Pressure and pull to cool your food products

Specific materials become colder or hotter depending on whether they are pressed together or stretched apart. Now scientists plan to use the materials to build refrigerators and air conditioning that…

Specific materials become colder or hotter depending on whether they are pressed together or stretched apart. Now scientists plan to use the materials to build refrigerators and air conditioning that is good for the climate.

That’s why you should read the article

Special materials can generate cold or heat with minimal electrical consumption.

Refrigerators, heat pumps and air conditioners are examples of appliances that this technology can improve.

Take a rubber band between your fingers and pull it out as far as you can.

If you do this fast enough, the elastic will become hotter than it was when it was in your palm. When you stop stretching it, it cools back down to its original temperature.

Stretching is an example of the so-called elastocaloric effect that science is currently working with. New types of materials are subjected to mechanical action – for example by being pressed together or pulled apart – and can thus change the temperature by up to 30°C.

In recent years, scientists from China, the USA and Spain have managed to create a temperature difference of 31.5° in a chemical mixture composed of nickel and manganese.

If physicists manage to increase the temperature difference even more and solve some technical hurdles, this elastocaloric material can be used to build green and climate-friendly refrigerators, heat pumps and air conditioning systems.

Rubber molecules lengthwise

The heating and cooling of the elastic that was described in the example above may seem strange, but there are well-researched physical laws from the world of thermodynamics that underlie these temperature fluctuations.

By being pressed together or pulled apart, the material can change the temperature by up to 30°C.

Thermodynamics is one part of physics that, among other things, describes how heat, pressure and energy act on and interact with each other in various matters.

One of the pioneers of thermodynamics was the French physicist Nicolas Léonard Sadi Carnot (1796-1832), who, among other things, used thermodynamics to come up with a theory about the theoretically most efficient motor. At the time, Carnot’s discoveries were used to improve the performance of steam engines.

In an engine, energy in the form of heat is converted into mechanical energy in a piston that uses axles to turn wheels and get the train moving.

If we return to the example of the rubber band that was stretched, the opposite happens. Then it is the mechanical movement of the rubber that generates heat.

The explanation for this can be found in the phenomenon of chaos, which tells how much chaos exists in the molecules of matter.

The second law of thermodynamics says that the disorder in, for example, a glass of water will always naturally stabilize or increase. The system thus receives more chaos than it was in the beginning if its state changes – like melting ice.

Pressure and heat change shape

Chaos is a term from thermodynamics and refers to the state of matter. Pressure, temperature and volume are crucial to the situation.

  • Melting ice adds to the mess

Ice cream melting in a glass or sugar dissolving in a cup of coffee are examples of chaos. The glass with the ice will release cold (mess increases) until the glass reaches room temperature when the ice is melted.

  • Molecules fall into disarray

The ice and the sugar keep from order to disorder and from molecules in an orderly structure to disrupted chaotic forms. The process is also known as the second law of thermodynamics.

If we think back to the elastic example, the turbulence will decrease and the elastic will cool as it is stretched. But it also means that the chaos increases in a corresponding way elsewhere in the system to preserve the total level of chaos. The second part of the stretch will be hotter.

Metals change crystal form

But what does this example have to do with the refrigerators of the future? Physicists certainly do not use elastics in the laboratory when working to develop efficient and reliable elastocaloric materials. Here, instead, so-called Shape Memory Alloys are leading the way.

These are mixtures of different metals, for example nickel, manganese and titanium, but when these alloys are subjected to high forces in the form of mechanical pressure or torque, they can change temperature due to the elastocaloric effect.

Danmarks Tekniske Universitet (DTU) has developed a heat pump in which elastocaloric materials alternately release heat (the tube at the bottom right) and cold (the tube at the top right).

The name “shape memory alloys” comes from the fact that the alloys can “remember” the original state of their molecules when the pressure or torque disappears.

In practice, the metals change the form of crystals when they are, for example, subjected to pressure. Physicists call this moving from an austenitic morphology, where the molecules are in a cubic shape, to a martensitic morphology, where the shape is like a cylinder or a diamond.

The change from one state to another causes the alloys to heat up, but a gaseous liquid circulating in the system takes the heat and carries it forward.

The next step is to remove the pressure from the alloys, which cools them down again – but has now become colder compared to their original state, and this process can be used, for example, to cool our food products.

Technology could improve refrigerators

In normal refrigerators, pressure pumps greenhouse gases around in a cycle. Along the way, it changes from a liquid to a gas, and that process creates cold.

In today’s refrigerators, this process has been greatly improved, but there is still a long way to go to reach maximum theoretical efficiency. Currently, the process yields about a fifth of what is maximally possible in refrigerators.

Chemical press cold in the refrigerator

When elastocaloric material is compressed and released again, it creates a cooling circuit that can replace the equipment currently in refrigerators.

1. Pressure heats matter

A piston presses against an elastocaloric metal from the outside. The pressure creates chaos in the structure of the material and causes it to change its shape, which creates heat in the chamber. The heat causes the gaseous liquid surrounding the material to expand.

2. The fluid is forwarded

A one-way valve opens to the right in the pressure chamber so that the liquid can float forward. The valve ensures that the liquid flows in one direction only. Thereby, the expanded liquid floats forward in the system in order to equalize the pressure.

3. The material cools again

The piston now pulls back out and the chamber cools down. The pressure decreases and thus the elastocaloric material in the chamber also cools. The cell remains cold because the molecules return to their previous shape and disorder is reduced.

4. Cold shoots out

The pressure falls to a lower level than in the previous chamber. Now the single-flow valve on the left of the system opens and the cold liquid is now able to cool the chamber again, but then the process starts again from the beginning.

With elastocaloric materials, experts hope to be able to improve the efficiency of cooling in refrigerators by 10 – 20% compared to the best refrigerators currently on the market.

This can make a significant difference to electricity consumption in the world, as refrigerators, air conditioning systems and heat pumps take up to 25 – 30% of the combined electricity consumption of people.

In Europe, about 70% of all food products are now chilled or frozen, and elsewhere in the world, where refrigerators and freezers are much older, the improvement can be even greater.

Another advantage of these elastocaloric materials is that there is no need for greenhouse gases in the refrigeration cycle, and in addition, elastocaloric refrigerators will be almost silent and completely vibration free.

But scientists still need to clear some technical hurdles before we can order an elastocaloric refrigerator online.

Temperature difference over 35 degrees

In previous prototypes from, for example, the Danish company DDU, the materials could only withstand 6,000 – 7,000 cycles in the cycle, where the material is subjected to high pressure.

The alloy has since been improved so that it can withstand about 100,000 cycles.

Elastocaloric refrigerators will be almost silent and completely vibration-free, just like today’s refrigerators.

Based on this, scientists believe that it will be no problem to reach up to one million or even ten million cycles, and these are the values that the materials must be able to withstand in order to be usable in the real world.

Another challenge is the temperature difference – called delta T by scientists – where the record currently stands at 31.5 degrees.

The temperature difference must ideally exceed 35 degrees to be able to use the equipment in refrigerators.

Other researchers are also working with other possible solutions such as so-called barocaloric materials where it is not mechanical pressure but changes in air pressure that create temperature differences.

A team of scientists from Spain, France and Great Britain has already managed to turn chaos into matter to such an extent that it can be compared to the efficiency of current refrigerators.

In this way, people hope that the refrigerators of the future will not have harmful greenhouse gases, but will have compressed and disassembled metal alloys that will make the cooling of the food much more environmentally friendly.

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