Most PCMs have low thermal conductivity, which can seriously affect the storage system charge and discharge heat transfer rates. To address this limitation, extended metal surfaces [7], conductive powders [5] or conductive matrices [6] have proven to be effective in increasing the PCMs heat transport properties, leading to a more uniform temperature distribution within the PCM and consequently better charge and discharge performances for the latent heat storage container/system. Other thermal enhancement solutions improve heat transfer between the flow and the PCM through direct contact between the PCM and their respective heat transfer fluid, allowing higher convective currents within the solid PCM whilst preventing flow mixing in the molten PCM due to different densities [110].
Another technique could be shape stabilization through the use of polymeric adsorbents such as polyethylene or polystyrene [111], that would prevent leakage of molten PCM. On the case of salt hydrates, the use of inorganic adsorbents, such as
diatomaceous earths containing silica [69], could prevent water separation from the inorganic salt, maintaining its phase change enthalpy.
2.4.1
Heat transfer enhancement using extended metal sur-
faces
One of the most widely used heat transfer enhancement techniques is the addition of extended metal surfaces, fins, to increase the heat exchange area between the PCM and the flow. Numerous studies on modelling the phase change process with different fin geometries have already been made [112, 113]. Agyenim [66] tested a compact horizontal tube in tube container with erythritol as the PCM, using longitudinal fins to enhance heat transfer.
(a) longitudinal fins [66] (b) annular fins [5]
Figure 2.1: Schematic representation of extended metal surfaces in a tube in tube.
They concluded that the system with longitudinal fins had an adequate melt- ing/solidifying process that could provide the required heat flux suitable to act as the heat source for driving Li-Br absorption cooling systems. Figure 2.1a and 2.1b illustrates two common fin geometries widely used in the literature [5].
2.4.2
Heat transfer enhancement using carbon
Exfoliated graphite, also known as expanded graphite (EG) is presented in figure 2.2a, 2.2b and 2.2c. It has a thermal conductivity ranging from 24 to 470 W/m.K depending on the measuring direction, with the potential to increase thermal con- ductivity within the PCM [88] using low volume ratios (usually around 10-15% [59]). EG is generally obtained from the corrosion of natural graphite with a mixture of nitric and sulphuric acid, followed by drying and rapid heating in a furnace at 800 to 900circC to obtain the desired volumetric expansion. To produce EG/PCM com- posites, the PCM is impregnated into the EG in vacuum to suppress the formation of air gaps within the composite material [69, 5]. This technique is the most ef- fective procedure currently used to enhance the PCM thermal conductivity [94]. It could also provide a shape-stabilized (SS) form to the PCM since pore cavities can withstand the thermal expansion typical during phase change and prevent leakage of molten PCM [69].
(a) carbon fiber cloth (b) carbon fibre brush (c) Expanded graphite
Figure 2.2: Three examples of heat transfer enhancement using carbon [5]
The container studied by Nakaso [6] presented in figure 2.3 was predicted to double its heat output (from 25 to 50kWth) if a carbon fibre cloth comprising 0.8% of the storage volume was incorporated into the system. The system would then provide a nearly constant heat output of 50kW for around 10h and 20 minutes.
Figure 2.3: A schematic diagram of the 18 770L compact latent heat storage unit designed by Nakaso [6], comprised of 18 parallel 28mm copper tubes, each having 14 passes through the PCM volume.
2.4.3
Heat transfer enhancement using metal matrices
Using sparse metal matrices is another way to increase thermal conductivity within the bulk PCM, providing also multiple nucleation points. The impregnation of a PCM medium with steel wool can be more cost effective than using EG, although the potential risk of thermal gaps due to poor contact between conductive surfaces can reduce their thermal conductivity improvement. Due being a sparse matrix and not a rigid and porous foam as expanded graphite, it would not provide shape stabilization of the molten PCM.Including small percentages by volume of metallic particles such as lessing rings, presented in figure 2.4a and 2.4b, could increase thermal diffusion within low thermal conductivity PCMs [5], would have the benefit of increasing the number of nucleation points, potentially enhancing crystallization within the PCM. However, the risk of thermal gaps would be even higher than using steel wool, due to their sparse distribution and also the conductive material could lose its miscibility when the PCM is in its molten state due to differences in density, separating from the storage material and sinking to the base of the container. This could be prevented by including gelling agents in the PCM [69], with a consequent reduction in the PCM volume ratio.
(a) aluminum lessing rings (b) stainless steel lessing rings
Figure 2.4: Two examples of metal matrices, from Agyenim [7]
2.4.4
Direct heat transfer techniques
Another technique to improve heat transfer is to provide direct contact between the heat transfer fluid and the PCM. It effectively increases heat transfer during the melting process due to the additional flow mixing between the heat transfer fluid and molten PCM, increasing the convective heat transfer on the solid-liquid PCM interface [8]. Weilong [114] studied the performance of a direct contact latent heat storage container using erythritol and a heat transfer oil, seen in figure 2.5a to 2.5c, concluding that at the beginning of the melting process the oil has a low flow rate due to the blockage of solid erythritol, the top surface of the PCM melts faster than the bottom due to the higher heat transfer rate and the melting time varies significantly with the oil flow rate.
(a) initial stage (b) middle stage (c) final stage
Figure 2.5: A temporal variation of the melting process in a direct contact heat transfer container using erythritol as PCM and oil as heat transfer fluid, from Wei- long [8].
To overcome the initial blocking of the fluid flow path when the PCM is in the solid state, Shaopeng [9] studied the insertion of electric heaters, seen in figure 2.6, and concluded that the overall energy spent on melting the initial flow pathways was 5% of the total thermal energy stored.
Figure 2.6: Schematic cross section showing the locations of the inlet pipes electric heaters, studied by Shaopeng [9]
2.4.5
Shape stabilization
Shape-stabilized PCMs are obtained adsorbing the molten PCM into the structure of a porous adsorbent. Common inorganic adsorbents such as some diatomaceous earths containing silica (SiO2) are generally used to adsorb salt hydrates due to their affinity with water [65]; however their maximum adsorption capability is usu- ally below 50% of its weight. Organic polymeric adsorbents such as high-density polyethylene, polystyrene [111] and polyvinyl chloride (PVC) are generally used to adsorb paraffin waxes, fatty acids and their blends and can generally adsorb up to 80% of its weight [115].
Alkan [116] prepared shape stabilized fatty acid PCMs by blending them with solutions of Poly(methyl methacrylate) (PMMA) in chloroform. They concluded that the composite PCM could maintain its shape up to 80%wt of PCM and that they could be considered as candidates for latent heat storage systems integrated into underfloor space heating applications.