Next renewable energy source: Ocean thermals

Place a heat engine between warm water at the ocean's surface and cold water pumped from the deep ocean and you have yourself an ocean thermal energy conversion plant. The greater the temperature difference, the stronger the flow of heat that can be used to do useful work such as spinning a turbine and generating electricity.

OTEC sounds like a great idea for generating energy if you can find a spot where there is enough of a temperature difference in nearby ocean waters. It turns out that Hawaii is just such a place.

But here’s the problem: No one has ever built a real OTEC plant, or more precisely, an OTEC plant that is anything but a proof-of-principle demonstration. The biggest OTEC plant so far was one that operated between 1993 and 1998 on the Big Island. The facility still holds the world record for OTEC power production, with its turbo-generator output exceeding 250 kW. It exported just a bit more than 100 kW of net power to the grid, about as much as one medium-sized wind turbine operating at full speed.

OTEC schemes us a low-pressure turbine operating in a Rankine cycle. The Hawaiian plant was an open-cycle engine that used the water heat source as the working fluid. It is also theoretically possible to devise such systems as closed-cycle engines using refrigerant working fluids such as ammonia or R-134a.

Now there’s some interest in resurrecting the OTEC effort in Hawaii. Researchers at the University of Hawaii have analyzed data from the National Oceanic and Atmospheric Administration's National Oceanographic Data Center. They say the western side of the Hawaiian islands look particularly promising because there is about a one-degree additional temperature differential there, compared to other near-by areas. This small difference translates to 15% more power for an OTEC plant.

University of Hawaii researcher Gérard C. Nihous recently described his work in the online Journal of Revewable and Sustainable Energy. He says practical temperature differences for OTEC are only of the order of 20 °C, with much of this resource needed in the heat exchangers. The thermodynamic efficiency of OTEC processes is of the order of 3%, he explains, and as a result, large seawater flow rates are needed to produce significant amounts of electricity (about 3 m³/sec of deep cold seawater and at least as much surface warm seawater per net megawatt of electrical power)

You can read a full transcript of his article here:
http://jrse.aip.org/jrsebh/v2/i4/p043104_s1?view=fulltext