Comment by adrianmonk | Hacker News Reader

Comment by adrianmonk | Hacker News Reader

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Can't you boost the efficiency by using heat recovery?

The steam is very hot, and the condensed fresh water is also pretty hot. You can use a heat exchanger to transfer this heat into your 16°C ocean water, preheating it so the sun doesn't have to do all the work.

This wouldn't mean magically getting free energy. Your output fresh water would otherwise be really hot, and that's where the energy comes from.

Obviously there are limits. If you have 100°C fresh water coming out and 16°C ocean water going in, a heat exchanger will at best average their temperatures to 58°C. You could in theory overcome this with a heat pump, but that's pretty elaborate.

(I'm not trying to design the perfect desalination system here. The point is there may be some easy efficiency wins.)

A countercurrent heat exchanger could transfer up to 99% of the heat from the steam to the ocean water: https://en.m.wikipedia.org/wiki/Countercurrent_exchange

That's obviously the way to go, but… Where does the 99% come from?

The theoretical efficiency could almost reach 1 with an infinite tube. In practice I've seen ~80%.

It sounds to me like the 'condenser' is perhaps wrapped around the water intake.

Do we want to heat up the oceans that much though?

I believe the heat is being transferred to the ocean water entering the desalination process, which reduces the energy required for desalination, so no transfer of heat to the ocean itself.

A simple system can do much better than the average. Picture the water flowing in a "U", with the hottest water at the bend. The coldest incoming water is warmed just a bit by the coldest outgoing water, then the slightly warmer incoming water is warmed by water a bit further upstream from the output. Carry that logic through and you recapture the heat with a fairly simple system (give or take longer pipes to let enough heat exchange happen).

A multi-stage or continuous heat exchanger can transfer a lot more than half the heat.

if you can do it at lower pressure, boiling and evaporation happen at a lower temperature.

Good idea, but it probably makes a very cheap system more expensive — kitchen-equipment expensive.

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