Phase change materials can store and release the large amount of heat required to change from solid to liquid phase. The energy required to melt a material (latent heat) is much higher than the energy to raise the temperature of the material (sensible heat). As the material is being melted or frozen, the temperature remains the same. In contrast, a concrete wall will absorb a lot of heat, but its temperature will change in doing so.
The simulations run for this article use PCM-impregnated drywall, which has a thermal capacity of 22 Btu/ft.sup2 (while remaining constant at 77°F). A 6-inch-thick concrete wall would have its temperature change by 1.5°F to absorb the same amount of heat. There are other subtle differences in how each thermal capacitor performs. For instance, the thickness of the material affects how quickly its average temperature changes due to internal resistance.
If you are designing a new structure, it will most likely be less expensive to build-in thermal capacity by placing high-mass materials such as concrete in contact with the building’s interior. However, for retrofits, adding a concrete wall, floor, or ceiling may not be practical. That is why building scientists are so interested in phase change materials.
If we are interested in storing heat or cooling, we can store much more energy per volume or mass with PCM than with sensible heat. Ever wonder why air conditioning is rated in tons? It takes 1 ton of cooling running for 24 hours to freeze 1 ton of water. 2,000 lbs. × 144 Btu/lb. ÷ 24 hours = 12,000 Btu/ hr. = 1 ton AC.