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OTEC (Ocean Thermal Energy Conversion) utilizes the temperature gradient between cold deep ocean water, and warmer water to do work. I understand the pressure in the depths may be as high as a couple of orders greater than surface atmospheric pressure.

I also remember, vaguely, that a fluid moves from an area of high pressure to low pressure; wouldn't a sealed pipe merely need valves at the top to control the flow? Does water need to be pumped up out of the deeps?

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up vote 5 down vote accepted

Typically, yes, the water does need to be pumped up.

Because if it released energy by rising, it would already have risen to the surface.

OTEC depends on a high-enough temperature difference between the lower-depth water intake and the higher-depth one, for that temperature difference to do enough work to provide some surplus power, in addition to the power needed to pump the water up.

Tepco's OTEC plant on Nauru (1982-3) reportedly generated 120kW electricity gross, of which 90kW was needed to operate the plant. The surplus 30kW was fed into the grid.

More context: OTEC is estimated to be viable with a ${\Delta}T$ of 20 Kelvin, so definitely the tropics, and predominantly the western Pacific. Unsurprisingly, Japan has been particularly active in OTEC. The global harnessable resource is estimated at $10^{13}W$, which is the same order of magnitude as total global energy consumption.

Map of the potential OTEC resource

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Thank you (+: I may be wrong but it would all have moved up if there was a significant pressure gradient between adjacent layers... In this case, the bottom of the sealed pipeline is easily several hundred psi higher than the top acting as a siphon. Incidentally, how does one pump water up from such depths? –  Everyone Aug 30 '12 at 19:44
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The pressure gradient is generated by the weight of the water itself: if you pushed an empty pipe 1km long and sealed at the bottom into the ocean and then unsealed it, the pressure would be just enough to push water all the way up to sea level, not more. That same pressure will keep replacing any water you remove from the top, so pumping does not have to have any particular difficulty. –  Jaime Aug 30 '12 at 22:23
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