▁However , ▁only ▁solid → liqu id ▁and ▁liquid → sol id ▁phase ▁changes ▁are ▁practical ▁for ▁PC M s . ▁Although ▁liquid – gas ▁trans itions ▁have ▁a ▁higher ▁heat ▁of ▁transformation ▁than ▁solid – liqu id ▁trans itions , ▁liquid → gas ▁phase ▁changes ▁are ▁imp ract ical ▁for ▁thermal ▁storage ▁because ▁large ▁volumes ▁or ▁high ▁press ures ▁are ▁required ▁to ▁store ▁the ▁materials ▁in ▁their ▁gas ▁phase . ▁ ▁Sol id – sol id ▁phase ▁changes ▁are ▁typically ▁very ▁slow ▁and ▁have ▁a ▁relatively ▁low ▁heat ▁of ▁transformation . ▁ ▁Initial ly , ▁solid – liqu id ▁PC M s ▁behave ▁like ▁sensible ▁heat ▁storage ▁( SH S ) ▁materials ; ▁their ▁temperature ▁r ises ▁as ▁they ▁abs orb ▁heat . ▁Un like ▁conventional ▁S HS ▁materials , ▁however , ▁when ▁PC M s ▁reach ▁their ▁phase ▁change ▁temperature ▁( the ir ▁mel ting ▁point ) ▁they ▁abs orb ▁large ▁amounts ▁of ▁heat ▁at ▁an ▁almost ▁constant ▁temperature ▁until ▁all ▁the ▁material ▁is ▁m elt ed . ▁When ▁the ▁ambient ▁temperature ▁around ▁a ▁liquid ▁material ▁falls , ▁the ▁PC M ▁solid ifies , ▁re le asing ▁its ▁stored ▁lat ent ▁heat . ▁A ▁large ▁number ▁of ▁PC M s ▁are ▁available ▁in ▁any ▁required ▁temperature ▁range ▁from ▁− 5 ▁up ▁to ▁ 1 9 0   ° C . ▁Within ▁the ▁human ▁comfort ▁range ▁between ▁ 2 0 – 3 0   ° C , ▁some ▁PC M s ▁are ▁very ▁effective , ▁storing ▁over ▁ 2 0 0 ▁k