Estrategia de diseño de sistemas combinacionales con ayuda de un lenguaje de

PCMs with low thermal conductivity are not adequate for latent heat thermal energy storage (LHTES),this prolongs the charging and discharging time (Tian and Zhao

(2009), Chiu (2011)) .The higher the thermal conductivity, the higher the heat transfer in the medium which reduces the charging and discharging period. To enhance their thermal conductivity various methods have been utilized by researchers; Colella et al. (2012) added naturally expanded graphite matrix to paraffin (RT 100) to achieve the heat fluxes needed for a medium scale LHTES for district heating systems. Heat transfer enhancement is dependent on the type of application. Velraj et al. (1999) stated that in an application, such as water heat recovery which involves a process that is intermittent; for heat to be recovered in a short time, heat transfer enhancement required is for charging. If the application involves a case where the heat is available at a constant rate for a longer period of time and needed to be released quickly as in the case of a solar domestic hot water application, then heat transfer enhancement required is for solidification.

Weinstein et al. (2008) mentioned that the effect of the low thermal conductivity of PCMs reduces their effectiveness in a TES application with large volumes by creating larger thermal differences within the PCM. Weinstein et al. (2008) used graphite nanofibers (GNF) to enhance the thermal conductivity of the PCM with a phase change temperature of 56°C. The thermal performance was found to be dependent on the GNF loading. As the GNF weight loading increased, the thermal response increases initially, but at high GNF loading the effect on the thermal response is negligible. However, it was discovered that certain portion of the PCM did not undergo phase change, which means the latent heat of fusion was not fully utilized in the LHTES. Arasu and Mujumdar (2012) reported that by adding a certain amount of alumina particles to paraffin wax, the thermal conductivity could be increased. (Halalwa (2005), Farid (1990)) proposed ensuring minimal distance between PCM and fins of heat exchangers or pipes, which requires using thin plate arrangement to enhance heat transfer. Shatikian et al. (2005) carried out a numerical study using fin spacing of 0.5 to 4 mm. The fins were made from aluminium with a thickness of 0.15 mm to 1.2 mm. Fins are mostly made from copper, steel, aluminium or magnesium. They are used to enhance the thermal conductivity into and out of the PCM. High thermal conductivity of the fins and large surface area help to enhance the performance of a latent heat thermal energy storage. Frazzica et al. (2017) compared two different heat exchangers based on the latent heat storage concept. The heat exchangers were an asymmetric plate heat exchanger and a fin and tube heat exchanger. The plate spacing for the plate

that the spacing dimensions were chosen in order to have an effective heat transfer between the heat exchangers and PCM. The fin and tube heat exchanger with welded fins performed better than the plate heat exchanger in terms of heat transfer. This is attributed to the presence of fins, which enhance heat transfer between the heat exchanger and the PCM. ACT (2018) stated that an optimal fin spacing of 1.27mm to 2.54 mm was used for a PCM heat sink for advanced cooling technique (ACT). It is observed that it takes a shorter time for the PCM to melt when there are more fins, and it takes a longer time for the PCM with the same volume to melt, when there are fewer fins.

For this research, the choice of using a plate heat exchanger made from polypropylene with a plate spacing of 4mm on the water side is based on the fact that an effective heat transfer would exist between the heat exchanger and water. The PP sheet as shown in Figure 6.4 provides a thin walled structure (0.3mm) which is suitable for effective heat transfer between the HTF, wall and PCM on top of the PP sheet during charging and discharging process. PP sheet is chemically resistant to attack from PCMs and due to the thin walls, it will be useful as an effective PHE.

There have been reports by some researchers that heat transfer could be increased by addition of nanoparticle copper to the PCM (nano fluid). Elgafy and Lafdi (2005) used carbon nano fibres (CNF) to enhance the thermal conductivity of a paraffin wax with a melting temperature of 67°C. Reddy (2007) used different number of fins attached to the PCM-water system to enhance heat transfer of a solar-integrated collector storage water heating system with PCM. However Chiu (2011) stated in his thesis that having larger number of fins to enhance thermal conductivity of PCM, could lead to reducing the IPF(Ice packing factor) of the PCM, which in turn reduces the energy storage capacity. The increase in cost of fabricating fins and difficulty of filling the PCM in the system prompted Chiu to state that the storage design is vital in improving the thermal performance of the PCM used. Dutil et al. (2011) stated that more fins also decreases the volume of PCM. This research looks at using low cost plate heat exchanger concept using polypropylene sheet with channels and a serpentine heat exchanger to enhance heat transfer in the thermal store.

Hassan (2014) studied three different models with pipes that have no fins, four fins and eight fins. He discovered the heat transfer was enhanced for the pipes with fins

compared to the pipe without fins as shown in Figure 2:2. Also, using multiple loop arrangement of pipes with different pipe diameters (40mm, 42.4mm and 46.64mm) in a 3D model, it was discovered that the largest pipe melted faster compared to the other two. Hassan’s simulations are not backed up with experiments. He claims the only way to improve the heat transfer of a PCM with low thermal conductivity was by increasing the heat transfer area. Increasing the HTF inlet temperature, varying the mass flow rate, geometry etc. was not considered in his analysis.

Figure 2:2: Melting phase comparison for the different models.Hassan (2014) The type of geometry has been known to affect the overall heat transfer performance of a thermal storage unit. Various types of geometries exist such as plate heat exchanger, shell and tube heat exchanger, U-tube, U-tube with inline fins, U tube with staggered fins and festoon design etc. Figure 2:3 shows an image of various geometries used for thermal storage.

The melting time of each of these designs for thermal energy storage varies. Kurnia et al. (2013) concluded that the serpentine design shown in Figure 2:2 (d) offers the best heat transfer when compared to other geometrical design; U-tube, U-tube with inline fins, U tube with staggered fins. Paraffin wax was used for the study and the mathematical model was based on conjugate heat transfer between the heat transfer fluid (water) and PCM. The U tube with fins shown in Figure 2:2(c) improved heat transfer compared to the U tube without fins shown in Figure 2:2(a), which fortifies the advantages the use of fins brings to a TES. The addition of fins enhances the heat transfer surface, thus improving heat transfer. However, the conclusion by Kurnia cannot be validated because it was based on computational analysis and without any experimental analysis.

Figure 2:3: Schematic drawing of a) U-tube, b) U-tube with fins, c) U-tube with staggered fins and d) festoon design. Kurnia et al. (2013)