sol id ▁phase ▁change , ▁there ▁is ▁no ▁visible ▁change ▁in ▁the ▁appearance ▁of ▁the ▁PC M , ▁and ▁there ▁are ▁no ▁problems ▁associated ▁with ▁handling ▁liqu ids , ▁e . g . ▁contain ment , ▁potential ▁leak age , ▁etc . ▁Currently ▁the ▁temperature ▁range ▁of ▁solid - sol id ▁PC M ▁solutions ▁sp ans ▁from ▁- 5 0   ° C ▁(- 5 8   ° F ) ▁up ▁to ▁+ 1 7 5   ° C ▁( 3 4 7   ° F ). ▁ ▁Selection ▁criteria ▁ ▁The ▁phase ▁change ▁material ▁should ▁possess ▁the ▁following ▁therm od ynamic ▁properties : ▁ ▁Mel ting ▁temperature ▁in ▁the ▁desired ▁operating ▁temperature ▁range ▁ ▁High ▁lat ent ▁heat ▁of ▁fusion ▁per ▁unit ▁volume ▁ ▁High ▁specific ▁heat , ▁high ▁density , ▁and ▁high ▁thermal ▁conduct ivity ▁ ▁Small ▁volume ▁changes ▁on ▁phase ▁transformation ▁and ▁small ▁v apor ▁pressure ▁at ▁operating ▁temper atures ▁to ▁reduce ▁the ▁contain ment ▁problem ▁ ▁Cong ru ent ▁mel ting ▁ ▁Kin etic ▁properties ▁ ▁High ▁nucle ation ▁rate ▁to ▁avoid ▁super co ol ing ▁of ▁the ▁liquid ▁phase ▁ ▁High ▁rate ▁of ▁cry stal ▁growth , ▁so ▁that ▁the ▁system ▁can ▁meet ▁dem ands ▁of ▁heat ▁recovery ▁from ▁the ▁storage ▁system ▁ ▁Chem ical ▁properties ▁ ▁Chem ical ▁stability ▁ ▁Complete ▁revers ible ▁free ze / m elt ▁cycle ▁ ▁No ▁de grad ation ▁after ▁a ▁large ▁number ▁of ▁free ze / m elt ▁cycle ▁ ▁Non - cor ros iveness , ▁non - to xic ,