Magnetic cool

Researchers at Imperial College London are in the process of developing environmentally friendly magnetic refrigerators and air conditioning systems.

The academic team claim magnetic refrigeration technology could provide a ‘green’ alternative to traditional energy-guzzling gas-compression fridges and air conditioners.

It is predicted they would require 20 to 30 per cent less energy to run than the best systems currently available, and would not rely on ozone-depleting chemicals or greenhouse gases.

Refrigeration and air conditioning units are believed to make a major contribution to the planet’s energy consumption.

In the summer months in the US it is estimated they account for approximately 50 per cent of the country’s energy use.

A magnetic refrigeration system works by applying a magnetic field to a magnetic material, such as a metallic alloy, causing it to heat up.

This excess heat is removed from the system by water, cooling the material back down to its original temperature.

When the magnetic field is removed, the material cools down even further, and it is this cooling property that researchers hope to harness for a wide variety of cooling applications.

The technology, based on research funded by the EPSRC, has proved possible in the lab, but researchers are still looking for improved materials that provide highly efficient cooling at normal room temperatures, so that the technology can be rolled out from the lab to people’s homes and businesses.

The team believes the best material will exhibit dramatic heating and cooling when a magnetic field is applied and removed.

It must operate in normal everyday conditions, and not lose efficiency when the cooling cycle is repeated time after time.

The Imperial researchers recently discovered that a pattern of crystals inside different alloys - otherwise known as their microstructure - has a direct effect on how well they perform at the heart of a magnetic fridge.

The team claims this could help them in the future to custom-design the best material for the job.

Lesley Cohen, one of the researchers, explained that by using probes designed at Imperial, her team, led by James Moore, was able to analyse what happens to different materials on a microscopic level when they are magnetised and demagnetised.

This enabled them to pinpoint what makes some materials better candidates for a magnetic fridge system than others.

‘We found that the structure of crystals in different metals directly affects how dramatically they heat up and cool down when a magnetic field is applied and removed,’ said Cohen, from Imperial’s department of physics.

‘This is an exciting discovery, because it means we may one day be able to tailor-make a material from the bottom up, starting with the microstructure, so it ticks all the boxes required to run a magnetic fridge.

'This is vitally important, because finding a low-energy alternative to the fridges and air conditioning systems in our homes and work places is vital for cutting our carbon emissions and tackling climate change.’

This new research follows up work conducted by the same Imperial group.

With that research, the group used similar probing techniques to precisely measure the temperature changes that occur when different materials are removed from a magnetic field, and used this information to analyse the different ways they occur.

Lead scientist Kelly Morrison found that at the molecular level, two temperature change processes, known as first and second order changes, happen simultaneously in each material.

The team thinks that the extent to which each of these two processes features in a material also affects its cooling capabilities.

Cohen said this means that while the majority of research to perfect magnetic refrigeration worldwide has tended to involve analysing and testing large samples of materials, the key to finding a suitable material for everyday applications may lie in the smaller detail.

‘Our research illustrates the importance of understanding the microstructure of these materials and how they respond to magnetic fields on a microscopic level,’ she said.

The research was carried out in collaboration with the Ames Laboratory in the US.