Solid state cooling - the discovery of the Multicaloric Effect

Today’s modern refrigeration technology is based almost entirely on a compression / expansion cooling cycle. This is rather old technology and, despite major advances, still energy inefficient, achieving Carnot efficiency of <60%. Excluding commercial and industrial refrigerators, there are currently 1.4 billion household refrigeration units all round the world. The average annual consumption of all these cold appliances amounts to about 453 kWh, resulting in annual electricity consumption of 634 TWh. In fact, refrigeration accounts for about 14% of total domestic energy consumption causing worldwide annual greenhouse gas emissions of 450 million tons of CO2eq [www.bigEE.net]. Moreover, the compression / expansion cooling technology works with refrigerants that are usually hazardous chemicals, ozone depleting and greenhouse gases. The use of vapour compressors limits how small a practical, cost effective refrigerant unit can be manufactured, which makes their integration into silicon chips impossible. Other limitations in terms of operation temperature make compression / expansion technology typically unsuitable for ultra low (mK - K) cryogenic cooling, such as cooling of detectors for space applications or other specialized ultra low temperature metrologies. Hence, a number of major disadvantages are driving the market away from this technology.
Solid-state cooling offers an elegant solution to all these issues. It is clean and energy efficient technology that utilizes the adiabatic relaxation of an order parameter in solids (electric, magnetic or elastic) to produce a temperature reduction of the solid refrigerant. Carnot efficiency approaching 75% has been demonstrated, with additional potential energy saving of up to 25%.
Most common solid-state cooling technologies to date are based on the electro-caloric or magneto-caloric effect. Magneto-caloric materials have already made it into industrial applications being traditionally used for space cooling. Cooling temperatures down to 10 - 40 mK are easily achievable using Cerium Magnesium Nitrate (Ce2Mg3(NO3)12∙24H2O paramagnetic salts, and temperatures as low as 22 K have been reported in PrNi5 paramagnetic intermetallic alloys. Gadolinium based compounds that allow tuning of Curie point temperatures to near room temperature, have facilitated the introduction of room temperature solid-state cooling based on magneto-caloric materials. Good magneto-caloric materials have 6 - 13 K adiabatic temperature lift and 100 - 200 mJ/cm3∙K entropy change. However, to achieve such performance they require large applied magnetic fields (50 - 100 kOe). To meet capacity requirements without superconducting magnets, a large amount of magnetocaloric material must be pushed quickly through a relatively small magnetic field generated by permanent magnets, typically around 10 – 15 kOe. Therefore, magneto-caloric cooling can only be competitive with vapor compression if new magnets are developed to provide large magnetic fields at reasonable cost, or if new caloric materials and effects are discovered.
Recent developments in solid-state caloric effects led to the realization that traditional solid-state cooling via electro-caloric, magneto-caloric or elasto-caloric materials could be further improved when cross-coupling effects between the ferroic phases within these materials stimulate large entropy changes. Dr Vopson at the University of Portsmouth has discovered a novel solid state caloric effect, The Multicaloric Effect, which shows promising advantages against all competitor technologies and has the potential to replace all existing refrigeration technologies in future.

1) The theoretical discovery of the Multicaloric Effect.
2) The proposal of a multicaloric effect in bi-layer multiferroic composites.
3) The publication of the generalized theory of giant caloric effects in solids.
4) The thermodynamic solution of a induced field paradox in multiferroics.