Elastocaloric Cooling

Current heat pumping technologies use a vapor compression cycle with a liquid refrigerant. In recent years there has been an increased interest in systems using a solid refrigerant instead, eliminating the negative environmental impact of today’s refrigerants. It is common that they either deplete the ozone layer or contribute to the global warming. This occurs when the refrigerant leaks, a phenomenon that you won’t have with a solid refrigerant. A group of interesting solid-state materials for this purpose are so-called caloric materials.

That all sounds fine. But what are caloric materials, and how could they be put into cooling and heating practice? Their most important characteristic is that their temperature can increase without thermal energy being added (an adiabatic process), if an external field of magnetism, electricity or stress is applied. With the external field held constant, a heat exchange will occur over time between the caloric material and its surroundings. This phenomenon can be made useful in heat pumps. In the examples here a stress field is applied; this sub-group of materials is called elastocaloric. Two different ways to integrate such materials in different configurations of a system is shown.

In the first example two sets of caloric materials are used and the stress is applied to one of them at a time, in a cycle. The stress is firstly applied to material set 1, until maximum stress is reached. This causes a temperature rise in the material. Still under stress, the heat in the material is exchanged to a fluid which is then pumped towards a heat sink, where the heat is released. Thus, material set 1 cools off. It can be cooled off even further through heat exchange with material set 2. The stress is then removed from set 1, and the process moves in the opposite direction, with the material first cooling down and then heating again, until it reaches its original state. This is done simultaneously for both material sets, so that one cools off when the other is heating up.

In the second example the elastocaloric material is used for heat storage for heat exchange, through a so-called regenerator wall. Basically, the principle is to let cold and hot fluids flow alternatively through the same flow passages, resulting in an intermittent heat transfer. The hot fluid will lose thermal energy to the regenerator wall, and the cold fluid will absorb it. This will result in a temperature difference along the regenerator, where the high temperature end is connected to the heat sink and the cold end to the heat reservoir.

The technologies are under development. As an example, the largest temperature lift noted so far for example 2 is 19.9K. Even though the cooling capacity of the prototype systems is still far from the requirements of a commercial application, the temperature lift performance is improving and getting closer to the minimum requirements needed.

David Catalini, Nehemiah Emaikwu, Yunho Hwang and Reinhard Radermacher, USA (Center for Environmental Energy Engineering, University of Maryland). Ichiro Takeuchi, USA (Department of Materials Science and Engineering, University of Maryland)

The text has been shortened by the HPC team

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