Rebecca Douglas explains how refrigeration works, and what new technologies are on their way to help improve and refine the process.
These days we take refrigeration for granted. Whether it’s a nice cool glass of beer from your fridge after a long day or simply the ability to store perishable foods, refrigeration is a technology most of us couldn’t do without.
The technology is simple: For a liquid to evaporate it must absorb some heat from its surroundings.
So, for example, if you drop a little rubbing alcohol on your hand, you’ll quickly feel the cold as the alcohol evaporates, taking heat from your skin to do so.
Traditional refrigerators use a liquid “refrigerant” which functions in the same way as the alcohol does to cool your skin. The difference is that the refrigerant is kept inside a series of coils. Cooling works like this:
1. The compressor in the refrigerator compresses the gaseous refrigerant; the refrigerant heats up as its compressed.
2. The coils on the back of the fridge let in the now warm, high pressure refrigerant and it starts to dissipate its heat.
3. The high pressure refrigerant flows through a small hole. On one side of the hole is all the high pressure refrigerant and on the other side the pressure is much lower because the compressor is sucking all the refrigerant back out
4. When the refrigerant enters this lower pressure area it boils, taking heat from the inside of the fridge to do so, cooling your food.
5. The cold refrigerant gas is sucked up by the compressor and the cycle starts again.
Originally people used ammonia as a refrigerant but ammonia is toxic so leaks are dangerous. We moved on to chemicals known as “chlorofluorocarbons” or CFC’s. They’re not as toxic to humans but they do destroy ozone, so we stopped using those too. Now we use stuff that isn’t toxic or as immediately damaging to the environment. To do this we needed new technology – we needed to exploit the “magnetocaloric effect.”
This is safer, more compact, more efficient and more environmentally friendly because it avoids the use of CFCs. It works similarly to traditional refrigerators but it uses magnets, and carefully chosen materials. This time the process goes like this:
1. The carefully chosen material is placed in an isolated environment and the magnet is turned on. This makes all the atoms in the material align and this warms the material up.
2. The magnet is kept on while a liquid is passed over it, absorbing its heat – this liquid is then separated from the material so the heat can’t flow back.
3. The magnetic field is reduced and all the aligned atoms start to misalign, they require heat energy to do this and this heat energy is taken from the inside of the fridge.
4. The magnetic field is increased and the cycle repeats.
This won’t work with all materials though – you have to use a magnetic solid. Some magnetic solids work better than others.
For a while gadolinium, a silvery-white rare earth metal, was the best available choice. It has many practical applications, from shielding in nuclear reactors to contrast agents for MRIs. It was quickly discovered that certain gadolinium alloys work even better for the magnetocaloric effect than pure gadolinium does. Other rare earth alloys can also produce the same effect with great efficiency.
However, much like ammonia and CFCs, the use of rare earth metals comes at a cost. Toxic acids are needed in the refining processes and badly managed mines and refineries are prone to releasing toxic waste into the water supply.
That isn’t the only concern. Whilst not all rare earth metals are technically “rare” the supply isn’t limitless. There are more frivolous uses than refrigeration; rare earth metals are used extensively in tablets and games consoles. On the other hand, perhaps shielding for nuclear reactors is probably more important than ultra-efficient cold beer and long-lasting deli meats.
Reducing the use of rare earth metals is therefore urgent. Researchers at the Rochester Institute of Technology (RIT) have been investigating “high entropy alloys” – metals combining four or five different elements. Several alloys have been developed with magnetocaloric properties without the need for a large supply of rare earth metals. Although they say in their paper that only a small range of possibilities have been explored, Casey Miller, head of materials science and engineering at RIT, has said that they have already “identified hundreds of new alloy combinations that could be useful.”
It’s good to know that the future of cold beer is safe and that we won’t have to sacrifice our environment (or our games consoles) to get it.