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Ocean Thermal Energy: The unseen sea opportunity


This article is to suggest that there be some strong grassroots support in Boulder and across America clamoring for prompt federal action—by Congress and the Obama Administration—to make ocean thermal energy a reality.  Although ocean thermal energy is not a panacea, it is the only remaining vast, untapped source of renewable energy. And, besides fueling hurricanes, it can do a lot toward replacing oil and coal, and toward mitigating global warming, which results can benefit everyone on the planet, landlocked or not.

There is a vast ocean thermal energy resource in the major oceans, resulting from global effects caused by solar radiation.  One key part of it consists of a thick slab of warm water in the surface layers of the tropical and subtropical oceans.  The other key part consists of a large supply of cold water, near freezing, at kilometer depths, conveyed to lower latitudes by the circulation of arctic and antarctic seawater.  The temperature difference between those two bodies of water can be converted to electricity by circulating large amounts of warm and cold seawater through a floating power plant which resembles a heat pump or refrigerator being run backwards.

This map shows oceanic regions (the redder the better) where these temperature differences are optimum for extracting this energy:

The map depicts contours of annual average temperature difference, “delta T,” in degrees Celsius, between surface seawater and cold seawater at a depth of 1,000 meters.  If the delta T parameter equals or exceeds 20 degrees Celsius, it appears likely that commercial ocean thermal power plants can economically convert such delta Ts into electricity.

These two ocean thermal resources, known as a “heat source” and a “heat sink,” remain at constant temperature throughout the day and night, although the temperature of the warm seawater can change somewhat seasonally.  Thus ocean thermal is a continuous (24/7) source of electricity, hence known as a “baseload” source.  This baseload feature of ocean thermal is in contrast to the intermittent nature of most sources of renewable energy.

Conventional power plants are also run on delta Ts.  Usually they operate from a heat source obtained by burning fossil or nuclear fuel under an evaporator, thereby boiling water into steam.  Then the expanding steam turns a turbogenerator, generating electricity, and the spent vapor is converted back to liquid in an air-cooled or water-cooled condenser.  Thus the air or water used for cooling is serving as a heat sink.  An ocean thermal plant operates the same way, except that no fuel is required, only the circulation of large quantities of warm and cold seawater.  For an ocean thermal plant, liquid ammonia, rather than water, is vaporized, then condensed.

Ocean thermal electricity can be generated on floating power plants located offshore, and delivered via submarine electric cable to islands and the mainland.  Since there is an economic limit to how far offshore such power plants can be situated, there is a vast amount of ocean thermal resource that is too remote for cable use.  To utilize these vast amounts of electricity located beyond economic cable access to shore, the electricity must be stored, then transported, in the form of an energy-carrier (such as hydrogen or ammonia), or in the form of an energy-intensive product (such as ammonia for fertilizer).  The ocean platforms, or floating factories, to be used for such power manufacture and conversion are referred to as “plantships.”

The likely market chronology for ocean thermal commercial development is that there will be an early market for ocean thermal electricity to displace oil-derived electricity in places like Hawaii and Puerto Rico, the Caribbean, and in many developing nations around the world.  Each megawatt of ocean thermal electricity can save 40 BBL of oil daily.  Then there will be a near-term market for electricity cabled to shore; for example, from Gulf Coast sites to the mainland U.S. electrical grid, at entry points extending from Brownsville to New Orleans, to Tampa, and Key West; and from sites off Baja California to, e.g., San Diego.  The vast longer-term market is to transport energy carriers and energy-intensive products to users from a fleet of plantships grazing the high seas.  Both ocean thermal power plants and plantships have the capability to produce copious amounts of fresh water as co-products.

The technology requirements for realizing viable ocean thermal power plants are largely available, although the cold water pipe that extends to about 1,000 meters depth presents some new challenges.  Thanks to the remarkable, innovative technology developed by the offshore oil industry, their engineering of ocean platforms has greatly reduced the risks of operating during storms and for surviving severe storms and hurricanes.  What now remains in the development of commercial ocean thermal technology is the successful demonstration of a multi-megawatt pilot plant, prior to the operation of a first-of-a-kind commercial plant.  The market-entry hurdle is that the pilot plant will be sub-economic, hence will have to be subsidized, en route to spawning a vast ocean industry.

In the United States, Lockheed Martin (LM) is designing a 5 to 10 megawatt pilot plant for operation off Pearl Harbor by 2015.  The LM team expects to use data from successful operation of that plant to design a first-of-a-kind 100 megawatt commercial plant for operation off Honolulu, Hawaii in 2020.  Rough, preliminary capital-cost estimates by LM of constructing that initial 100 megawatt commercial ocean thermal power plant range from about $1 Billion to $1.5 billion, or $10 to $15 per watt.  So, to make a crude comparison, the projected capital cost of this immature technology (i.e., for this first-of-a-kind baseload power plant), of $10 to $15 per watt, compares favorably to the capital cost, per time-averaged watt (since they are intermittent sources), of today’s wind and photovoltaic (PV) power systems, which employ mature technology.  Wind and PV system capital costs per nameplate watt are about $4 and $7, respectively, which, to enable making this crude comparison, must be multiplied by a factor of three, since these intermittent sources are available only about one-third of the time.

The French government and its shipbuilding arm, DCNS, are planning a 10 megawatt pilot plant for operation off Réunion Island by 2015, to be preceded in 2011 by a land-based prototype.

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One Response to “Ocean Thermal Energy: The unseen sea opportunity”

  1. […] The ocean platforms, or floating factories, to be used for such power manufacture and conversion are referred to as “plantships.” The likely market chronology for ocean thermal commercial development is that there will be an early … visit. […]

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