PCMs are phase change materials that melts like ice and solidify like water maintaining a constant temperature. In BTS the air-condition is set at say 20C. when the power is off, the temperature starts rising, making it very hot like 50C or more. The PCM will melt at 25C or 29C depending upon the target set and maintain the BTS temperature. There is a chapter on BTS on the website of pcmenergy.com. PCMs probably have no other use in BTS.