## 2 Materials and Methods
### Methods  
Two types of analysis were carried out, one regarding the thermal properties (melting temperature and melting enthalpy) of the PCM itself and another regarding stability (no material degradation) against thermal cycles. Finally, a corrosion analysis was carried out to check the compatibility with different steels for the container. **Characterization of the PCM thermal properties (melting temperature and melting enthalpy). **  
The PCM was thermally characterized (melting temperature, enthalpy and stability over time) by using a DSC/TGA of the brand TA Instruments, model SDT Q600. The test was carried out in a nitrogen atmosphere with a 10 \({}^{\circ}\)C/min heating rate within the range of 500 to 750 \({}^{\circ}\)C.  
**PCM stability against thermal cycles when enclosed into a stainless-steel container. **  
To check the PCM thermal stability, a thermal cycle was designed and run in the muffle furnace, which consisted of increasing the temperature above the PCM melting point followed by a cooling-down stage. This way, the PCM energy absorption time in the melting and crystallization plateaus was obtained. Thermal cycles were carried out using a Hobersal brand muffle, model HD 230. To collect the data and monitor the process, an Almemo 8590-9 data logger from the AHLBORN brand was used, as well as type K thermocouples, from the Recmann brand. The test temperature range was from 580 to 800 \({}^{\circ}\)C.  
Several containers in AISI 304 stainless steel were manufactured. Two models of cylindrical containers were designed, one 50 mm high, 43 mm in diameter and 1.5 mm thick and the other 76 mm high, 76 mm in diameter and 1.5 mm thick. Both containers \begin{table}
\begin{tabular}{c c} \hline
**Salt System** & **Melting Point (\({}^{\circ}\)C)** \\ \hline
**Li2 CO3** & **723** \\ Na2 CO3 & 854 \\ K2 CO3 & 891 \\ L2 CO3–Na2 CO3 (52–48 mol\%) & 501 \\ Li2 CO3–K2 CO3 (62–38 mol\%) & 498 \\ Na2 CO3–K2 CO3 (56–44 mol\%) & 710 \\ \hline Li2 CO3–Na2 CO3–K2 CO3 (43.5–31.5–25 mol\%) & 397 \\ K2 CO3–MgCO3 (57–43 mol\%) & 460 \\ \hline \end{tabular}
\end{table} 
Table 1: Melting points of single alkali carbonate salts and common carbonate eutectic mixtures [5]. possessed two holes in the top cover, one located in the central area and the other between the central area and the outer edge. These holes were used to place the temperature control thermocouples and to fill in the container with PCM. As lithium carbonate comes in powder form, the first step for its use was to melt it in order to deaerate it. The PCM filling was performed in several steps (filling, melting and refilling and melting until the PCM melted mass is obtained). To do this, small amounts of material were introduced into the container, as shown in Figure 2, and then melted at 750 \({}^{\circ}\)C so that the material compacts and eliminates the air; then, more PCM powder was added and melted. This operation was carried out as many times as necessary until the container was filled up with the required PCM mass, in this case 160.8 g. Then, the sample was ready for the thermal stability test. The location of the high-temperature \"K\"-type thermocouples in the device is shown in Figure 3. The temperature in the center and side of the container as well as the outside temperature (oven temperature) were recorded every second for a greater control of the process. Figure 4 shows the assembly of the test in muffle. Figure 3: Assembly of the device with thermocouples. Figure 2: Filling the container with lithium carbonate following several steps to assure the PCM deaeration. Corrosion test to check the compatibility of the PCM with different steels for the container. After the stability against thermal cycles test, the containers were cut to see whether there had been corrosion in the container material (stainless steel) that could have damaged it. Given the importance of selecting a container material resistant to contact with carbonates at high temperature, a corrosion test was run for two stainless steels, AISI 304 and AISI 310.