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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Thermal cloak passively keeps electric vehicles cool in the summer and   
   warm in the winter    
      
     Date:   
         July 11, 2023   
     Source:   
         Cell Press   
     Summary:   
         When an electric vehicle is parked outside, its temperature can   
         swing wildly from day to night and season to season, which can   
         lead to deterioration of the battery. To dampen these fluctuations   
         and extend the battery's lifespan, researchers have designed an   
         all-season thermal cloak that can cool an electric vehicle by   
         8DEGC on a hot day and warm it by 6.8DEGC at night. The cloak,   
         made predominantly of silica and aluminum, can do so passively   
         without outside energy input and operates without any modification   
         between hot or cold weather.   
      
      
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   When an electric vehicle is parked outside, its temperature can   
   swing wildly from day to night and season to season, which can lead to   
   deterioration of the battery. To dampen these fluctuations and extend the   
   battery's lifespan, researchers have designed an all-season thermal cloak   
   that can cool an electric vehicle by 8DEGC on a hot day and warm it by   
   6.8DEGC at night. The cloak, made predominantly of silica and aluminum,   
   can do so passively without outside energy input and operates without   
   any modification between hot or cold weather.   
      
   This prototype is described July 11 in the newly launched Device, an   
   application-oriented sister journal to Matter,Joule, and Cell.   
      
   "The thermal cloak is like clothes for vehicles, buildings, spacecrafts,   
   or even extraterrestrial habitats to keep cool in summer and warm in   
   winter," says senior author Kehang Cui, a materials scientist at Shanghai   
   Jiao Tong University.   
      
   To dampen natural temperature fluctuations, the cloak isolates the car --   
   or any other object beneath it -- from the surrounding environment. The   
   cloak has two components: an outer layer which efficiently reflects   
   sunlight and an inner layer that traps heat inside. Whatever heat the   
   outer layer does absorb is emitted in such a way that it can be readily   
   dissipated to outer space. This design earns it the name of Janus thermal   
   cloak, inspired by the two-faced Roman god Janus.   
      
   "The cloak works basically the same way the earth cools down, through   
   radiative cooling" says Cui. "The earth is covered by the atmosphere,   
   and the atmosphere is transparent to a certain range of electromagnetic   
   energy we radiate."  While this process is desirable in the summer, it   
   would make the car colder during winter months. "You have to develop   
   something that can turn on and off by itself without external energy   
   input, and that's extremely difficult," says Cui.   
      
   Cui and his team designed the cloak to automatically counteract this   
   effect in the winter. The cloak employs an effect called "photon   
   recycling" - - essentially, any energy that is trapped under the cloak   
   will bounce back and forth between the car and cloak rather than escape   
   to the surroundings outside.   
      
   To assess the performance of the thermal cloak, the researchers conducted   
   tests on electric vehicles parked outside under typical ambient conditions   
   in Shanghai. While the cabin temperature of an uncovered car reached   
   50.5DEGC at mid-day, the cabin of the cloak-covered car reached 22.8DEGC   
   -- 27.7DEGC lower than the uncovered car and 7.8DEGC lower than the   
   temperature outside. At midnight, the covered car stayed 6.8DEGC higher   
   than the temperature outside, never dropping below 0DEGC.   
      
   "This is the first time that we could achieve warming above the ambient   
   temperature by almost 7DEGC during winter nights," says Cui. "This is   
   also kind of surprising to us -- there's no energy input or sunshine and   
   we can still get warming."  The outer component of the cloak is made of   
   thin fibers of silica that were then coated in flakes of hexagonal boron   
   nitride, a ceramic material similar to graphite that enhances the fibers'   
   solar reflectivity. These fibers are then braided and woven together into   
   a fabric and adhered to the inner layer, which is made of aluminum alloy.   
      
   The team purposefully designed the cloak to make scaling up production   
   easier in the future. For example, using thinner silica fibers would have   
   increased solar reflectivity, but they would be weaker and couldn't be   
   made with high- volume, industrial-level production techniques already   
   available. In addition, the materials used, including the aluminum,   
   silica, and boron nitride, are all low-cost and make the cloak   
   lightweight, durable, and fire-retardant.   
      
       * RELATED_TOPICS   
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             o Battery_electric_vehicle o Season o Temperature o Fuel_cell o   
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   Story Source: Materials provided by Cell_Press. Note: Content may be   
   edited for style and length.   
      
      
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   Journal Reference:   
      1. Huaxu Qiao, Zhequn Huang, Jianming Wu, Jie Shen, Heng Zhang, Qixiang   
         Wang, Wen Shang, Wei Tang, Tao Deng, Hongxing Xu, Kehang   
         Cui. Scalable and durable Janus thermal cloak for all-season   
         passive thermal regulation. Device, 2023; 100008 DOI:   
         10.1016/j.device.2023.100008   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230711133111.htm   
      
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