I am looking at Phase Change Materials as a building fabric option and from reading up on the topic theres something I don’t understand. Hopefully somebody can help explain.

I read that when a substance melts it stores latent energy, otherwise charging the material. When the material freezes again it releases this ‘stored’ energy and this is what is utilised for heating or cooling. However, I’ve also read that some substances can store energy during the freezing process and this is where it loses me...

Am I misunderstanding the process or is this specific to certain materials?

Thanks in advance

Kind Regards


  • 1
    $\begingroup$ Change « freezing » for solidifying... $\endgroup$
    – Solar Mike
    Apr 8 '18 at 20:09
  • $\begingroup$ Which substances are you referring to that store energy during freezing? $\endgroup$
    – paparazzo
    Apr 9 '18 at 7:41
  • $\begingroup$ It was a general statement I had read that indicated you could use free cooling to cool the PC and ‘freeze it’, thus charging. Then when warmer ambient conditions met the PC, it would melt and thus discharge the energy. However, Ive seen other descriptions that say the reverse i.e charge when melt and discharge when freeze. Neils answer below made sense. $\endgroup$
    – 85l00k85
    Apr 10 '18 at 4:22

Let's take water as an example. to freeze water into ice, you must extract from it a certain amount of heat energy per unit of mass, which is called the specific heat of fusion. This same amount of heat must be added to the resulting ice in order to melt it back into water again. In this sense, the work it took to draw the heat of fusion out of the liquid water and freeze it is "stored" in the resulting chunk of ice, and is available to perform refrigeration work for you at a later time.

Alternatively, the heat that the chunk of ice pulled out of its surroundings as it melted is similarly "stored" in the resulting body of melt water. A device called a heat pump can then be used to pull that absorbed heat back out of the liquid water and exhaust it into a space where heating is needed, thereby turning the water back into ice.


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