Power from the Sea—Innovation at the crossroads of global security and green energy

[OKIsItJustMe]

http://www.ornl.gov/info/ornlreview/v44_3_11/article09.shtml

Power from the Sea

Innovation at the crossroads of global security and green energy

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.

Power from the sea

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 commercialscale OTEC power plant would require at least 20 very large heat exchangers. That's where the graphite-foam-based heat exchangers developed by Klett and his research team come 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. Alternatively, the same size heat exchangers could produce twice the power for the same cost.

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[johnd83]

1. Low thermal efficiency

The efficiency of a turbine depends on the difference in temperature between hot and cold sides (among a huge number of other variables). The low temperature difference means that there will be very low efficiency, and there is a large energy usage to pump the water. If they dropped this thing near a geothermal vent it may be more practical.