5 ms·

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 effic

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
  - hinkley 3y ago
    https://en.wikipedia.org/wiki/Legionnaires%27_disease https://en.wikipedia.org/wiki/Legionnaires%27_disease I didn't know that legionella can kill macrophages from the inside. That's nasty.
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.

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
- hinkley 3y ago
  https://en.wikipedia.org/wiki/Legionnaires%27_disease https://en.wikipedia.org/wiki/Legionnaires%27_disease I didn't know that legionella can kill macrophages from the inside. That's nasty.

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.