## 3 Results  
### The Experimental Tests  
Using the test rig, a wide experimental campaign was carried out in order to assess the charging and discharging efficiency of the system for different operative conditions. The experimental data which were collected have been used for the following validation of the mathematical model. The efficiency of charge and discharge phases is evaluated as follows:  
\[\eta_{charge\ phase}=\frac{Q_{sens}+Q_{des}}{Q_{HTT\_c}} \tag{15}\]  
\[\eta_{discharge\ phase}=\frac{Q_{HTT\_d}}{Q_{sens}+Q_{ads}+Q_{E/C}} \tag{16}\]  
where \(Q_{sens}\) is the sensible heat that is stored/extracted from the zeolite, bath, stainless-steel case, and water adsorbed during the charge/discharge phase thanks to their temperature variation during the processes. \(Q_{des}\) and \(Q_{ads}\) are the thermochemical energy that is provided to and extracted from the zeolite for the desorption and adsorption processes. They are calculated as follows:  
\[Q_{ads/des}=|x_{start}-x_{end}|\cdot m_{z}\cdot\Delta H_{ads/des} \tag{17}\]  
where \(x_{start}\) and \(x_{end}\) are the water uptake (kg water/kg dry zeolite) at the start and end of each process, and \(m_{z}\) is the mass of dry zeolite within the bed. \(Q_{HTT\_c}\) and \(Q_{HTT\_d}\) are the energy that is transferred from/to the thermal fluid during the charging/discharging phases. Finally, \(Q_{E/C}\) is the thermal energy that is provided to the pool during the adsorption for the vapour production. This contributes to the energy input during the discharging phase. The energy provided to the cold water flowing through the pool and cool plates for the vapour condensation is not computed as a useful effect during the charging phase because its temperature is too low to assure a useful exploitation. Table 1 reports the data and results of the tests that are presented in this paper. Considering the accuracy of the instruments for the measurement of HTF mass flow and temperatures, the accuracy of the estimation of the charge/discharge efficiency is about 5% with respect to the values which are reported in Table 1. No replications of the same tests were executed because it was very complex and unreliable to obtain the same starting conditions in terms of pressure and temperature field within the zeolite bed. Concerning the desorption, we tested both the possibility to use contemporarily (Test 1) the cooling plates and the pool for the vapour condensation, which is requested for the pressure control inside the vacuum chamber, and the use of the sole pool (Test 2). In this way, we investigated the effect of the cooling plates on the temperature trend of the zeolite bed. In Test 3, we investigated the effect of the vacuum pump for the vapour extraction from the chamber as a method for the vacuum control without using the cool plates and pool. The aim of this test is to obtain a benchmarking with the typical configuration of thermochemical storage with zeolite, where the vapour condensation and evaporation are obtained in a different chamber (phase danger configuration). During the discharging phase, when the stored energy is collected by the end-user, we compared the effectiveness of the pool heating for the vapour production, which is necessary for the zeolite adsorption. In Test 4, the pool is not heated, and the vacuum induced by the vapour adsorption assures the vapour evaporation from the pool. In Test 5, during the first 600 min, hot water periodically feeds the pool to produce the vapour required by the absorption and, consequently, to assure the maintained pressure of the vacuum  
chamber within the range 13-15 mbar. The charging and discharging efficiency of all these tests is reported in Table 1.  
Within the bed, four temperature thermocouples assure the measurement of the temperature profile of the zeolite. Figures 6 and 7 show the trends of the mean value of these four measures during the desorption and adsorption tests, respectively.