Having a cold drink or enjoying an environment at a comfortable temperature on a hot summer day seem like almost banal activities. More essential needs are taken for granted, such as the transport and storage of perishable goods and the maintenance of suitable conditions in inhabited spaces, industry and the information technology sector.
However, all these features are first-world luxuries that require refrigeration technologies at a high environmental cost, particularly due to their mass use and continuous growth.
Today, it is estimated that there are 3.6 billion air conditioning units installed worldwide, a number that will increase four-fold in the next thirty years due to the growth of middle classes in developing countries.
Artificial cold already represents 20% of the total energy consumed in buildings, both in dwellings and in commercial establishments, so that any improvement in the efficiency of refrigeration devices is highly desirable.
In addition, more extensive use of refrigeration technology involves another problem of great magnitude due to the use of fluids that are extremely harmful for the environment. It has been over two decades since chlorofluorocarbons were banned due to their capacity to damage the ozone layer. However, the compounds that are used currently, hydrochlorofluorocarbons and hydrofluorocarbons, have a potential greenhouse effect up to a thousand times greater than carbon dioxide.
It is estimated that in 2050 the accidental but frequent release of these gases will represent almost 10% of total CO₂ equivalent emissions, which is why the European Union is planning their gradual elimination over the next decade. Clearly, then, the fight against climate change requires the development of more efficient technologies that are more environmentally friendly.
Alternatives to current methods
The most commonly used refrigeration technology today is based on a cycle of compression and expansion of a fluid, during which it undergoes a process of condensation and evaporation. This method can be used to control the heat exchange associated with the transformation of liquid to gas through the external work of compression. It was conceived and developed during the nineteenth century due to the formulation of thermodynamics. Its success lies in the fact that the heat of evaporation is high and therefore the refrigerating effect is also great.
However, due to the disadvantages described above, potential alternatives have been researched recently. In a 2014 report by the US Department of Energy to identify options, elastocaloric and magnetocaloric methods were proposed as two of the most promising concepts. These are solid-state caloric methods, which avoid the use of harmful fluids and are more efficient.
In a way that is analogous to gas compressors, these techniques consist of controlling the heat exchange associated with transformations that occur in the solid state by the application of an external field. The nature of this field will be determined by the physical magnitude of a change during the transformation and will determine the name of the corresponding caloric effects.
Thus, magnetocaloric, electrocaloric, elastocaloric and barocaloric effects refer to those obtained by the application of a magnetic, electric, force or hydrostatic pressure field, respectively. The effects can be obtained in transformations that involve changes in magnetisation, polarisation, deformation and volume. Currently, there are prototypes based on these magnetocaloric, electrocaloric and elastocaloric effects, but they are still far from potential commercial implementation.
Research into barocaloric effects is more recent and prototypes have not been developed yet. However, significant advances have been made in this field.
The special features of solids
As with gas compressors, the technique itself is as important as the solid material used in the cycle. This has led to the search for optimum materials for this purpose. One of the main, most widespread problems of these materials is that in transformations in solids considerably less energy is exchanged than in transformations involving liquid and gas, so the refrigerating capacity of solids is far less than that of current fluids.
However, a family of materials known as plastic crystals has been identified recently that, in some cases, have heat transformation comparable to that of evaporation. The energy mainly comes from changes in the molecular arrangement produced when pressure is applied or an electric or magnetic field. Given that the transformation of the resulting material also leads to a substantial change in volume, it can be controlled by application of pressure and therefore leads to colossal barocaloric effects.
Finding an alternative to the current gas compressors continues to be a scientific and technological challenge of the first order. However, the environmental needs mean research must continue in this area. The recent observations of vast barocaloric effects in plastic crystals represents a great quantitative leap forward that could accelerate the development of refrigeration technologies based on the solid state.
Josep Lluís Tamarit, Researcher in the Materials Characterisation Group (GCM), Universitat Politècnica de Catalunya – BarcelonaTech and Pol Lloveras, Researcher in the Department of Physics (FIS), Barcelona East School of Engineering, Universitat Politècnica de Catalunya – BarcelonaTech.