# Experimental Study on Two PCM Macro-Encapsulation Designs in a Thermal Energy Storage Tank
## 4 Conclusions  
Macro-encapsulation of phase change materials (PCM) represents one of the most widely used techniques for the implementation of latent heat thermal energy storage systems. The design of the macro-encapsulation is fundamental to archive the best compromise between optimal heat transfer performance and energy stored. However, current literature lacks experimental data on the effect of macro-encapsulation in the performance of latent heat thermal energy storage. This paper analyzed, through an experimental study, the effect of the design of macro-encapsulated PCM on the thermal behavior of a latent heat thermal energy storage tank during both the charging and discharging processes. In this study, external dimensions of the energy storage tank were fixed and two different types of commercial slabs with different thickness filled with the same PCM were tested. The results could be particularly useful to evaluate the best configuration of storage medium when the storage tank is limited with a fixed volume. The results were compared in terms of temperature profile, heat transfer rate, and energy stored/released. The results and the conclusions obtained from this study can be applied to similar configuration of the PCM storage that aim to use rectangular macro-encapsulated slabs as storage medium. The lesson learnt from this study suggests that macro-encapsulation design has a relevant impact on the heat transfer during both charging and discharging processes, so the design of the TES unit should be done and analyzed according to the requirements of the application. The use of a thinner macro-encapsulation design (ThinICE) allowed fitting a larger number of slabs inside the tank. However, the higher amount of encapsulation material and the larger distance between the slabs (i.e., higher HTF channels height) resulted in a 30% less amount of PCM introduced inside the tank with this encapsulation design. With ThinICE slabs, the temperature profiles were less affected by the influence of the mass flow rate, promoting a stratified temperature profile inside the tank in both the charging and discharging processes. Using FlatICE, this effect is more pronounced at low flow rates due to the smaller height of the channels that obstructed the flow at the bottom of the tank during charging and at the top of the tank during discharging. However, at high flow rates, the stratification is reduced with the use of thicker slabs, especially during the discharging process. In all the discharging tests, when the outlet temperature of the tank reached 7 \({}^{\circ}\)C, approximately 85% of the energy previously stored in the tank was discharged. The effect of increasing the heat transfer surface using ThinICE slabs on the power delivered by the storage tank is mostly appreciated at a higher flow rate where the heat transferred by convection is higher. Furthermore, using thinner slabs, the higher heat transfer surface area achieves a higher discharging power but is delivered for a shorter period of time. Therefore, for longer discharging periods and for higher storage capacity given a fixed volume of storage tank, the use of FlatICE should be preferred. Figure 10: Total energy released by the PCM storage tank during the discharging processes of the four study cases presented in this study. **Author Contributions:** Conceptualization, D.V., E.B., and L.F.C. ; methodology, D.V., E.B., and G.Z. ; formal analysis, D.V., E.B., A.C., B.D.M., A.d.G., G.Z., and L.F.C. ; investigation, D.V., A.C., and E.B. ; resources, L.F.C. ; data curation, L.F.C. ; writing--original draft preparation, D.V. and E.B. ; writing--review and editing, D.V., E.B., A.C., B.D.M., A.d.G., and G.Z., L.F.C. ; visualization, D.V. ; supervision, L.F.C. ; project administration, L.F.C. ; and funding acquisition, L.F.C. All authors have read and agreed to the published version of the manuscript. **Funding:** This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 764025 (SWS-HEATING).