Electricity From the Sea

U.S. Navy Plans Ocean Thermal Energy Conversion Plants
By william matthews
Published: 1 June 2009

Heat from the tropical sun warms the ocean surface off Guam to an inviting 80 degrees Fahrenheit. But 3,000 feet below, the ocean remains a bone-chilling 40 degrees.

That temperature difference promises to provide the U.S. Navy with a limitless supply of electricity.

Good thing, too – the U.S. military plans to move 8,000 Marines and 17,000 family members to Guam from Okinawa, Japan, by 2014. But these new residents and the expanded military installations are likely to overwhelm Guam’s power grid, which today generates all of its electricity from imported oil.

The Navy thinks “ocean thermal energy conversion” may be the answer to Guam’s future electricity needs – and Diego Garcia’s, Kwajalein’s and Hawaii’s, too.

Turning the ocean’s temperature differentials into electricity is not a new idea. The concept was developed in 1881, and the first functioning ocean thermal energy conversion (OTEC) power plant was built in Cuba in 1930.

But the economics and politics of oil and OTEC have never aligned quite right for the thermal technology to take off – until, possibly, now. With oil prices uncertain, supplies limited and demand growing, a power plant that runs on warm and cold water has fresh appeal.

The concept sounds relatively simple. Surface water warmed by the sun is pumped through a heat exchanger, where it heats a fluid with a very low boiling point. Ammonia works, as do fluorinated carbons and some hydrocarbons. The fluid expands rapidly as it vaporizes, and pressure from the expanding vapor turns a turbine that turns a generator that produces electricity.

Once it passes through the turbine, the vapor is piped into a condenser, where cold water from deep in the ocean chills it, returning it to its liquid state. The liquid is then pumped back to the warm water heat exchanger to repeat the cycle.

That’s it. There’s no fuel other than temperature differentials. There’s no pollution. And OTEC offers an alternative to dependence on foreign oil.

Interest in OTEC was high in the early 1970s, when an Arab oil embargo against the United States drove oil prices up and made supplies scarce. But when the embargo ended in 1974, interest in OTEC and other alternative energy sources waned.

Interest is reviving today, but ocean thermal energy conversion still faces some big hurdles. One is initial cost.

An OTEC plant that generates 100 to 200 megawatts of electricity could produce power at a cost that’s competitive with imported oil, said Robert Varley, who manages OTEC business ventures for Lockheed Martin. That’s enough electricity for 25,000 to 50,000 homes. Such a plant would cost $1.5 billion – a lot more than anyone so far has been willing to invest, Varley said.

And before building a 100-megawatt plant, it would be smart to build a smaller plant – say, 10 megawatts – to prove the concept and answer engineering questions, he said. That, too, has been prohibitively expensive.

“You can’t recoup the cost” of a 10-megawatt plant because it produces too little power to be profitable, Varley said. So for decades, ocean thermal energy conversion has remained an intriguing on-again, off-again science experiment.

But that, too, may be changing. In April, the U.S. Navy announced plans to award a contract late this year for OTEC plant designs to be used at its base on Diego Garcia in the Indian Ocean and at other Navy locations.

And in February, the Navy began feasibility studies for an OTEC plant to meet “a significant amount of the current and future electrical power needs” for Guam. The Navy wants plants that can also produce fresh drinking water.

That’s a slightly different design from the OTEC plant described above. That first plan is a “closed-cycle” generator, so-called because the fluid that is converted into vapor and back into liquid is used over and over again and never leaves the system.

The Navy favors an “open-cycle” system in which warm seawater is injected into a low-pressure chamber. There, because of the low pressure, a portion of the water flashes into steam, which drives a turbine to generate electricity.

When it turns into steam, the seawater leaves its salt behind, and when it leaves the turbine, it can be condensed into fresh water for drinking, irrigation and other uses.

In 1993, an experimental land-based open-cycle OTEC plant in Hawaii was able to produce 50 kilowatts of “net power,” according to the U.S. Energy Department. Net power is the total power produced minus the electricity that’s needed to keep the power plant running.

The pumps that OTEC plants use to draw cold water up from the deep require a lot of electricity. In 1979, for instance, a 50-kilowatt OTEC plant mounted on a Navy barge moored off Hawaii produced 52 kilowatts of electricity, but consumed all but 15 kilowatts.

Although the science is sound, there remain some technical challenges to building a large OTEC power plant. One is building a 3,000-foot-long, large-diameter, cold-water pipe that can withstand the ocean environment.

Lockheed is working under a $1.2 million contract with the Energy Department to build prototype pipes out of fiberglass and composite materials. “From an engineering standpoint, it’s doable,” Varley said. “The actual metric is how cheap can we build it.”

Passing electrical current through water breaks the water molecules into hydrogen and oxygen, which are then captured. Hydrogen can be used as a fuel or it can be combined with nitrogen from the air to make ammonia. Ammonia is also a fuel – it has been used to power buses and rocket planes – and has a plethora of industrial purposes.

“If we got significant funding soon, we could have a pilot OTEC plant in the water in 2013,” said Dennis Cooper, Lockheed’s OTEC program manager.

That wouldn’t be soon enough to meet the electricity needs of the Marines arriving on Guam. But it could start a process that within a generation could have ocean thermal energy conversion providing all the electricity Guam – or Hawaii or Diego Garcia – needs, Lockheed officials say.

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