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.