Traditional vapor-compression refrigeration relies on hydrofluorocarbons (HFCs) as refrigerant fluids — a class of compounds with global-warming potentials (GWP) thousands of times higher than that of CO2. A new approach from scientists at Lawrence Berkeley National Laboratory (LBL; Berkeley, Calif.; www.lbl.gov) demonstrates cooling without the use of high-GWP fluids using what the researchers call the “ionocaloric” effect.
In ionocaloric cooling, ions mixing with a solid material induce a phase transition in the solid by lowering its freezing point, similar to the freezing-point depression that occurs when salt is used to melt road ice. Cooling occurs because the lowering melting point moves through the solid-to-liquid phase transition temperature. “Ionocaloric refers to a thermal response to an applied ionic field, or electrochemical potential,” explains Drew Lilley, co-founder of Calion Technologies, a company formed to develop the technology. Lilley’s co-founder is LBL scientist Ravi Prasher.
To elicit the cooling effect, the salt is mixed with the solid form of the pure solvent ethylene carbonate, which has a melting point around 35°C. The temperature of the mix decreases through the solid-to-liquid phase transition of ethylene carbonate The temperature decreases to 6.4°C with 23% NaI, the researchers say, with heat absorption occurring to convert the solid to liquid, thus giving rise to a cooling effect.
The key to the technology, however, is making the phase transition reversible. To do that, the researchers apply a voltage to separate the ions from the solvent through a membrane, similar to what occurs in a battery or fuel cell. Separating the ions from the solvent raises its melting point again, and the ethylene carbonate re-crystallizes, releasing heat.
The researchers have built one prototype system, and are working on a larger-scale prototype to refine the ion-solvent separation step. “Our current focus is on improving the engineering,” Lilley says. “We are really looking to flesh out the potential of this kind of cooling for a host of different applications.”