Why do heating and cooling systems emit so much CO2? The answer lies in the outdated technologies that we continue to rely on. Gas boilers for heating, and air conditioners and fridges for cooling that run on vapour compression, have been around for over a century. These technologies are inefficient and problematic, and efforts to solve one issue often lead to new challenges.

Take CFCs, for instance: they were phased out after it became clear they were depleting the ozone layer. Their replacements – HCFCs and HFCs – do not harm the ozone but are significant contributors to global warming. Today, there is a push towards so-called natural refrigerants, such as butane, CO2, and ammonia, which have a much lower global-warming potential. However, these come with their own risks, as many are toxic, flammable, or even explosive.

Decarbonising heating and cooling involves more than just eliminating harmful gases. Refrigerant leakage accounts for only a third of emissions from vapour-compression systems. The other two-thirds come from energy use, and the energy efficiency of these systems is notoriously low.

That is why my research focuses on entirely cutting out gases and developing new technologies based on solid materials, known as caloric materials. Eliminating gases instantly reduces one-third of emissions, but the real potential lies in creating a system that is two to three times more energy efficient than vapour compression. It would be transformative.

Caloric materials have been around for a long time – NASA developed the first magnetocaloric cooling prototype in the 1970s. However, solids have always underperformed compared to fluids, which, due to the free movement of their molecules in the gas state, release a lot of heat when compressed. By contrast, it is much harder to manipulate solid materials in a way that produces similar results. Most research has focused on magnetocaloric materials, which release heat when exposed to a magnetic field. But after 50 years of research, the best magnetocaloric materials only achieve a temperature change of three degrees Celsius under magnetic fields generated by permanent magnets. By comparison, refrigerant gases in vapour-compression systems can easily achieve 40 degrees on compression.

In 2013, we began exploring alternatives and identified new families of organic solids, where molecules sit in a crystalline lattice. Under normal atmospheric pressure, these molecules rotate freely. Two years later, we demonstrated that when applying pressure, the molecules stop rotating – and this achieves the same thermal changes as the gases that we use in vapour compression. This was a big breakthrough.

Our spinout company, Barocal, has already developed its second-generation prototypes based on our barocaloric solid refrigerants, which perform on par with the vapour-compression systems currently on the market. We are now working on the third generation, which will surpass these conventional systems. Developing a completely new cooling and heating system is both challenging and exciting: we developed new materials and tools, made mistakes, and learned from them.

The second law of thermodynamics tells us that you can’t achieve cooling without heating something else. The hotter the planet gets, the more people will install air-conditioning systems, which warm the planet further – so we need a solution, fast. My hope is that we will have these systems in your home, in your car, and in your fridge in five to ten years’ time.