Okinotorishima Ocean Thermal Energy Conversion
The Daily Yomiuri reports today that the Japanese government is considering installing an ocean thermal energy conversion (OTEC) system on Okinotorishima. (The title of the article is "Ocean power plan mooted for island"; I have no idea what they meant by that. I suspect it's a mistranslation of some sort.) Okunotorishima is an atoll 1,740 kilometers south-southwest of Tokyo, in the general area of Iwo Jima. It's part of what the Japanese call the Senkaku Islands and the Chinese call the Diaoyutai Islands. Japan and China are still fussing over who owns them; they're just rocks, barely above high tide, but there's good fishing there, and believed to be substantial natural gas deposits.
Tokyo's right-wing governor, Shintaro Ishihara, has made development and defense of the Senkaku Islands one element of his platform. They technically fall under the administration of Tokyo-to, which Ishihara is governor of; Tokyo-to is something like the U.S. District of Columbia. (This is answer to a geography trivia question I like: "What is the geographic extent of Tokyo, north to south?" No one has ever been within a factor of five. Even just considering the inhabited Ogasawara islands, Tokyo is more than a thousand km north-south.) He convinced the Prime Minister, Koizumi, to support the project, and about two million dollars was allocated for a study in this year's budget.
Anyway, politics aside, the OTEC would use the 20 degree Celsius thermal gradient between the deep water and the surface to boil ammonia, which drives turbines, then is sent to the depths for cooling and recondensation. The technology has been proven in other demonstration projects, but I doubt it's economically viable when other power sources are available. The article doesn't say how much power would be generated, but does say the total cost would be "tens of billions of yen" (hundreds of millions of dollars). Quite a bit to power a small marine observation station, a few seasonally inhabited structures, and some ice-making equipment for the fishermen.
This website says the U.S. National Renewable Energy Lab is no longer working on OTEC, but doesn't say why. This article has a map showing that Okinotorishima actually is near the edge of a large swath of ocean north of the Great Barrier Reef that has very high thermal gradients, making it appropriate for this kind of power generation.