The energy from the sun heats the surface water of the ocean. In tropical regions, the surface water can be 40 or more degrees warmer than the deep water. This temperature difference can be used to produce electricity.

Photo: Hamed Saber

Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) has the potential to produce more energy than tidal, wave, and wind energy combined. The OTEC systems can be open or closed.

In a closed system, an evaporator turns warm surface water into steam under pressure. This steam spins a turbine generator to produce electricity. Water pumps bring cold deep water through pipes to a condenser on the surface. The cold water condenses the steam, and the closed cycle begins again. In an open system, the steam is turned into fresh water, and new surface water is added to the system.

A transmission cable carries the electricity to the shore. The OTEC systems must have a temperature difference of about 25 degrees Celsius to operate. This limits OTEC’s use to tropical regions where the surface waters are very warm and there is deep cold water. Hawaii, with its tropical climate, has experimented with OTEC systems since the 1970’s.

Photo: wikimedia.org

Today, there are several experimental OTEC plants, but no large operations. There are many challenges to widespread use. The OTEC systems are not very energy efficient. Pumping the water is a giant engineering challenge.

Ocean Thermal Energy Conversion (OTEC) was conceived of by the French engineer Jacques D’Arsonval in 1881. However, at the time of this writing the Natural Energy Laboratory of Hawaii is home to the only operating experimental OTEC plant on the face of the earth. OTEC is a potential alternative energy source that needs to be funded and explored much more than it presently is. The great hurdle to get over with OTEC implementation on a wide and practically useful level is cost.

It is difficult to get the costs down to a reasonable level because of the processes presently utilized to drive OTEC. Ocean thermal energy would be very clean burning and not add pollutants into the air. However, as it presently would need to be set up with our current technologies, OTEC plants would have the capacity for disrupting and perhaps damaging the local environment.

Photo: zebu.uoregon.edu

Kinds of OTEC

1. Closed Cycle OTEC uses a low-boiling point liquid such as, for example, propane to act as an intermediate fluid. The OTEC plant pumps the warm sea water into the reaction chamber and boils the intermediate fluid. This results in the intermediate fluid’s vapor pushing the turbine of the engine, which thus generates electricity. The vapor is then cooled down by putting in cold sea water.
2. Open Cycle OTEC is not that different from closed cycling, except in the Open Cycle there is no intermediate fluid. The sea water itself is the driver of the turbine engine in this OTEC format. Warm sea water found on the surface of the ocean is turned into a low-pressure vapor under the constraint of a vacuum. The low-pressure vapor is released in a focused area and it has the power to drive the turbine. To cool down the vapor and create desalinated water for human consumption, the deeper ocean’s cold waters are added to the vapor after it has generated sufficient electricity.
3. Hybrid Cycle OTEC is really just a theory for the time being. It seeks to describe the way that we could make maximum usage of the thermal energy of the ocean’s waters. There are actually two sub-theories to the theory of Hybrid Cycling. The first involves using a closed cycling to generate electricity. This electricity is in turn used to create the vacuum environment needed for open cycling. The second component is the integration of two open cycling such that twice the amount of desalinated, potable water is created that with just one open cycle.

Moreover, to being used for producing electricity, a closed cycle OTEC plant can be utilized for treating chemicals. OTEC plants, both open cycling and close cycling kinds, are also able to be utilized for pumping up cold deep sea water which can then be used for refrigeration and air conditioning. Furthermore, during the moderation period when the sea water is surrounding the plant, the enclosed are can be used for mariculture and aquaculture projects such as fish farming.

Photo: eurocean.org

There is clearly quite an array of products and services that we could derive from this alternative energy source. In addition, the electricity must be transported to land. It will probably be 10 to 20 years before the technology is available to produce and transmit electricity economically from OTEC systems.

Source: mms.gov

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