Sadi Carnot

“Mercurious, the theoretical limit I saw was 6-7% efficiency. It does seem that moving all that water is a problem.”

If the differential temperature is 24 K and the surface temperature is 300 K, the Carnot limit – the absolute maximum efficiency – is only 8%, or as low as 3.3% for a temperature difference of 10 K. In practice, a real heat engine can’t reach that limit. I would assume that the quoted efficiency of 6-7% is based on the assumption of a temperature difference of 24 K – which is really the practical maximum you can find – and it doesn’t even begin to factor in the energy input to the pumps.

If instead the ammonia were in a valve controlled (to prevent backflow) closed loop inside an open top canopy/sheath, extending deep into the cooler waters, cold water confined by the sheath would be heated by descending ammonia, rising through the sheath and drawing more cold water in. The ammonia would cool and liquefy, being forced by pressure up the other arm of the loop where it would then start to boil and transition to gas, driving turbines at the top.

The nutrient rich deep ocean water drawn up through the sheath as a result of being heated would, instead of being dumped back into the depths as in your diagram, fertilise the upper ocean waters creating a localised biological oasis in the surrounding watery desert. Increased biological diversity and fishing productivity is a by-product of the design.

I think this design was actually implemented in the Caribbean sometime in the 1920′s.

It’s true that too many nutrients are damaging the reef but that’s partly because they are a concentration of certain nutrients, rather than the mix from deep water that ocean life has evolved to deal with. It’s the difference between feeding a visiting possum refined white bread and a diverse diet more similar to the one it’s used to in the wild.

And of course one of the worrying aspects of climate change is a reduction in mixing between the surface and the deep ocean, although the areas this would operate in are probably not the places where the fall off is occuring.

There is a preference for building the plants on land, which would need deep water within 10 kilometres. Four sites are identified near the USA:

* the island of Hawaii
* Providence Island in the Bahamas (Nassau)
* St. Croix in the Virgin Islands
* Grand Cayman

If, like me, you have a few shares in Geodynamics, you have to keep an eye out for competition that might blow them out of the water, as it were. I don’t think OTEC is it. Still it could be the go in places like Hawaii and the US naval base in Diego Garcia where one is being built. As Robert said, in these places the usually ship in diesel.

Why get it from bananas or ammonia when we may be able to get so much more from appreciating fishies?

That’s cool, but like a lot of sea based stuff, it still suffers from the inevitable degradation and buildup of sea gunk that makes submerged moving parts impractical over the longer term. How do you clean the thing? How long before it becomes a massive oyster hatchery? Mmmm, oysters.

I can image the hoohah trying to lay subsea cables through the great Barrier reef Marine Park.

Of all* the recent new cutting edge renewables I’ve read about, this one struck me as the most exciting.

* true, it’s not that many