Green material for refrigeration identified

air conditioner
Credit: CC0 Public Domain

Researchers from the UK and Spain have identified an eco-friendly solid that could replace the inefficient and polluting gases used in most refrigerators and air conditioners.

When put under pressure, crystals of neopentylglycol yield huge effects—enough that they are competitive with conventional coolants. In addition, the material is inexpensive, widely available and functions at close to . 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 .

"Refrigerators and 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 "" 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.


Explore further

Pressure makes best cooling

More information: Nature Communications (2019). DOI: 10.1038/s41467-019-09730-9
Journal information: Nature Communications

Citation: Green material for refrigeration identified (2019, April 18) retrieved 20 October 2019 from https://phys.org/news/2019-04-green-material-refrigeration.html
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Apr 18, 2019
The link for the paper in Nature Communications does not work. It comes up as "DOI not found" on DOI.org.

Apr 18, 2019
"This change can be achieved by applying a magnetic field, an electric field or through mechanic force."
"The temperature change achieved is comparable with those exploited commercially in HFCs and HCs."

This is all well and good but just how will the air conditioner, using this material, compare in electrical efficiency, to the AC units being built today?

Apr 18, 2019
DOI Link now working, and document is *free access*.

Apr 18, 2019
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.


Incorrect. Most conventional cooling technologies rely on the phase change from liquid to gas. The earliest versions used to pump air through an expansion nozzle, which would cause it to drop in temperature, but that was highly inefficient and noisy, and prone to getting blocked. Almost immediately after inventing the process, they switched from expanding gasses to boiling liquids.

Put a bottle of water in a wet sock, and the evaporation of the water cools it some few degrees below ambient. The same process operates in a refrigerator, only the hydrocarbon fluid is forced to evaporate by drawing a vacuum on it.

Apr 18, 2019
@Eikka: "Incorrect. Most conventional cooling technologies rely on the phase change from liquid to gas."

You are correct. Compressed fluids describes a hydraulic system, Not refrigeration.

Apr 19, 2019
You are correct. Compressed fluids describes a hydraulic system, Not refrigeration


"Fluids" in thermodynamics can mean liquids or gas, so not necessarily a hydraulic system. It just describes something that flows: is fluid.

But the method of operation is still fundamentally different. It is possible to make a refrigerator by expanding fluids, and this is used in certain applications, but it's less efficient to do so.

The difference between a boiling liquid or vapor compression cycle, and gas cycle refrigeration is the difference between the ideal heat engines of Brayton and Rankine. These fellows described two different thermodynamic cycles that result in the output of work from a flow of heat, and when you reverse their operation you get a refrigerator.

Apr 19, 2019
@Eikka makes a good point. The changes of phase absorb additional heat beyond that needed to merely heat the mass; this is called "latent heat." That this proposed device doesn't include this makes it questionable.

You'd need a lot of heat density change to make up for the lack of latent heat.

What happens in a refrigerator is that the substrate is compressed, then cooled, then allowed to collect both mass heat and latent heat from its surroundings in the presence of the cold box, then allowed to radiate to the environment, then compressed and cooled again. It's a heat pump.

Modern refrigerators are of course more complex, directing drafts of air to various portions of the cold box and creating two different cold boxes at different temperatures, but the basic theory of operation remains the same.

Apr 19, 2019
The other questionable claim is that modern refrigerators are "relatively inefficient".

If you take a device such as an air-to-air heat pump used for domestic heating, it may have a CoP of 6-7 over a temperature difference of 40 degrees. That is, when it's -20 C outside, it is still pumping in heat at +20 C and putting out 6-7 times the power that was used to run the pump. Alternatively, when you use it in a freezer, the same applies when it's -20 C inside the box and +20 in the room outside.

The Coefficient of Power is the reciprocal of the efficiency the same system would achieve when you run it as a heat engine instead of a heat pump (reverse operation). The Carnot efficiency over the same range is about 13%, the reciprocal of which is 7.7 which means these devices are already operating very close to the theoretical maximum (78 - 91% from ideal). The conventional refrigerators are in fact highly efficient, and doing any better is a very difficult task.

Apr 19, 2019
The explanation to the above is to understand the relationship between a heat pump and a heat engine.

A Carnot cycle is the best physically obtainable, ideal, heat cycle. Doing any better would be creating energy out of nothing. Logically, if you couple a Carnot heat pump to a Carnot heat engine, the temperature difference caused by the pump is enough to drive the engine with just enough power to run the pump, no more no less.

So if for a temperature difference of -20 to +20 C the Carnot efficiency is 13%, no refrigerator may pump more heat than 7.6923... times the input energy, because in turn, the Carnot heat engine that runs at the output would transform that flow of heat back into mechanical power at 13% efficiency to run the pump.

If you multiply the Carnot efficiency over the temperature difference with the coefficient of power of the heat pump, and arrive at a number greater than 1, you've invented free energy - a perpetual motion engine.

Apr 19, 2019
Incidentally, if you want to save on heating in the winter, you should take buckets of water from the tap and put them in your freezer. Once frozen, chuck the cylinders of ice out the door and repeat the procedure.

Why? Because the fridge is a heat pump, and the water is drawing heat from the ground. In effect, you'd be operating a ground-source heat pump because the water that comes into the house is at the average temperature of the pipeline - something like +12 - 15 degrees, and freezing it releases all that energy, plus the latent heat required to freeze the ice.

The latent heat is actually quite a lot. For a bucket of water you get on the order of 1 kWh of heat out by freezing it, and you only need around 0.1 kWh of electricity to extract it. You'd save 90% on your heating bills, provided you keep freezing enough water. By the end of January, you could build a whole ice castle out of it.

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