# Experimental Study on Two PCM Macro-Encapsulation Designs in a Thermal Energy Storage Tank
## 3 Results and Discussion
### Heat Transfer Rate Evolution and Total Energy Stored in the Charging Process  
Figure 6 presents the evolution of the heat transfer rate (HTR) during the charging process of the four studied cases. Due to the characteristics of the experimental set-up, at the beginning of the experiment, the inlet temperature of the tank oscillated \(\pm\)2 \({}^{\circ}\)C with respect to the desired temperature, affecting the initial peak of the heat transfer rate. However, the inlet temperature stabilized (with Tin standard deviation lower than 0.3) before the temperature inside the tank reaches the latent range of the PCM. The HTR profiles showed an exponential behavior with significantly higher values during the first 20 min of the process when the heat is mainly transferred to the HTF inside the tank and, therefore, rapidly increases its temperature. Afterwards, while the PCM temperature increases, the values of the heat transfer exponentially decrease until minimum values.  
Figure 5: Charge PCM slab temperature profile for different slabs and HTF flow rates: (**a**) C_ThinICE and 2 L/min, (**b**) C_FlatICE and 2 L/min, (**c**) C_ThinICE and 4 L/min, and (**d**) C_FlatICE and 4 L/min. Note: The red line denotes the end of the charging experiment (T_out = 25 \({}^{\circ}\)C). The time axis is not presented on the same scale in all the figures.  
During the first 1.5 h of operation, ThinICE_2L and FlatICE_2L showed similar HTR values, which indicates that, due to the low flow rate, the heat transfer by convection is low. Therefore, the higher heat transfer surface area existing with ThinICE slabs is not fully exploited. Moreover, after 1.5 h the HTR delivered to the ThinICE_2L decreases faster than the one delivered to the FlatICE_2L due to the higher amount of PCM introduced into the tank with FlatICE slabs. At a higher flow rate, heat transfer by convection increases. Therefore, during the first 1.5 h of operation, the bigger heat transfer surface area present with ThinICE_4L slabs increases its HTR over FlatICE_4L. After this period, and similar to the results at 2 L/min, the power delivered to the ThinICE_4L decreased faster than the one delivered to the FlatICE_4L.  
When analyzing the effect of the flows in each slab type, the influence is greater in the tank with ThinICE slabs obtaining, after the initial peak, up to 0.4 kW more in ThinICE_4L than in ThinICE_2L. In the case of the tank with FlatICE, this increase drops to 0.1 kW when comparing FlatICE_4L vs FlatICE_2L. The latter results corroborate the statement above; the increase in heat transfer by convection, as the flow rate increases, is more pronounced in the tank with ThinICE slabs due to the larger heat transfer surface and the lower thickness of the PCM layer using this type of slab.  
Figure 7 reports the total energy stored for each experiment condition. The results with ThinICE slabs show that the flow variation did not affect the total stored energy, suggesting the correct utilization of the energy storage capacity of the PCM. Conversely, when analyzing the tank with FlatICE slabs, the charging experiments at 4 L/min stored 10% less energy than the same experiment at 2 L/min. This is due to changes in the flow rate distribution between the slab channels inside the tank when increasing the flow rate. At the end of the experiment, (T_out 25 \({}^{\circ}\)C) with FlatICE at 4 L/min, the PCM in the bottom slabs of the tank had not completed the phase change (Figure 5) and therefore stored 8% less energy.  
Figure 6: Evolution of the HTF heat transfer rate during the charging processes of the four study cases presented in this study.