Green material for refrigeration identified

British and Spanish researchers have identified an ecological solid that could replace the inefficient and polluting gases used in most refrigerators and air conditioners.

When pressurized, neopentyl glycol plastic crystals produce enormous cooling effects, enough to be competitive with conventional coolants. In addition, the material is inexpensive, widely available and operates near ambient temperature. The details are published in the journal Nature Communications.

The gases currently used in the vast majority of refrigerators and air conditioners – fluorinated hydrocarbons and hydrocarbons (HFCs and HCs) – are toxic and flammable. When they flee in the air, they also contribute to global warming.

"HFC and HC refrigerators and air conditioners are also relatively inefficient," said Dr. Xavier Moya, of the University of Cambridge, who led the research with Professor Josep Lluís Tamarit, of the University of Cambridge. 39, Polytechnic University of Catalonia. "This is important because refrigeration and air conditioning currently absorb one fifth of the world's energy produced and the cooling demand is only increasing."

To solve these problems, materials scientists around the world have been looking for solid replacement refrigerants. Moya, a researcher at the Royal Society of Cambridge's Department of Materials Science and Metallurgy, is one of the leaders in this field.

In their new publication, Moya and collaborators of the Political University of Catalonia and the University of Barcelona describe the enormous thermal changes under pressure obtained with plastic crystals.

Conventional cooling technologies rely 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 decreases in temperature and cools the environment.

With solids, cooling is achieved by modifying the microscopic structure of the material. This change can be achieved by applying a magnetic field, an electric field or through a mechanical force. For decades, these caloric effects have lagged behind the thermal changes available in fluids, but the discovery of colossal barcaloric effects in a plastic crystal of neopentyl glycol (NPG) and other related organic compounds leveled 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. This is not only widely available, but is also inexpensive.

NPG molecules, composed of carbon, hydrogen and oxygen, are almost spherical and interact only weakly. These loose bonds in its microscopic structure allow the molecules to rotate relatively freely.

The term "plastic" in "plastic crystals" does not refer to its chemical composition, but to its malleability. Plastic crystals are at the boundary between solids and liquids.

NPG compression causes thermal changes of unprecedented magnitude due to molecular reconfiguration. The temperature change obtained is comparable to that commercially exploited in HFCs and HCs.

The discovery of colossal barcaloric effects in a plastic crystal should bring barcaloric materials to the forefront of research and development in order to achieve safe and environmentally friendly cooling without compromising the performance.

Moya is now working with Cambridge Enterprise, the marketing arm of Cambridge University, to commercialize this technology.