If you’ll humor me by remembering your last chemistry class, you’ll remember that table salt, made of sodium chloride (NaCl), likes to dissolve in water. True enough, it’s the very reason why we don’t just get our drinking water from the nearest beach. There’s much more happening inside your last salt sprinkle into the pot of your last dinner than just disappearing salt, though.
To make it short, the dissolution of salts (like NaCl) in polar solvents (like water) involve three steps: 1) the breaking of ionic bonds between ions in salts; 2) the breaking of intermolecular attraction between water molecules; and 3) the formation of intermolecular forces between ions and water molecules. Steps 1 and 2 absorb energy, while step 3 releases energy. Different salts dissolve in water in different ways, and release different amounts of energy per step.
When a specific salt absorbs more energy than it releases upon dissolving (meaning steps 1 and 2 absorb more energy than step 3 can release), then the dissolution is said to be endothermic, and the resulting process takes energy from its surroundings. When the opposite happens, the dissolution releases more energy than it absorbs (step 3 releases more energy than what steps 1 and 2 can absorb), and the dissolution is said to be exothermic, releasing that energy into its surroundings.
It’s this very phenomenon that makes the proposition of a research team, led by King Abdullah University of Science and Technology professor Peng Wang, tantalizing. They proposed a method of cooling habitable and productive spaces like houses and offices. Their new research is published in the journal Energy & Environmental Science.
Wang and team are currently developing a model wherein they will make use of the tendency of some salts to absorb energy when it dissolves in water. After a tedious process of choosing the right salt, the team ended up choosing ammonium nitrate (NH4NO3) as their salt of choice, due to its capacity to rapidly cool its surroundings upon dissolution into water. Additionally, NH4NO3 can dissolve up to 208 g of itself in just 100 g of water; this, combined with its relatively cheap price and ubiquity in industry as a fertilizer component, meant it was perfect for the team’s desired application.
NH4NO3 was dissolved in water, which was then placed inside a metal 3D solar regenerator shaped like a bespoke cup. This cup would then be sealed inside a polystyrene foam box, keeping the metal cup insulated from its environment. This allowed Wang and team to isolate the cooling effect of the NH4NO3 dissolution on the air inside the foam box. The team chose a metal for the cup that can absorb as much of the solar spectrum, or all light released as energy by the sun, as possible.
In their setup, the temperature of the cup dropped from around 25 °C (77 °F) to around 3.6°C (38 °F) within 20 minutes. The cup also managed to stay at a temperature of about 15 °C (59 °F) over the next 15 hours. Wang and team then exposed the metal cup to solar energy, which proceeded to heat the water. The water evaporated, leaving the salts behind as crystalline deposits on the walls of the metal cup. In essence, the NH4NO3 crystals left behind serve as “storage” for the solar energy gathered by the metal cup and the water.
According to the team, applying technologies to recollect the evaporated water, like a solar still, can recapture any water that has evaporated from the cup, making the system reusable. Of particular note is the fact that this entire proposed system requires no electricity, as all the energy needed for it will be supplied by the Sun. This, they say, makes the concept applicable for use in hot regions of the Earth.
Said Wenbin Wang, a postdoctoral student from Prof. Wang’s laboratory: “Hot regions have high levels of solar energy, so it would be very attractive to use that solar energy for cooling.” Prof. Wang continued: “We conceptualized an off-grid solar-energy conversion and storage design for green and inexpensive cooling.”
(For further reading, check out how your future house can save even more on your electric bill by using your own footsteps.)
References
- Coxworth, B. (2021, September 20). Sunlight and salt water join forces in electricity-free cooling system. New Atlas. https://newatlas.com/good-thinking/sunlight-salt-water-electricity-free-cooling-system/
- Lumen Learning. (n.d.). Heat of solution. Retrieved October 5, 2021, from https://courses.lumenlearning.com/introchem/chapter/heat-of-solution/
- Wang, W. (2021, September 19). Strong sunlight powers passive cooling device. https://discovery.kaust.edu.sa/en/article/1174/strong-sunlight-powers-passive-cooling-device
- Wang, W., Shi, Y., Zhang, C., Li, R., Wu, M., Zhuo, S., Aleid, S., & Wang, P. (2021). Conversion and storage of solar energy for cooling. Energy & Environmental Science. https://doi.org/10.1039/D1EE01688A
