## 2 Thermochemical Energy Storage Design
### Experimental Procedure  
The reactor can work in two modes: (i) as a fixed packed bed and (ii) as a moving bed. The rotary valve (see Figure 3) is permanently disabled in the mode (i) and all the solid material in the reactor is hydrated progressively from the initial state \(x_{0}\) to the maximum allowed state \(x_{max}(t)\). For the experiments in the mode (ii), the rotary valve is actioned automatically by the stepper motor every \(\Delta t_{s}\) seconds, making the hydrated material fraction to be evacuated from the reactor, while the new anhydrous fraction refills the reactor from the upper hopper (see Figure 3). Therefore, there is a hydration gradient \(x=x(z,t)\) along the vertical axis \(z\) inside the reactor in the mode (ii), with the boundary conditions at the reactor inlet \(x(t)=x_{0}\), \(z=0\) and at the reactor outlet \(x(t)=x_{max}\), \(z=L\), where the overall hydration time is \(t=\sum_{i=1}^{N}\Delta t_{s}\) and \(N\) is total number of valve turns. Moreover, the air-to-air heat exchanger HX2 can be by-passed by the valve V3 (see Figure 2), so that the reactor R1 can be run with or without heat recovery from the air-to-air heat exchanger. The operations used for the experimental protocol in the mode (i) and (ii) were identical, with the only difference that the rotary valve remained permanently disabled in the mode (i). Primary to experiments, the material was dehydrated in the laboratory oven at 150 \({}^{\circ}\)C for 24 h. Before loading the material to the upper hopper, the centrifugal fan P3 and the hydraulic pump P1 were running at least 20 min in order to check the measurement equipment and to reach the isothermal conditions. The valve V3 was positioned in such a way that the air-to-air heat exchanger HX2 was by-passed. Once the isothermal conditions were reached, the fan P3 was briefly off, and the material was loaded to the \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Value** & **Method, Apparatus Used** \\ \hline Specific surface area (m\({}^{2}\)/g) & 75 & Brunauer-Emmett-Teller method, \\  & & BELSORP-max [35] \\ Total pore volume (cm\({}^{3}\)/g) & 0.38 & N\({}_{2}\) sorption measurement at 77 K, \\  & & BELSORP-max [35] \\ Packing density of anhydrous material (kg/m\({}^{3}\)) & 703 & Bulk measurement [35] \\ Reaction heat (kJ/kg) & 2757 & Efficiency test using TGA/DSC 111 \\  & & connected to Wetsys unit (Setaram) [35] \\ Apparent thermal conductivity of material at & 0.16 & Transient hot bridge method in VTRK300 \\ water mass uptake \textgreater{} 0.14 kg/kg (W/m-K) & & climate chamber (Heraeus Vötsch) [36] \\ \hline \end{tabular}
\end{table} 
Table 2: Physical and thermal properties of SG/CaCl\({}_{2}\) (43 wt.%) composite. Figure 4: Characteristic curve of sorption of water vapor on SG/CaCl\({}_{2}\) (43 wt.%) composite [35]. reactor through the upper hopper. The experiment started immediately once the centrifugal fan was again on and all the measured data were logged by the data acquisition system to the computer. For the experiments with reactor running in the moving bed mode, the rotary valve was manually turning 3 to 5 to turns to let the material to be better packed in the reactor, afterwards the rotary valve worked in the automatic mode every \(\Delta t_{\mathrm{s}}\) seconds. All the experiments started with the heat exchanger HX2 set in the by-pass mode. The heat recovery by the air-to-air heat exchanger HX2 was activated if the air temperature rejected at the hot side was higher than the air temperature at the reactor inlet. The activation of the heat recovery was done by turning the valve V3 in the direction of the heat exchanger HX2. The decision to stop the experiment was taken with regards to the temperature difference threshold at the cold side of the air-to-water heat exchanger (<1 \({}^{\circ}\)C) and the air humidity threshold (\(\sim\)30% of R.H. at 20 \({}^{\circ}\)C) at the outlet of the reactor. The performance parameters of the prototype in mode (i) and (ii) were evaluated using the following equations.