Unless the condensing coils transferred heat efficiently enough to boil the incoming water, that would barely make a dent in efficiency.

> [water's] enthalpy of vaporization, 40.65 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 °C to 100 °C (cp = 75.3 J/(K mol))

https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#Therm...

Efficient heat transfer just means using a big exchange surface versus a small material thickness and/or good heat conductivity of the material. But the problem you probably have in mind is that the vapor can't drive the boiling as for the vapor to transfer its energy it has to be condensed, at which point the question becomes why would the vapor condense and boil the water on the other side versus simply boil itself. The answer here should be a pump, so as to make the pressure on the condensation side higher than the pressure on the boiling side. The pump will need energy to maintain the cycle, but it will be (much) less than the energy needed to boil off water in the absence of the pump and condenser. The question becomes open again, I think, why would this be less efficient than reverse osmosis?

Sufficient pressure is actually exactly the problem, but it shouldn't be necessary to think about that. I'm just saying, intuitively, could you imagine pumping steam through condensing coils and the water around it boiling as a result? Like say you're boiling some water on the stove and steam is rising out of it. If you held a cup of water in that steam, how much steam would it take to boil that cup of water?

Keep in mind water vapor is lighter than dry air of the same temperature; it takes up about 2000x as much volume as the same mass of liquid water. The vast majority of the steam that goes into a practical condensing coil will come right out the other side, only a tiny amount of the steam that's in very close proximity to the surface of the coil will actually condense, which like, if you imagine steam going into a curly metal pipe immersed in water, I'd hope it's intuitive that tons of steam would make it out.

You'd need crazy long and thin pipes, crazy high pressure, and crazy pipe configurations so there's way more steam surface area than water being heated, in order to boil water with the equivalent amount of steam.

There's other tradeoffs here, of course. Both solar panels and reverse osmosis machines are expensive, they're like fancy machined semiconductors and ceramics and stuff, whereas an evaporator is some metal pipes and tanks and maybe some glass or mirrors, so depending on application they may be much more cost effective. But in terms of efficiency, there's no comparison.