## 4 Energy Storage Density  
In order to evaluate the improvement of the reaction kinetics achievable by the foamed structure compared to the bulk MgSO4 7H2O, as well as to evaluate the TES storage density of the investigated materials, TG/DSC analyses under pure water vapor conditions during a dehydration process were performed for the pure salt, FOAM50, FOAM60, and FOAM70 samples. Figure 5 compares the DSC thermograms and the TG evolution obtained under the mentioned working conditions. The first observation is that the three foamed samples clearly show desorption peaks, while the pure salt only shows a very small desorption peak. This behavior can be attributed to a kinetic hindering the desorption process due to the bulk state of the MgSO4. This is already a positive feature confirming the usefulness of the proposed approach to overcome issues related to the pure salts. Analyzing the foamed samples, it is evident that in the investigated temperature range, there are three distinct reactions, whose peak temperatures can be identified at about 65 \({}^{\circ}\)C, 111 \({}^{\circ}\)C, and 146 \({}^{\circ}\)C, respectively. Nevertheless, about 90% of the overall energy is stored during the first reaction, thus confirming the applicability of this material in a low/medium temperature range. Finally, as expected, FOAM70 shows wider peaks compared to FOAM60 and FOAM50, since the heat flow is normalized over the composite mass. Accordingly, the role of the active material (i.e., the salt) is more evident for the samples with higher salt content. In order to analyze the actual energy associated with the process, the integration of each thermogram was performed. Table 1 summarizes the results obtained by the data analysis. At first, the actual salt content in the three foamed samples was evaluated, by comparing the theoretical and the actual mass loss encountered during the test. As it can be highlighted, in both cases, the amount of salt inside the porous structure was lower than the theoretical one. There was a discrepancy of about 16-18% compared to the nominal content. This could be due to the sample handling process, during which the salt deposited close to the surface, so not very well embedded inside the foam, may be partially lost. Nevertheless, the achieved sorption enthalpy for the whole temperature range, 1012.8 J/g and 786.2 J/g, for FOAM70 and FOAM50, respectively, are quite attractive and in line with the data reported in the literature in [30]. The FOAM60 batch showed intermediate values compared to the other composite foams. As already mentioned before, most of the energy associated with the process is released during the first stage, where, according to [30], the equivalent of 5.9 water molecules are released. Referring to the measured density of the samples, which is in the range of 330-470 kg/m\({}^{3}\) (with highest density for FOAM70 and lowest one for FOAM50), it was also possible to evaluate the achievable energy density considering a low/medium charging temperature (e.g., around 100 \({}^{\circ}\)C), thus involving only the first reaction occurring. The calculated volumetric energy densities evidenced that the highest value is obtained for FOAM70 where a volumetric energy density equal to 0.48 GJ/m\({}^{3}\) was observed. The foams with lower salt contents highlighted a less effective volumetric energy density. In particular, FOAM50 showed a value of 0.26 GJ/m\({}^{3}\). The achievable TES density of the developed foams was compared to the reference sensible water storage. Figure 6 reports the energy density increase of the foamed samples against the sensible water as a function of the temperature difference under which the sensible TES can operate. For very small temperature ranges, i.e., 20 and 30 K, the increase reached up to 470%. These operating conditions can be assumed when a low temperature floor heating system is employed.