Solid- or liquid-state cooling strategies often rely on caloric effects in which materials are taken through some sort of phase change. Researchers from the Lawrence Berkeley National Laboratory and the University of California, Berkeley found that ions in solution can be used to control the melting and crystallization of a material, creating what they refer to as an ionocaloric cycle.
Ionocaloric refrigeration would eliminate the risk of greenhouse gases escaping into the atmosphere by replacing them with solid and liquid components. Image credit: Jenny Nuss / Berkeley Lab.
Finding a solution that replaces current harmful refrigerants is essential for countries to meet climate change goals, such as those in the Kigali Amendment.
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 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,” said Drew Lilley, a Ph.D. candidate at the University of California, Berkeley and the Lawrence Berkeley National Laboratory.
“We think the ionocaloric cycle has the potential to meet all those goals if realized appropriately.”
The new ionocaloric cycle joins several other kinds of ‘caloric’ cooling in development.
Those techniques use different methods 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 colleagues calculated that ionocaloric cycle has the potential to compete with or even exceed the efficiency of gaseous refrigerants found in the majority of systems today. They also demonstrated the technique experimentally.
They used a salt made with iodine and sodium, alongside ethylene carbonate, a common organic solvent used in lithium-ion batteries.
“There’s potential to have refrigerants that are not just GWP (global warming potential)-zero, but GWP-negative,” Lilley said.
“Using a material like ethylene carbonate could actually be carbon-negative, because you produce it by using carbon dioxide as an input. This could give us a place to use carbon dioxide from carbon capture.”
Running current through the system moves the ions, changing the material’s melting point.
When it melts, the material absorbs heat from the surroundings, and when the ions are removed and the material solidifies, it gives heat back.
The first experiment showed a temperature change of 25 degrees Celsius using less than one volt, a greater temperature lift than demonstrated by other caloric technologies.
“There are three things we’re trying to balance: the GWP of the refrigerant, energy efficiency, and the cost of the equipment itself,” said Dr. Ravi Prasher, a researcher at the University of California, Berkeley and the Lawrence Berkeley National Laboratory.
“From the first try, our data looks very promising on all three of these aspects.”
“We have this brand-new thermodynamic cycle and framework that brings together elements from different fields, and we’ve shown that it can work.”
“Now, it’s time for experimentation to test different combinations of materials and techniques to meet the engineering challenges.”
The team’s work was published in the journal Science.
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Drew Lilley & Ravi Prasher. 2022. Ionocaloric refrigeration cycle. Science 378 (6626): 1344-1348; doi: 10.1126/science.ade1696
