The barocaloric effect: a new opportunity for sustainability in refrigeration?

The barocaloric effect is currently one of the most studied thermodynamic phenomena in the alternative cooling technologies sector.  In a global context characterised by growing demand for air conditioning and refrigeration, the quest for more efficient solutions with less environmental impact has become a strategic priority.

This is supported by a recent discovery, (published in the journal Nature), by a team led by Professor Bing Li of the Institute of Metal Research at the prestigious Chinese Academy of Sciences (top accademic institute in the People’s Republic of China and leading global research organisation).

The study describes a new mechanism defined as the ‘barocaloric dissolution effect’ observed in acqueous solutions of ammonium thiocyanate (NH₄SCN).  This innovative breakthrough could mark a turning point for low emission refrigeration, thanks to the combination of high thermal performance and functional simplicity.

What is the barocaloric effect?

To better understand the significance of this discovery, it is important to start from the rule of physics which forms the basis of the barocaloric effect.  It is a process by which a material undergoes a variation in temperature when exposed to a change in pressure.  In almost adiabatic conditions, this rise or fall in pressure causes an entropic variation, resulting in the heating or cooling of the system.

This process belongs to the wider family of caloric effects, including those induced by magnetic or electric fields, or by mechanical pressure.  The peculiarity of the barocaloric contribution lies in the fact that the pressure (at a relatively easy level to apply and control in an industrial setting), can generate significant thermal variations, without the use of traditional, high global warming potential refrigerant fluids.

Historically, most studies have focused on solid materials which present structural phase transitions.  In these cases, the variation in temperture is associated with crystalline reorganisation, induced by compression.  Nevertheless, these materials also present some limits regarding heat transfer and the mechanical complexity of the systems required to apply the pressure.

The new barocaloric dissolution effect

The research carried out by the Chinese group introduces a different model.  Instead of being based on solid to solid transitions, the system exploits the balance between the crystallisation and dissolution of a salt in an acqueous solution.

The mechanism is as effective as it is simple.  When pressure is increased, the ammonium thyocianate tends to crystalise.  This process is exothermic and causes the release of heat towards the outside.  In a subsequent phase, when pressure is reduced, the salt dissolves rapidly in the solution.  The dissolution is endothermic and absorbs heat from the surrounding environment, generating a marked drop in temperature.

The cycle of crystallisation and dissolution is therefore controlled entirely by the level of pressure and is at the heart of the new barocaloric effect.  Unlike traditional solid materials, in this case the fluid simultaneously takes on the role of active material as well as the vehicle to convey heat.  This elimates many of the inefficiencies typical of systems based on solid elements, where thermal transfer often results in a bottleneck situation.

One of the most surprising effects to emerge from the study involes the range of variation in obtainable temperatures.  At room temperature, the NH₄SCN solution is capable of reducing its own thermal condition by around 26.8–30 °C in just 20 seconds during the depressurisation phase (a higher level than that recorded in several solid barocaloric materials analysed in related literature).

From an energy point of view, simulations of the cycle show that a single interaction can absorb up to 67 Joules of heat per gramme of solution.  Even more significant is the data relating to second-law efficiency, which reaches 77%.  This parameter measures how closely the real cycle matches ideal thermodynamic behaviour, providing a solid indication of the system’s potential for use.

These results sugggest that the barocaloric dissolution effect should not be considered just a curious scientific fact, but a technology with genuine potential for use.

Comparison with steam compression refrigeration

Conventional refrigeration is dominated by the steam compression cycle, an extremely widespread technology which has been finely tuned in over a century since it was first developed.  Nevertheless, it is based on refrigerant fluids which, despite their evolution towards less impactful formulations, are still associated with significant environmental issues.

According to the report ’Global Cooling Watch 2025’ released by the United Nations Environment Programme, global demand for cooling systems could triple by 2050 compared to 2022 levels, with a potential doubling of emissions linked to air conditioning.  In this scenario, solutions based on the barocaloric effect could make a real contribution to reducing the sector’s carbon footprint.

A system based on the pressure variation of a liquid solution, without the use of high GWP fluorinated gases, could offer an attractive alternative.  Furthermore, the combination of an active material and a heat transfer fluid enables simpler architectures to be used compared to systems based on solids, with potential advantages in terms of compactness and maintenance.

Structure of the operative cycle and possible applications

The proposed refrigeration cycle is divided into four main phases, which follow traditional thermodynamic cycles very closely.  In the first phase, pressurisation causes crystallisation of the salt and the release of heat.  Subsequently, this heat is dissipated towards the external environment.  In the third phase, depressurisation starts the rapid dissolution of the solute, resulting in heat absorption.  Finally, the refrigeration capacity generated is used to cool the room or desired process.  The rapidity with which the phenomena of crystalisation and dissolution take place is a key factor in ensuring fast cycles and high refrigeration capacity.

Consequently, there are many potential applications of the barocaloric dissolution effect.  In data centres (where heat management accounts for a significant proportion of overall energy consumption), such high efficiency technology could considerably reduce operating costs and associated emissions.  Similarly, in energy hungry industrial processes, the adoption of alternative cooling systems could contribute to improving environmental performance.

In HVAC for use in large commercial buildings or key infrastructures too, the opportunity to achieve large variations in temperature by using moderate levels of pressure offers interesting prospects for the design of innovative systems.