## 3 Results
### Composite Foams Obtained through Almost Free Expansion Process
#### 3.1.2 Adsorption Capacity  
As shown in Figure 3, water adsorption isotherms of pure SAPO34 and of Z20, Z40, and Z60 have been measured at 30 \({}^{\circ}\)C and 70 \({}^{\circ}\)C. The sharp uptake rise at low relative humidity (RH \(<\) 15-20%) and, more generally, the isotherm shape of the pure material is similarly present also in the foams proposed in this work. At each step of relative humidity, sorption capacities of Z20, Z40, and Z60 are lower than one of pure SAPO34. This effect is related to the presence of silicone (included in the reference mass of each sample) that does not participate in the sorption process. As discussed in a previous research [22], the reduced adsorption capacity is in agreement with the zeolite mass fraction. In fact, if the adsorption capacity were referred only to the dry mass of zeolite of each sample, adsorption isotherm curves of pure desiccant and of foams would be very close (maximum reduction around 14%).  
Based on obtained results, Z60 foam keeps a high sorption capacity of the SAPO34 and, therefore, can be effectively used to develop innovative components for heat storage applications.  
Experimental data at 30 \({}^{\circ}\)C and 70 \({}^{\circ}\)C and additional ones at 50 \({}^{\circ}\)C have been used to fit parameters of sorption isotherm curves proposed by [PERSON] (D-A) [25]:  
\[W_{ads}=W_{0}\;e^{-(\frac{4}{E})^{n}}=W_{0}\;e^{-(\frac{-RT\;h(RH)}{k})^{n}} \tag{4}\]  
where \(W_{0}\) is the maximum sorption capacity, \(A\) is the adsorption potential, \(E\) is the characteristic D-A parameter and \(n\) is an exponent describing the surface heterogeneity. The adopted fitting approach has already been used by other researchers [26; 27] for SAPO34, with satisfactorily results. Parameters can be fitted by using entire data between minimum and maximum sorption capacity (obtaining one set of parameters) [26] or by dividing the uptake curve in different sections (obtaining multiple sets of parameters) [27]. Authors adopted the first approach because it is particularly simple and obtained correlation can be easily implemented in phenomenological models of the device, although the second one generally leads to a more accurate correlation. Obtained parameters of Z60 are \(W_{0}\) = 0.1486 kg kg\({}^{-1}\)  
Figure 3: Adsorption capacity of pure SAPO34 and zeolite foams at different temperature.  
E = 420 kJ kg\({}^{-1}\) and n = 4.01 with R-square equal to 0.9376. Measured adsorption equilibrium curves and numerical results obtained through Equation (4) are reported in Figure 4.  
Finally, according to the adopted D-A approach, it is worth specifying that the heat of adsorption can been calculated as:  
\[Q_{\text{ads}}=\lambda+A \tag{5}\]  
where \(\lambda\) is the enthalpy of evaporation of water at a given temperature and \(A\) is the adsorption potential. In Figure 5, the heat of adsorption of Z60 is calculated and plotted against the sorption capacity for different temperatures (by using Equations (4) and (5)).  
Figure 4: Adsorption capacity Z60 zeolite foam at different temperatures as a function of the adsorption potential.  
Figure 5: Z60 zeolite foam: heat of adsorption as a function of adsorption capacity at different temperatures.  
Although the evaluation of the adsorption kinetics is not the scope of this work, it is worth specifying that silicone foam does not affect significantly adsorption kinetics. Sorption characteristic time of pure zeolite and foamed samples is of the same magnitude order.