The world's largest untapped source of solar energy doesn't lie on the vast sands of the Sahara or even atop the high chaparral of the desert Southwest. Instead, it stretches across at least 23 million square miles of earth's tropical oceans; the uppermost layers of which make a prime natural source of thermal energy.

Regardless of time of day or cloud cover, Ocean Thermal Energy Conversion (OTEC) promises to harness this thermal sea-based resource year round.

OTEC production converts heat energy from seawater into kinetic energy using the ocean’s naturally steep temperature gradient.  It’s this juxtaposition of tropical (and sometimes subtropical) subsurface seawater at temperatures typically above 80 degrees F. and below 40 degrees F. that makes OTEC possible.  

An OTEC plant literally pumps the warm surface seawater through a heat exchanger connected to a closed circuit filled with several hundred tons of liquid ammonia.  Since ammonia boils at lower temperatures and at lower pressures than water, once the warm seawater hits the heat exchanger, it causes the ammonia to vaporize and expand in volume.  As this ammonia vaporizes, it creates pressure to run a turbine coupled to a generator.  In most cases, the resulting electricity would be delivered onshore via an undersea cable. 

Once this ammonia vapor exits the turbine, it flows through a second heat exchanger that is connected to a cold water pipe carrying tons of seawater pumped from depths of 3000 ft.  This cold seawater, in turn, condenses the spent ammonia vapor back into liquid and the whole OTEC process begins again.  

But despite the fact that the idea for the technology is more than a century old; to date, OTEC has only been successfully demonstrated on small scales of less than a quarter of a megawatt (MW) and has yet to produce utility-scale power.  

“Funding certainly is the biggest obstacle for OTEC,” said Gerard Nihous, an ocean engineer at the University of Hawaii at Manoa. “While nothing we have learned in the past suggests that OTEC has major technological hurdles left to clear, OTEC cannot be considered ready for commercialization.  A multi-year operational record at sea would help resolve lingering uncertainties and fix the design 'bugs' that are bound to be revealed.”

Such sea operations would begin aboard a stationary floating plant that would skim off a small percentage of the surface layer to use as the heat source.  Auxiliary power sources would get the OTEC process and the pumps started; then the plant would generate enough energy to power itself.  But even so, an OTEC plant’s real-time operating efficiency is expected to reach only a few percent.   

“The heat exchanger cost-efficiency and turbine cost-efficiency tradeoff is different with OTEC than in a conventional steam power plant,” said Chris Barry, an Annapolis-based naval architect and the ocean renewable energy panel chair for the Society of Naval Architects and Marine Engineers.  “We have to squeeze everything we can out of the energy we have.  Each element of an OTEC plant has to be incredibly efficient, because you’ve got very little to work with.” 

Even so, all U.S. territories in the tropics would be prime locations for OTEC, including, Puerto Rico, the U.S. Virgin Islands, Guam, and American Samoa. 

In the continental U.S., Florida is seen as a potential prime OTEC producer. Florida Atlantic University (FAU) in Boca Raton, in collaboration with Lockheed Martin Corporation, did an ocean thermal resource assessment off the state’s southeast coast.  As a result, Howard Hanson, chief scientist at the Southeast National Marine Renewable Energy center at FAU, now says he initially envisions three 100-MW plants operating just offshore feeding 300 MW of power into south Florida’s electrical grid. But not everyone is convinced that south Florida’s top ocean layer would be warm enough for year-round OTEC. 

Meanwhile, the Baltimore-based OTEC International, LLC (OTI) is planning on the 2014 completion of a small 1-MW land-based plant to demonstrate the technology.  To be located on the Kona coast of the Big island of Hawaii, its cost will run in the tens of millions.  But thus far, all of OTI’s efforts are being funded by Baltimore’s Abell Foundation. 

Although located onshore, Barry Cole, OTI executive vice president and its director of technology development in Baltimore, says that the 1-MW demonstration plant will use an existing infrastructure of more than 10,000 feet of pipes to tap into the requisite offshore warm and cold water reserves needed for OTEC production. 

Lockheed Martin did not respond to requests for comment on their own OTEC initiatives, but they have been actively working on Hawaii-based OTEC plans with Makai Ocean Engineering, Inc. of Honolulu. * (see 1st comment below)

Customer end user costs for OTEC power in Hawaii are expected to be in the neighborhood of 0.25 to 0.35 cents per kWh which is in line with current residential rates now. 

“Everybody’s pushing for large OTEC plants to produce electricity at an attractive rate,” said Joe Van Ryzin, a vice president at Makai Ocean Engineering, Inc. “OTEC’s potential is absolutely huge; build enough 100-MW OTEC plants in Hawaii and you could provide all of its electricity needs.”   

Van Ryzin says Lockheed Martin has conceptual designs for a variety of OTEC pilot plants.  One of those is a 5- to 10-MW pilot plant, which could see construction off Oahu by 2015; with a 100-MW commercial plant to follow by as early as 2020.  Some cost estimates for a 5 -10 MW plant run as little as $300 million.  However, a 100-MW plant may hit $1.5 billion.

From a distance of 10 miles off shore, such a floating plant would look like one of the ships that routinely bring fuel oil to Hawaii’s ports.  But its generated electricity would arrive onshore via undersea cable.

“The offshore oil industry has done us a great favor by putting billions of dollars into development of these floating platforms,” said Robert Cohen, an independent OTEC consultant in Boulder, Colorado.  “They’ve done all the engineering.  So we don’t have to build a fancy platform; we just have to build an OTEC power module that will work.”  

Once built, such floating OTEC plants would operate much like offshore oil rigs.  However, Cole says the ultimate future of OTEC, for the U.S. at least, is the production of concentrated liquid energy; such as ammonia and hydrogen using electricity generated far from land in the ocean’s equatorial belt.  These liquefied chemical energy carriers could then be readily shipped back to the mainland U.S. via tanker.     

Hydrogen can be readily made by the electrolysis of seawater into hydrogen and oxygen.  Ammonia (NH3), in turn, can be manufactured by combining atmospheric nitrogen with hydrogen from seawater.    

“Ammonia is an easy high-energy chemical to make and can be used as a primary fuel potentially even in ammonia-burning hybrid vehicles,” said Barry.    

OTI is currently negotiating with Caribbean Utilities Company, LLC. of the Cayman Islands about construction of OTI’s first commercial 25 MW OTEC plant at a still undisclosed location.  

But Cole says the plant is expected to see construction by 2018 at a cost of several hundred million dollars.  OTI is also in negotiations with the Hawaiian Electric Company for a follow-on 100-MW OTEC plant to be located off Oahu which is hoped to also be completed by 2018.

For all its potential, however, OTEC still remains largely overlooked by the larger renewable energy community. 

“OTEC is the renewable energy elephant in the room; huge and hard to ignore, although many do,” said Van Ryzin.  “Today, we are focused on renewable low-hanging fruit, on what we can conveniently do now.  But as a nation, are we addressing our major long-range [energy] problems?”