# Techno-Economic Analysis of a Solar Thermal Plant for Large-Scale Water Pasteurization
## 3 Plant Model Design
### Lumped-Component Plant Model  
A dedicated lumped-component model of the solar thermal water treatment plant has been developed in the Simulink(r) environment implementing the equations previously described, in order to simulate the performance of the plant during a whole year (see Figure 3 for an overview of the complete model). The model allows to evaluate the main operative parameters, such as temperatures, water flow rates and heat exchanged inside the various components using iterative procedures. In this study, four water treatment capacities have been investigated for the simulated plant, namely 50 L s\({}^{-1}\) (i.e., 4320 m\({}^{3}\) day\({}^{-1}\)), 100 L s\({}^{-1}\) (i.e., 8640 m\({}^{3}\) day\({}^{-1}\)), 250 L s\({}^{-1}\) (i.e., 21,600 m\({}^{3}\) day\({}^{-1}\)) and 500 L s\({}^{-1}\) (i.e., 43,200 m\({}^{3}\) day\({}^{-1}\)).  
Figure 4a shows the contribution of the solar field with respect to the total thermal energy required for the water treatment in terms of the \(f_{s}\) factor, which is defined as [43]  
\[f_{s}=\Phi_{sf}/(\Phi_{sf}+\Phi_{gb}), \tag{21}\]  
and the obtained treatment temperature during the year for the considered pasteurization protocol (114 s at 75 \({}^{\circ}\)C). According to the design condition imposed, the treatment temperature is always above 75 \({}^{\circ}\)C, and its nearly constant trend proves that the activation logic of the gas burner correctly provides the auxiliary thermal power to the thermal storage system when the solar field is not enough for driving the water pasteurization. In detail, the major contribution from the solar field is obtained during the summer period (i.e., approximately from May to September). Instead, during the winter period (from September to May), lower values in the available hourly solar irradiance for the chosen location of the plant (Torino, Italy) imply larger utilization of the gas burner, with a consequent higher value of the power provided by the natural gas usage and reduction of the \(f_{s}\) factor. The widedistribution of the \(f_{s}\) factor during the day would imply continuous switch-on/switch-off of the gas burner; however, the adopted system for thermal storage has been thought indeed to avoid this continuous transient operation of the gas burner.  
On the other hand, Figure 4b shows the temperature of the treated water during its passage throughout the treatment line. First, the water temperature rises quickly in the plate heat exchanger during the pre-heating phase, from the ambient temperature to 70 \({}^{\circ}\)C. The temperature is then further increased inside the treatment coil, where water is maintained above 75 \({}^{\circ}\)C for about 170 s, to comply with the target pasteurization protocol. Finally, the treated water exits the circuit through the heat exchanger, where temperature drops to ambient conditions as heat is recovered and used to pre-heat the incoming raw water inflow.  
Figure 4: Results of the numerical model: (**a**) \(f_{s}\) factor (blue dots) and treatment temperature (red solid line); (**b**) mean temperature of the water to be treated during its passage throughout the plant. The red and blue sections highlight respectively the pre-heating and post-cooling of the fluid through the plate heat exchanger, which acts as an economizer. The results refer to the 250 L s\({}^{-1}\) plant capacity.  
Figure 3: Overview of the Simulink® lumped- model of the solar pasteurization plant. Solar field, gas burner, heat exchanger and storage tank subsystems are identified with yellow, red, blue and green, respectively.