Scientists develop first-of-its-kind air conditioner that uses solid refrigerants

The world is getting hotter by the day. It is now 1.1 degrees Celsius warmer on average than it was before the Industrial Revolution. This means that cooling, in general, has percolated into our lifestyles, almost essential for our survival.

However, the irony is as the planet warms, the technology we seek refuge in can only contribute to climate change, making the climate hotter. Room air conditioners are expected to quadruple to 4.5 billion by 2050, according to Scientific American.

Now, cooling an environment needs an enormous amount of energy. Our energy grids primarily rely on fossil fuels, and any energy that is used to reduce temperature emits greenhouse gases. Refrigerants, chemicals that are used to reduce temperatures, have high global warming potential. When these leak out into the atmosphere, it massively impacts the climate. The hydrofluorocarbon refrigerants in these and other cooling devices are potent greenhouse gases and major drivers of climate change.

Therefore, we must start looking for environmentally-friendly solutions before it's not too late.

Now, scientists have created a prototype device that could someday replace existing "A/Cs". The new version is more environmentally friendly and uses solid refrigerants to cool a space, according to a release.

The researchers presented their results at the fall meeting of the American Chemical Society.

"Just installing an air conditioner or throwing one away is a huge driver of global warming," said Adam Slavney, Ph.D., who presented the work at the meeting.

Solid refrigerants to the rescue

Traditional cooling systems, such as air conditioners, 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. A compressor that works at about 70–150 pounds per square inch (psi) (483 to 1034 kilopascals) turns the gas back into a liquid, releasing heat.

With air conditioners, this heat is directed outside the home. This cycle seems effective but concerns about climate change and strict regulations on hydrofluorocarbon refrigerants are triggering the search for more environmentally responsible ones.

An ideal solution could be solid refrigerants as, unlike gases, they wouldn't leak into the environment from A/C units. The researchers found that barocaloric materials, a class of solid refrigerants, work similarly to traditional gas-liquid cooling systems. Though they use pressure to go through heat cycles, it drives a solid-to-solid phase change.

Barocaloric solid materials comprise long, flexible molecular chains that are "typically floppy and disordered" but under pressure, they become more ordered and rigid - releasing heat. According to Jarad Mason, Ph.D., the project’s principal investigator, who is at Harvard University, the process of going from an ordered to a relaxed structure is like melting wax, but without it becoming a liquid. When that pressure is released, the material reabsorbs heat, completing the cycle.

However, barocaloric systems have their disadvantages. Most of these materials require massive pressures to drive heat cycles. To produce these pressures, expensive, specialized equipment is needed. These are not practical for real-world applications.

Using the machine as a testbed to find even better materials

But, Mason and his team recently reported that barocaloric materials that can act as refrigerants at much lower pressures. The refrigerants, which are called metal-halide perovskites, can work in a cooling system built from scratch. "The materials we reported are able to cycle at about 3,000 psi, which are pressures that a typical hydraulics system can work at," said Slavney.

Their first-of-its-kind prototype can demonstrate the use of these new materials in a practical cooling system. The device has three main parts: a metal tube packed with the solid refrigerant and an inert liquid — water or oil, a hydraulic piston that applies pressure to the liquid, and the liquid helps transfer that pressure to the refrigerant and helps carry heat through the system.

"Our system still doesn’t use pressures as low as those of commercial refrigeration systems, but we’re getting closer," said Mason. According to the team, this is the first working cooling system using solid-state refrigerants that rely on pressure changes.

The team now 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," said Slavney, including ones that work at lower pressures and that conduct heat better. The researchers believe solid-state refrigerants could become a suitable replacement for current air conditioning and other cooling technologies.

Study Abstract:

Vapor-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.