Energy in water (in the form of kinetic energy, temperature differences or salinity gradients) can be harnessed and used. Since water is about 800 times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy.
There are many forms of water energy:
Hydroelectric energy is a term usually reserved for large-scale hydroelectric dams. Examples are the Grand Coulee Dam in Washington State and the Akosombo Dam in Ghana. Micro hydro systems are hydroelectric power installations that typically produce up to 100 kW of power. They are often used in water rich areas as a Remote Area Power Supply (RAPS). There are many of these installations around the world, including several delivering around 50 kW in the Solomon Islands. Damless hydro systems derive kinetic energy from rivers and oceans without using a dam. Ocean energy describes all the technologies to harness energy from the ocean and the sea.
Marine current power, similar to tidal stream power, uses the kinetic energy of marine currents. Ocean thermal energy conversion (OTEC) uses the temperature difference between the warmer surface of the ocean and the colder lower recesses. To this end, it employs a cyclic heat engine. OTEC has not been field-tested on a large scale. Tidal power captures energy from the tides. Two different principles for generating energy from the tides are used at the moment.
Tidal motion in the vertical direction – Tides come in, raise water levels in a basin, and tides roll out. Around low tide, the water in the basin is discharged through a turbine, exploiting the stored potential energy. Tidal motion in the horizontal direction – or tidal stream power. Using tidal stream generators, like wind turbines but then in a tidal stream. Due to the high density of water, about eight-hundred times the density of air, tidal currents can have a lot of kinetic energy. Several commercial prototypes have been build, and more are in development.
Wave power uses the energy in waves. Wave power machines usually take the form of floating or neutrally buoyant structures which move relative to one another or to a fixed point. Wave power has now reached commercialization. Osmotic power or salinity gradient power, is the energy retrieved from the difference in the salt concentration between seawater and river water. Reverse electrodialysis (PRO) is in the research and testing phase. Vortex power is generated by placing obstacles in rivers in order to cause the formation of vortices which can then be tapped for energy. Deep lake water cooling, although not technically an energy generation method, can save a lot of energy in summer. It uses submerged pipes as a heat sink for climate control systems. Lake-bottom water is a year-round local constant of about 4°C.
Wave Farms Expansion
Portugal now has the world’s first commercial wave farm, the Agucadoura Wave Park, officially opened in September 2008. The farm uses three Pelamis P-750 machines generating 2.25 MW.Initial costs are put at 8.5 million. A second phase of the project is now planned to increase the installed capacity to 21MW using a further 25 Pelamis machines.
Funding for a wave farm in Scotland was announced in February, 2007 by the Scottish Government, at a cost of over 4 million pounds, as part of a £13 million funding packages for ocean power in Scotland. The farm will be the world’s largest with a capacity of 3MW generated by four Pelamis machines.
The major advantage of hydroelectric systems is the elimination of the cost of fuel. Other advantages include longer life than fuel-fired generation, low operating costs, and the provision of facilities for water sports. Operation of pumped-storage plants improves the daily load factor of the generation system. Overall, hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy, and areas with abundant hydroelectric power attract industry.
However, there are several major disadvantages of hydroelectric systems. These include: dislocation of people living where the reservoirs are planned, release of significant amounts of carbon dioxide at construction and flooding of the reservoir, disruption of aquatic ecosystems and birdlife, adverse impacts on the river environment, potential risks of sabotage and terrorism, and in rare cases catastrophic failure of the dam wall.
Hydroelectric power is now more difficult to site in developed nations because most major sites within these nations are either already being exploited or may be unavailable for other reasons such as environmental considerations