December 22, 2010 - Ocean thermal energy conversion

Wednesday, December 22, 2010

As any sailor knows, the Earth's oceans are enormous and powerful. About seventy percent of the Earth’s surface is covered by its oceans. These oceans contain vast amounts of energy - many times more than is consumed by mankind. Through the confluence of water's physical properties and natural energy dynamics, combined with a considerable amount of technology, much of this energy can be harnessed. Whether or not it is cost-effective is dependent on the specifics of each project and its technology, as well as whether any governmental incentives or subsidies exist.

A summer view from the dock at Great Cranberry
Island across to Mount Desert Island.
When we think of the potential of ocean energy, offshore wind development often comes to mind. Offshore wind may rely on oceanic siting and conditions, but its primary energy source is the wind. The waters of the ocean itself are understood to contain even more energy, in the form of moving water like currents, tides, and waves, as well as in subtler embodiments such as gradients of temperature and salinity.

Ocean thermal energy conversion, sometimes called OTEC, is a form of ocean energy extraction that is subtle yet potentially significant. The temperature difference between warmer shallow water and cooler deep water can drive a heat engine, usually using the Rankine cycle. The basic concept isn't new; the idea appears to have been first proposed in France in 1881, with the first operating plant (22 kW) built in Cuba in 1930. Since then, a number of small projects have been developed, mostly as research or demonstration projects, but the technology hasn't seen widespread adoption. Cost is the key driver here; because OTEC facilities generally have a high cost per unit of power produced, interest in OTEC tends to follow cost spikes in oil and energy pricing.

It may not mean energy prices are on the rise, but the U.S. Naval Facilities Engineering Commission has issued a total of $12.5 million in contract and grant funding to Lockheed Martin Corp. to design and commercialize a 10 MW pilot project off the Hawaiian coast of Oahu. $8.1 million of this money was awarded in 2009, with the remaining $4.4 million grant coming in last month. Hawaii is already home to an OTEC facility: the Natural Energy Laboratory of Hawaii Authority at Keahole Point. Hawaii is ideal for OTEC: larger temperature differences mean greater efficiency (important, where the theoretical maximum efficiency may only be about 7%). It will be interesting to see if the proposed new OTEC plant is built, as well as what we learn from the experience.  Will commercial-scale OTEC soon make economic sense?  What will it take to get there?

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