Thermal energy storage systems consist of three major components or parts;
Storage medium
Heat transfer fluid
Container system
The storage medium describes the type of storage material used, whether it is a sensible or latent storage material. For this research, the storage medium used is PCM. The heat transfer fluids mostly used are water, air, steam and thermal oil. The container system describes the type of geometry used for the thermal energy storage unit.
Various geometries are used for thermal energy storage, they include: Shell and tube geometry, rectangular geometry, spherical geometry. The geometries have PCM or HTF flowing through the tubes as shown in Figure 1:3. Figure 1:3(a) shows the HTF flowing through the tubes and the space within the main shell filled by PCM. Figure 1:3(b) describes a case where the PCM is within the tube and the space within the shell is filled by the HTF. Also, it may flow through channels that have a rectangular cross- section (e.g. plates). Wang et al. (2015b) carried out research which involved the HTF flowing through zigzag channels and Kurnia et al. (2013) carried out computational fluid dynamics analysis on research using serpentine tubes. Other geometries used include; straight tubes, pipes, rectangular channels etc. Much research has been done with various geometries as shown in Figure 1:4, different arrangements of the PCM and HTF.
Qianjun (2016) mentioned that the tube in shell is the most commonly used storage tank in CSP plants. Several researchers utilise the shell and tube geometry, it is also the most applied in industry. To enhance heat transfer, some applications use finned tube in shell. There is also rectangular shaped system, though often popular for use with sensible heat storage.
Figure 1:3: Tube in shell arrangement. Qianjun (2016)
Liu (2012) mentioned that the material used for the compartment containing the PCM should also be taken into consideration since it should be compatible (chemically and mechanically) with the PCM being used; for instance the container should be resistant to the corrosive nature of the PCM. Dincer and Rosen (2011) cited an example of the tendencies of paraffin based PCM to soften some plastics, but are compatible with most metals. However, metal containers may easily corrode when exposed to salt hydrates. Containers also have to be capable of withstanding the expansion of the PCM when it melts and the commensurate reduction in volume when it solidifies.
Trp (2005) carried out experimental and numerical investigation of transient behaviour of PCMs in a shell and tube LHS. The model developed was based on enthalpy formulation for non-isothermal behaviour of PCM. The results were validated with paraffin used as the PCM. Ismail and Henriquez (2002) used a numerical model to simulate heat transfer in a LHS, using ethylene glycol as its heat transfer fluid and a packed bed of spherical capsules filled with water as the PCM. The effect of varying the HTF inlet temperature, mass flow rate, material used for the spherical capsule were investigated experimentally and numerically to understand their effect on the storage unit’s overall heat transfer performance. Barba and Spiga (2003) used three different geometries for the PCM container to analyse the behaviour of encapsulated salt hydrates (PCM) in a domestic hot water tank. Their findings showed the spherical geometry showed better response time in charging and discharging when compared to the tray and cylindrical geometry. The spherical geometry also produced the largest energy density when compared to the others. Belleci and Conti (1993) used the enthalpy method to numerically study the behaviour of PCM solar shell and tube
energy storage system. Esen et al (1998) studied the behaviour of solar water heating systems incorporated with a latent heat system with a cylindrical geometry. Different PCMs were analysed using the same geometry.
Figure 1:4: Different types of PCM storage geometries D’Avignon (2015). The thermal behaviour of the phase change material and type of geometry used as a heat exchanger is vital to the performance of the thermal energy storage. The charging and discharging is highly affected by these factors. Thus it is important that the heat exchanger used should have a large surface area for effective heat transfer between the storage material, geometry used and the heat transfer fluid (Abhat (1980).
Campos-Celador et al. (2014) compared a conventional 500 litre hot water tank used for domestic application with a rectangular finned plate heat exchanger and discovered that the plate heat exchanger allows the required volume to be reduced to half of the conventional hot water tank. Also, the advantage of producing the plate heat exchanger using a simple manufacturing process, high modularity, high surface to volume ratio and the ability to easily integrate the rectangular shaped plate heat exchanger in spaces within domestic homes makes them suitable over conventional cylindrical shaped heat storage sources. Flat plate heat exchangers can easily be integrated in structures of the building and, with the concerns of space in residential homes, it provides a good advantage over other types of geometry used to produce heat exchangers. Due to the modular nature of the plates, it is possible to arrange the plates in parallel or in series based on the kind of arrangement desired. Liu et al. (2014) concluded that flat plate LHTES systems are adaptable and heat enhancement structures can easily be integrated. Polymers are suitable for use with PCMs because they do not react with the PCM and are resistant to pitting, bubbling and precipitation (Dinker et al. (2017). Based on such findings, the polypropylene plate heat exchanger was studied in this
research. The PP sheet with a thin walled plate spacing of 4mm and narrow channels which allows HTF to flow through is used to construct the PHE thermal store. This allows for effective heat transfer between the wall of the PP sheet, HTF and PCM. The modular nature of the PP sheet makes it possible to arrange them in series or parallel. Stacks of the PP sheet can be arranged to obtain the required capacity from the PHE store. Parameters such as PCM thickness, flow rate and thermal properties of the PCM are analysed in this thesis.