Experimental and Numerical Analysis of a PCM-Supported ...

liquid to gas or reverse. The phase change involved takes place without a change in
the temperature of the material itself. Figure (2.1) depicts the mechanisms of heat
absorption and release in LHES materials. During the heat transfer to the solid
phase, as long as the matrix structure is strong enough to stand the rising kinetic
energy due to the higher oscillation amplitude of each element in the matrix; the
matrix structures does not change and the change of internal energy can be
monitored by a rise of temperature. Therefore, initially solid-liquid PCM perform like
conventional storage materials; their temperature rises as they absorb heat, then
change has yet a sensible component.
At the melting point there is a change to the next higher level of entropy, the binding
forces in the matrix are no longer strong enough to stand the risen kinetic energy of
its elements. As the temperature of the PCM rises, their chemical bonds break up
gradually as the material changes phase from solid to liquid. The phase change is a
heat seeking (endothermic) process and therefore, the PCM absorbs large amount
of heat without getting hotter, i.e. while storing heat, the temperature of the PCM
stays nearly constant until the melting process is completed.
(a)
(b)
Figure 2.1: Phase change behavior of solid-liquid latent heat storage
systems; (a) melting at a sharp point, (b) melting over a temperature range
The energy stored during the phase change process is called “latent heat” or “heat of
fusion”. As the materials cool off (at night or on cloudy days) they subsequently
release the stored heat while solidifying in the same fashion.
The storage capacity of the LHES system with a PCM medium is defined by the
following equation:
Q
T
T
1
2
cs( Tm
T1)
mH
m
cl
( T2
Tm
)
m
T
T
m
16