Phase Change Materials (PCMs) do a strange and amazing thing. They do not change temperature during the change of phase from a solid to a liquid or vice versa. Water is a common Phase Change Material (PCM). Water has the ability to hold one BTU of heat energy per pound per degree F. Water in the solid phase (ICE) also has the same ability to hold one BTU of heat energy per pound per degree F.
The strange and wonderful thing is that during the phase change from liquid to solid, water gives up 144 BTUs before it can pass from 32°F liquid to 31°F ice. This is 143 times the amount of BTUs it takes to change the temperature of water or ice by one degree F at other than 32°F temperatures. It works the same in the other direction from a solid phase melting into the liquid phase. This massive amount of heat storage capacity only occurs at 32° F and is called “Latent Heat Capacity”. Unlike Sensible Heat, Latent Heat does not cause a change in temperature of the substance.
Sensible and Latent Heat
Sensible Heat is ordinary heat and it determines the temperature of a material. If you add thermal energy the temperature increases, if you remove the thermal energy the temperature falls.Latent Heat is harder to understand. It is the heat that is released when a PCM changes phase from a liquid to a solid. To melt the PCM and change it back to a liquid requires the same amount of extra Latent Heat energy. The total amount of energy required varies with different PCMs. In the case of water/ice it is 143 times as much to Phase Change than to raise the temperature outside of the phase change temperature range. This means a small amount of PCM at the phase change temperature can control a relatively large amount of heat energy.
The temperature difference across the internal “R” Value of a wall or a ceiling has never been challenged and was long thought to be immutable. However, by using Phase Change Materials (PCMs) it is possible to control this temperature difference for a time. The length of the time period is determined by the amount of PCM, the temperature difference, and the “R” Values on both sides of the PCM. A small amount of PCM to control the temperature difference across a wall for 24 hours is all that is necessary. During daylight hours the PCM is melting and each night it will refreeze. The PCM changes to solid at night and to liquid during the day. All the while that this is going on day after day, THE PCM REMAINS AT A CONSTANT TEMPERATURE! It is this characteristic of the PCM that we have seized upon.