7.12 c > The Pelamis wave energy transformer floats on the ocean like a giant sea snake. It consists of several segments which move against each other and build up hydraulic pressure. This in turn drives a turbine. A new Pelamis generation is currently under construction.
An unretrieved treasure troveThe oceans are teeming with energy. Tidal forces move immense masses of water. Strong winds build up mighty waves. Almost 90 per cent of global wind energy is contained in the turbulence above the world’s oceans. Wind, waves and currents together contain 300 times more energy than humans are currently consuming. For a long time, this abundance went untapped. In recent years, however, we have begun to harness this energy. The first offshore wind farms were built. Hundreds of power plants have been and are being built to convert ocean current and wave energy to electricity. The key renewable marine energies are:
- Wind energy
- Wave energy
- Tidal energy
- Ocean current energy
- Energy derived from temperature differences at various ocean depths (ocean thermal energy conversion – OTEC)
- Energy derived from the different salt content of freshwater and saltwater (osmotic power)
7.12 e > Offshore wind turbines produce electricity in many places around the world. One of the largest offshore wind farms, consisting of 48 turbines, is located in the Baltic Sea between Denmark and Sweden. A transformer station has been constructed here to feed power into the Swedish grid.
7.13 > Wind turbines are normally erected at a maximum water depth of 45 metres; otherwise, the cost of the towers is prohibitive. One alternative is to anchor floating turbines to the sea bed with holding cables. The first prototypes are currently being tested.
Offshore windWind energy is currently at the most advanced stage of development, and the signs are extremely promising. Experts estimate that offshore wind power alone could in future supply about 5000 terawatt-hours (TWh) of electricity a year worldwide – approximately a third of the world’s current annual electricity consumption of about 15,500 terawatt-hours (1 terawatt-hour = 1 trillion watts). It is anticipated that offshore wind energy plants (WEPs) alone in Europe will supply about 340 terawatt-hours a year by 2015. About 40 offshore wind energy projects have so far been implemented worldwide, most of them in the United Kingdom, Denmark, the Netherlands and Sweden. Two trends are clear. One, that the facilities are getting bigger all the time, and two, that we are constantly venturing into deeper waters, which will allow the construction of wind farms over far greater areas. Whereas at the beginning of this century we were building in coastal areas at depths of 2 to 6 metres, wind turbine towers are now anchored to the ocean floor at depths of more than 40 metres. Floating offshore concepts are also being developed for even deeper waters. The world’s first floating wind energy plant was recently constructed off the coast of Norway by a Norwegian-German consortium. Backed by the experience of hundreds of thousands of onshore WEPs, wind energy has become a mature technology. The high wind speeds and harsh environmental conditions at sea, however, mean that some technological improvements are required, a fact borne out by the problems encountered by the first large-scale wind farm in Denmark. For this reason only twelve wind turbines from different manufacturers were initially built and tested at Germany’s first offshore wind farm “Alpha Ventus”. Located in the North Sea about 40 kilometres off the island of Borkum, the farm was sponsored by the German Federal Ministry of Economics. Offshore plant is still considerably more expensive to construct than onshore due to the challenging foundation work and complicated connection to the power grid. Nonetheless, according to experts, offshore wind energy, supported by feed-in payments and support measures, will continue to grow substantially in the coming years.
Wave energyThe global technical potential of wave energy is estimated at 11,400 TWh per year. Its sustainable generating potential of 1,700 TWh per year equates to about 10 per cent of global energy needs. There are various different concepts for generating power from wave energy, most of which can be classified in three basic types:
- The “Oscillating Water Column” principle: Wave action causes water to move up and down in an air-filled chamber. The air is displaced and forced through a turbine which generates electricity. Pilot plants of this type were set up in recent years in Portugal, Scotland and Japan.
- The “Oscillating Bodies” principle: Facilities of this type use the motion of ocean waves to generate electricity. They include semi-submerged generators on which a float on a fixed counterbearing moves either sideways or up and down. Other systems consist of flexible mounted components that move against each other, putting hydraulic oil under pressure. The oil in turn drives a turbine. Recently, the British “Pelamis” system, a type of sea snake composed of several segments that float on the water’s surface, created quite a stir. Pelamis, the world’s first wave energy system, was established off the coast of Portugal in 2008 and is connected to the power grid by an undersea cable. Similar farms are planned in Spain and Portugal.
- The “Overtopping” principle: Similar to a dam, overtopping devices have a reservoir that is filled by incoming waves to levels above the surrounding ocean. The energy of the water falling back to the ocean is used to drive a turbine. Prototypes of both floating and fixed systems have already been installed in Denmark and Norway. >