Civil Engineering Reference
In-Depth Information
9.3 Thermal Properties of PCM Doped with Nanomaterials
The heat transfer performance of the PCM-based LTES system largely depends on
the thermal properties of PCM, which includes thermal conductivity, latent heat of
fusion, phase transition temperature, degree of supercooling, heat storage and
release characteristics, thermal stability and thermal reliability. Interestingly, the
thermal properties of pure PCM can be tuned for achieving the improved TES
capability using the nanomaterials.
The incorporation of nanomaterials and their effect on the inherent thermal
properties of PCM are briefly discussed in the following sections. The thermal
conductivity of the PCM in an encapsulated storage plays a significant role in
determining its heat transfer ability towards the temperature differential between
the PCM and the surrounding heat transfer fluid with respect to the wall thickness.
Generically, the salt hydrate type of PCMs exhibit a higher thermal conduc-
tivity than that of the organic PCMs, but the former may get destabilized over
periodic thermal processes. Thus, the organic PCMs with low thermal conduc-
tivity, but are commercially viable, can be mixed with the proper proportions of
nanomaterials. The nanoparticles embedded into the pure PCM may create more
thermal conductive interfaces within the PCM matrix layers.
When the PCM is subjected to heat or cold energy, these thermal interfaces can
serve as thermal energy carriers to transfer the heat energy effectively throughout
the PCM, thereby enabling the PCM to undergo congruent melting and freezing
processes. The inherent Brownian motion, diffusion and surface charge effects of
the nanoparticles have a direct influence on improving the thermal conductivity of
PCM. In recent times, many research works have been performed to improve
the thermal conductivity of PCMs using a variety of nanostructured materials
(Parameshwaran et al.
2012
). The SEM and TEM images of nanomaterials and
PCM embedded with nanomaterials are shown in Fig.
18
.
The latent heat of fusion and phase change temperature are the most vital
parameters of PCM, which decide its applicability for enabling TES facility in
buildings. It is well known that water or ice has the maximum latent heat of
enthalpy of 330 kJ/kg K when compared to the conventional organic and other salt
hydrate PCMs.
But, the phase change characteristics and supercooling of water or ice resist
them from utilizing for short-term TES applications in buildings. Precisely, the
doping of nanomaterials in PCM at proper weight proportions may lead to the
condition of congruent freezing and melting during phase change.
As the size of the nanoparticles are comparatively larger than the PCM mol-
ecules, the presence of individual nanoparticles or a cluster of nanoparticles can
act as the nucleation sites for the growth of the ice-like crystals in the PCM during
freezing process. Similarly, during the melting process, the nanomaterials effi-
ciently transfer the heat energy absorbed within the PCM matrix, enabling the
formation of liquid front in the PCM.
