▁the ▁addition ▁of ▁just ▁ 4 . 1 8 ▁J / g . ▁ ▁Water / ice ▁is ▁therefore ▁a ▁very ▁useful ▁phase ▁change ▁material ▁and ▁has ▁been ▁used ▁to ▁store ▁winter ▁cold ▁to ▁cool ▁buildings ▁in ▁summer ▁since ▁at ▁least ▁the ▁time ▁of ▁the ▁A cha emen id ▁Empire . ▁ ▁By ▁mel ting ▁and ▁solid ifying ▁at ▁the ▁phase ▁change ▁temperature ▁( P CT ), ▁a ▁PC M ▁is ▁capable ▁of ▁storing ▁and ▁re le asing ▁large ▁amounts ▁of ▁energy ▁compared ▁to ▁sensible ▁heat ▁storage . ▁ ▁He at ▁is ▁absor bed ▁or ▁released ▁when ▁the ▁material ▁changes ▁from ▁solid ▁to ▁liquid ▁and ▁vice ▁vers a ▁or ▁when ▁the ▁internal ▁structure ▁of ▁the ▁material ▁changes ; ▁PC M s ▁are ▁accordingly ▁referred ▁to ▁as ▁lat ent ▁heat ▁storage ▁( L HS ) ▁materials . ▁▁ ▁There ▁are ▁two ▁principal ▁classes ▁of ▁phase ▁change ▁material : ▁organ ic ▁( car bon - cont aining ) ▁materials ▁derived ▁either ▁from ▁pet role um , ▁from ▁plants ▁or ▁from ▁animals ; ▁and ▁salt ▁hyd r ates , ▁which ▁generally ▁either ▁use ▁natural ▁sal ts ▁from ▁the ▁sea ▁or ▁from ▁min eral ▁depos its ▁or ▁are ▁by - products ▁of ▁other ▁processes . ▁A ▁third ▁class ▁is ▁solid ▁to ▁solid ▁phase ▁change . ▁ ▁PC M s ▁are ▁used ▁in ▁many ▁different ▁commercial ▁applications ▁where ▁energy ▁storage ▁and / or ▁stable ▁temper atures ▁are ▁required , ▁including , ▁among ▁others , ▁he ating ▁p ads , ▁cool ing ▁for ▁tele phone ▁switching ▁boxes , ▁and