## 3 Results
### Measurement Result of the System under Outdoor Conditions
#### 3.2.2 Power Consumption on the Methanation System  
Thermocatalytic conversion of CH4 is a highly exothermic reaction and consequently it may result in reactor overheating and low conversion of CO2. Reactor cooling is critical to the methanation process to avoid catalyst deactivation. Operating temperature is the most critical parameter for optimizing the methanation system. Additional, critical operational factors such as catalyst deactivation, CO formation, CO2 conversion, and CH4 yield depend upon the operating temperature. The temperatures of the reactors should be maintained at a sufficiently high temperature to maintain catalytic reactions but also sufficiently low to avoid CO formation [25]. Heat management of the methanation system, specifically, reactor cooling and utilizing the reaction energy for external power to the reactors, is an active area of research in StM systems. The heat released from the reaction must be removed from the system and subsequently may be utilized in the electrolysis process [15,25]. Heat management is an important factor in reactor design. Optimization of heat removal, to ensure uniform temperatures within the reactors, is a currently unsolved concern. Cooled-packed reactor systems are a possible solution for heat management that is currently being investigated and is costly. The most known Sabatier reaction is shown below in Equation (2):  
\[\text{CO}_{2}+4\text{H}_{2}\rightleftharpoons\text{CH}_{4}+2\text{H}_{2}\text{O }\Delta\text{H}^{\circ}{}_{298}=-164.9\text{ kJ}/\text{mol} \tag{2}\]  
The reaction is significantly exothermic. The reaction consumes 4 moles of H2 for each mole of CH4 produced; 2 moles of H2O are generated as a byproduct. Energy is required to initiate the reaction. Increasing reaction time provides some heat to perpetuate the reaction. The energy required to maintain the reaction decreases from the initial amount once the reaction becomes stable. Reactor temperature  
Figure 5: (a) Measured hydrogen generation rate (\(GR_{\text{H2}}\)) compared to direct normal irradiance (DNI) and (b) conversion of CO2 to CH4 compared to reactor temperature. Data in both plots were obtained on a clear, sunny day (15 th November 2019).  
was regulated by a proportional, integral, derivative (PID) controller. When reactor temperature increases because of the exothermic reaction, PID control switches regulate power depending upon reactor temperature. As a result, heat from the reactors provided the required reaction energy, and the power consumption of the methanation system was reduced. The total power consumption of the reactors was measured on a clear, sunny day from 10:00 a.m. to 2:00 p.m. local time. Figure 6a shows the reactor total power consumption compared with time. Although the reaction initiates in reactor 1, generated gases, residual, and unreacted gases, maintain the reaction in reactor 2. The reaction rate of reactor 2 was lower than that of reactor 1. Therefore, the total power consumption of reactor 2 was higher than that of reactor 1. It may be observed that after the initial energy was provided, the total power consumption of reactor 1 stabilizes as a result of the Sabatier effect. The heat of reaction for reactor 1 reduces power consumption of reactor 1, whereas for reactor 2, the reaction rate will be reduced because the majority of the reaction occurs in reactor 1. Therefore, more energy was consumed by reactor 2 to maintain the temperature and reaction. A total of 0.145 kWh was required for operation under sunny conditions. Figure 6b indicates the total power consumption of the reactors under overcast conditions. The DNI was negligible as was the generation of H\({}_{2}\). The reactors required more energy under cloudy conditions due to low reaction rates. The total power consumption of 0.281 kWh was observed under overcast conditions.