4 ms·

It does share those limitations because physical reality requires it to! There is no technological way out. Thermodynamics asks for some amount of energy in e

It does share those limitations because physical reality requires it to! There is no technological way out. Thermodynamics asks for some amount of energy in exchange for some amount of condensed water. The amount of energy is astronomical compared to the meager amount of water. You see the giant ass heatsink at the bottom of the image? Why do you think that exists? The water is condensing on a peltier heat pump. In a desert, to get a single bottle of water you're going to have to blow hundreds of thousands of liters of air (at perfect efficiency) over the pump. And that's for a single water bottle! Oh and this is assuming that your peltier device can get cold enough to produce a 100% humidity atmosphere. Because if it doesn't you get no water.

adrianmonk 9y ago
"They have developed a completely passive system that is based on a foam-like material that draws moisture into its pores and is powered entirely by solar heat." http://news.mit.edu/2017/MOF-device-harvests-fresh-water-from-air-0414 http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro...
- tytytytytytytyt 9y ago
  > The current version can only operate over a single night-and-day cycle with sunlight, Kim says, but “continous operation is also possible by utilizing abundant low-grade heat sources such as biomass and waste heat.” Why can it only un for a single day on solar?
  - hexane360 9y ago
    It can operate for multiple days, but it only generates water once a day. At night the foam absorbs moisture, and then sunlight provides energy to release it as water.
nkurz 9y ago
The water is condensing on a peltier heat pump. What makes you so confident that they are using a Peltier? Here's their description of the operation of the device: During adsorption, air is circulated around the MOF layer and water from air is adsorbed. Passive radiative cooling lowers the MOF layer temperature below the ambient by dissipating thermal radiation to the clear cold sky to increase the effective RH for adsorption. During water production, the OTTI aerogel is stacked on top of the MOF layer to suppress convective heat loss from the solar absorber. The desorbed vapour is condensed on a condenser and the heat of condensation is rejected to the ambient by a heat pipe heat sink. And here's the more complete description of the condenser: The condenser of the device was fabricated with a copper plate (4 cm by 4 cm and 0.6 cm thick) attached to a commercial air-cooled heat sink (NH-L9x65, Noctua) to efficiently dissipate the heat from condensation to the ambient. Are you still sure it's condensing on a Peltier cooler? The paper never mentions "Peltier". If the "condenser" is actually an electrically driven Peltier, this would seem like a fraudulently bad description, justifying retraction of the paper. Is it possible that you are wrong? Still, I agree that you might be right about the physical limitations of scaling. You seem knowledgeable about the field, and I'd be interested to hear your impression after you read the actual paper: https://www.nature.com/articles/s41467-018-03162-7 https://www.nature.com/articles/s41467-018-03162-7. (I am genuinely interested in hearing your opinion about what they are doing, but would strongly suggest less overconfidence and more humility when offering bombastic pronouncements on papers you haven't read.)
- flukus 9y ago
  > What makes you so confident that they are using a Peltier? The article contains a link to an older one and says the device is based on that: http://news.mit.edu/2017/MOF-device-harvests-fresh-water-from-air-0414 http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro... . Notice the heat sink? Notice quotes like: In fact, the system doesn’t even require sunlight — all it needs is some source of heat, which could even be a wood fire. “There are a lot of places where there is biomass available to burn and where water is scarce,” Rao says. That sure sounds and looks like a peltier device to me. MIT is gaining a reputation for crackpot stuff like this.
  - nkurz 9y ago
    Yes, there are previous devices that use a Peltier cooler to condense water directly out of the air. The article you link mentions these as a contrast to what this device is doing: "Another method of obtaining water in dry regions is called dew harvesting, in which a surface is chilled so that water will condense on it, as it does on the outside of a cold glass on a hot summer day, but it “is extremely energy intensive” to keep the surface cool, she says, and even then the method may not work at a relative humidity lower than about 50 percent. The new system does not have these limitations." This approach uses a "sorbent" that adsorbs water at night, and then then uses sunlight to generate heat to drive the water out of the sorbent during the day. It does not use a thermoelectric cooler: 'The new system, by contrast, is “completely passive — all you need is sunlight,” with no need for an outside energy supply and no moving parts.' Rather than requiring active cooling, this approach requires heat to release free the water from the sorbent. The passive heat sink is used to help capture the water after the heat from the sunlight forces it out of the sorbent: "The desorbed vapour is condensed on a condenser and the heat of condensation is rejected to the ambient by a heat pipe heat sink." So as best as I can tell, your theory that the paper (which you haven't read) is simply lying about what they are doing? Not impossible, but I think it would require some greater level of evidence the "Notice the heat sink". And while we're at it, how does "biomass available to burn" imply that they are using a thermoelectric aka Peltier cooler? https://en.wikipedia.org/wiki/Thermoelectric_effect https://en.wikipedia.org/wiki/Thermoelectric_effect (Yes, I agree that it would be nice if the press release would actually link to the paper that correctly describes the apparatus, but its regrettable failure to do so does not give license to make up your own details as to how it works.)

adrianmonk 9y ago

"They have developed a completely passive system that is based on a foam-like material that draws moisture into its pores and is powered entirely by solar heat." http://news.mit.edu/2017/MOF-device-harvests-fresh-water-from-air-0414 http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro...

tytytytytytytyt 9y ago
> The current version can only operate over a single night-and-day cycle with sunlight, Kim says, but “continous operation is also possible by utilizing abundant low-grade heat sources such as biomass and waste heat.” Why can it only un for a single day on solar?
- hexane360 9y ago
  It can operate for multiple days, but it only generates water once a day. At night the foam absorbs moisture, and then sunlight provides energy to release it as water.

nkurz 9y ago

The water is condensing on a peltier heat pump. What makes you so confident that they are using a Peltier? Here's their description of the operation of the device: During adsorption, air is circulated around the MOF layer and water from air is adsorbed. Passive radiative cooling lowers the MOF layer temperature below the ambient by dissipating thermal radiation to the clear cold sky to increase the effective RH for adsorption. During water production, the OTTI aerogel is stacked on top of the MOF layer to suppress convective heat loss from the solar absorber. The desorbed vapour is condensed on a condenser and the heat of condensation is rejected to the ambient by a heat pipe heat sink. And here's the more complete description of the condenser: The condenser of the device was fabricated with a copper plate (4 cm by 4 cm and 0.6 cm thick) attached to a commercial air-cooled heat sink (NH-L9x65, Noctua) to efficiently dissipate the heat from condensation to the ambient. Are you still sure it's condensing on a Peltier cooler? The paper never mentions "Peltier". If the "condenser" is actually an electrically driven Peltier, this would seem like a fraudulently bad description, justifying retraction of the paper. Is it possible that you are wrong? Still, I agree that you might be right about the physical limitations of scaling. You seem knowledgeable about the field, and I'd be interested to hear your impression after you read the actual paper: https://www.nature.com/articles/s41467-018-03162-7 https://www.nature.com/articles/s41467-018-03162-7. (I am genuinely interested in hearing your opinion about what they are doing, but would strongly suggest less overconfidence and more humility when offering bombastic pronouncements on papers you haven't read.)

flukus 9y ago
> What makes you so confident that they are using a Peltier? The article contains a link to an older one and says the device is based on that: http://news.mit.edu/2017/MOF-device-harvests-fresh-water-from-air-0414 http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro... . Notice the heat sink? Notice quotes like: In fact, the system doesn’t even require sunlight — all it needs is some source of heat, which could even be a wood fire. “There are a lot of places where there is biomass available to burn and where water is scarce,” Rao says. That sure sounds and looks like a peltier device to me. MIT is gaining a reputation for crackpot stuff like this.
- nkurz 9y ago
  Yes, there are previous devices that use a Peltier cooler to condense water directly out of the air. The article you link mentions these as a contrast to what this device is doing: "Another method of obtaining water in dry regions is called dew harvesting, in which a surface is chilled so that water will condense on it, as it does on the outside of a cold glass on a hot summer day, but it “is extremely energy intensive” to keep the surface cool, she says, and even then the method may not work at a relative humidity lower than about 50 percent. The new system does not have these limitations." This approach uses a "sorbent" that adsorbs water at night, and then then uses sunlight to generate heat to drive the water out of the sorbent during the day. It does not use a thermoelectric cooler: 'The new system, by contrast, is “completely passive — all you need is sunlight,” with no need for an outside energy supply and no moving parts.' Rather than requiring active cooling, this approach requires heat to release free the water from the sorbent. The passive heat sink is used to help capture the water after the heat from the sunlight forces it out of the sorbent: "The desorbed vapour is condensed on a condenser and the heat of condensation is rejected to the ambient by a heat pipe heat sink." So as best as I can tell, your theory that the paper (which you haven't read) is simply lying about what they are doing? Not impossible, but I think it would require some greater level of evidence the "Notice the heat sink". And while we're at it, how does "biomass available to burn" imply that they are using a thermoelectric aka Peltier cooler? https://en.wikipedia.org/wiki/Thermoelectric_effect https://en.wikipedia.org/wiki/Thermoelectric_effect (Yes, I agree that it would be nice if the press release would actually link to the paper that correctly describes the apparatus, but its regrettable failure to do so does not give license to make up your own details as to how it works.)