In winter, you will often see municipal trucks spraying salt on roads and sidewalks to clear them of snow. Even if no heat is added, dissolving salt effectively lowers the freezing point of water, which explains why ice melts even at subzero temperatures. And because the natural world is driven by energy conservation, melting ice cools its surroundings.
Remarkably, no energy input is required for this to happen, which gave some scientists an interesting idea. Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory used this precise physical principle to develop a new refrigeration device that cools objects in a completely different way than your kitchen refrigerator.
This new method is believed to be not only more energy efficient, but also much more environmentally friendly, as conventional refrigeration relies on extremely potent greenhouse gases as a cooling agent.
How Ionocaloric Cooling Works
Similar to conventional refrigeration, the new cooling technique in question, called “ionocaloric cooling”, exploits the fact that heat is absorbed or released when materials change phase, for example from solid ice to liquid water. For ice to melt it must absorb heat from its surroundings and when water freezes it releases heat into the surroundings.
In an ionocaloric device, this phase change occurs not by pressurizing or heating a material, but rather by the flow of ions, which are electrically charged atoms or molecules.
When an electric field is applied to a material, the ions in that material experience a force and begin to move. This movement causes a change in the entropy of the material, which results in a change in temperature. By applying and removing an electric field in a controlled manner, it is possible to use the ionocaloric effect to produce cooling or heating. Tada!
But is this method really feasible? After refining the theory behind the ionocaloric cycle and demonstrating the technique experimentally, Berkeley Lab physicists certainly think so.
“The refrigerant landscape is an unsolved problem: No one has successfully developed an alternative solution that makes things cold, works efficiently, is safe and doesn’t harm the environment,” said Drew Lilley, a graduate research assistant at Berkeley Lab and a doctoral student at UC Berkeley who led the study. “We believe that ionocaloric cycling has the potential to achieve all of these goals if done appropriately.”
The ionocaloric device is a solid-state technology that uses ionic movement, unlike conventional cooling which relies on the phase change of a liquid to absorb heat and produce cooling. No compressors, expansion valves, or moving parts are required, meaning an ionocaloric refrigerator is potentially more energy efficient, more environmentally friendly, able to heat or cool quickly depending on the situation, and can cover a wider temperature range.
| Functionality | Ionocaloric cooling | Conventional refrigeration |
|---|---|---|
| Energy efficiency | Potentially more effective | May be relatively ineffective |
| Environmental impact | Does not depend on refrigerants | May have significant environmental impact if refrigerants are released into the atmosphere |
| Temperature range | Not limited | Typically used for cooling temperatures between -20 and 40°C |
| Speed | Can be activated and deactivated quickly | May take a while to cool or reheat |
| Size | More compact and portable | Can be relatively large and bulky |
The future of cooling
The research team performed experiments using a salt composed of iodine and sodium to melt crystals of ethylene carbonate, an organic solvent commonly used in Li-ion batteries. Using a single volt of charging, the system temperature difference reaches a whopping 25 degrees Celsius (45 degrees Fahrenheit).
The new study also mentions that the ionocaloric cycle has the potential to compete with, or even exceed, the efficiency of gaseous refrigerants found in the majority of systems today. Additionally, by using a material such as ethylene carbonate, which can be produced using carbon dioxide as an input, the refrigerant could have a negative global warming potential, meaning it could actually remove carbon from the atmosphere.
This is particularly important given how many countries are struggling to meet climate change goals, such as those in the Kigali Amendment (agreed to by 145 parties, including the United States, in October 2022). The agreement commits signatories to reducing the 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.
“It is possible to have refrigerants that not only have zero GWP (global warming potential), but negative GWP,” Lilley said. “Using a material like ethylene carbonate could actually be carbon negative, because you’re producing it using carbon dioxide as an input. This could give us a place to use CO2 from carbon capture.”
The ionocaloric cycle can also be reversed to produce heating for residential and industrial applications. Currently, the team is working to optimize the ionocaloric cycle to improve its efficiency as well as its scalability potential so that it can support the large amounts of cooling demanded by the industry.
“We have this whole new thermodynamic cycle and framework that brings together elements from different fields, and we’ve shown that it can work,” said Ravi Prasher, a research affiliate in Berkeley Lab’s Energy Technologies area and assistant professor of mechanical engineering at UC Berkeley. “Now is the time to experiment to test different combinations of materials and techniques to address engineering challenges.”
The results were published in the journal Science.
This article originally appeared in January 2023 and has been updated for clarity, including new content.
