## 2 Methodology
### PCM Selection Methodology  
The first step of the selection methodology consisted of a thorough review of potential PCM candidates available in the literature, whether commercial or not, for both MED and CON systems. Therefore, the initial search was performed focusing only on the phase change temperature of the PCM. A wider temperature range, from 0 \({}^{\circ}\)C to 7 \({}^{\circ}\)C and from 48 \({}^{\circ}\)C to 68 \({}^{\circ}\)C, was initially considered for the MED and CON systems, respectively, due to some uncertainties regarding the optimal phase change temperature at the very preliminary stage of the project development, when the PCM selection process began. As a second step of the selection methodology, given the relatively large number of possible PCM candidates found in the first step, a pre-selection was performed in order to exclude PCMs that were not suitable either because of health hazard issues, corrosion with the tank container material (aluminum), or very bad thermophysical properties. Likewise, when two or more different PCM had similar features, only the most suitable of them was chosen. Once a reduced group of potential PCM candidates was obtained, the third step of the methodology consisted of the development of a decision matrix as a tool to make a final selection based on objective criteria. Since there is no standard selection procedure, different selection criteria can be used, depending on the focus of the application. General properties are usually taken into account, such as phase change enthalpy, width of the phase change range, subcooling, hysteresis, thermal conductivity, density, availability, toxicity, or cost. Some of these properties were used in the pre-selection step to exclude some of the potential PCM candidates, therefore the decision matrix was built to compare the different valid options and to find the most promising ones for the two HYBUILD systems. Hence, the following parameters were considered in the decision matrix: width of the phase change range, melting enthalpy, availability, price, and maximum working temperature in the case of the CON system. The selection of these parameters is justified by their direct impact on the system viability from the operational and economic points of view, and the relative easiness in obtaining their values. A score was assigned to each of the above decision parameters for each PCM candidate taking into account the criteria shown in Table 1.  
Next, a total score was calculated for each of the PCM, based on a weighted average of the partial scores obtained for each of the decision parameters. With regards to the values chosen for the weight, two scenarios were considered, as shown in Table 2: Scenario 1, in which the value of each decision parameter was selected based on authors criteria based on their experience, and Scenario 2, in which all decision parameters were given the same weight. The weighting of the parameters in Scenario 1 is the one that suits the main requirements of the systems, such as compactness and cost-effectiveness of the TES solution, which make it so PCM enthalpy and price have a higher weight with respect to the other decision parameters. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Temperature Range** & \multicolumn{2}{c}{**Enthalpy**} & \multicolumn{2}{c}{**Availability**} & \multicolumn{2}{c}{**Price**} & \multicolumn{2}{c}{**Maximum**} \\ (T, in \({}^{\circ}\)C) & & **(h, in kJ/kg)** & & **(-)** & & **(P, in \&/kg)** & **Temperature** \\ \hline T \(<\) 2 & 3 & h \(>\) 250 & 3 & Yes & 3 & P \(<\) 2.5 & 3 & T\({}_{\text{max}}>\) 120 & 3 \\
2 \(<\) T \(<\)3 & 2 & 200 \(<\) h \(<\) 250 & 2 & No & 0 & 2.5 \(<\) P \(<\) 5 & 2 & T\({}_{\text{max}}<\) 120 & 0 \\
3 \(<\) T \(<\)4 & 1 & 150 \(<\) h \(<\) 200 & 1 & - & - & 5 \(<\) P \(<\) 10 & 1 & - & - \\ T \(>\) 4 or n.a. & 0 & h \(<\) 150 or n.a. & 0 & - & - & P \(>\) 10 or n.a. & 0 & - & - \\ \hline \hline \end{tabular} n.a.—not available.  
\end{table} 
Table 1: The scoring criteria applied to each decision parameter.