Sustainable Electricity-Free Air Conditioner Can Harness Solar Energy To Reduce Temperatures on Hot Days

Strong Sunlight Powers Passive Cooling Device

KAUST scientists have developed a simple cooling system based on solar energy and the cooling effect of saltwater evaporation that could be used for refrigeration in hot regions with limited access to electricity. Credit: © 2021 KAUST; Veronica Moraru

Strong Sunlight Powers Passive Cooling Device

A simple cooling system driven by the capture of passive solar energy could provide low-cost food refrigeration and living space cooling for impoverished communities with no access to the electricity grid. The system, which has no electrical components, exploits the powerful cooling effect that occurs when certain salts are dissolved in water. After each cooling cycle, the system uses solar energy to evaporate the water and regenerate the salt, ready for reuse.

“Hot regions have high levels of solar energy, so it would be very attractive to use that solar energy for cooling,” says Wenbin Wang, a postdoc in Peng Wang’s lab. In many parts of the world, there is a greater need for cooling because of climate change, but not every community can access electricity for air conditioning and refrigeration. “We conceptualized an off-grid solar-energy conversion and storage design for green and inexpensive cooling,” Professor Wang says.

Passive Cooling System

The cooling system designed by KAUST engineers could be used to cool rooms in households. Credit: © 2021 KAUST; Wenbin Wang

The team designed a two-step cooling and regeneration system, with the cooling step based on the fact that dissolving certain common salts in water absorbs energy, which rapidly cools the water. After comparing a range of salts, ammonium nitrate (NH4NO3) proved to be the standout performer, with a cooling power more than four times greater than its closest competitor, ammonium chloride (NH4Cl). The ammonium nitrate salt’s exceptional cooling power can be attributed to its high solubility. “NH4NO3’s solubility reached 208 grams per 100 grams of water, whereas the other salts were generally below 100 grams,” Wenbin says. “This salt’s other advantage is that it is very cheap and already widely used as fertilizer,” he adds.

The system has good potential for food storage applications, the team showed. When the salt was gradually dissolved in water in a metal cup placed inside a polystyrene foam box, the temperature of the cup fell from room temperature to around 3.6 degrees Celsius (38.5 degrees Fahrenheit) and remained below 15 degrees Celsius (59 degrees Fahrenheit) for over 15 hours.

Once the salt solution reached room temperature the team used solar energy to evaporate the water using a bespoke cup-shaped 3D solar regenerator. The cup was made from a material designed to absorb as much of the solar spectrum as possible. As the water evaporated,  the NH4NO3 crystals grew over the cup’s outer wall. “The crystallized salt can be collected automatically as the salt drops off due to gravity,” Wenbin says.

Once collected, the salt effectively represents a stored form of solar energy, ready to be reused for cooling again when required.

Reference: ” Conversion and storage of solar energy for cooling” by Wenbin Wang, Yusuf Shi, Chenlin Zhang, Renyuan Li, Mengchun Wu, Sifei Zhuo Sara Aleida and Peng Wang, 1 September 2021, Energy & Environmental Science.
DOI: 10.1039/D1EE01688A

8 Comments on "Sustainable Electricity-Free Air Conditioner Can Harness Solar Energy To Reduce Temperatures on Hot Days"

    • The government should be building prototypes for all of these different types of storage systems. We need to get back to being the technical leader of the world.we are not in a war anymore so use that money to be the best and most successful scientifically advanced place on the planet and the world leader again rerun the new deal like what ended the great depression and build up the renewable energy industry, rebuild our roads and power grid systems and let the world follow or not. Take care of our country and leave the world alone for the next few years and concentrate on making our country the best it can be. Raise the standard of living for all of our citizens. Quit giving our money away and get our country straightened out until we have the highest quality of life in our borders for at least 10-20 years and ignore the world turn our borders over to the military to maintain our immigration process. Quit worrying about what the world says about us and make sure we are the best version of our country . restart our space race and get out into the universe and make the star’s red,white and blue. Build a city on the moon and Mars and start mining the asteroid belt to build space stations and ships to go to the stars.we could have the first city on the moon within 10 years, Mars in 15 years and then make the jump to the nearest stars in 20yrs and the universe in less time than you think because the technology will advance incredibly faster than anyone thinks. We can go looking for alien races to trade with then we won’t be confined to one little planet. Turn our nuclear weapons into space ships and get out into the universe and go see what is out there.

  1. mmm.


  3. An explosive innovation.

  4. Beirut is a great example of why storing large amounts of ammonium nitrate inside residences is a terrible idea.

  5. Seems like this system will require a lot of water, no? Another thing many hot places lack…

  6. Definitely need to couple this to a water reclamation system. And test a full size prototype instead of just a bench scale system. Cool idea though!
    The explosive nature of ammonium nitrate is definitely a concern with actually rolling this out, but there may be another chemical that can be added to mitigate that risk.

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