One possibility for future energy production involves harvesting the warmth of Earth's tropical oceans, using the natural heat differentials in the water to drive turbines. It would be relatively simple if you didn't need a ludicrously large piece of pipe, 33 feet in diameter and stretching a kilometer beneath the water. To put that in context, that's a New York subway tunnel wide and two and a half Empire State Buildings high.
"To produce sizable amounts of power, ocean thermal energy conversion plants have to move rivers of water," said Laurie Meyer, chief technologist for OTEC at Lockheed Martin, which is building the pipe for a new 10-megawatt pilot plant. This structure also needs to withstand the constant rocking pressure of ocean currents, atmospheric pressures at 3,200 feet deep and the variable temperature of the water it's designed to harvest - so building the pipe has been a challenge. Here's how Lockheed is doing it.
The oceans are huge reservoirs of warmth, especially in the tropics, where the temperature differential between warm surface water and cool deep water can reach 40 or more degrees Fahrenheit. This differential can be harnessed to run a heat engine, using some principles from utility plants, plus some from refrigeration. Warm surface water goes into a heat exchanger, which vaporizes an ammonia solution; the resulting steam drives a turbine, and the ammonia is re-condensed using cold ocean water. The water is then pumped back into the ocean, a few degrees warmer than when it came in. But to reach the energy production scales of a traditional fossil fuel-fired power plant, 100 megawatts or more, you need a whole lot of water.
Lockheed first explored OTEC in the 1970s, and it's resurrecting the concept now in the face of rising energy prices. A 10-MW plant is expected to be operational later this year. Under a U.S. Navy grant, Lockheed designed a new composite material for the pipe, and engineers have been building prototypes at the company's plant in Sunnyvale, Calif. Full-scale OTEC pipes will be made right at the site of the power plant by extruding them right into the water.
A 3,200-foot-long, 33-foot wide pipe is not something you could build in a factory, haul out to sea and drop into the water, Meyer explained. Aside from the logistical challenges of moving it along railways or barges, it would probably be impossible to raise it to the right angle and drop it down to the proper depth. Instead, Lockheed will build it in place, using techniques the company first developed for spacecraft construction.
...For the 10-MW plant, Lockheed is building a 4-meter diameter pipe (about 13 feet). For a full-scale, 100-MW plant, the pipes would be 10 meters, or 33 feet. The length of the pipe will be determined by the depth of the cold water - in some potential OTEC sites, chilled water may lurk around 1,000 meters deep, and in others, it may be shallower. The world has plenty of areas where OTEC could work, mostly around the equator.
