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Scientists have developed a more environmentally friendly air conditioner

Scientists have developed a greener prototype device that could one day replace existing air conditioners.

Summer is in full swing in the United States, and people are turning down the thermostats on their air conditioners to beat the heat. However, hydrofluorocarbon refrigerants in these and other cooling devices are potent greenhouse gases and major contributors to climate change. Today, scientists report a prototype device that could replace existing air conditioners in the future. It uses solid refrigerants to effectively cool the room and is much more environmentally friendly.

The researchers presented their findings yesterday (August 23, 2022) at the fall meeting of the American Chemical Society (ACS). ACS Fall 2022 is a hybrid meeting held virtually and in person August 21-25 with on-demand access August 26-September 9. The meeting will feature about 11,000 presentations on a wide range of scientific topics.

“Simply turning on the air conditioner or throwing it away is a huge contributor to global warming,” says Adam Slovene, Ph.D., who is presenting the work at the meeting. As a greenhouse gas, the refrigerants used in these systems are thousands of times more powerful than carbon dioxide. They can accidentally leak out of systems when they are processed or disposed of.

Environmentally friendly conditioner

The prototype cooling system uses new solid-state barocaloric materials. Author: Adam Slovene

Refrigerants in conventional refrigeration systems, such as air conditioners, function by switching between gas and liquid states. When the liquid turns into a gas, it expands and absorbs heat, cooling the room or interior of the refrigerator. Operating at 70 to 150 pounds per square inch (psi), the compressor forces the gas back into liquid, releasing heat. This heat is directed outside the home, in the case of air conditioners. When using the air conditioner, this heat is removed outside the house. Although this cycle is efficient and effective, global warming concerns and tightening restrictions on hydrofluorocarbon refrigerants are driving the search for more environmentally friendly alternatives.

Solid refrigerants can be an ideal solution. Unlike gases, solids will not escape into the environment from air conditioning units. One class of solid refrigerants, called barocaloric materials, works similarly to traditional gas-liquid refrigeration systems. They use pressure changes to undergo thermal cycling, but in this case the pressure causes a solid-to-solid phase change. This means that the material remains solid, but the internal molecular structure changes.

A key structural aspect of these barocaloric solids is that they contain long flexible molecular chains that are typically flexible 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 similar to melting wax, but it doesn’t turn into a liquid, says Jarrod Mason, Ph.D., the project’s principal investigator, who is at Harvard University. When the pressure is released, the material absorbs the heat again, completing the cycle.

However, barocaloric systems have a major drawback. Namely, most of these materials require massive pressure to handle heat cycles. To generate such pressures, systems require expensive specialized equipment that is impractical for real-world cooling applications. Recently, Mason and his team reported barcaloric materials that can act as refrigerants at much lower pressures. Now they have shown that refrigerants called metal halide perovskites can work in a cooling system they built from scratch. According to Slawney, “the materials we reported are capable of operating at pressures of about 3,000 psi, which is the pressure at which a conventional hydraulic system can operate.”

The research team has now created 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 of them 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 fluid. Finally, the fluid helps transfer this pressure to the refrigerant and helps transfer heat through the system.

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

With the device now in hand, the researchers plan to test a variety of bar caloric materials. “We really hope to use this machine as a test bed that will help us find even better materials,” says Slawney, including ones that work at lower pressures and conduct heat better. Scientists believe that with the optimal material, solid-state refrigerants could become a viable replacement for modern air conditioners and other cooling technologies.

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

Name
Materials for practical solid-state barocaloric cooling: a chemist (re)invents air conditioning

Abstract
Over the last century, the air conditioning system based on vapor compression has evolved into a very efficient technology that is essential to modern life. However, the hydrofluorocarbon refrigerants that are central to this technology are powerful greenhouse gases – one to five thousand times more effective than CO2. The unintentional release of these refrigerants into the atmosphere during air conditioner installation, maintenance and disposal currently causes approx. 4% of planet-wide global warming and is expected to rise to 10% of total warming by 2050. To address this source of emissions, we are focused on developing solid-state barcaloric materials that can serve as direct replacements for hydrofluorocarbons in air conditioners and other heat pump applications . These solids operate on the same pressure-driven thermodynamic cycle as vapor compressors, but use 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, but the combination of low junction pressure sensitivity and high junction hysteresis means that most require impractically high pressures—above 1000 bar—to achieve effective cooling. We recently discovered a promising new family of barocalorics: layered halide perevskites with long alkalimous tails. They undergo solid-solid, order-disorder transitions within the alkyl sublattice, which are similar to the melting of simple n-alkanes, although limited in two dimensions by the layered structure of perovskite. Layered perovskite transitions occur near ambient temperature with high pressure sensitivity and extremely low hysteresis while maintaining a moderately high transition entropy. This combination of properties makes it possible to realize efficient barocaloric cooling with layered perovskites with pressure fluctuations of 200 bar or less, which is achievable with standard hydraulic systems. To demonstrate this in practice, we designed and built a custom barocaloric prototype device and achieved efficient barocaloric cooling at moderate pressure for the first time. I will discuss our current progress, current challenges, and future directions for this work.

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