How to extract energy from the ocean

How to extract energy from the ocean

All that water in the ocean holds a lot of heat, but it’s not spread out. Just a few feet down below the temperature drops considerably, and it gets even colder at a few hundred feet. Scientists want to tap on this huge thermal differential to power energy plans.

This is how it works:

Surface water heats a fluid with a low boiling point, such as ammonia. The resulting gas drives a power-generating turbine. The gas is then cooled by passing it through cold water pumped up from the bottom of the ocean via massive fibreglass tubes that suck up cold water. The gas condenses back into a liquid that can be used again, and the water is returned to the deep ocean.

Scalability seems to be a problem:

“A 100 MW plant might have a pipe 30 feet in diameter suspended 3000 feet. That’s not a small challenge. You’ve got this huge structure vertically suspended. You’ve got a lot of stresses and strains from current, from the movement of platform on the surface”.

And small designs also had trouble in the past:

In 2003, Indian engineers building a 1 MW ocean thermal plant attempted to lower an 800-metre cold water pipe into the ocean from a barge in the Bay of Bengal only to lose the pipe in 1100 metres of water. A new pipe met the same fate the following year.

Besides the obvious technical issues (how to keep these massive pipes in place in the ocean, etc), I see problems with the energy used to pump cold water from the bottom to condensate the working fluid. How does that fit in with Conservation of Energy principles? If we want to break the laws of Physics and build a perpetual machine, might as well go with a known, guaranteed source like magnetic motor.