## 2 Thermochemical Energy Storage Design  
### Configuration Concept and Working Principle  
The configuration concept is based on an open sorption process of water vapor on a composite material in a vertical moving bed reactor. The concept is shown in Figure 2 The full details about the used material and the designed reactor are described in the following sections. The idea of the moving bed reactor is to generate a constant thermal power, which is not possible with the fixed packed bed configuration. Moreover, the use of a single bed, instead of multiple beds, aims at improving the overall system compactness. All the solid material is processed by a reactor R1 and stored separately in storage vessels S1 and S2. However, the main challenge consists to design the solid feed/extractor mechanism (elements V8 and V9 in Figure 2).  
During the thermal discharge, the ambient cool moist air E1 is supplied to the cold side of the heat exchanger HX2 by the fan P3, where it can be preheated from the hot side of HX2 before entering to the reactor R1. The heat exchanger HX3 is by-passed with valves V1 and V2, and the valve V3 is open in the direction of HX2 (see Figure 2). The hot and dried air at the reactor outlet is introduced to the air-to-water heat exchanger HX1 for the immediate charge of the water tank U1. Finally, the cooled and dried air is rejected from HX2 to the atmosphere. The water pump P1 is normally activated and the water pump P2 is deactivated.  
There is no evaporator module in this configuration, that makes the system to be more compact. Such a decision is exclusively reserved to the use of a highly hygroscopic composite material. However,  
Figure 2: Combined solar thermochemical energy storage system configuration.  
this designer's step sets the operational constraints, especially for the outdoor temperature below 5 \({}^{\circ}\)C and the air moisture content lower than \(5.0\times 10^{-3}\) kg/kg. In such conditions, the charge of the water tank U1 can be ensured by its direct heating with the solar collectors E2 by turning the valves V4, V5, V6, and V7, and activating the pump P2. Moreover, a back-up electric heater E3 can be installed in the water tank too.  
The thermal charge operation uses the solar energy E2. The type of solar thermal collectors determines the dehydration temperature that can be supplied to the reactor. [PERSON] et al. [34] showed that the dehydration temperature above 100 \({}^{\circ}\)C is achievable with evacuated tube or concentrated solar collectors, which improves the reactor energy storage density by more than 50% in comparison with the dehydration between 50 and 80 \({}^{\circ}\)C with glazed solar thermal collectors.  
During the thermal charge, the outdoor air E1 is blown through the cold side of the air-to-air heat exchanger HX2, after what it receives the necessary heat from the solar thermal collectors E2 through the water-to-air heat exchanger HX3. The valves V1 and V2 stay open in the direction of HX3 and R1 units. Afterwards, the hot air is distributed to the reactor module R1. At the same time, the hot water tank U1 can be also charged from the air-to-water heat exchanger HX1, thus reusing a part of heat rejected from the reactor R1.