Greener fridges

Modern refrigerants rely on hydrofluorocarbons, potent greenhouse gases. New legislation means alternatives are urgently sought. So, could solid refrigerants be the answer? Jasmin Fox-Skelly reports

Developed in the 1800s, refrigeration relies on a process called vapour compression to keep food items chilled. A cool gas circulates through coils inside the wall of the refrigerator. As it passes through the inside of the fridge, it absorbs heat from food items and travels to a compressor below, which squeezes the gas, turning it into a liquid. When this happens, the fluid gets very hot, and heat is released into the surrounding room. Next, the fluid is expanded so that it becomes a gas again. This causes the gas to get very cold, beginning the whole process again. 

While early refrigerants were flammable, toxic, and limited to industrial use, this changed with the advent of chlorofluorocarbons (CFCs). Initially considered the perfect refrigerants, CFCs evaporated and condensed at exactly the right temperatures, were unreactive, non-toxic and non-flammable. They enabled every family to have a fridge in their home for the first time.[1]

Unfortunately, however, the chemical stability of CFCs turned out to be their downfall. CFCs from abandoned fridges travel up as far as the stratosphere, where they are broken down into chlorine atoms. Chlorine atoms react with and destroy ozone, a protective zone around the Earth that absorbs harmful UV rays.

Adopted in 1987, the Montreal Protocol to phase out CFCs to protect the ozone layer, helped usher in a related class of chemicals to fill the gap.[2] Hydrofluorocarbons (HFCs) contain no chlorine and are much more reactive in air, so they never reach the stratosphere to deplete ozone. However, their molecular structure means they absorb infrared light, making them powerful greenhouse gases. HFC molecules absorb roughly 1000–3000 times as much infrared as carbon dioxide. In fact, a recent study showed that HFCs leaking from discarded fridges and air conditioning units are responsible for 4% of global greenhouse gas emissions – about twice as much as aviation.[3]

For this reason, a further amendment to the Montreal Protocol, known as the Kigali Amendment, was negotiated in 2016. It calls for an 85% reduction in HFCs by 2036 in developed countries.

So, what can we use instead? Alternatives to HFCs have existed for over a century. These ‘natural refrigerants’ include carbon dioxide, ammonia, water vapour, and hydrocarbons. But they are not without their downsides. Ammonia is extremely toxic and ignites easily. Hydrocarbons are also flammable, and this greater reactivity means plumbing must be more resistant to corrosion to prevent dangerous leakage of gases. These drawbacks have traditionally limited the use of natural refrigerants to industrial systems.

Hydrocarbons are used for refrigeration in oil refineries where the entire plant is designed to handle a flammable product. Carbon dioxide is used in some supermarkets, ice rinks, car air conditioning systems, and industrial freezers.

‘Natural refrigerants can be flammable, corrosive and/or toxic,’ says Xavier Moya, a materials scientist at the University of Cambridge, UK. ‘This doesn’t matter so much in commercial or industrial applications where you can keep the refrigerant outside your plant, but if you want to use it in your house then safety considerations are very important. In the last few years there have been a number of deaths due people inhaling refrigerant gases, so you need to be careful with what you use.’

Vapour compression technology is much more efficient than it used to be, but this increase in efficiency is plateauing. When you look at emissions associated with cooling, only one third of emissions are down to the refrigerant itself leaking into the atmosphere, while two thirds are due to the energy consumption needed to run the air conditioning system.
Xavier Moya a materials scientist at the University of Cambridge, UK

Hydrofluoroolefins (HFOs) are another option. HFOs contain unsaturated organic compounds of hydrogen, fluorine, and carbon. Because the compounds are unsaturated, they break down quickly in the atmosphere, making them safer for the environment as they neither trap heat nor attack the ozone layer. Originally developed by Honeywell and DuPont – two of the world’s largest refrigerant producers – examples include HFO-1234yf, which has a Global Warming Potential of less than one and is approved for certain uses in some chillers. HFO-1234yf is also used in most new car air-conditioning systems and is expected to be nearly universally used in the coming years in the EU, US, and Japan.

However, the problem with gases such as HFOs is that they are not particularly efficient. ‘With many refrigerants, as you reduce the global warming potential it comes at the cost of performance,’ says Moya. ‘If you lose performance then your refrigeration system becomes more inefficient, and then the CO2 emissions associated with energy consumption become more relevant.’

These emissions are already staggering. Together with air conditioners and heat pumps, fridges are estimated to consume between 25 and 30% of the world’s electricity. Much of this is down to the inefficiency of the vapour compression system. A lot of energy is needed to pump gases around the system, and as fridges need to be left on permanently, this adds up to a lot of electricity. For these reasons, many scientists believe that it is time to stop using gaseous refrigerants completely.

‘Vapour compression technology is much more efficient than it used to be, but this increase in efficiency is plateauing,’ says Moya. ‘When you look at emissions associated with cooling, only one third of emissions are down to the refrigerant itself leaking into the atmosphere, while two thirds are due to the energy consumption needed to run the air conditioning system.’

Plastic crystals

So, could we find a new, more efficient, technology? One option is solid state refrigerants. These would stay solid throughout the whole refrigeration process, so there would be no gas leakage, and hence no direct carbon emissions. In theory, cooling systems based on solid state refrigerants could also be more efficient than vapour compression systems because they can recover energy during their cooling cycles.

Barocal, a start-up company founded by Xavier Moya, is developing cooling systems that use solid refrigerants made from plastic crystals.[4] The crystals comprise cheap and readily available elements such as carbon, hydrogen and oxygen. The crystals rapidly switch between a disordered solid-state phase and an ordered solid–state phase depending on the pressure applied to them. When the crystals change phase, they either absorb or release heat. So as the crystals are squashed, they heat up, and when the pressure is relaxed, they cool down – cooling the environment around them as they do so. A cycle of compression could then be used to absorb heat from the inside of a fridge, before dumping the heat into the external surroundings.

‘Our material consists of small molecules sitting within a crystal lattice,’ says Moya. ‘At atmospheric pressures these molecules can rotate almost freely so they are highly disordered, but when you apply a pressure to them these rotations stop, so you go from a highly disordered phase to a highly ordered phase. This causes a big change in temperature.’

The principle is similar to the vapour compression system, where in the gas phase molecules can move around freely, while in the liquid phase they cannot. Like the vapour compression system, to create a fridge you would need to cycle between the two states. ‘When you have a transition from the low-pressure phase to high pressure phase the material gets hot, and when you drive the transition back, the material gets cold,’ explains Moya. ‘If you can go back and forth between the two different phases then you can get your cooling system up and running.’

Also developing plastic crystal refrigerants is Bing Li, a materials scientist at the Chinese Academy of Sciences’ Institute of Metal Research in Shenyang.[5] One type of plastic crystal, neopentyl glycol, is particularly promising. However, Li acknowledges that more work is needed to reduce heat loss and maximise the energy efficiency of the system if it is to compete with existing liquid-to-gas refrigerants.

‘We are still optimising the materials, making them more sensitive to pressure, more conductive to heat and more fatigue-resistant,’ says Li. ‘We also have to design a machine that makes the best use of the material, and we are currently designing this proof-of-principle machine in our lab.’

Magnetic fridges

Another idea is to use magnets to alternately heat up and cool a material. The magnetocaloric effect occurs when certain metals are exposed to a fluctuating magnetic field. When the magnetic field is increased, the magnetic moments – effectively, little magnets – within the metal align with the magnetic field. This creates a more ordered state, which causes the material to heat up. Then when the magnetic field is reduced, the moments within the metal become more disorientated and disordered – causing the material to rapidly cool down.

The magnetocaloric effect is strongest in materials containing the rare earth metal gadolinium, alloys of praseodymium and nickel, or lanthanum-iron-silicon alloys. However, these materials are extremely expensive, scarce, difficult to extract and in some cases harmful to the environment. Currently, the cost of a magnetic refrigerator is thought to be five to ten times more expensive than a standard fridge, limiting commercialisation. So far, the only company that has produced a magnetic fridge is Magnotherm, a spin-out of the Technical University of Darmstadt, Germany. The fridge, Polaris, is a magnetic beverage cooler that can cool as many as 150 beverages to 5°C.

This is where researchers at the University of Sheffield, UK, hope to help. Led by Nicola Morley, Professor of material physics, the team are hunting out new affordable materials that exhibit magnetocaloric properties – the key step needed to make widespread magnetic refrigeration a reality. Alloys under consideration include those containing combinations of iron, silicon, aluminium, chromium, nickel and copper. Compared with rare earth metals, these alloys are cheap and sustainable.

‘With magnetocalorics, you’ve got to think of cost. You could have something that’s amazing, but if its 20% more expensive than a bog-standard fridge then no one is going to buy it,’ says Morley.

To find the alloys that display the greatest magnetocaloric effect, the team is creating computer models of various alloys to see how they might behave. Those with the greatest potential are then synthesised for physical testing. The group is also deploying a technology called high-throughput combinatorial fabrication and characterisation to find new materials. This involves creating thin films of alloys and applying a magnetic field across their width. By using this method, it is possible to investigate many tens of compositions in a relatively short time, highlighting those worthy of further exploration. So far, the leading candidate contains a mixture of cobalt, iron, nickel, chromium and aluminium.

‘It’s okay but not brilliant because the temperature range it works in is just above room temperature, which isn’t ideal,’ says Morley. However, if they can find new materials that work and are safe, the efficiency savings from magnetic fridges would be huge. According to Morley, theoretically magnetic fridges could be up to 40% more efficient than conventional systems.

Although the field of solid-state refrigeration is still in its infancy, with much work needed to refine the raw materials and design whole new fridges to work with them, the benefits to the environment, once realised, are so large that it’s easy to see why scientists are so excited about the new technology.

‘With global warming increasing we really need to find a solution that is truly low carbon,’ says Moya. ‘We need to find a new technology that is more efficient and that doesn’t rely on gases, and that means solid state refrigerants.’

References
1 G. Constable, et al; A Century of Innovation: Twenty Engineering Achievements that Transformed our Lives. John Henry Press, 2003.
2 J.C. Farman, et al, Nature, 1985, 315, 207.
3 Pallav Purohit and Lena Höglund-Isaksson, Atmos. Chem. Phys., 2017, 17, 2795.
4 P. Lloveras, et al, Nature. Commun. 2019, 10, 1803
5 B. Li, et al, Nature; 2019, 567, 506.