Civil Engineering Reference
In-Depth Information
Zhao and Zhang
2011
). Several forms of bulk encapsulated PCMs have been
developed for active and passive solar applications in building including direct
heat gain. However, the surface area of most encapsulated commercial products
has been inadequate to deliver heat to the building after the PCM melted by direct
solar radiation. One problem to solve in some PCM applications is the liquid
migration, using some kinds of packing.
Microcapsules consist of little containers, which pack a core material with a
hard shell. Microencapsulating PCM brings some more important advantages like
that microcapsules can handle phase-change materials as core, as far as, they
tolerate volume changes.
The microencapsulated phase-change material is defined as composing of
phase-change materials (PCMs) core and a polymer or inorganic shell to maintain
the shape and prevent PCM from leakage during the phase-change process.
Microencapsulated phase-change material overcomes the following problems in
comparison with the convectional PCM: corrosive to metal, decomposition, sub-
cooling and leakage.
2.3 Phase-Change Slurries
Recently, a new technique has been proposed to use phase-change materials in
thermal storage systems, heat exchangers and thermal control systems. This new
technique consists of forming a two-phase fluid, from the mixture of a fluid, such
as water, and a phase-change material, such as paraffin, resulting in a latent heat
storage fluid (Delgado et al.
2012
; Youssef et al.
2013
; Chen and Fang
2011
;
Zhang and Ma
2012
; Zhang et al.
2010
).
Among the latent thermal fluids, five types of fluids are mentioned:
1. ice slurries;
2. Phase-change material microemulsions, in which the PCM is dispersed in water
through an emulsifying agent;
3. microencapsulated PCM slurries, where the PCM is microencapsulated in a
polymeric capsule and dispersed in water;
4. clathrate hydrate PCM slurries, where the clathrate hydrates are composed of
water molecules (host molecule) forming a weaved structure where the mole-
cules of the other substance (guest molecule) are accommodated, constituting a
special molecular structure where the heat associated with the chemical reac-
tion of formation and dissociation of clathrate hydrate is greater than that of ice
melting; and
5. shape-stabilised PCM slurries (ssPCM slurries), based on ssPCM, these can
consist of paraffin infiltrated in high-density polyethylene, with a melting
temperature higher than of the paraffin. In this way, the paraffin is retained
inside the structure of high-density polyethylene, avoiding the leak of the PCM.
Figure
1
shows a schematic draw of the different types of PCM slurries,
although the case of clathrate hydrate PCM slurries does not appear drawn.
