Ocean Thermal Energy Conversion

September 23rd, 2011 by Breath on the Wind

OTEC is a technology that has been discussed and extensively researched until recently by the US government. Now OTE Corporation (@OTEcorporation on twitter) and Bahamas Electricity Company, announced that they signed a memo of understanding for the further development of the world’s first two commercial ocean thermal energy conversion (OTEC) plants, sited in the Bahamas. OTE Corporation has been in the news this year as it fills corporate positions. With the announcement of this agreement, it begins its core mission to bring OTEC to the nearly 100 tropical regions around the world, where land-based commercial OTEC power plants are now an economically viable solution.

Tropical Islands

Islands present a special problem for electric power. You may have the sun and some wind, but these power sources are not constant. On the mainland, we can use the grid as a buffer between supply and demand, but islands have to resort to expensive energy storage or imported supplies of fossil fuels. And so, it is not surprising that we find electrical storage batteries and some of the most expensive electric rates on islands. Tropical islands also have another largely untapped resource. This is the difference between surface water temperatures and that found in the deep ocean.

How It Works

A typical power plant uses a heat source to boil water. The heat source could be coal, nuclear or even a solar collector (Power tower or trough CSP.) Steam then drives a generator through a turbine. This is the Rankine cycle.

OTEC uses a smaller difference in ocean temperatures to generate electricity. It is an adaptation of the technology used in ice houses, solar ponds, and Dry Well Geothermal: the organic Rankine Cycle. In this instance, a different fluid is substituted for water. Ammonia is often used due to its lower boiling point. To cool the liquid and restart the cycle, deep ocean temperatures are used. Where does this energy actually originate? It is the Sun that heats the ocean surface and is being harnessed by OTEC.

History

OTEC was first considered almost 140 years ago. In 1974, the US, through the National Energy Research Labratory (NREL), was conducting the world’s most intensive research at a site in Hawaii.

Advantages

Unfortunately, while the organic rankine cycle allows us to use lower temperatures it is not as efficient as using super-heated steam. Inefficiency at first may appear to put the OTEC at an economic disadvantage to other forms of power generation. However, a facility can make up for this deficiency by using additional economic streams including:

1. Supplying potable fresh water

Theoretically, an OTEC plant that generates 2-MW of net electricity could produce about 4,300 cubic meters (14,118.3 cubic feet) of desalinated water each day.

2. In addition to providing power and water, OTE Corporation can provide cooling to areas near its plants powered by the deep cool ocean. This may simply be very economic cooling for nearby buildings or may be promoting aquaculture and unusually temperate plants grown in a tropical environment:

OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics.

What is done on land can also be done with aquaculture using cool water to support lobsters and other species not normally grown in the tropics.

Future Possibilities

Islands are not the only places that need water. California has already built desalination plants to augment its water supply. Any tropical coastal region could use OTEC. OTEC is a clean, renewable energy source that has the potential to free many economies from their dependence on oil.

Photo Credits: OTEC

For additional reading/sources: NREL overview











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