When put under pressure, plastic crystals of neopentylglycol yield huge cooling effects – enough that they are competitive with conventional coolants. In addition, the material is inexpensive, widely available and functions at close to room temperature. Details are published in the journal Nature Communications.
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 Dr Xavier Moya, from the University of Cambridge, who led the research with Professor Josep Lluís Tamarit, from the Universitat Politècnica de Catalunya. “That’s important because refrigeration and air conditioning currently devour a fifth of the energy produced worldwide, and demand for cooling is only going up.”
To solve these problems, materials scientists around the world have sought alternative solid refrigerants. Moya, a Royal Society Research Fellow in Cambridge’s Department of Materials Science and Metallurgy, is one of the leaders in this field.
In their newly-published research, Moya and collaborators from the Universitat Politècnica de Catalunya and the Universitat de Barcelona describe the enormous thermal changes under pressure achieved 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, cooling its surroundings.
With solids, cooling is achieved by changing the material’s microscopic structure. This change can be achieved by applying a magnetic field, an electric field or through mechanic force. For decades, these caloric effects have fallen behind the thermal changes available in fluids, but the discovery of colossal barocaloric effects in a plastic crystal of neopentylglycol (NPG) and other related organic compounds has levelled the playfield.
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, plasticisers and lubricants. It’s not only widely available, but also is inexpensive.
NPG’s molecules, composed of carbon, hydrogen and oxygen, are nearly spherical and interact with each other only weakly. These loose bonds in its microscopic structure permit the molecules to rotate relatively freely.
The word “plastic” in “plastic crystals” refers not to its chemical composition but rather to its malleability. Plastic crystals lie at the boundary between solids and liquids.
Compressing NPG yields unprecedentedly large thermal changes due to molecular reconfiguration. The temperature change achieved is comparable with those exploited commercially 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 safe environmentally friendly cooling without compromising performance.
Moya is now working with Cambridge Enterprise, the commercialisation arm of the University of Cambridge, to bring this technology to market.
P. Lloveras et al. ‘Colossal barocaloric effects near room temperature in plastic crystals of neopentylglycol.’ Nature Communications (2019). DOI: 10.1038/s41467-019-09730-9
DdayJ on April 19th, 2019 at 12:51 UTC »
While some refrigerants are flammable, such as propane (R290) and ethane (R170), and some are toxic, such as ammonia (R717), the refrigerants most commonly used in residential refrigeration units are Chlorodifluoromethane (R22) and R410a, which is a blend of Difluoromethane (R32) and Pentafluoroethane (R125). R22 is an HCFC (HydroChloroFluoroCarbon) and while being non toxic (unless you're huffing it, in which case it's a nervous system depressant), non flammable, and having a very low ozone depleting potential (0.055, compare that to R13, which has a factor of 10), due to the Montreal Protocol's plan for completely phasing out HCFC's (due to the chorine content, which is the cause of ozone depletion), R22 must be phased by about 2020, by which point it will no longer be able to be manufactured. In response, R410a was developed, which, as an HFC (HydroFluoroCarbon) azeotropic blend, has no ozone depletion factor due to the refrigerants not containing chlorine (although it is a slightly worse greenhouse gas), it is also non flammable and non toxic.
The articles claim that the refrigerants used in most applications are toxic and flammable (while may be true in some niche applications) is simply not the case for the broader consumer market, and a blatant misconception of the standards set by ASHRAE in today's HVACR industry.
Orwellian1 on April 19th, 2019 at 12:05 UTC »
So thinking practically, I am having a hard time thinking of a system design that would effectively use a solid refrigerant. There is no free lunch, so any heat absorption done (plus mechanical heat gained from compression) has to be rejected outside the conditioned space. Into the outside air for most ACs and refrigeration systems, or into the ground for geothermal.
With a gas/liquid refrigerant, that is relatively easy. Pump it inside at high pressure as a liquid, drop the pressure and force evaporation which absorbs heat. Then it continues back outside as a gas with all of the heat it absorbed. Compress back into a liquid, blow outside air across the lines to get rid of the extra heat, and the cycle repeats.
With a solid refrigerant you aren't going to be moving it back and forth. It will have to alternate between absorbing and rejecting heat in place. It would likely use water, but to stick with the previous analogy. You would blow air across the solid for air conditioning for a while, and then switch to outside air blowing across it to cool it back down???
Efficiency is incredibly important in refrigeration. As the article points out, it is a major energy hog. That being said, just because the solid refrigerant has an equitable heat absorption efficiency as HCFCs, doesn't mean a system can be designed with an equitable practical efficiency.
Minor quibble with the article: Most refrigerants used are not flammable in a material way, and most are not toxic. While their greenhouse potential is high, there is long standing regulation requiring recovery and recycling. I have been trying to find atmospheric measurement studies tracking release for many years, but it doesn't seem to be an area of interest post "ozone hole" era.
I am a touch skeptical of the movement to ban current refrigerants due to greenhouse potential without that data, and the fact that Honeywell and DuPont are leading that environmental push.
agate_ on April 19th, 2019 at 10:35 UTC »
Interesting. However, reading the article, there are two huge problems:
the material needs to be solid to work, so the "refrigerator" wouldn't be a simple plumbing and pump arrangement, you'd need to build some sort of complicated hydraulic press. The material needs to cycle through very high pressure, around 250 MPa GPa (2500 atmospheres), about ten times the pressure of a scuba tank. Making it safe for home use would not be easy.https://www.nature.com/articles/s41467-019-09730-9/tables/1
Edit: meant to write MPa instead of GPa, but I think the other comparisons, and general conclusion about safety, are correct.