When a solid PCM is heated up and
reaches its melting point, it goes through a phase change, from solid
to liquid. During this process the material absorbs a certain amount of
heat, known as melting enthalpy. Despite the heat input, the temperature
of the material stays at a relatively constant temperature, even though
phase change is taking place. We thus speak of latent (concealed) heat
having been taken up by the material. Equally, when the phase change process
is reversed, that is from liquid to solid, the stored latent heat is released,
again at a nearly constant temperature.
Many heat storage systems, like a conventional hot water storage unit, use sensible heat only, i.e. the temperature change of water. The heat capacity of a material operating on the basis of sensible heat can be calculated as follows:
Q = m · cp · delta T
| Q | amount of heat |
| m | mass |
| cp | specific heat capacity |
| delta T | temperature difference |
In a latent heat storage unit, the sensible heat is augmented by the latent heat. In this case, the heat capacity is defined by the following equation:
Q = m · cp · delta T + m delta hmelt
where delta hmelt is the specific melt enthalpy
When operating at low working temperature differences
(delta T < 15 K) a latent heat storage unit provides a very effective
storage of thermal energy, provided that the latent heat storage material
goes through its phase change within this working temperature difference.
 |
 |
 |
 |
 |
 |
 |
 |
 |
(see: heat storage unit evaluation)
RUBITHERM GmbH has therefore developed a range of PCM's having melting temperatures of between approx. -3 °C and 100 °C. These PCM's are sold under the registered trade name of RUBITHERM.
|
Glossary: • The
Latent Heat Storage Process |