As David Mackay has pointed out, the risk with ground source heat pumps is that you eventually extract enough heat to freeze the ground around you. Essentially, the ground is a good store of heat because of it's low thermal conductivity, but for the same reason it's also hard to replenish the store. Which means that if you're constantly take heat from the same hole in the ground, eventually you get to a point where you've taken it all.
Of course in the ocean you have a different situation, first of all because you're working with a liquid heat store, and secondly because you can more easily float your power station around to different locations. In the video clip I have linked to above, Dr Ted Johnson, Director of Alternative Energy Development at Lockheed Martin who are developing the prototype, says:
"I dream of thousands of floating OTEC ships, roaming the seas of the world and providing an inexhaustible supply of clean energy, fuel and water for all people of the world"Well, it's a wonderful vision and I agree with that. But presumably the problem is how to store the electric power that the floating heat pumps generate, or more precisely how to do that without destroying the economics of the thing completely. Bear in mind that electric storage is REALLY expensive: even a small 24 KWh battery for a state of the art electric car costs about $10,000 - $15,000, and Ted is talking about ocean pumps with 10 or 100MW capacity. So even without having done the math on this (which at some level is against all my principles), I'd assume that the energy storage costs do this project in.
BUT, hang on a moment. Things would be different, wouldn't they, if you had something useful you could do with all that power right there in the ocean? And if you think for a minute about the applications of clean energy generation in close proximity to the ocean... well, it doesn't take long for desalination to jump to mind, i.e. the highly power-intensive conversion of salt water to drinking water.
Perhaps Dr Johnson has come up with something unbelievable after all, the answer to the world's water wars of the future. And clean water is of course, much easier to store and move around than clean energy.
* Of course, I noticed that this technology is not exactly a heat pump, but it's a close cousin. So let's get on with it and not be too pedantic.
