A cool new idea from British scientists: the magnetic fridge

The solid-state fridge could halve your energy bills and help to save the environment

Do you like fridge magnets? A new technology being developed by British scientists could put an even bigger one inside your refrigerator. Once Karl Sandeman, a physicist at Cambridge University, has helped resolve the practical issues, the cooling power of the 21st century fridge will come from a 19th century discovery - and it promises to cut energy consumption by 40% and save the ozone layer.

The key is a material that cools when it is put in a magnetic field. The idea - which is ambitious, but feasible - is to replace the present system used by refrigerators the world over. Your kitchen fridge has a compressor, which turns a gas into a liquid, releasing heat (which you'll feel at the back of the fridge). The liquid is then pumped round the inside walls of the fridge, where it draws heat from the contents; that turns it into a gas, which is pumped on to the compressor.

But what if you could replace the fluid with a magnet? "The amazing thing about magnetism is that it's actually a quantum mechanical phenomenon," says Sandeman. "It's all down to something mysterious called spin. The electrons act almost like a miniature bar magnet."

Temperature change

As a quantum mechanical property of the electron, spin is usually taken to mean its rotational momentum (like the Earth rotating around its axis). That momentum - described as "up" or "down" - creates a tiny magnetic field. When all the electrons in a material spin in the same way, their fields combine to create what we perceive as magnetism. However, an iron magnet heated to 700C will "disorder" and lose its powers, known as a magnetic phase transition.

In 1881, the German physicist Emil Warburg put a block of iron into a strong magnetic field and found it increased very slightly in temperature. Scientists now know the electrons pivot in the field to align at a lower energy state, releasing surplus energy. The metal warms up in what's known as the magnetocaloric effect, which is greatest near the magnetic phase transition temperature.

"If you can suddenly alter the degree of ordering of all these little spins, then you get a large response," says Sandeman. For iron at room temperature, the response is just 0.1C. Some materials cool in a magnetic field, a property that's used in low temperature research. Finding the right room temperature material is the key to a magnetic fridge, where the cooling power is derived from a positive magnetocaloric effect coupled to heat exchange.

One material works nicely: the element gadolinium (Gd). It's a silvery-white metal that's strongly attracted by a magnet, has a magnetic disordering temperature of 20C, and a giant magnetocaloric effect of several degrees. A waste product from permanent magnet manufacture, gadolinium costs around £100 per kg; a magnetic fridge would use 0.15kg. Sandeman's current research, however, is looking at other possibilities.

"The quest is to get away from these expensive rare earth materials and look for magnetic materials which have a phase transition at room temperature," says Sandeman, whose research job at Cambridge University is funded by the Royal Society. He also works with Professor Derek Fray, a leading expert in materials chemistry. "What I'm actually working with is an alloy of two magnetic materials, cobalt and manganese," says Sandeman.

When these elements are mixed with non-magnetic "spacers" like silicon, the cost falls to £5 per kg. Strangely, his latest experimental alloy has a negative magnetocaloric effect - it cools in a magnetic field. This could also be harnessed for fridges through a heat exchange process.

A Cambridge University spin-out company, Camfridge Ltd, has built two prototype magnetic fridges that use gadolinium. While the latest one is little more than fridge innards, the team - which includes Sandeman as chief scientific officer, Fray and experienced business people - is striving to develop the revolutionary effect for commercial exploitation in fridges and, perhaps, air conditioning.

"It's a sea change in thinking," says Sandeman. "It never ceases to amaze me how you can take a block of this stuff and stick it into a [magnetic] field. The prototype is operational and has achieved a large temperature span."

A magnetic fridge works like this. Powdered gadolinium (with coarse grains for good heat transfer qualities) is put into a magnetic field. It heats up as the randomly ordered magnetic moments - the electrons with spin - are aligned, or "ordered", by the field. The newly-acquired heat - a boost of between 2-5C, depending on the gadolinium's original temperature - is removed by a circulating fluid, like a conventional fridge.

The magnetic field is removed and the gadolinium cools below its starting temperature as the electrons resume their previously disordered state. Heat from the system to be cooled - your fridge interior - can then be transferred to the now cooler metal. Then all you do is endlessly repeat. But unlike conventional fridges, which need very toxic chemicals, the only liquid needed for heat transfer is water, alcohol or, more likely, antifreeze.

Cutting energy use

A more advanced prototype next year will optimally bring together three elements - temperature span, cooling power and efficiency - along with a faster motor. This will allow less gadolinium to be used with a smaller magnet, saving materials costs. Camfridge's managing director, Neil Wilson, says: "In terms of technical specification, that prototype will get us to a domestic fridge. Commercial manufacturers have hit the wall; there is not much more they can do. We're wanting to cut the energy use by half."

Professor Stephen Blundell, of Oxford University, also understands the issues well, as he's written a textbook on magnetism and researches magnetic properties in materials. Magnetocaloric effects are becoming more practical, he thinks, thanks to improved magnet technology and new materials. A magnetic fridge would be compact, less noisy and won't need harmful gases.

"I think this technology has real potential, but it is still at the early stages. The claims of 40% efficiency savings seem a little speculative, though not completely unreasonable," says Blundell.

Some 15% of UK energy is used in refrigeration and cooling for air conditioning, and much more in warmer countries. Garry Staunton, head of low carbon research at the Carbon Trust, which is financially supporting the magnetic fridge's development along with Cambridge University and other investors, says that 22m tonnes of UK carbon dioxide emissions annually are due to refrigeration and air conditioning. Efficiency improvements to domestic fridges since 1990 have seen a 27% reduction in their energy use.

Increasing energy efficiency with new technology is the key to stabilising and reducing carbon emissions. Consumers seem willing to support and demand new energy-efficient appliances in their homes, while everyone has felt the sharply rising cost of electricity. As the fridge magnet moves inside the fridge, it may become the exciting new green technology of the 21st century.
