Summer is in full swing in the United States, and people are turning down the thermostats for their air conditioners to beat the heat. However, 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 replace existing “A/Cs” in the future. It uses solid refrigerants to efficiently cool a space and is much more environmentally friendly.

The researchers presented their results yesterday (August 23, 2022) at the fall meeting of the American Chemical Society (ACS). ACS Fall 2022 is a hybrid meeting being held virtually and in person on August 21–25, with on-demand access available from August 26–September 9. The meeting features nearly 11,000 presentations on a wide range of science topics.

“Just installing an air conditioner or throwing one away is a huge driver of global warming,” says Adam Slavney, Ph.D., who is presenting this work at the meeting. As greenhouse gases, the refrigerants used in these systems are thousands of times more potent than carbon dioxide. They can accidentally leak out of systems when they are being handled or disposed of.

Refrigerants in conventional cooling systems, such as those in air conditioners, function by cycling between the states of being a gas and a liquid. When a liquid turns into a gas, it expands and absorbs heat, cooling a room or a refrigerator’s interior. The gas is forced back into a liquid by a compressor that operates between 70 and 150 pounds per square inch (psi), which releases heat. This heat is directed outside the home, in the case of air conditioners. This heat is sent outside the house when an air conditioner is used. Despite the fact that this cycle is efficient and effective, worries about global warming and tighter restrictions on hydrofluorocarbon refrigerants are driving a quest for more ecologically friendly alternatives.

An ideal solution could be solid refrigerants. Unlike gases, solids won’t leak into the environment from A/C units. One class of solid refrigerants, called barocaloric materials, work similarly to traditional gas-liquid cooling systems. They use pressure changes to go through heat cycles, but in this case, the pressure drives a solid-to-solid phase change. That means the material remains a solid, but the internal molecular structure changes.

The key structural aspect of these barocaloric solid materials 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 it becoming a liquid, says Jarad Mason, Ph.D., the project’s principal investigator, who is at Harvard University. When the pressure is released, the material reabsorbs heat, completing the cycle.

However, there is a big disadvantage to barocaloric systems. Namely, most of these materials require massive pressures to drive heat cycles. To produce these pressures, the systems need expensive, specialized equipment that’s not practical for real-world cooling applications. Mason and his team recently reported barocaloric materials that can act as refrigerants at much lower pressures. They’ve now shown that the refrigerants, which are called metal-halide perovskites, can work in a cooling system they’ve built from scratch. According to Slavney, “The materials we reported are able to cycle at about 3,000 psi, which are pressures that a typical hydraulics system can work at.”

The research team has now 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 packed with the solid refrigerant and an inert liquid — water or an oil. Another piece 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 carry heat through the system.

After solving several engineering challenges, the team has shown that the barocaloric materials work as functional refrigerants, turning pressure changes into full temperature-changing cycles. “Our system still doesn’t use pressures as low as those of commercial refrigeration systems, but we’re getting closer,” says Mason. To the team’s knowledge, this is the first working cooling system using solid-state refrigerants that rely on pressure changes.

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

Support and funding for the research came from the Harvard University Materials Science Research and Engineering Center, the Harvard Climate Change Solutions Fund, and the Arnold and Mabel Beckman Foundation.

TitleMaterials for practical solid-state barocaloric cooling: A chemist (re)invents an air conditioner

AbstractVapor-compression based air conditioning has matured over the last century into a highly efficient technology which is essential to modern life. However, the hydrofluorocarbon refrigerants central to this technology are potent greenhouse gases—one to five thousand times more effective than CO2. The unintentional release of these refrigerants to the atmosphere during air conditioner installation, maintenance, and disposal is currently responsible for ca. 4% of planetwide global warming and is expected to rise to 10% of all warming by 2050. To eliminate this source of atmospheric emissions, we are focused on developing solid-state barocaloric materials which can serve as direct replacements for hydrofluorocarbons in air conditioners and other heat-pump applications. These solids operate with the same pressure-driven thermodynamic cycle as vapor compressors but utilize a solid-solid phase transition to store and release heat rather than the traditional liquid-vapor transition. Many different compounds have been proposed as possible barocaloric materials, however a combination of low transition pressure sensitivity and high transition hysteresis means that most require impractically high pressures—in excess of 1000 bar—to achieve efficient cooling. We have recently discovered a promising new family of barocalorics: layered halide perovskites with long alkyl ammonium tails. These undergo solid-solid, order-disorder transitions within the alkyl sublattice which are analogous to the melting of simple n-alkanes, albeit confined to two dimensions by the layered perovskite structure. Layered perovskite transitions occur near ambient temperature with high pressure sensitivity and extremely low hysteresis, while maintaining moderately high transition entropies. This combination of properties enables layered perovskites to realize efficient barocaloric cooling with a pressure swing of 200 bar or less, which is achievable with standard hydraulic systems. To demonstrate this in practice, we have designed and constructed a custom barocaloric prototype device and achieved efficient barocaloric cooling at moderate pressures for the first time. I will discuss our current progress, ongoing challenges, and future directions of this work.