Technology Review, August 13, 2001

Oceans of Power

Is ocean thermal energy conversion finally ready to pump up?

By Eric Bender

Once upon a time the U.S. suffered a real energy crunch, the president declared national energy independence "the moral equivalent of war," the feds started spending real money on offbeat energy sources, and about the most offbeat was ocean thermal energy conversion (OTEC).

The basic idea, proposed a century earlier, was simple: vast stretches of tropical and subtropical seas show a difference of around 20 °C between warm surface waters and near-freezing waters a kilometer or two below. That temperature difference could drive massive turbines, producing electrical power around the clock with minimal environmental degradation.

Twenty years ago the Department of Energy threw more than $200 million at the idea. Researchers dreamed up a fleet of grazing OTEC plantships, each steadily grinding out as much as 500 megawatts of power. But the federal program expired with few visible results aside from stacks of engineering reports.

Devotees in the U.S. and overseas kept nudging the idea forward, though, mostly focusing on small, island-based variants. Next year, if all goes well, construction will begin on the first commercial ocean thermal plant, a one-megawatt operation at the Natural Energy Laboratory of Hawaii Authority on the island of Hawaii.

Electrical Power Plus

Unlike the earlier work, today's ocean thermal projects don't stop at electrical power but focus on a mix of products appropriate for a given site, says Hans Krock, professor of ocean engineering at the University of Hawaii-Manoa. These could include:

* fresh water (often in high demand)

* use of nutrient-rich deep water for aquaculture (including farming of coldwater fish)

* coastal cooling (running cold water across heat exchangers to drive air conditioners may prove cost effective against high-priced island electricity)

A case in point is the Natural Energy Laboratory site at Keahole Point in Hawaii, home of pioneering ocean thermal work. The lab shut down its last OTEC prototype two years ago. But it continues to pump up deep seawater for commercial aquaculture operations and coastal cooling.

The lab is also adding another deepwater pipe, big and deep enough to handle the new ocean thermal system that will generate 1 to 1.4 megawatts of power. The 140-centimeter-wide pipe is being made in a single piece almost three kilometers long, says Tom Daniel, NELHA's scientific and technical director. The $11.2-million pipe project is scheduled for completion next summer.

Still, towing the pipe to the site and carefully sinking it is "a very high-risk operation," Daniel comments. (An Indian government project to build a floating one-megawatt ocean thermal plant apparently dropped and lost its pipe, he says.)

Cycling Ahead

Because ocean thermal plants run on a relatively small temperature difference compared to conventional steam-driven plants, engineers have cooked up many ingenious designs to wring out the most efficiency, especially for the crucial heat exchangers and low-pressure turbines.

The Hawaii plant will run on the Kalina cycle, with its working fluid a sealed-off mixture of ammonia and water. Warm seawater vaporizes the mixture, which drives a turbine. The mixture is separated into ammonia-rich and ammonia-poor streams, condensed by cold deep water and then combined for another round.

Kalina technology is widespread in new conventional power plants. Exergy of Hayward, CA, has commercialized it in other kinds of plants with relatively small temperature differences, including geothermal plants and steel mills. "Every piece of the technology is off the shelf, and it works," says Krock.

An alternative approach comes from Sea Solar Power, a controversial ocean thermal firm based in Baltimore, MD. Its heat exchanger design uses propylene as the working fluid and a turbulent-flow process inspired by refrigeration techniques, says president Robert Nicholson.

Sea Solar is about halfway through a two-year $20 million project to optimize the heat exchanger and other components, with funding from Baltimore's Abell Foundation, Nicholson says. However, outsiders view the company with a mixture of respect for its founders' engineering prowess, exasperation at its refusal to detail its technology, and caution, since none of its commercial projects have yet come through.

Water Everywhere, and Some to Drink

A third option, "open cycle" designs that use seawater as the working fluid, has been studied primarily for its ability to produce fresh water. Krock says that two main problems-condensing gases released by the vaporization process and the need for specialized turbines-have been overcome.

He and his students have proposed a plant for Oahu, where "the freshwater resources are close to being tapped out," he says. At a cost of about $80 million, the plant would produce five million gallons of freshwater a day, 8 to 10 megawatts of power, and 20 megawatts-worth of coastal cooling.

Islands in the (Coldwater) Stream

Sea Solar Power has signed a memorandum of understanding with Guam to create an ocean thermal plant and is negotiating with other potential buyers, says Nicholson. He claims that funding is available to build a first 10-megawatt plant, which he puts at around $45 to $50 million.

Krock's group also has churned out proposals for other island-based ocean thermal systems. One for the Navy base at Diego Garcia in the Indian Ocean, for example, could save 30 percent of the cost of generating power and fresh water, he estimates.

"The Gulf of Mexico is a perfect place to do OTEC," Krock adds, given the new deepwater petroleum platforms. "Ironically, oil operators are inevitably the inheritors of this technology."

Saltwater Pipe Dreams?

Yet dreams of the grazing plantships still linger, although viewed with skepticism by many veteran ocean engineers.

Sea Solar Power's Nicholson says it could assemble multiple OTEC power units into a 100-megawatt ship that's one-eighth the size and cost of the behemoths envisioned by the federal research. "We're ready to build 100-megawatt plants now," he declares.

Other experts don't buy such claims. "That's ridiculous," says NELHA's Daniel. "You've got to scale up first."

Further off on the horizon, Krock suggests that OTEC plantships could crank out hydrogen as the world economy starts to shift toward that fuel. Using the cold water as a heat sink could aid the process of liquefying hydrogen, he points out.

Robert Cohen, a Boulder, CO, consultant who was program manager for the Department of Energy's ocean energy program, retains his enthusiasm for ocean thermal energy. "OTEC could eventually provide a significant fraction of global energy needs," Cohen says, both by generating electricity and in creating energy-intensive fuels such as hydrogen.

Cohen notes, though, that the technology has suffered from a history of grand goals and claims. "OTEC seems to bring out extremely subjective opinions from two groups, which I call the skeptics and the zealots," Cohen says, while the truth "tends to lurk somewhere between the extremes."

Eric Bender is editor of technologyreview.com.