Complex sentence: Liquid-gas phase changes are not practical for use as thermal storage on account of the large volumes or high pressures required to store the materials when in their gas phase, although liquid--gas transitions have a higher heat of transformation than solid--liquid transitions.
Split into simpler sentences: Liquid-gas phase changes are not practical for use as thermal storage due to the large volumes or high pressures required to store the materials when in their gas phase. Liquid--gas transitions do have a higher heat of transformation than solid--liquid transitions.