Caloric cooling represents an environmentally friendly alternative to vapour compression refrigeration as it does not require refrigerants with high global warming potential. Instead, it exploits the phase transitions of solids, driven by magnetic fields or mechanical forces, which are associated with absorption or release of heat. In particular, elastocaloric cooling is attractive as it relies on relatively easy-to-operate actuators. Yet, the cooling performance is still poor. Additionally, refrigeration devices present a trade-off between temperature difference between the hot and cold side of the device and cooling power; for example, a large temperature difference is needed for air humidification and high power is required for turning on air conditioners. Now, Ichiro Takeuchi and colleagues across the USA and China design a device based on the compression of nickel–titanium tubes that can be operated efficiently in two modes: one mode delivers large temperature difference, and the other high cooling powers.
The difference between the two modes is in the amount of heat-exchange fluid displaced from the nickel–titanium tubes (to and from which the material rejects or absorbs heat) during each refrigeration cycle. To achieve large temperature differences, a small portion of the fluid is displaced from the caloric material: the latent heat released during phase transition increases the fluid temperature and creates a large temperature gradient along the flow direction. Instead, to obtain high cooling powers, a large portion of the fluid is displaced to transfer the latent heat to the heat sink. The researchers achieve a 22.5 K temperature difference and a cooling power of 260 W with the first and second operating modes, respectively. Both values are among the highest for elastocaloric cooling and compare well with magneto-caloric cooling that requires expensive magnetic fields to operate. The approach of Takeuchi and team is a step forward towards a practical caloric cooling prototype with improved performance in a wide range of working conditions.
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