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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Your misunderstanding is built upon the false assumption that water must be boiling hot in order to evaporate. If that was true, we couldn’t dry our clothes outdoors.
Consider if they built just a single cell of this apparatus: one side is heated by the sun and draped with soaked fabric, the other side is like a glass of cold water which forms dew. The glass is kept cold by the incoming water. However, having just a single cell with a drastic temperature difference isn’t the most efficient, so they stack several cells after one another, and just cool the final cell with incoming water. Having one very cold cell at the rear, and one very hot cell at the front, ensures that every cell gets a bit of a temperature difference from front to back, and a temperature difference is all that is needed to drive evaporation and condensation.
As far as I understand it, this much gentler cycle also concentrates the salt to such a small degree that it diffuses out of the soaked fabric at night, instead of gumming up the works.
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