## 6 Modeling and Simulation Results
### Hybrid Power Plant (Solar-Biogas) While Heating the AD with Condensation Heat  
When the condensation temperature increases to achieve a higher temperature for the AD to boost biogas production, the efficiency of the thermodynamic cycle decreases, and energy production theoretically declines. However, biogas production increases, meaning that the Hydraulic Retention Time decreases, reducing biogas costs. Based on the steam calculations application [20], Table 6 illustrates the theoretical annual electricity production and the new cost of the produced biogas to be used later for calculating the total electricity cost produced for the studied cases. It is worth noting that case 1 is the reference case for biogas LCOE.  
Figures 5-9 illustrate the LCOE produced for the five studied cases (solar-biogas), where \(t_{\rm AD}\): average digester temperature \({}^{\circ}\)C, and \(t_{\rm c}\): corresponding condensation temperature \({}^{\circ}\)C. It was found that the lowest LCOE = 0.1283 EUR/kWh was achieved at SM = 1 and FLHs = 0.  
Figure 4: LCOE generated by the solar plant, with variations in SM at different FLHs (storage capacities).  
Figure 3: Annual net electricity generated by the solar plant, with variations in SM at different FLHs (storage capacities).  
By reading the figures, the following could be noticed:  
For cases 1, 2, 4, and 5, the LCOE values are somewhat close, between 0.156 and 0.2 EUR/kWh for SM = 1; FLHs = from 1 to 10. This means that the hybrid power plant could produce electricity with a low LCOE and the possibility of storage for a few hours compared to solar nets.  
The LCOE for case 3 (the best case) ranges from 0.1283 to 0.1447 EUR/kWh (SM = 1; FLHs = from 1 to 10). This enables the hybrid power plant to operate at sunset using solar storage heat with a low LCOE.  
Figure 5: LCOE produced by the hybrid plant at different FLHs and SM for case 1 (reference) (t\({}_{\rm AD}\) = 33.17 \({}^{\circ}\)C; t\({}_{\rm c}\) = 41.5 \({}^{\circ}\)C).  
Figure 6: LCOE generated by the hybrid station for case 2 (t\({}_{\rm AD}\) = 37.5 \({}^{\circ}\)C; t\({}_{\rm c}\) = 57.5 \({}^{\circ}\)C).  
Figure 8: LCOE produced by the hybrid station for case 4 (t\({}_{\rm AD}\) = 45.87 \({}^{\circ}\)C; t\({}_{\rm c}\) = 65 \({}^{\circ}\)C).  
In all cases, it is advised to operate the hybrid power plant with values of SM no greater than 2.5. This reduces the investment cost in the solar field and the area of land needed.  
The hybrid solar-biogas system achieved a significant reduction in LCOE compared to pure solar energy or pure biogas energy, and at the same time, it achieved a competitive price compared to the prices of the electricity grid in Idlib [33].  
Hybrid Power Plant with Heating the AD to the Optimal Temperature in the High-Temperature Range (Thermophiles)  
Referring to Figure 2, it is clear that the optimal temperature for achieving the highest growth rate of gas-producing bacteria in the high range is approximately 62 \({}^{\circ}\)C, which results in a doubling of the growth of gas-producing bacteria. This requires a minimum condensation temperature of about 70 \({}^{\circ}\)C. Table 7 shows the projected annual production of electricity and LCOE for case 6. Figure 10 shows the LCOE produced according to SM at different FLHs; it is clear that the lowest LCOE = 0.0824 EUR/kWh was achieved at SM = 1 and FLHs = 0. Figure 10 shows that the optimal SM value for obtaining the lowest LCOE is SM = 1 for all FLH values, with the LCOE ranging from 0.0832 to 0.1004 EUR/kWh. This allows the hybrid power plant to function after sunset using solar storage heat, resulting in a reasonably low LCOE. Also, there is no pointed change in the LCOE at the same SM (if SM > 4) by changing the FLHs.  
Figure 9: LCOE produced by the hybrid station for case 5 (t\({}_{\rm AD}\) = 49.6 \({}^{\circ}\)C; t\({}_{\rm c}\) = 70 \({}^{\circ}\)C).