## 4 Discussion  
Selecting sorption reactions for sorption reactors involves collecting relevant vapor pressure lines. Fortunately, collections of parametrized vapor pressure are available [5; 10; 11], but it is not always easy to check the validity of the available data, as the original sources and with that the experimental methods and conditions are often difficult to access. For that purpose, we studied the thermodynamic background of vapor pressure lines in order to validate collections by mutual comparison of the data contained. For liquid-vapor equilibria, we considered the van der Waals equation, and for solid sorption, the Polanyi-Wigner equation. In particular, we examined conditions for [PERSON]'s rule (1884) to hold, as this would provide a particularly useful method to examine the validity of vapor pressure lines compared to an existing collection. It is known that for liquid-vapor equilibria, [PERSON]'s rule cannot be derived from a law of corresponding states such as the van der Waals equation [17; 24], but one can argue from the rapid increase of vapor pressure lines that [PERSON]'s rule may roughly hold for collections of common fluids. For solid sorption, the validity of [PERSON]'s rule seems to depend on the entropy or free space of sorbent molecules in the bound state, suggesting that for subgroups with certain types of bonds, [PERSON]'s rule will hold. Indeed, for the chloride ammoniate and the sulfate hydrate subgroups, [PERSON]'s rule was particularly accurate, with a high regression coefficient. When adding other salts to  
Figure 12: Heat storage densities \(Q/V\) (GJ/m\({}^{3}\)) for salt hydrates, obtained for the same collection as Figures 10 and 11 for cases for which we found hydrate densities \(\rho\) (kg/m\({}^{3}\)). Orange points are transitions with the lowest and highest transition entropies. We see that due to hydrate densities and the amount of water in the transition, \(Q/V\) shows no clear correlation with \(T_{d}\), according to \(r^{2}=0.19\). Figure 13: Heat storage densities \(Q/V\) (GJ/m\({}^{3}\)) for salt hydrates, including the volume of process water in \(V\) for the same data as for Figure 12. Orange points are transitions with the lowest and highest transition entropies. Including process water in the storage density is fair for closed systems as in Figure 1 where process water is kept for rehydration. Consequently, the resulting heat storage density is smaller. In addition, the correlation heat between storage and temperature is more accurate, with \(r^{2}\) = 0.64 compared to \(r^{2}\) = 0.19 for Figure 12.  
the hydrate collections, the regression coefficient becomes considerably smaller, although the fitted line seems about the same. Possible explanations for this observation may be the limited accuracy of the additional data for other salts or a fundamental difference in the binding of water to the other salts. We note that the entropy difference of evaporation of the work fluid, e.g., water or ammonia, considerably differs from the corresponding entropy differences of sorption in salts. This is different for adsorption in, for example, activated carbon, silica gel, and zeolite, and for condensation in salt solutions, where these entropy differences are approximately equal [15; 16]. It would be interesting to keep track of [PERSON]'s rule when new sorption measurements on ammoniation or hydration transitions are done or when old measurements are verified. Depending on the fundamental accuracy of [PERSON]'s rule, not only data validation can be done but also predictions of vapor pressure lines for transitions of which only the latent heat is known. [PERSON]'s rule sets a limit to the possible heat storage density for THS systems for given desorption and sorption temperatures in terms of heat per mole sorbent. In terms of heat per volume of hydrated salt, no clear correlation with hydration and dehydration temperatures was found.