I would agree Hermit. Sea Solar Power is ready to build one. (www.seasolarpower.com). Now. You are correct that the efficiency is low, but I'd add that that is irrelevant. Combining it with wind, wave and current power is also feasable on a large platform. Essentially unlimited fresh water is a by product. H2 would be a primary product. JTEC is a new technology that could also be incorporated. Open ocean aquaculture and seafood processing are also compatible (using deep (cold) ocean water to aid flash freezing).

The seastead needs a raison d'etre to ever become reality. My entirely unsolicited opinion is that Seastead needs to focus on becoming a business with viable products. The USA,  for instance is just not repressive enough to inspire competent people to choose anarchy over prosperity. Anarchy or freedom is a fine ancilliary goal, but without an economically based focus, it's just a hobby.

“Prosperity is only an instrument to be used, not a deity to be worshiped.”

Solar assisted OTEC-

Thought it up and googled it. The energy island guys aready though of it in a different form. 

OTEC operates on a temperature differential of about 40 deg F between the cold deep water and warm surface water. Increasing the temperature difference increases the output or reduces the size/cost of the plant. Using a floating breakwater of the dissipation type (seawalls are impractical in the open ocean) as a solar preheater increases the effeciency of OTEC.

OTEC is one of those perennial favorites of ocean-based community projects, but it has huge problems that rarely recieve the attention they deserve.

1. It is very imature technology. Someone is going to need to drop a huge pile of loot into R&D to get to full scale, production ready hardware. (Think 100's of millions of dollars)

2. OTEC requires very warm surface water to be efficient, which limits seastead location. (equatorial ocean only)

3. OTEC requires very deep water to be efficient, which limits seastead location. (Usually WAY offshore)

4. Efficiency increases with size, so good OTECs are very large and don't scale down easily. You need a large seastead to make it worthwhile.

5. Fresh water is a byproduct of the more technically challenging open cycle design. The more near term closed cycle OTEC does not produce fresh water directly.

6. Very few sites are suitable for land-based OTECs, which means if you drop the $$$ to develop one, there is not much of an export market.

My opinion: for near term off the shelf power go with diesel fuel generators for baseload power supplemented by as much wind and solar as you are willing to waste your $$ on. (Diesel will always cost less than the renewables once you factor in $300/sq ft for top deck space and huge costs for battery storage) If you have megabucks and want to develop a new power souce from scratch, design a gas turbine based nuclear particle bed reactor. They work 24 hrs per day everywhere on earth, scale down well, and would be a hot export item.

In Anguilla the electric company uses diesel and the cost of electricity here is $0.45 US per kwh.   I aircondition a well insulated dome and my electric bill is about $2,000 US per month.   I was very happy this week when I found out there are now airconditioners that are about twice as efficient as the one I am currently using (currently 5 tons at 10 SEER and going to 5 tons at 21 SEER - Carrier Infinity series).  

In the US government subsidies covering about half the cost of solar power make it competative with $0.11 per Kwh electricity.   But since our rates are about 4 times the US rates, we don't need any subsidy to make solar win out.  So I fully expect to put in solar sometime in the next few years.

I think solar prices are going to drop enough over the next year or two that I am not in a big rush to buy stuff now.

Anyway, I think diesel is not going to be the way to go for seasteads.   I think solar will be.

Power and potable water from of-shore OTEC plants (as well as other product streams) could potentially be provided via a long-term utility purchase contract with private owner operators.  The owner/operators themselves would provide the capital to build the plant (no cost to the utility users).  The users would essentially sign a long term utilities purchase agreement from the owner operators of the plant.  This type of arrangement is being considered for Naval Support Activity Diego Garcia in the Indian Ocean.  This type of acquisition strategy avoids the high capital costs of building a plant while simultaneously benefitting from utilities and energy rates that are more competitive than importing hydrocarbon fuels to generate electricity and produce freshwater.  However, the utility users must be able to guarantee a long term utilities demand sufficient enough to make the return on invetsment favorable.  Concurrent other Deep Ocean Water resource product streams may also make the return on ivestment even more attractive for owner/operators. 

Since the fuel input is free (cold water), the efficiency alone is not what kills you.  Efficiency is part of why the economics don't work out well though.   You have a capital investment to set everything up.  Then you have to use high grade energy to pump the cold water up and then get low efficiency on converting that to high grade energy.    Given all the costs and how long things last,  so far I think you are better off spending your money on solar.    I suspect it will always be true, but the economics can change as technology changes.  

-interesting article from www.marshalislandsjournal.com copied below.

The Marshall Islands government is making headway to get both landowner and US agreement for long-term use of Kwajalein — but Foreign Minister Tony deBrum said there is no question that to get a new land use agreement (LUA) signed by the landowners, the military use and operating rights agreements (MUORA) in the Compact will have to be changed.

DeBrum, in an exclusive interview with the Journal, said numerous proposals are being discussed by the landowners “to break the logjam,” though he acknowledged there is much to do before a new LUA is a possibility.The landowners are currently facing a December 18 deadline to sign a new LUA or risk losing $20 million in rental payments that have accumulated in an escrow account.

DeBrum said he is hopeful that the LUA issue can be resolved by then or in the alternative that if substantive progress toward a resolution is demonstrated the deadline can be removed by the US.

DeBrum, who is one of three senators representing Kwajalein Atoll, said that the “landowners are not attracted by the $20 million, they are insulted by it.

“Al Staymen (a key staff person with the US Senate Energy Committee) put (the December 18 deadline) in the original act (law) and he can remove it more simply than we can change the MOURA,” deBrum said. “We’ve been very forthcoming with the US about working to get agreement with the landowners.”

Kwajalein people are not against long-term use by the US, but they are not prepared to accept an agreement that perpetuates the current situation at Ebeye, deBrum said. DeBrum sees a proposal for inexpensive power as “an imaginative way to try to break the impasse (over the LUA).”

He is pushing a plan for an Ocean Thermal Energy Conversion (OTEC) power system for Ebeye and Kwajalein. “This would add a $200 million asset to the landowners' portfolio and provide virtually free power and water (for the atoll),” he said.

I know this discussion is about energy, but even if OTEC is not efficient enough, it seems to me that the real advantage is the cold water from the deep. What if you scrap using the cold water for energy production and instead use it for energy usage reduction. For example, 40F is more than cold enough to supply your airconditioning needs, which is a huge energy hog. You could also use it to improve the efficiency of refridgerators and freezers. If you have a larger thermal source of energy, like a solar thermal plant, you could still use the cold water on the condensing side to improve efficiency. I also believe that this colder water could still be used in the production of freshwater via condensation, especially if combined with solar. Then there is the minerals in the deep water, which I understand may have good value in aquaculture/fish farming.

The question then is how much cold water do you need for these purposes and is it cost efficient to pump that quantity to the surface when you compare the energy used for pumping versus the energy savings potential.

As for the cost of the pipe, I would think it would be much lower due to the much smaller quantity of cold water needed for the above purposes versus OTEC.

What kind of pipe is needed? If you put the pump at the bottom, could you use a very thin walled plastic/fabric due to the positive pressure inside? That way you could easily pull it to the surface if the water is to shallow, etc.

I don't know if these ideas have merit, but I would think this is possible to research. If you are going to live on the ocean, you will have to use all of the resources available, and that 40F water seems to be a very valuable resource all by itself.

Sorry, I'm in the Philippines, so my thinking is definately more equitorial! I know there are a lot of people here that want to be in the more northern latitiute, say SF bay area, but in my mind that is not practical for many reasons. Time will tell.

You may get what you want, but will you want what you get?