power
The largest wave energy potential is in the Atlantic Ocean and in the Pacific Ocean between 400 and 650 latitude. Here there is approximately 50 to 100 kW per meter breadth of the wave summit (wave front). Close to land, energy density diminishes because the waves are hindered by islands and mainland.
In addition to this, energy is lost through friction against the bottom in ground waters. Energy in waves is equally distributed between potential energy (due to water lifted up from the through of the wave up to the wave summit) and kinetic energy (due to the water’s changing speed). A number of players have established test- and pilot projects. Some of these have been running for many years without having gone further than the pilot phase.
Areas of application
Ocean waves are a clean and renewable energy source, formed by the reshaping of wind energy when wind blows over the ocean surface. Wind energy in its turn comes from solar energy, because solar heat creates high pressure and low pressure. Energy transport is condensed through both of these energy conversions. Directly under the surface, average wave energy transport is typically five times denser than wind energy transport 20 meters above water and 10-30 times denser than the intensity of sun radiation.
The average values of wave energy transport vary to a certain degree from one year to another. Values vary more strongly between seasons. The level of wave energy (and wind energy) is higher in winter than in summer.
Since there can be waves (swells) even when the wind is calm, wave energy is more stable than wind energy. Most often, wave power must be exploited outside the established infrastructure. Wave power plants can be located offshore, close to shore, or on land.
Offshore plants have by far the largest energy potential and are the least controversial among environmentalists. However, plants require large investments in cables and plants for connecting to the grid ashore. System enlargements can reduce costs related to grid connection down to an acceptable level.
Plants that are close to the coast can be visible from land, and coastal traffic limits area utilization. Energy density in the waves near the coast is lower compared to further out to sea. However, investment costs in plants that are close to shore are lower than for offshore plants, and access for control and maintenance are simpler.
Wave energy must be transferred to energy in a swinging system that reacts with the waves. The swinging system can be an oscillating water column in a liquid or a stationary chamber. The energy must also be converted to useful mechanical energy with the help of turbines or other hydraulic or pneumatic engines. Finally the energy is converted to electricity through a generator.
A simple and popular concept for exploitation of wave energy is the Oscillating Water Column (OWC). This technology is often used at plants on land. Through continuous changes in the level of liquid, the waves create alternating air pressure inside a chamber that again runs the air turbine. When water rises in the chamber, excess pressure is created. When water sinks again, a vacuum is created. These differentials in pressure drive air currents in and out of the chamber. Wells turbines are suited to utilize this air current, because the turbine turns in the same direction regardless of the direction of the air current.
Another concept with increasing popularity is the point absorber. Here the unit floats in or under the surface, moored to the bottom. A pump is fastened to the mooring line, and the movements of the waves run the pump.
A hybrid plant can pump sea water to a high pressure tank or an elevated reservoir on shore. An aggregate (turbine and generator) can produce electricity by leading water back to sea.
Technology status
Exploitation of wave energy is still in an early stage. Wave energy is competitive in some niches, for example in the operation of navigation buoys, desalinization of sea water and power supply to isolated island communities where they only have expensive electricity from diesel aggregates.
Wave power plants are in such an immature phase that it is impossible to give any general cost estimates. These are parameters that must be calculated for each single project. They depend on wave resources, technology lifetime and grid connection. Among the relevant players is Norwegian Fred Olsen (Fobox) who has expressed a goal to bring down the price to 2.7 c€/kWh. However, they have expressed that there is a long way to go before they reach this. Australian Energtech believes that power from the first plants will cost approximately 7 c€/kWh, and in the long term, their goal is to bring the price down to 2.9 c€/kWh.
A number of plants with basis in Norwegian technology and Norwegian companies are being planned and constructed:
Norwegian Pelagic Power bases their technology on a variant of the wave pump technology. Several pumps will be moored to the seabed, and floaters will run the pumps. The units will pump the sea water to a central turbine and generator that produces electric power. The plant will be built in a simple way and with economical materials, and the company will concentrate on low production costs rather than high efficiency. Pelagic Power will install a 1:3 pilot installation in 2007, and plans a full-scale installation in 2009.
FO3 is the Norwegian wave power project that has come the furthest. The ship owner and investor Fred Olsen, who is behind the company Fobox AS, financed the project. They have an installation outside Jomfruland, in the outer parts of the Oslo fjord, which is the laboratory platform "Buldra", built in the scale of 1:3. The wave-powered generator here is integrated in a floating platform construction, which is built in composite. Under the platform there are a series of plastic bridging boats that move with the waves. The bridging boats run a hydraulic system that again generates electric energy. The full-scale platforms will be 36 metres wide and 18 metres high. They will be placed together with common monitoring and control systems and common connection to the distribution grid on shore through a deep-sea cable. Installed output from the plant is planned to be 1,5 MW per platform. The platforms will be unmanned and have an expected service life of minimum 15 years.
Wave Energy AS is another Norwegian company that has developed a wave power concept with background in the oil industry. The wave-powered generator is built over an incline with several floors where the principle is to let water from three different basins run a number of impellers on the same shaft. In this way, waves on all levels are exploited. Technology will prevent start and stop sequences for the turbine, even though water is only added to the impeller on one level. THis increases supply stability of electricity and the generator’s service life. Tests carried out at the University of Aalborg show that a wave power plant can exploit 50 per cent of the wave’s energy. Wave Energy AS has bought the patent "Seawave Slot-cone Generator" (SSG) which has the advantage that the water energy is held in a number of reservoirs, one above the other. This increases the hydraulic effect.
The company has also developed the technology "Multistage turbine" (MST), which uses different heights of waterfalls on the same turbine wheel. The concept can be used in the area close to the sea as well as in floating devices. Wave energy is planning a full-scale prototype generator at Kvitsøy in Rogaland, and the project has received financial support from the EU.
In 2007 Ocean Power Delivery Ltd. (OPD) will establish the commercial wave-powered generator Pelamis off the coast of Northern Portugal. The project has had prototype testing and try-out at the Orkney Islands. The full-scale project will have a total installed output of 2.25 MW. If the project is successful, OPD plans to add 30 more production units, with a total output of approximately 20 MW. Pelamis WEC (Wave Energy Converter) consists of two cylindrical steel sections that are hinged together. This gives it a "worm-like" structure that floats semi-submersed in the sea. When Pelamis is exposed to waves, the sections move in relation to one another. These movements run hydraulic pistons on the vertical as well as the horizontal level. The pistons pump liquid with high pressure through accumulators through hydraulic motors, which again run generators. The Pelamis-units are moored in a way that they can be adjusted to all types of waves. The depth during construction is 50-60 metres, and the distance to shore is 5-10 kilometres. This makes it possible to exploit the energy in the swells, unaffected by the seabed. The electric power is transferred to shore in cables through the seabed. Pelamis can utilize a large spectrum of wave heights and frequencies. At the same time, the plant will adjust to very large waves by reducing production. The construction will be 120 metres long and 3.5 metres in diameter. The company is partly owned by Norsk Hydro.