A more environmentally friendly air conditioner

Summer is in full swing in the US and people are turning on their air conditioners to beat the heat. But the hydrofluorocarbon refrigerants in these and other cooling devices are potent greenhouse gases and major drivers of climate change. Today, scientists report a prototype device that could one day replace existing “A/Cs.” It is much more environmentally friendly and uses solid refrigerants to effectively cool the space.

The researchers will present their results today at the fall meeting of the American Chemical Society (ACS).

“Just installing an air conditioner or throwing it away is a huge driver of global warming,” says Adam Slavny, Ph.D., who is presenting this work at the meeting. The refrigerants used in these systems are thousands of times more powerful than carbon dioxide and can accidentally leak from the systems when they are handled or disposed of.

Traditional cooling systems, such as air conditioners, work by causing the refrigerant to cycle between gas and liquid. When a liquid turns into a gas, it expands and absorbs heat, cooling a room or the inside of a refrigerator. A compressor, operating at about 70–150 pounds per square inch (psi), turns the gas back into a liquid, releasing heat. With air conditioners, this heat is directed outside the home. Although this cycle is efficient, concerns about climate change and stricter regulations on hydrofluorocarbon refrigerants are driving the demand for more environmentally responsible ones.

Solid refrigerants can be an ideal solution. Unlike gases, solids will not leak into the environment from air conditioning units. One class of solid refrigerants, called barocaloric materials, work similarly to traditional gas-liquid refrigeration systems. They use changes in pressure to go through thermal cycles, but in this case the pressure causes a phase change from solid to solid. This means that the material remains solid, but the internal molecular structure changes. The key structural aspect of these barocaloric solids is that they contain long, flexible molecular chains that are typically floppy and disordered. But under pressure, the chains become more ordered and rigid, a change that releases heat. The process of going from an ordered to a relaxed structure is like melting wax, but without turning into a liquid, says Dr. Jarrad Mason, the project’s principal investigator, who is at Harvard University. When the pressure is released, the material reabsorbs the heat, completing the cycle.

A disadvantage of barocaloric systems, however, is that most of these materials require enormous pressure to drive heat cycles. To produce these pressures, the systems require expensive, specialized equipment that is not practical for real-world cooling applications. Mason and his team recently reported barocaloric materials that can act as refrigerants at much lower pressures. Now they have shown that the refrigerants, which are called metal halide perovskites, can work in a cooling system they built from scratch. “The materials we reported can move at about 3,000 psi, which is the pressure at which a typical hydraulic system can operate,” Slavni says.

The team has already built a first-of-its-kind prototype that demonstrates the use of these new materials in a practical cooling system. The device has three main parts. One is a metal tube filled with a solid refrigerant and an inert liquid – water or oil. Another part of the device is a hydraulic piston that applies pressure to the liquid. Finally, the liquid helps transfer that pressure to the refrigerant and helps transfer heat through the system.

After solving several engineering challenges, the team showed that barocaloric materials work as functional refrigerants, converting pressure changes into full cycles of temperature change. “Our system still doesn’t use as low a pressure as commercial refrigeration systems, but we’re getting close,” says Mason. As far as the team knows, this is the first working cooling system using solid-state refrigerants that rely on pressure changes.

With the device now in hand, the team plans to test different barocaloric materials. “We’re really hoping to use this machine as a testing ground to help us find even better materials,” Slavni says, including ones that work at lower pressures and that conduct heat better. With an optimal material, the researchers believe that solid refrigerants could become a viable replacement for current air conditioning and other cooling technologies.