Geothermal power generators use temperature differences between the surface and areas deeper in the earth to move a gas or liquid through a loop and drive a turbine. (One example is the Chena Geothermal Power Plant in Alaska. See the related project at the Chena Hot Springs Aurora Ice Museum, which uses 165°F water from a geothermal well to help keep the ice museum cold.)
Ocean thermal power systems work on similar principles, but, to state the obvious difference, underwater rather than underground. A story in the LaCrosse Tribune (Wisconsin) discusses work by an 80-year old Wisconsin engineer who obtained some patents on related inventions in the early 1980s, but saw interest in ocean thermal generation drop off as energy prices fell.
The system uses the difference between the heat of the ocean’s surface water, about 80 degrees in the tropics, and the colder water deeper down to force ammonia through a turbine that turns a generator to produce electricity.
The electricity then can be converted into various sources of power, such as hydrogen, and then used to operate something like a hydrogen-cell car.
…The proposal describes the energy system as free of pollutants and, like the wind, cost-free as well.
“This can work 24 hours a day, 365 days a year, because of the enormity of this source,” Foust said. “It goes all the way around the earth, deep and wide.”
The term “cost-free” is incorrect, of course, both in reference to ocean thermal and wind energy power systems. The system cost something to build, cost something to maintain, and occupy real space in the water that would otherwise be available for other uses. Which is why, as inventor Foust is quoted as saying later in the article, “These alternate energy programs are only viable when the cost of energy is high.”
A recent report by the Texas Comptroller of Public Accounts assessing energy resources for the state was dismissive of the prospects for ocean thermal energy conversion, saying, “ocean thermal energy conversion (OTEC) is the least accessible form of ocean power, and perhaps the least useful for the U.S.” (From the Texas state government’s The Energy Report 2008.) But all such assessments depend on the particular resources and technologies assumed, and both technology and our understanding of resources constantly changes.
In particular, the report observed that the relatively shallow waters in the Texas Gulf do not have sufficient temperature differences between the surface and the ocean floor. Offshore Hawaii, on the other hand, has more potential. Another alternative would be to use temperature differences between surface water and geothermal sources beneath the ocean floor. For example, use offshore oil and gas wells in the Gulf to gain access to higher temperatures under the ocean.
At some price for electricity, such possibilities become economical.