Phase Change Materials

 

Changes of phase of materials involve large amounts of energy, that is latent heat.  Energy must be input on melting and is released on solidification.  Phase change materials (PCMs) absorb heat at their melting point upon an increase in surrounding temperatures and release the heat upon cooling.  PCMs are available with a range of melting points for varying applications.  For use in construction, PCMs would have a melting point just above room temperature such that they are still solid at room temperature.  Any increase in temperature causes them to melt, thus absorbing heat from the surroundings.  A reduction in temperature would result in the heat being released to the surrounding air again.  Consequently, the use of PCMs aids in removing fluctuations in room temperature, thus reducing heating and cooling loads.

 

Phase change materials can be incorporated into the building envelope, for example in both interior and exterior walls as well as in the floor, combined with under-floor heating, and with other insulating materials.  With under-floor heating, the function of PCMs is to steadily release heat to a room as it is required.  There are many other applications of PCMs under investigation.  An investigation of particular significance is that into the use of PCMs as a high density heat storage method for heat gained, for example, from solar collectors.

 

There are two main forms of PCMs, parafins and inorganic hydrated salts.  Research is currently underway into the benefits of each, however hydrated salts are seen to have greater thermal conductivity together with a greater latent heat storage density.  Parafins are also flammable whilst hydrated salts are not.  When applied to the walls of buildings, PCMs are not known to give off any harmful gases, but parafins are significantly more toxic than hydrated salts.  Depending on the source of information, that is its bias towards either form of PCM, the stability of each form is scrutinised although many sources agree that the stability of hydrated salts exceeds that of parafins.