Does water need to pumped up from deep ocean?

Question

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?

Answer 1

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.