I’m sure some other folks have heard of the excitement over highly efficient solar stills designed recently. [Link to paper](https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee04122b).
Unfortunately every article either dives into deep details or completely skims over operating principles. It seems like the idea is that solar heat evaporates some water vapor. Then the water vapor condenses and is collected, and releases heat. That much makes sense. But these ‘multistage’ stills seem to be based on the idea of collecting the heat of condensation evaporate more water?
What I don’t understand is–for the water vapor to condense, it has to be touching a surface below the boiling point. Ergo, if the water is hot enough to evaporate, the water vapor can’t condense and add more heat to it. If it’s cool enough for the water vapor to condense on it and warm it up, it’s because the temp is low enough that evaporation isn’t occurring…
I must be misunderstanding something here at a fundamental level but I have no idea what it is. Any help understanding this better is much appreciated!
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You can recycle heat even when the waste heat you are recycling is at a lower temperature than the temperature you require.
When something cools, something else warms. So, when you condense the water by cooling it, the heat goes into the water that you have not yet purified. That water is then easier to heat to boiling, requiring less new heat.
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