Solid state cooling with elastocaloric materials (Coolem)

Around 20% of the world's electricity is used for cooling and consumption is rapidly increasing. Current cooling devices are based on old technology with low coefficient of performance (COP), using working fluids that may be harmful and leak to the environment. This is about to change, as part of a silent revolution: inventions dating back to the first industrial revolution, such as internal combustion engines and incandescent lamps are being replaced by alternatives based on solid state technology, such as electric motors and LED lighting. A similar trend can be expected for heat pumps: compressor-based devices will be replaced by more efficient and reliable solid-state technologies based on physical mechanisms like magnetocaloric, electrocaloric and elastocaloric effects. These can convert magnetic, electrical or mechanical energy into heat currents, i.e. solid state cooling. This can have a significant effect: upgrading cooling technology has been identified as the single measure that can provide the greatest reduction in greenhouse gas emissions globally (Project Drawdown). The Cool'em project aims to develop novel elastocaloric materials with unprecedented cooling efficiency, using advanced experimental and theoretical tools. Three different material groups have been studied, with different types of activity: 1. Alloys of Cu, Al and Mn are studied to understand how grain boundaries in the materials affect the properties under repeated cycles of mechanical stress. Pillars around one micrometer in diameter have been subjected to micromechanical testing, and detailed studies of such pillars have been performed with transmission electron microscopy. 2. Quaternary alloys based on TiNi are studied by depositing thin films with graded composition on a patterned substrate. This is used to investigate phase change temperatures and mechanical properties as a function of composition. This can lead to materials that can work at several different temperatures, providing a cascade of different elastocaloric materials that together provide a large cooling effect. 3. Bulk materials of Cu, Al and Zn are made with different microstructures (single crystals, polycrystals and deformed crystals) to systematically investigate how the reversibility of such materials depends on the microstructure.

The Cool'em project aims to develop a superior cooling technology based on the elastocaloric effect. This is based on a thermodynamic cycle in which solid state phase transformations release and absorb heat. The transformations are between a high-symmetry austenite and low-symmetry martensite phase. Long-term operation without loss or fatigue is a prerequisite of a functioning cooling device; we coin this as superreversibility. This can be achieved by so-called supercompatibility of the two phases, which is described by mathematical correlations between their lattice parameters. Optimized microstructure is an alternative path, in which advanced processing is used to maximize reversibility of the phase transformation. An important goal of Cool'em is to identify new superreversible materials with a high cooling coefficient of performance (COP). A larger selection of superreversible materials will open up new temperature regimes for elastocaloric cooling, will increase the maximal COP, and will facilitate the search for a cascaded cooling device. This allows very high overall COP and a large temperature difference between the cold and warm side. In order to accomplish these very ambitious goals, a carefully selected set of tools will be employed or developed. The latest available theoretical tools will be used to predict new, supercompatible martensitic phase transformations and elastocaloric materials with very high COP. A range of advanced synthesis and processing techniques will be used to vary and optimize the microstructure of selected compounds. State-of-the-art characterization and testing tools will be used to establish correspondence between processing parameters and functional behaviour. Digitalization of experiments will speed up and target the fabrication of materials towards optimal ones. All aspects of the activities will pursue the principles of Responsible Research and Innovation through integration of this in all work packages.