Researchers in the UK and Spain have identified an environmentally friendly solid that could replace the inefficient and polluting gases used in most refrigerators and air conditioners.
When put under pressure, the neopentyl glycol plastic crystals produce enormous cooling effects – enough to be competitive with conventional refrigerants. In addition, the material is inexpensive, widely available and works close to room temperature. Details are published in the journal Communications of nature.
The gases currently used in the vast majority of refrigerators and air conditioners – hydrofluorocarbons and hydrocarbons (HFCs and HCs) – are toxic and flammable. When they leak into the air, they also contribute to global warming.
"Refrigerators and air conditioners based on HFCs and HCs are also relatively inefficient," said Xavier Moya of the University of Cambridge who led the research with Professor Josep Lluís Tamarit of the Universitat Politècnica de Catalunya. "This is important because refrigeration and air conditioning currently consume one-fifth of the energy produced worldwide, and the demand for cooling is only increasing."
To solve these problems, materials scientists around the world have sought alternative solid refrigerants. Moya, a research fellow at the Royal Society at the Department of Materials Science and Metallurgy at Cambridge, is one of the leaders in this field.
In his recently published research, Moya and colleagues at the Universitat Politècnica de Catalunya and the University of Barcelona describe the enormous thermal changes under pressure obtained with plastic crystals.
Conventional cooling technologies depend on the thermal changes that occur when a compressed fluid expands. Most cooling devices work by compressing and expanding fluids such as HFCs and HCs. As the fluid expands, it lowers the temperature, cooling its surroundings.
With the solids, the cooling is obtained by changing the microscopic structure of the material. This change can be achieved by applying a magnetic field, an electric field or through mechanical force. For decades, these caloric effects have fallen behind the thermal changes available in fluids, but the discovery of colloquial barocaloric effects on a plastic neopentyl glycol (NPG) crystal and other related organic compounds level the playing field.
Due to the nature of their chemical bonds, organic materials are easier to compress and NPG is widely used in the synthesis of paints, polyesters, plasticizers and lubricants. It is not only widely available, but it is also cheap.
NPG molecules, composed of carbon, hydrogen, and oxygen, are almost spherical and interact only weakly. These loose bonds in their microscopic structure allow the molecules to spin relatively freely.
The word "plastic" in "plastic crystals" refers not to its chemical composition, but to its malleability. Plastic crystals lie on the border between solids and liquids.
NPG compaction produces major unprecedented thermal changes due to molecular reconfiguration. The temperature change achieved is comparable to those commercially exploited in HFCs and HCs.
The discovery of colossal barocaloric effects in a plastic crystal should bring barocaloric materials to the forefront of research and development to achieve environmentally safe cooling without compromising performance.
Moya is now working with Cambridge Enterprise, the marketing arm of Cambridge University, to bring this technology to market.
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