Neil Wilson, Camfridge
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from the show Chilling Out - The Science of Cryogenics
Neil - The basic challenge with domestic refrigeration is trying to get more and more efficient appliances. An average domestic fridge at home is probably about 10% efficient and the European Union has introduced a new set of legislation, essentially asking fridge manufacturers to go from 10% to 20%. The challenge they have is that they can do this, but they need to do a lot of work in changing the insulation. They need to use vacuum panels, which are fragile and expensive, and also require them to redesign the appliance. Manufacturers are keen not to do this and so they're looking to our technology to allow them to introduce the highest efficiency devices without having to change the way they make and construct an appliance.
Ben - So, your device does the job of the compressor that we would normally see in the back of a fridge?
Neil - Yes. If you look at the back of your fridge, you'll see a big metal black box, and that's the gas compressor. The engine inside compresses the gas which liquefies, and that then gets pumped around, where the gas can evaporate. When it evaporates, it absorbs heat and cools down the milk. At the other side of the cycle, the gas recondenses into liquid and emits heat. So at the back of your fridge, you feel heat coming out – that's the gas recondensing, and inside your fridge of course is cold and that's where the liquid is evaporating.
Ben - How are you seeking to replace it or improve on it?
Neil - We’re using a completely different approach to creating a cooling cycle using magnets and special metal alloys. In some sense conceptually, it is a very similar sort of process. In the gas compressor, you rely on the liquid gas transition. In the magnetic solution, we’re relying on a very similar change from a ferromagnetic phase where the electrons inside this metal are all nicely organised and aligned. In that state it’s attracted to magnets, and by changing the magnetic field, you can make it switch to a paramagnetic phase where the electrons are completely disordered, and no longer attracted to external magnetic fields.
Ben - So how does a change in the magnetic field or the magnetic structure of a metal lead to a change in temperature?
Neil - We’re using special materials called magnetocaloric alloys, and these are materials that change temperature when exposed to a magnetic field, and that forms the basis for magnetic cooling. Now this effect has been known about for some time, in fact, I think it was discovered in the late 19th century in iron at several hundred degrees Celsius. All what we’re really doing is using this effect, but using it at room temperature in order to exploit the temperature change.
Ben - So what actually is that metal? I can see you have a piece, roughly a square centimetre of it with you here. What’s it made of?
Neil - This particular alloy is 95% iron, but it’s doped with lanthanum, silicon, and cobalt. This has been designed to have a Curie temperature around room temperature. What I mean by that is when the material is below its Curie temperature, it’s attracted to this magnet. As you can see, it’s sticking onto the magnet quite happily, but when the material goes above its Curie temperature, it ceases to become magnetic. If I turn on the fan here and just heat it...
Ben - It almost immediately falls off!
Neil - Yes, as soon as it was pushed above its Curie temperature, it fell off the magnet, and it’s these magnetic properties that we’re exploiting in a magnetic cooling engine and combining that with the magnetocaloric effect, the actual temperature change. You can actually create four sides of a refrigeration cycle and it’s those four process – the temperature change, and then the way the material changes its properties when exposed to heat - that you can use to actually pump heat from cold to hot or from hot to cold.
Ben - So how would we then integrate this into the existing design of a fridge?
Neil - In an existing fridge, you have the gas compressor, and the gas compressor absorbs heat from the interior of the fridge and it also emits heat from a hot exchanger at the back of the fridge. In a magnetic cooling system, it’s somewhat different because our refrigerant isn't a gas or a liquid that's pumped around, our refrigerant is solid and so it sits inside our device. So in order to couple the refrigerant to the hot and cold exchanger, we use a liquid - but that liquid is nothing special, it’s basically water. So in fact, with our magnetic solution, not only have we got rid of the often toxic gases that are used in gas compressors, we have a solid so it cannot leak and at the same time the liquid that we’re using to move the heat around is a safe, nontoxic fluid like water.
Ben - So in terms of efficiency, you said the aim with this is to allow a new generation of extremely efficient fridges. How does it compare to what we’ve already got? How much bang for your buck do you get?
Neil - Roughly speaking, if you take a standard A-plus fridge using a gas compressor, and replace that gas compressor with our magnetic engine, you will double the efficiency of your fridge without having to make any other changes to the appliance. So it’s a factor of two improvement.
Ben - What else can we refine to try and make this even better?
Neil - Well, the first area is we can improve the refrigerant materials. The lanthanum, iron, silicon, cobalt I talked about earlier, that's a very nice compound. However, down the road in the next 12 months, there will be a more powerful version of this material, and that will allow us to make the magnetic field smaller, so the device can be even smaller, even lighter, and cost less. The second thing that is key is to be able to run the machine faster. If you run the machine faster, you either create more cooling power or, again, require less material or less magnet. Those two factors combined will allow us to either make the solution for the domestic fridge that's expensive and increasingly competitive or alternatively, it will allows us to make bigger versions in terms of cooling power for the technology that might be applicable for supermarkets or for car air conditioning.