5 ms·

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 dev

by 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.)
  - extrapickles 9y ago
    The technique described sounds similar to how desiccant dehumidifiers work, where you capture moisture in a desiccant then remove the moisture by heating the desiccant. The energy required is similar to the chiller based technology due to the laws of thermodynamics. I have a one that has a descant drum, and it slowly rotates as it blows air through it. A small section of the drum is heated to release water into a separate air stream and that air is then ran through a radiator to cool it, releasing water into a storage container. It costs $0.5-1/L to generate water with it, since the heat comes from electricity. Heat could easily come from solar, leaving ~20w for a fan.
rukittenme 9y ago
> What makes you so confident that they are using a Peltier? Because you can buy the exact device on Amazon (Noctua heat sink sold separately)[1]. Compare that image to the images shown in this MIT news article[2]. > 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. I would agree. > 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. This describes a Peltier device exactly. Pass a current through it and one side will get hot and one side will get cold. Hence the heat sink. You need someway to dissipate that electrical energy. Ask yourself, if the system works passively (i.e. ambient temperature), why do they need a heat sink? Would attaching a heat sink to my shed cool it off? I think the answer is clear. Ignoring everything I said, to condense water from air you need two things. Air with water in it and a temperature differential. How is the differential generated? According to the paper, a condenser. How does every condenser generate a temperature gradient? Electrical current. 1. https://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Cooler-Volt/dp/B002UQQ3Q2 https://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Coo... 2. http://news.mit.edu/2017/MOF-device-harvests-fresh-water-from-air-0414 http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro...
- nkurz 9y ago
  As long as you are clear that you are accusing the authors of outright fraud, I appreciate and applaud your logic! I think you are wrong, though. Ask yourself, if the system works passively (i.e. ambient temperature), why do they need a heat sink? Because as you say, they need a cooler surface than the ambient for the water to condense on. Ambient temperature in this case refers to the temperature inside the solar chamber containing the saturated sorbent and insulated by the translucent aerogel. In the "legit" view, the heatsink is cooled relative to this ambient by being thermally coupled to the cooler outside air. In the paper, Figure 3 shows the temperature differential as about 40C: https://www.nature.com/articles/s41467-018-03162-7/figures/3 https://www.nature.com/articles/s41467-018-03162-7/figures/3 Would attaching a heat sink to my shed cool it off? Well, if the inside of the shed has been heated by the sun so that it is warmer than the outside air, then yes. The heatpipes are effectively windows for heat to escape from the relative hot interior to the relatively cool exterior. Isn't this exactly how a passive heatpipe cooler like the Noctua works when installed as designed to cool a CPU? Some airflow over the radiating fins doesn't hurt, but convection takes care of this if the surface is large enough.

by nkurz 9y ago

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.)
- extrapickles 9y ago
  The technique described sounds similar to how desiccant dehumidifiers work, where you capture moisture in a desiccant then remove the moisture by heating the desiccant. The energy required is similar to the chiller based technology due to the laws of thermodynamics. I have a one that has a descant drum, and it slowly rotates as it blows air through it. A small section of the drum is heated to release water into a separate air stream and that air is then ran through a radiator to cool it, releasing water into a storage container. It costs $0.5-1/L to generate water with it, since the heat comes from electricity. Heat could easily come from solar, leaving ~20w for a fan.

rukittenme 9y ago

> What makes you so confident that they are using a Peltier? Because you can buy the exact device on Amazon (Noctua heat sink sold separately)[1]. Compare that image to the images shown in this MIT news article[2]. > 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. I would agree. > 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. This describes a Peltier device exactly. Pass a current through it and one side will get hot and one side will get cold. Hence the heat sink. You need someway to dissipate that electrical energy. Ask yourself, if the system works passively (i.e. ambient temperature), why do they need a heat sink? Would attaching a heat sink to my shed cool it off? I think the answer is clear. Ignoring everything I said, to condense water from air you need two things. Air with water in it and a temperature differential. How is the differential generated? According to the paper, a condenser. How does every condenser generate a temperature gradient? Electrical current. 1. https://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Cooler-Volt/dp/B002UQQ3Q2 https://www.amazon.com/TEC1-12706-Thermoelectric-Peltier-Coo... 2. http://news.mit.edu/2017/MOF-device-harvests-fresh-water-from-air-0414 http://news.mit.edu/2017/MOF-device-harvests-fresh-water-fro...

nkurz 9y ago
As long as you are clear that you are accusing the authors of outright fraud, I appreciate and applaud your logic! I think you are wrong, though. Ask yourself, if the system works passively (i.e. ambient temperature), why do they need a heat sink? Because as you say, they need a cooler surface than the ambient for the water to condense on. Ambient temperature in this case refers to the temperature inside the solar chamber containing the saturated sorbent and insulated by the translucent aerogel. In the "legit" view, the heatsink is cooled relative to this ambient by being thermally coupled to the cooler outside air. In the paper, Figure 3 shows the temperature differential as about 40C: https://www.nature.com/articles/s41467-018-03162-7/figures/3 https://www.nature.com/articles/s41467-018-03162-7/figures/3 Would attaching a heat sink to my shed cool it off? Well, if the inside of the shed has been heated by the sun so that it is warmer than the outside air, then yes. The heatpipes are effectively windows for heat to escape from the relative hot interior to the relatively cool exterior. Isn't this exactly how a passive heatpipe cooler like the Noctua works when installed as designed to cool a CPU? Some airflow over the radiating fins doesn't hurt, but convection takes care of this if the surface is large enough.