Ocean Thermal Energy Conversion And How it Works
By: Jeremy Feakins
Introduction •
Most of the electricity we use comes from heat engines of one kind or another. A heat engine is a machine that cycles between two different temperatures, one hot and one cold, usually extracting heat energy from a fuel of some kind. In a steam engine or a steam turbine, for example, coal heats water to make hot, high-pressure steam, which is then allowed to expand and cool down to a lower temperature and pressure, pushing a piston and turning a wheel as it does so..
Ocean Thermal energy cOnversiOn and hOw iT wOrks
Closed cycle Open cycle Land- and seabased OTEC
Closed cycle • In closed-cycle OTEC, there is a long, closed loop of pipeline filled with a fluid such as ammonia, which has a very low-boiling point (−33°C or 28°F). (Other fluids, including propane and various low-boiling refrigerant chemicals, have also been successfully used for transporting heat in OTEC plants.) The ammonia never leaves the pipe: it simply cycles around the loop again and again, picking up heat from the ocean, giving it up to the OTEC power plant, and returning as a cooled fluid to collect some more.
How closed-cycle OTEC works Ammonia (or another low-boiling, heat-transport fluid) flows around a closed loop at the heart of the system. That's the white square in the center of this illustration. Hot water enters a completely separate pipe near the surface of the ocean and is piped toward the central loop containing the ammonia. The hot water and the ammonia flow past one another in a heat exchanger, so the hot water gives up some of its energy to the ammonia, making it boil and vaporize. The vaporized ammonia flows through a turbine, making it spin. The turbine spins a generator, converting the energy to electricity.
• In open-cycle OTEC, the sea water is itself used to generate heat without any kind of intermediate fluid. At the surface of the ocean, hot sea water is turned to steam by reducing its pressure (remember that a liquid can be made to change state, into a gas, either by increasing its temperature or reducing its pressure). The steam drives a turbine and generates electricity before being condensed back to water using cold water piped up from the ocean depths.
Land- and sea-based OTEC • Open- and closed-cycle OTEC can operate either on the shore (land-based) or out at sea (sometimes known as floating or grazing). Both have advantages and disadvantages, which we'll consider in a moment. Land-based OTEC plants are constructed on the shoreline with four large hot and cold pipelines dipping down into the sea: a hot water input, a hot water output, a cold-water input, and a coldwater output.
• OTEC systems are, for the most part, environmentally benign. Although accidental leakage of closed cycle working fluids can pose a hazard, under normal conditions, the only effluents are the mixed seawater discharges and dissolved gases that come out of solution when sea water is depressurized. Although the quantities of outgassed species may be significant for large OTEC systems, with the exception of carbon dioxide, these species are benign.
• Jeremy Feakins is an entrepreneur who provides the help you define and execute key growth strategies, deliver a great launch experience, bring new products and services to market, and advise you on finding investment capital. He is currently, CEO of the Ocean Thermal Energy Corporation.