## 4 Discussion  
Figure 11 presents a summary of the KPIs used to compare the three modules when operated as heat exchangers (in three-fluids HEX mode), and as TES modules (in charging and discharging mode). The y-axis (in the left) presents the KPIs, which include the COP, the effectiveness, the energy level at thermal equilibrium (in three-fluids HEX mode), the discharging power, the charging power, and the energy storage capacity. The y-axis (in the right) presents the absolute value of each KPI as normalized by volume, whereas the x-axis presents the ratio of each KPI type (i), to the maximum value (KPI\({}_{i,\text{max}}\)) obtained from either of the three modules. The maximum KPI value among the three modules is registered as 100%, whereas the other values are reported as its ratio highlighting a broader aspect in the comparison of the modules and the KPIs. Figure 10: Results of three-fluids HEX mode: (**a**) energy level at thermal equilibrium and effectiveness and (**b**) COP of the refrigeration system against time. Figure 9: A summary of the results three-fluids HEX mode (**a**) temperature at the condenser, (**b**) pressure at the condenser and evaporator, and (**c**) cooling load at the evaporator. In the first case, when operating the modules as TES modules, TES\({}_{1}\) registered the highest charging and discharging power, but with the lowest energy storage capacity (the experimental value of the energy recovered from the modules during the discharging process from \(-4\)\({}^{\circ}\)C to \(12\)\({}^{\circ}\)C). TES\({}_{2}\) registered the second in all the KPI values. It registered a discharging power 6% lower than the maximum, and 21% lower charging power to the maximum (that of TES\({}_{1}\)). The energy storage capacity was 8% lower than the maximum (that of TES\({}_{3}\)). TES\({}_{3}\) registered the lowest charging and discharging power around 33% lower than the maximum value. However, the high energy storage capacity of TES\({}_{3}\) is expected because of its high PCM density in comparison with TES\({}_{1}\), which has less PCM density since it contains more refrigerant and HTF channels. The high charging power obtained in TES\({}_{1}\) could be argued to result from its high heat transfer surface area, which is 2.3 times higher than that of TES\({}_{2}\), and 2.8 times higher than that of TES\({}_{3}\). Likewise, the high discharging power obtained in TES\({}_{1}\) is accounted by the high active surface area for heat exchange, which is 2.5 times higher than that of the other modules. However, despite the same surface area for heat transfer and amount of PCM, TES\({}_{2}\) presented 25% higher discharging power than that of TES\({}_{3}\), meaning that the arrangement of the HTF channels allows the PCM to discharge faster in TES\({}_{2}\) than in TES\({}_{3}\). Moreover, it was noted that to achieve a complete phase change, it was required to charge TES\({}_{2}\) and TES\({}_{3}\) to lower temperature values due to the nonhomogeneous charging of the modules. However, operating the refrigeration systems at extreme low-temperature values is not a desirable feature and one must take care to avoid raising issues such as defrosting and compromising the COP. In the second case, when the modules were operated as heat exchangers, the KPIs discussed include the COP, the effectiveness, and the SOC (energy level in the module) at thermal equilibrium. TES\({}_{1}\) registered the highest value for the three KPIs. TES\({}_{2}\) registered the second highest values with COP relatively similar to TES\({}_{1}\) (less than 5% of the maximum, 4.4), effectiveness around 24% lower, and SOC at thermal equilibrium around 38% lower, whereas TES\({}_{3}\) registered the lowest values with COP around 19% lower, effectiveness around 33% lower, and SOC at thermal equilibrium around 44% lower. Among the three KPIs, the change in design is observed to affect less the COP when compared with the other KPIs.