## 1 Introduction  
Here, we would like to share some results of our investigations on selecting salt hydrates for application in heat storage systems. Selection criteria are, for instance, a high heat storage density at favorable operating temperatures, sufficient output power, cycling stability, and low costs and low toxicity [1; 2; 3; 4; 5; 6; 7; 8; 9]. Examples of salt hydrates that could be used for heat storage are Na\({}_{2}\)S\(\cdot\)5H\({}_{2}\)O (cf. [1; 2; 3; 4]) and K\({}_{2}\)CO\({}_{3}\cdot\)1\(\times\)H\({}_{2}\)O [5], but there are many other candidate reactions [6; 7; 8; 9]. Selecting a suited salt hydrate depends on a number of its properties, which are obtained from experimental data. Validation of such data is our main objective here, and we will discuss the application of [PERSON]'s rule for that purpose. In order to properly introduce our research questions and their relevance, we start with a brief discussion on heat storage by salt hydrates. Salt hydrates can be used for heat storage by removing the contained crystal water molecules with the available heat, e.g., by the following transition:  
\[\text{K}_{2}\text{CO}_{3}\cdot\text{1}\text{2}\text{H}_{2}\text{O}+\text{Heat }\leftrightarrow\text{K}_{2}\text{CO}_{3}+\text{1}\text{2}\text{H}_{2}\text{O}.\]This is an example of a reversible thermochemical reaction, and K\({}_{2}\)CO\({}_{3}\)\(\cdot\)1\(\%\)H\({}_{2}\)O is therefore a thermochemical material (TCM). The reaction products K\({}_{2}\)CO\({}_{3}\) and H\({}_{2}\)O can be stored separately and the input heat can be recovered by rehydrating K\({}_{2}\)CO\({}_{3}\) with water vapor. This way of heat storage is virtually loss-free and therefore particularly suited for long-term storage. Figure 1 is a sketch of a system for thermochemical heat storage (THS), designed for seasonal storage of excess solar heat in summer for later use in winter [4]. For heat storage, heat from solar collectors of, e.g., 80-90 \({}^{\circ}\)C, is transferred to a hot water storage tank. When sufficient heat is collected, hot water is transferred by a heat exchanger (HX) to a reactor containing modules with TCM, e.g., a hydrated salt. With the available heat, the salt is dried while the desorbed water is captured by a pool condenser. Condensation heat is transferred to a reservoir, which may be a borehole energy storage (BES). During heat storage, the salt is kept dry by closing a valve to the condenser. For heat recovery, the valve is opened. Then, the pool acts as an evaporator providing water vapor for rehydration. The hydration heat is transferred to hot water storage for further application. Meanwhile, the BES provides 10-15 \({}^{\circ}\)C heat to the evaporator to prevent it from cooling down during evaporation. For this THS system, we need a salt hydrate dehydrating at 80-90 \({}^{\circ}\)C while the process water from the salt is condensed in the pool at, for example, 20 \({}^{\circ}\)C during summer. From the vapor-pressure lines (pT-lines) of water and a candidate salt hydration transition, one can determine whether the candidate transition is suited and indeed takes place at the given temperatures. As an example, Figure 2 shows the pT-lines of water (blue) and of the transition K\({}_{2}\)CO\({}_{3}\)-(1\(\%\)-0)H\({}_{2}\)O (green). The horizonal red line shows the vapor pressure of about 23 hPa in the condenser at 20 \({}^{\circ}\)C. According to the intersection with the pT-line of K\({}_{2}\)CO\({}_{3}\)-(1\(\%\)-0)H\({}_{2}\)O, the K\({}_{2}\)CO\({}_{3}\)-1\(\%\)H\({}_{2}\)O needs to be heated to at least 68 \({}^{\circ}\)C to dehydrate.