System combines salt and sunlight for off-grid cooling

Researchers in Saudi Arabia have developed a system that uses endothermic dissolution of a salt in water to cool its immediate surrounding area. Solar energy can then regenerate the solute and means the cyclic system can work without requiring an electricity supply, making it well-suited for users in remote areas.

Many areas have experienced record-breaking temperatures in recent years and the mortality risks associated with extreme heat means that demand for cooling technology is set to increase in the future. However, most cooling equipment relies on electricity-driven vapor compression, which is unviable for the hundreds of millions of people living in off-grid communities.

‘We believe that our system is capable of providing cooling power for remote regions or off-grid communities,’ says Peng Wang from King Abdullah University of Science and Technology.

The system developed by Wang’s team has two stages. The first stage involves dissolving ammonium nitrate salt in water in a metal cup, which requires energy. This energy is drawn from the immediate environment, cooling the air surrounding the cup as well as the solution. The second stage regenerates the solute by evaporating the water solvent and is achieved by wicking the solution vertically onto the surface of a 3D structure, which when exposed to simulated sunlight offers a large area over which the solvent can evaporate. This leaves behind a compact crystalline salt layer on the surface, from which larger crystals can drop-off over time, to be collected underneath the structure for re-use.

‘The system can deliver cooling whenever needed, all day long and all year round, while the solar solute regenerator works during daytime when solar energy is present,’ comments Wang.

The group found that by separating dissolution and solute regeneration, energy storage and cooling powers of up to 191W/m² could be used on-demand across the seasons. Separating the stages physically and timewise makes it possible to control the cooling rate. The amount of salt being dissolved can be adjusted to suit different purposes, and the regeneration rate can be controlled by influencing the solvent evaporation rate – for example by altering the available surface area for crystals to form on.

‘This system is not at all expensive as both the dissolution cooling system and solute regenerator are cheap,’ says Wang. However, there are some limitations to consider. ‘Compared to conventional air conditioning, this system still has a low cooling power, and it is only suitable for cooling small spaces.’

Such sustainable cooling technologies ‘hold significant promise to address increasing cooling demands while reducing energy consumption and global emissions,’ comments Evelyn Wang, who heads the department of mechanical engineering at MIT in the US. ‘In particular, the improved cooling power with the use of low-cost materials will have impact in a range of applications including building cooling and perishable substance storage.’

‘The lack of electricity input makes the concept promising for rural and off-grid applications.’ comments Renaldi Renaldi, a fellow of the Oxford Martin programme on the future of cooling in the UK. ‘It would be interesting to learn more on its scaling-up potential, safety, and reliability.’