Ocean Thermal Energy
Most energy generated from water is tied in some way to the force of gravity. Tides rely on the gravity of the moon as it orbits the Earth while wave energy and traditional hydropower rely more directly on the energy created when water is lifted to a height and allowed to fall with the force of gravity. Ocean thermal energy, however, relies on the difference in temperature between the cool deep water of the ocean and warmer surface water.
The difference in temperature can actually provide power to a heat engine, which can in turn be used to generate electivity. The concept is relatively simple. Warm ocean water is either allowed to boil itself or is used to boil a secondary fluid. The gas that is generated is used to turn a turbine and generate electricity. Cold ocean water is then used to condense the gas back to a liquid so that the cycle can repeat itself.
The process is simple, but the implementation is not. Getting cold water to the surface requires energy, so some of the energy produced from these systems is lost, making them necessarily less efficient than wave, tidal, or traditional hydropower. On the other hand, one of the techniques for getting cold water to the surface involves desalination. Because desalinated water is less dense, it rises on its own. Desalinating water near the ocean floor can be used as a method of brining cold water to the surface. Because this cold water is now fresh water, once it is used to condense the gas in the heat engine it can be used for drinking water. In this way, ocean thermal energy may become attractive in densely populated coastal areas where freshwater is in limited supply.
Like tidal power, the French were also the first to implement technology based on Ocean Thermal Energy Conversion (OTEC). In 1881, Jacques Arsene d’Arsonval proposed using thermal energy to generate power. His student Georges Claude then went on to build the first OTEC plant in Cuba in 1930, producing 22 kilowatts of electricity. Another plant was constructed in 1935, but his time onboard a cargo vessel off of the coast of Brazil. Both systems were destroyed by ocean waves. In 1956, a 3 megawatt plant was constructed by French scientists in Ivory Coast, Africa, but was never finished.
The only other real implementation of OTEC technology is in the United States where the Natural Energy Laboratory runs a testing facility. The NEL is part of the Hawaii Authority and has developed several small OTEC experiments. The fist was in 1979 and the next in 1999. The 1999 test produced 250 kW and represented the largest implementation of OTEC closed-cycle technology to date.
Close cycle systems use low boiling point liquids (like ammonia) to generate steam. These systems are usually low pressure and so don’t generate a great deal of energy. The 250 kW system mentioned above was a closed-cycle system.
These systems use warmer ocean waters in the tropics by capturing it in a container that is reduced to below atmospheric pressure. The lower pressure allows the water to boil at a low temperature. The generated steam then moves a turbine. The steam is condensed back into water by cold water from the deep ocean. The water now contains no salt and is fit for drinking or irrigation. A 50 kW test plant has been constructed using this technology.
This system uses the low pressures of the open system and the low boiling point liquids of the closed system to produce energy. No major implementations of the hybrid system have been undertaken.