Adding salt to a road before a winter storm changes when ice will form. Researchers at the (US) Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have applied this basic concept to develop a new method of heating and cooling. The technique, which they have named ‘ionocaloric cooling’.
Ionocaloric cooling takes advantage of how energy, or heat, is stored or released when a material changes phase – such as changing from solid ice to liquid water. Melting a material absorbs heat from the surroundings, while solidifying it releases heat. The ionocaloric cycle causes this phase and temperature change through the flow of ions (electrically charged atoms or molecules) which come from a salt.
Researchers hope that the method could one day provide efficient heating and cooling, which accounts for more than half of the energy used in homes, and help phase out current “vapour compression” systems, which use gases with high global warming potential as refrigerants. Ionocaloric refrigeration would eliminate the risk of such gases escaping into the atmosphere by replacing them with solid and liquid components.
Commenting on their work, Drew Lilley, a graduate research assistant at Berkeley Lab and PhD candidate at UC Berkeley who led the study, said, “The landscape of refrigerants is an unsolved problem: No one has successfully developed an alternative solution that makes stuff cold, works efficiently, is safe, and doesn’t hurt the environment. We think the ionocaloric cycle has the potential to meet all those goals if realized appropriately.”
Finding a solution that replaces current refrigerants is essential for countries to meet climate change goals, such as those in the Kigali Amendment (accepted by 145 parties, including the United States in October 2022). The agreement commits signatories to reduce production and consumption of HydroFluoroCarbons (HFCs) by at least 80% over the next 25 years. HFCs are powerful greenhouse gases commonly found in refrigerators and air conditioning systems, and can trap heat thousands of times as effectively as carbon dioxide.
The new ionocaloric cycle joins several other kinds of ‘caloric’ cooling in development. Those techniques use different methods – including magnetism, pressure, stretching, and electric fields – to manipulate solid materials so that they absorb or release heat. Ionocaloric cooling differs by using ions to drive solid-to-liquid phase changes. Using a liquid has the added benefit of making the material pumpable, making it easier to get heat in or out of the system – something solid-state cooling has struggled with.
Lilley and corresponding author Ravi Prasher, a research affiliate in Berkeley Lab’s Energy Technologies Area and adjunct professor in mechanical engineering at UC Berkeley, laid out the theory underlying the ionocaloric cycle. They calculated that it has the potential to compete with or even exceed the efficiency of gaseous refrigerants found in the majority of systems today.
They have also demonstrated the technique experimentally. Lilley used a salt made with iodine and sodium, alongside ethylene carbonate, a common organic solvent used in lithium-ion batteries.
