74 ms·

I don't think they are boiling it. It read to me like they're just evaporating the water in some efficient way, rather than boiling. What would the math look l

I don't think they are boiling it. It read to me like they're just evaporating the water in some efficient way, rather than boiling. What would the math look like if they aren't necessarily going to boil?

scotty79 3y ago
Thermodynamics can't be cheated. If you want to turn some liquid into gas at given pressure you need to deliver specific amount of energy regardless of how you do it.
- mitthrowaway2 3y ago
  However, you also need to turn the same amount of gas back into liquid, just somewhere else without the salt. It does seem like there's good potential for recovering and reusing that energy.
Izkata 3y ago
From some quick searches, it looks like 2257 J the energy used by the phase transition, so that part will probably be the same.
AlexandrB 3y ago
Boiling is just evaporating water rapidly. No matter what you do, you need to put the same amount of energy in to convert some mass of water to vapor. Any efficiency gains would be in how you get this energy into the water (electric element vs. directly heated by the sun) not in the energy required.
- PicassoCTs 3y ago
  Ultrasound would do it without boiling?
  - AlexandrB 3y ago
    I don't think it matters whether you "boil" the water or not, you still need to put in enough energy to cause a state transition in the water you're evaporating. I see some papers on using ultrasonics to increase the efficiency of energy transfer from a heating element[1], but I don't think the second law of thermodynamics allows for a free lunch here. [1] https://www.sciencedirect.com/science/article/abs/pii/S1359431118300383 https://www.sciencedirect.com/science/article/abs/pii/S13594... Edit: If you're thinking of something like an ultrasonic humidifier, I don't think these actually evaporate the water[2]. The mist these produce would still contain salt if you tried to use them for desalination. [2] https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifiers https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifi...
  - abakker 3y ago
    Ultrasonic "steam" tends to aerosolize the total dissolved solids. Google "white dust" in regards to ultrasonic humidification. I suspect that this would not work for desalination. Aside: I have a home-built ultrasonic humidifier. If I run it with Boulder, CO tap water that is low in TDS, it only takes a day or so to have a PM2.5 >600 in my house. For this to work I had to install an RO filter in order to humidify with ultrasonic and not degrade air quality.
    - chipsa 3y ago
      Ultrasonic humidifiers need to be run with distilled or equivalent purity water, yes. Not just for the lack of salts being aerosolized, but also because anything that may incidentally grow in the water also will be aerosolized. Distilled water helps minimize growth.
    - hinkley 3y ago
      Also the total undissolved bacteria and ameoba.
  - mensetmanusman 3y ago
    That will give you bone lung
- jjk166 3y ago
  Technically evaporation takes less energy than boiling as your product is ambient temperature water vapor instead of hot water vapor. You also don't have to replace heat lost to the environment by the hot water and steam during the boiling process. The difference can be lessened by good insulation and heat regeneration, but they still can't be perfect. Of course on the flip side, your goal is fresh liquid water, so you need to condense the vapor. Condensing hot vapor is easy, just expose it to cooler ambient conditions. Condensing ambient vapor is harder, and will require you to run something like a refrigeration cycle or a chemical desiccant system which will need energy to be regenerated. Most commercial systems use vacuum distillation which boils water at low temperatures and pressures, which has its own drawbacks but is generally more efficient.
  - AlexandrB 3y ago
    Fair enough. I guess what I'm getting at is that the heat of vaporization sets a lower bound for how much energy you need to add, regardless of whether you boil the water or evaporate it.
    - DoctorOetker 3y ago
      You are correct that at ambient temperatures there is an ultimate lower bound for energy. You are incorrect that at ambient temperature the lower energy bound is set by the latent heat of vaporisation, as others have pointed out this is theoretically recoverable. At ambient temperature there is however still a fundamental physical limit: the solvation energy of the salt in the water: https://en.wikipedia.org/wiki/Solvation#Solvation_energy_and_thermodynamic_considerations https://en.wikipedia.org/wiki/Solvation#Solvation_energy_and... Now the whole globe does not have the same ambient temperature, and as you know about global warming it would be great to shed some energy in the form of heat. There are many forms of desalination. Another way to desalinate is freezing: when salty water freezes, it pushes out the salt, so while desalinated water ice forms, the liquid water surrounding the ice will increase in salinity and become brine. One could then use simple nets or grills to separate ice from brine. Suppose one has a space elevator, or even a tether from a balloon, but capable of carrying significant weight. The temperature falls roughly adiabatically with height. Above the tropopause the atmosphere is essentially cloud-free, CO2 free and below freezing point (say -60 deg C). Hence the latent heat of fusion (freezing) can be shed to outer space. So it should be possible to lift salty water up an elevator, allow it to freeze over, separate brine from ice at the top, then lower the separated brine and ice. The energy required to lift the brackish water is compensated by the energy released by lowering the freeze-distilled water and brine. What comes up must go down, so simplistically speaking a pulley in equilibrium, so that the only energy intentionally exerted is lost to pulley and air friction. Then one would be cooling the planet and receiving frozen freeze-distilled water at the same time. The law of conservation of misery is typically not a fundamental law of nature, but imposed by reluctance to study of those who dictate artificial laws.
fatnoah 3y ago
> I don't think they are boiling it. It read to me like they're just evaporating the water in some efficient way, rather than boiling There are definitely efficiencies to be had, though I don't know enough of the math to judge one vs. the other. During my brief patent career, I wrote the patents for a distillation system where the main elements involved heating water that was distributed across rotating blades (heat + surface area + air movement) to evaporate the water. When the water was collected, it passed through a heat exchanger that exchanged heat with the in-flowing water. The result was a very efficient system on a small scale, at least.
- genewitch 3y ago
  the swamp cooler panel seems more durable than rotating blades, maybe*. If i were going to desal it'd be with solar; which seems inefficient but one could precipitate "CO2" out of the water as calcium carbonate during the same process. Emergency water supply for tropical weather aftermath, during the quiet season park upstream from a coral reef that's in danger. *edit: although window and wall unit HVAC use the blades to fling water around so the condenser gets the coolest possible air
bertil 3y ago
They have to “boil” but not get the water to 100ºC. Water evaporates in the air at any temperature; it’s faster when the water is warm, and the air is warm and dry. Technically, that’s boiling, even if it’s not exactly like how your kettle does it. Essentially, they find an equilibrium between the cold water coming in, warming in the sun, an increasing amount evaporating into the warm, damp chamber, and the remaining brackish water being cooled by the new water.

scotty79 3y ago

Thermodynamics can't be cheated. If you want to turn some liquid into gas at given pressure you need to deliver specific amount of energy regardless of how you do it.

mitthrowaway2 3y ago
However, you also need to turn the same amount of gas back into liquid, just somewhere else without the salt. It does seem like there's good potential for recovering and reusing that energy.

Izkata 3y ago

From some quick searches, it looks like 2257 J the energy used by the phase transition, so that part will probably be the same.

AlexandrB 3y ago

Boiling is just evaporating water rapidly. No matter what you do, you need to put the same amount of energy in to convert some mass of water to vapor. Any efficiency gains would be in how you get this energy into the water (electric element vs. directly heated by the sun) not in the energy required.

PicassoCTs 3y ago
Ultrasound would do it without boiling?
- AlexandrB 3y ago
  I don't think it matters whether you "boil" the water or not, you still need to put in enough energy to cause a state transition in the water you're evaporating. I see some papers on using ultrasonics to increase the efficiency of energy transfer from a heating element[1], but I don't think the second law of thermodynamics allows for a free lunch here. [1] https://www.sciencedirect.com/science/article/abs/pii/S1359431118300383 https://www.sciencedirect.com/science/article/abs/pii/S13594... Edit: If you're thinking of something like an ultrasonic humidifier, I don't think these actually evaporate the water[2]. The mist these produce would still contain salt if you tried to use them for desalination. [2] https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifiers https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_humidifi...
- abakker 3y ago
  Ultrasonic "steam" tends to aerosolize the total dissolved solids. Google "white dust" in regards to ultrasonic humidification. I suspect that this would not work for desalination. Aside: I have a home-built ultrasonic humidifier. If I run it with Boulder, CO tap water that is low in TDS, it only takes a day or so to have a PM2.5 >600 in my house. For this to work I had to install an RO filter in order to humidify with ultrasonic and not degrade air quality.
  - chipsa 3y ago
    Ultrasonic humidifiers need to be run with distilled or equivalent purity water, yes. Not just for the lack of salts being aerosolized, but also because anything that may incidentally grow in the water also will be aerosolized. Distilled water helps minimize growth.
  - hinkley 3y ago
    Also the total undissolved bacteria and ameoba.
- mensetmanusman 3y ago
  That will give you bone lung
jjk166 3y ago
Technically evaporation takes less energy than boiling as your product is ambient temperature water vapor instead of hot water vapor. You also don't have to replace heat lost to the environment by the hot water and steam during the boiling process. The difference can be lessened by good insulation and heat regeneration, but they still can't be perfect. Of course on the flip side, your goal is fresh liquid water, so you need to condense the vapor. Condensing hot vapor is easy, just expose it to cooler ambient conditions. Condensing ambient vapor is harder, and will require you to run something like a refrigeration cycle or a chemical desiccant system which will need energy to be regenerated. Most commercial systems use vacuum distillation which boils water at low temperatures and pressures, which has its own drawbacks but is generally more efficient.
- AlexandrB 3y ago
  Fair enough. I guess what I'm getting at is that the heat of vaporization sets a lower bound for how much energy you need to add, regardless of whether you boil the water or evaporate it.
  - DoctorOetker 3y ago
    You are correct that at ambient temperatures there is an ultimate lower bound for energy. You are incorrect that at ambient temperature the lower energy bound is set by the latent heat of vaporisation, as others have pointed out this is theoretically recoverable. At ambient temperature there is however still a fundamental physical limit: the solvation energy of the salt in the water: https://en.wikipedia.org/wiki/Solvation#Solvation_energy_and_thermodynamic_considerations https://en.wikipedia.org/wiki/Solvation#Solvation_energy_and... Now the whole globe does not have the same ambient temperature, and as you know about global warming it would be great to shed some energy in the form of heat. There are many forms of desalination. Another way to desalinate is freezing: when salty water freezes, it pushes out the salt, so while desalinated water ice forms, the liquid water surrounding the ice will increase in salinity and become brine. One could then use simple nets or grills to separate ice from brine. Suppose one has a space elevator, or even a tether from a balloon, but capable of carrying significant weight. The temperature falls roughly adiabatically with height. Above the tropopause the atmosphere is essentially cloud-free, CO2 free and below freezing point (say -60 deg C). Hence the latent heat of fusion (freezing) can be shed to outer space. So it should be possible to lift salty water up an elevator, allow it to freeze over, separate brine from ice at the top, then lower the separated brine and ice. The energy required to lift the brackish water is compensated by the energy released by lowering the freeze-distilled water and brine. What comes up must go down, so simplistically speaking a pulley in equilibrium, so that the only energy intentionally exerted is lost to pulley and air friction. Then one would be cooling the planet and receiving frozen freeze-distilled water at the same time. The law of conservation of misery is typically not a fundamental law of nature, but imposed by reluctance to study of those who dictate artificial laws.

fatnoah 3y ago

> I don't think they are boiling it. It read to me like they're just evaporating the water in some efficient way, rather than boiling There are definitely efficiencies to be had, though I don't know enough of the math to judge one vs. the other. During my brief patent career, I wrote the patents for a distillation system where the main elements involved heating water that was distributed across rotating blades (heat + surface area + air movement) to evaporate the water. When the water was collected, it passed through a heat exchanger that exchanged heat with the in-flowing water. The result was a very efficient system on a small scale, at least.

genewitch 3y ago
the swamp cooler panel seems more durable than rotating blades, maybe*. If i were going to desal it'd be with solar; which seems inefficient but one could precipitate "CO2" out of the water as calcium carbonate during the same process. Emergency water supply for tropical weather aftermath, during the quiet season park upstream from a coral reef that's in danger. *edit: although window and wall unit HVAC use the blades to fling water around so the condenser gets the coolest possible air

bertil 3y ago

They have to “boil” but not get the water to 100ºC. Water evaporates in the air at any temperature; it’s faster when the water is warm, and the air is warm and dry. Technically, that’s boiling, even if it’s not exactly like how your kettle does it. Essentially, they find an equilibrium between the cold water coming in, warming in the sun, an increasing amount evaporating into the warm, damp chamber, and the remaining brackish water being cooled by the new water.