Cascading electrocaloric cooling units

Electrocaloric cooling could be efficient, compact, and even flexible. The temperature lift has been small. The cascade approach can resolve this issue, and additionally improve the COP.

Overheating of electronic parts can be annoying for personal electronics user. Without proper cooling, heat generated by the battery and CPU of a personal electronic device can quickly build up, and may lead to performance degradation, sometimes abrupt shutdown, or even explosion in a few extreme cases. The need for effective cooling is especially important for portable devices as their computing power escalates while size is further reduced. Conventional cooling systems, such as those used in air conditioners and refrigerators, are based on a process called vapor compression. These are simply too big; it is impractical to shrink their size to fit into personal devices. Also, the fluorocarbon coolant has thousands of times greater capacity to warm the atmosphere than carbon dioxide.

More than 5 years ago, we were charged to solve a different cooling need: to maintain sufficient personal thermal comfort while reducing the HVAC energy consumption for offices and buildings. The mission was to create a wearable cooler. The electrocaloric polymer introduced by Prof. Qiming Zhang of the Pennsylvania State University immediately caught our attention. Well, as polymer scientists, we may be biased toward using a polymer to solve practical problems. More importantly, polymer films are flexible. In collaboration with Roy Kornbluh of SRI International and Prof. Sungtaek Ju of the Mechanical and Aerospace Engineering Department of UCLA, we first verified the electrocaloric effect in the polymer film. To move heat in just one direction, an electrostatic actuation mechanism was innovated to toggle the polymer film between a heat source and sink (Science, 2017, 357, 1130). This device was thin, lightweight, and flexible. Pocket cooler is a fitting name.

The co-first authors, Dr. Rujun Ma and Dr. Ziyang Zhang have moved on to their next career stops. However, there are unsolved issues. The temperature lift of the first pocket cooler was rather low, limited by the stably operable electric field of the electrocaloric polymer film. This is a fundamental challenge. Voltage cascade is a successful approach to lifting the power conversion efficiency of solar cells. Heat regeneration has met its success in magnetocaloric cooling. The challenge is how to implement it in electrocaloric devices, without tradeoff of compact size. We ruled out the use of an external motor or pump. Dr. Yuan Meng, the first author of the work, spearheaded the cascade architecture to retain the compactness and relatively simple electrically driving scheme. Our solution was to pair the electrocaloric cooling element and operate the pair in antiphase. This way, the temperature span can be built upon one another. Similar pairs may be stacked to further increase the temperature span. This cascade cooling device does not necessarily enhance the cooling heat flux over its ‘unit cooling device’ counterpart, since the cooling capability of the material directly in contact with the heat source does not change. Another benefit of antiphase pairing is that the capacitive charges may be recycled between the paired units; the overall energy usage efficiency can be higher than a unit cooling device. We envision this design strategy to inspire the development of more cascade solid-state cooling devices. Hopefully, the optimized compact cascade cooling system will find applications such as in electronic device cooling, personal thermal management, emergency medical treatment.

For more details, check out our paper recently published in Nature Energy:

 A Cascade Electrocaloric Cooling Device for Large Temperature Lift

Yuan Meng, Ziyang Zhang, Hanxiang Wu, Ruiyi Wu, Jianghan Wu, Haolun Wang, Qibing Pei*

Nature Energy, 2020, DOI: 10.1038/s41560-020-00715-3

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