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Design and Development
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Power from the Sea
Ocean-borne robots and other vital equipment could one day recharge midway through their missions via a limitless source of energy, thanks to work underway at the U.S. Department of Energy’s Oak Ridge National Laboratory.
By Robotics Trends' News Sources - Filed Jan 10, 2012

Computer rendering of an OTEC system. (Credit ORNL)

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Editor’s note: This article originally appeared in Vol. 44, No. 3 issue of the ORNL Review. It is posted here courtesy of Oak Ridge National Laboratory.

Technological developments are generally incremental changes that slip mostly unnoticed into business practices, industrial processes and daily routines. Those aren't the kind of scientific advancements Lockheed Martin Corporation's Open Innovation Program seeks to address. Johnnie Cannon, who heads up ORNL's Global Security Directorate's collaboration with the program, says its goal is to develop "disruptive" technologies that leapfrog the competition, rather than making gradual, predictable progress. Currently Cannon's collaboration portfolio consists of active projects in a range of disciplines, including advanced materials, quantum computing, and ocean thermal energy conversion.

"One of our highest profile collaborations with the Open Innovation Program is Ocean Thermal Energy Conversion," Cannon says. "It represents a substantial investment by LMC over several years." OTEC can be used 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.

OTEC uses temperature differences in the world's oceans to create energy. In the tropics, the surface water temperature is about 25 degrees C; and at 3000 feet down it's about 5 degrees C. "That's a difference of about 20 degrees C, and can be used to generate power," says 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.

Given the state of current OTEC technology, a commercial scale OTEC power plant would require very large heat exchangers. That's where the graphite-foam-based heat exchangers developed by Klett and his research team comes in. Graphite foam combines a tremendous amount of surface area with a high capacity for conducting heat, enabling these heat exchangers to improve the performance of standard thermally conducting units while reducing their size and cost. Making heat exchangers twice as effective means an OTEC power plant could cut the size of its heat exchangers in half, shrinking the capital expenditure for the plant and making OTEC a much more practical green energy alternative.

Studies have estimated that the heat exchangers for a 100 MW OTEC power plant would account for significant costs, and the graphite-foam-based heat exchangers have the potential to reduce that figure by 50%. Because heat exchangers are a large part of an OTEC plant's footprint, and OTEC plants are located on offshore platforms like those used for oil and gas drilling, reducing the footprint is important.

Plentiful, reliable energy

"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. He notes that consumers don't always understand that the only kind of "green" energy that is currently available as "base power"—power that is available 24 hours a day, 7 days a week—is geothermal. "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."

Klett and his colleagues at Lockheed Martin are building a laboratory-scale heat exchanger that is 3 feet in diameter and 20 feet long and are shipping out to Hawaii to test at the National Energy Laboratory of Hawaii Authority (NELHA). Hawaii, which is also rich in geothermal resources, has made a commitment to eliminate its dependence on foreign energy sources by 40% in the year 2030. OTEC is viewed as a very attractive alternative to accomplish this goal.

Unique capabilities

Klett notes that the technology used to build heat exchangers for OTEC could 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 adds. ORNL's experience collaborating with Lockheed Martin through the Open Innovation Program has opened the door to working with them outside of the program on other research and development projects, Klett says. "The more they learn about us and the unique capabilities that we have, the more they come to us for help in areas outside of the Open Innovation Program.

As for the future of OTEC technology, Klett says, "I think this is a case where if we build it they will come. If we can build a power source that doesn't require fuel and only requires maintenance, then we won't have to worry about the price of fuel going up and down. The price of energy generated by an OTEC plant will be tied to the cost of maintenance—and if we come up with cheaper ways of maintaining the plant, the price of the OTEC energy could actually go down, and hopefully be competitive with conventional power plants."

"The demonstration of the project is scheduled for this spring in Hawaii," Cannon says. "If everything works as expected, it could be a game-changer in terms of generating green energy." —Jim Pearce

 


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