Regulators and manufacturers started phasing our chlorofluorocarbon gas refrigerants like Freon in the late 1970s after scientists discovered that they could deplete the ozone layer. But their replacements, hydrochlorofluorocarbons and hydrofluorocarbons, are greenhouse gases. Some scientists and engineers have suggested using solid-state refrigerants to avoid those environmental issues but their use has yet to be realized. Now researchers propose that so-called plastic crystal materials could be more effective refrigerants than previously studied solid-state materials (Nature 2019, DOI: 10.1038/s41586-019-1042-5). However, experts caution that major engineering challenges stand between these crystals and your fridge.
In conventional fridges, gaseous refrigerants change from liquid to gas, absorbing energy from the air inside the fridge and cooling it. A compressor then increases the pressure and temperature of the gas, dumping that absorbed energy outside the fridge as hot air as the refrigerant returns to the liquid phase. Previously proposed solid-state alternatives could go through similar energy absorbing and dumping cycles but through changes induced by electric or magnetic fields, instead of pressure.
Bing Li of Shenyang National Laboratory for Materials Science and colleagues are proposing that plastic crystals could participate in those cycles through pressure changes somewhat similar to conventional gas refrigerants. Plastic crystals are not new materials. Their molecules are regularly spaced as in other crystals, but the individual molecules can be oriented randomly. Li says his group is the first to explore their use as refrigerants, although researchers decades ago proposed plastic crystals could be used to store thermal energy.
Under pressure, the molecules in plastic crystals become more ordered. Imposing order means reducing the materials’ entropy. The resulting energy change could be harnessed for cooling effects. As a result, plastic crystals join other so-called barocaloric materials, including natural rubber. Through studies of the plastic crystal neopentylglycol, the group reported changes in entropic energy that are ten times greater than those observed in other barocaloric materials.
To better understand how the barocaloric material works, the researchers used X-ray diffraction, quasi-elastic neutron scattering, and inelastic neutron scattering analyses to observe the effects of pressure on neopentylglycol and determine at what pressures the energy changes occurred. Based on their calculations, neopentylglycol can induce a 50 K temperature change under 45 megapascals of pressure, which is 10 times as great as the pressures used in a typical fridge.
Michael Shatruk of Florida State University, who has studied magnetic refrigerant materials, calls the work intriguing but emphasizes there are still a lot of practical concerns to address. “We need to see more efforts directed at engineering practical refrigeration cycles with these materials,” Shatruk says. He points out that an entropic change may not mean a temperature change if the materials absorb a lot of heat or cannot transfer it efficiently.
Li agrees. His group is already working on those engineering questions, but he hopes many other researchers will join him in exploring how plastic crystals could be practical refrigerants.