Lockheed won’t release the cost of the project, but outside experts estimate that a 10-MW facility would cost roughly US $300 million to $500 million. However, experts say that a full-scale 100-MW plant would be more competitive at just $1.2 billion.
OTEC systems make use of the temperature differential in tropical areas between warm surface water and cold deep water. In most systems, ammonia, which has a very low boiling point, passes through a heat exchanger containing the warm water. The ammonia is vaporized and used to turn a turbine, and then it’s cycled past the cold water to recondense. This is a renewable energy technology with the rare capacity to supply base-load power, as water temperatures are fairly stable.
The ammonia passes through a closed loop, while the water comes and goes through massive pipes. The project in China may pump cold water up from a depth of about 1000 meters, using a pipe that’s 4 meters across. Varley says that some of the infrastructure can be borrowed from the offshore drilling industry: “We showed them our requirements for the platform, and they yawned and said, ‘Is that all you got?’ ” he says. “But then we showed them the pipe.” Attaching the massive pipe to a relatively small floating platform creates unusual stresses, Varley says. Lockheed also had to find materials for the pipes and the heat exchangers that could withstand the harsh marine environment.
The concept goes back to the 1880s, but harnessing such small temperature differentials — 20–25 degrees Celsius — has proven challenging. Early OTEC systems were 1–3 percent thermally efficient; the theoretical maximum is 6–7 percent.