3 ms·

Let me elaborate because this received a couple of downvotes. They only use the metal-organic framework to adsorb water vapor from the atmosphere during the nig

Let me elaborate because this received a couple of downvotes. They only use the metal-organic framework to adsorb water vapor from the atmosphere during the night and then use sunlight to desorb it during the day which leaves them with water vapor again. Now they still have to condense that into water and their 2017 paper [1] shows that they use a Peltier element paired with a big heat sink to do this. But this is the process that actually requires the most energy, disiapating 2.265 MJ/kg of water which is enough energy to heat the same amount of water by 541 K. Maybe I am missing something but to me it seems like that metal-organic framework at best provides a marginal benefit over simply condensing water on a Peltier element directly, which will, with an efficiency of 15 % [2], still require about 4.2 kWh/kg. [1] https://globalscience.berkeley.edu/sites/default/files/waterharvesting.pdf https://globalscience.berkeley.edu/sites/default/files/water... [2] https://en.wikipedia.org/wiki/Thermoelectric_cooling#Performance https://en.wikipedia.org/wiki/Thermoelectric_cooling#Perform...

jessaustin 9y ago
ISTM they're just waiting until night again? A simple mechanical design could contain water vapor in a chamber isolated from environmental atmosphere but not ambient temperature. Eventually it condenses and is harvested. The MOF acts as a sort of Maxwell's Demon, moving water from the environment to the isolated chamber. The trick would be to remove the MOF from communication with the chamber (e.g. at 2 PM) without allowing the chamber humidity to equalize with the environment. Probably there is some sort of airlock?
nkurz 9y ago
I downvoted the original comment because I thought it was factually accurate but misleading. The implication was that the amount of energy required made it obviously nonviable, but you didn't actually show the work to make that case. Without strong evidence, "go figure how viable this is" is an unhelpful thing to say when someone claims to have a working prototype. Your explanation here is much better (and I upvoted it), but I think it still misses the major point that the authors claim that the new device does not use a Peltier cooler. Instead, it's heated by the sun, and uses a passive heatsink. I think it's the same mentioned in the earlier paper as "We also report a device based on this MOF that can harvest and deliver water (2.8 L kg–1 day–1 at 20% RH) under a non-concentrated solar flux below 1 sun (1 kW m–2), requiring no additional power input for producing water at ambient temperature outdoors.". So I think the answer to your question is "Yes, it can be viable if you have a convenient source of heat that can produce the necessary energy". 1 MJ is about 270 Watt Hours, sunlight at noon is about 1000 W/m^2, and solar thermal collection can be more than 50% efficient. This doesn't mean it's viable, but as the new paper shows, if you have a square meter of sunlight available the energy necessary for condensation is probably less of a limiting factor than the collection of the moisture into the (currently exotic) sorbent.

jessaustin 9y ago

ISTM they're just waiting until night again? A simple mechanical design could contain water vapor in a chamber isolated from environmental atmosphere but not ambient temperature. Eventually it condenses and is harvested. The MOF acts as a sort of Maxwell's Demon, moving water from the environment to the isolated chamber. The trick would be to remove the MOF from communication with the chamber (e.g. at 2 PM) without allowing the chamber humidity to equalize with the environment. Probably there is some sort of airlock?

nkurz 9y ago

I downvoted the original comment because I thought it was factually accurate but misleading. The implication was that the amount of energy required made it obviously nonviable, but you didn't actually show the work to make that case. Without strong evidence, "go figure how viable this is" is an unhelpful thing to say when someone claims to have a working prototype. Your explanation here is much better (and I upvoted it), but I think it still misses the major point that the authors claim that the new device does not use a Peltier cooler. Instead, it's heated by the sun, and uses a passive heatsink. I think it's the same mentioned in the earlier paper as "We also report a device based on this MOF that can harvest and deliver water (2.8 L kg–1 day–1 at 20% RH) under a non-concentrated solar flux below 1 sun (1 kW m–2), requiring no additional power input for producing water at ambient temperature outdoors.". So I think the answer to your question is "Yes, it can be viable if you have a convenient source of heat that can produce the necessary energy". 1 MJ is about 270 Watt Hours, sunlight at noon is about 1000 W/m^2, and solar thermal collection can be more than 50% efficient. This doesn't mean it's viable, but as the new paper shows, if you have a square meter of sunlight available the energy necessary for condensation is probably less of a limiting factor than the collection of the moisture into the (currently exotic) sorbent.