Solid refrigerants help create eco-friendly cooling system
Image credit: Andrey Popov | Dreamstime
Scientists have unveiled a prototype device that uses solid refrigerants to efficiently cool a space – a more eco-friendly solution that could one day replace existing air conditioners.
People across the US are turning up their air conditioners to beat the summer heat. But the hydrofluorocarbon refrigerants in these products and other cooling devices are potent greenhouse gases and major drivers of climate change, experts have said.
“Just installing an air conditioner or throwing one away is a huge driver of global warming,” said Adam Slavney from Harvard University. “The refrigerants used in these systems are thousands of times more potent than carbon dioxide and can accidentally leak out of systems when they are being handled or disposed of.”
Traditional cooling systems such as air conditioners (A/Cs) work by causing a refrigerant to cycle between being a gas or a liquid. When the liquid becomes a gas, it expands and absorbs heat, cooling a room or the interior of a refrigerator.
A compressor that works at about 70-150lb per square inch (psi) turns the gas back into a liquid, releasing heat. Air conditioners direct this heat outside the home.
Though this cycle is efficient, concerns about climate change and stricter regulations on hydrofluorocarbon refrigerants are spurring the search for more environmentally responsible ones.
Solid refrigerants could be an ideal solution, researchers have said. 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.
Such systems 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, the researchers said. But under pressure, the chains become more ordered and rigid – a change that releases heat.
Going from an ordered to a relaxed structure is like melting wax, but without it becoming a liquid, said Jarad Mason, the project’s principal investigator from Harvard. When the pressure is released, the material reabsorbs heat, completing the cycle.
But a disadvantage of barocaloric systems is that most of these materials require massive pressures to drive heat cycles. To produce these pressures, the systems need expensive, specialised 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 have shown that the refrigerants, which are called metal-halide perovskites, can work in a cooling system they’ve built from scratch.
“The materials we reported can cycle at about 3,000 psi, which are pressures that a typical hydraulics system can work at,” said Slavney.
The team has now built a first-of-its-kind prototype that shows 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,” said 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 team plans 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, including ones that work at lower pressures and that conduct heat better,” said Slavney.
With an optimal material, the researchers believe solid-state refrigerants could become a viable replacement for current air conditioning and other cooling technologies.
In 2020, E&T spoke to Solar Polar, a start-up company based in Peterborough which developed a low-cost cooling solution that doesn’t require any electricity.
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