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Next Green Energy Game Changer? Sea Power

Any technology that uses heat exchangers, from heat pumps to LNG re-gasification to power plants, could benefit from the development of a new green energy alternative: ocean thermal energy conversion.

Computer rendering of an OTEC System. Source: Oak Ridge National Laboratory, ORNL Review
Computer rendering of an OTEC System. Source: Oak Ridge National Laboratory, ORNL Review

With energy conservation and management of increasing interest to manufacturers, solar panels and wind turbines are showing up on and around plants in increasing numbers. Soon oil platforms, seafood processing plants and other facilities located near an ocean may have an additional green power technology alternative: ocean thermal energy conversion (OTEC).

According to a recent issue (Vol. 44, No 2, 2011) of the ORNL Review, the Global Security Directorate of the Oak Ridge National Laboratory (ORNL) is collaborating with Lockheed Martin Corp.'s Open Innovation Program to make OTEC a reality. Johnnie Cannon, head of the ORNL group involved, says the technology used to build heat exchangers for OTEC could also be used to increase the efficiency of other types of power plants.

"Potentially, any technology that uses heat exchangers, from heat pumps, desalination, LNG re-gasification to power plants, could benefit from this development," Cannon says. If everything works as expected during a demonstration of the project scheduled for this winter in Hawaii, "it could be a game-changer in terms of generating green energy," he adds.

OTEC uses temperature differences in the world's oceans to create energy. In the tropics, the surface water temperature is about 25 degrees C; at 3,000 feet down it's about 5 degrees C. That 20-degree difference can be used to generate power.

According to James Klett of ORNL's Materials Science and Technology Division, the OTEC power generation system works by using this temperature difference to drive a closed-loop Rankine cycle power plant. The Rankine cycle begins by pumping the 25 degree C surface water through a heat exchanger to boil ammonia. The ammonia becomes a gas, which is used to spin a turbine-generator to produce power. Then, the 5 degree C water is used to cool the ammonia, which condenses to its liquid state within a heat exchanger, called a condenser, and the cycle starts over again.

OTEC power benefits
"There are several compelling advantages to the system," says Klett. "First, it produces totally green energy; there are no by-products. It's also very much like geothermal, solar or wind power in that it does not take any fossil fuel to drive it, so costs are limited to construction and maintenance."

In addition, Klett is particularly emphatic about the availability of OTEC power. "With other renewables," he says, "when the wind stops, you don't have power. If it's a cloudy day, you don't have power. Even hydroelectric power is at the mercy of fluctuating water levels. OTEC can actually be used for base power."

Estimates suggest that, in tropical latitudes, OTEC has the potential to generate 3 to 5 terawatts of power without affecting the temperature of the ocean or the world's environment. "That's more than the electric generating capacity of this country," he says. "If we can supply a large fraction of our base power needs with green energy, we can revolutionize power generation."

The challenge of the technology right now is that a commercial-scale OTEC power plant would require at least 20 very large heat exchangers, says Klett. To address this, the ORNL research team has developed graphite-foam heat exchangers that combine a large amount of surface area with a high capacity for conducting heat. That means they can improve the performance of standard thermally conducting units while reducing their size and cost.

Graphite foam's large surface area and high capacity for conducting heat boost the performance of heat exchangers. Klett says studies have estimated that the heat exchangers for a 100 MW OTEC power plant would account for roughly 25 percent of the total capital costs, and the graphite-foam-based heat exchangers have the potential to reduce that figure by 50 percent.

Cannon says Lockheed Martin Corporation (LMC) work with the Open Innovation Program "represents a substantial investment by LMC over several years." OTEC is initially being developed to address the U.S. military's energy needs in parts of the world where long supply lines or distant power-generation facilities make generating power problematic. ORNL's other active projects in the program cover advanced materials and quantum computing.

Renee Robbins Bassett,, is Managing Editor at AutomationWorld.

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