Tuesday, May 18, 2010

Ocean Thermal Energy Conversion

Chart of optimum ocean temperature differentials for OTEC

Chicago — For almost one hundred years the concept of Ocean Thermal Energy Conversion (OTEC) has been understood. OTEC generates electricity by utilizing the temperature differential between sun-warmed surface water and the cold ocean depths.

The concept of a heat engine is well understood in thermodynamics engineering, and much of the energy used around the world passes through a heat engine. A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one to the other, the engine converts some of the heat energy to work energy. This principle is used in steam turbines and internal combustion engines, while refrigerators reverse the direction of flow of both the heat and work energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.

Surface water is pumped through a heat exchanger, where it heats a liquid chemical with a low boiling point, such as ammonia, which then expands as it vaporizes. The vaporized gas drives turbine blades that generate electricity. The gas is then piped into a condenser back in the cold deep ocean water, which chills it, returning it to its liquid state so the cycle may be repeated.

Demonstration plants have been built over the years, but OTEC has never been put into commercial operation primarily because of its high cost.

The New York Times has reported this week on Pacific Otec and its efforts with OTEC. For OTEC to be efficient, the technology requires a temperature differential of at least 20 degrees Celsius (36 F), which is available over large expanses of tropical waters. “Every additional degree will help produce 15 percent more energy,” said Philippe Dubau, General Manager of Pacific Otec, a subsidiary of Pacific Petroleum, an oil product distributor in French Polynesia, New Caledonia and Vanuatu that has been moving into the renewable energy sector.

The attractive aspect to OTEC is that it would provide steady power over a 24-hour cycle in a never ending loop, as long as the temperature differential exists.

Pacific Otec, DCNS, the French government-owned naval architect and military shipbuilder, and Xenesys, a Japanese specialist in desalination and thermal energy conversion technology, are working on a feasibility study for a commercial OTEC plant in Tahiti. Financial backing for the project has been provided by the French and French Polynesian governments.

The project envisions building an offshore OTEC platform, with a 10 megawatt-hour generating capacity, which will be connected to the Tahiti power grid and could produce enough electricity to cover 10 percent of the islands’ needs, Mr. Dubau said. The offshore OTEC plant would be 25 meters (80 feet) high and submerged 25 meters below the surface to remain stable in heavy weather. One of the main attractive features of OTEC is the compact footprint it requires as compared to solar and wind power.

As reported by The New York Times, Mr. Dubau said. “This feasibility study is not about the technology; we know it works. We also know the design of the plant is correct. But what we need to do now is to design the optimal energy system, considering local environmental data; to design the integration of the process into the chosen platform type; and, of course, study the economic feasibility of the whole project.”

Richard Wottrich

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