## 6 Results and Discussions  
### System Performance  
As the wellhead condition varies, both the wellhead pressure and temperature vary correspondingly. The power production of the proposed TF-SF system under variable wellhead conditions and different separating temperatures are firstly investigated, since the power capacity is the primary objective of power plants in geothermal fields. And the freshwater production is analysed thereafter as extra benefits of this new design. During this simulation, the total-flow turbine efficiency has been conservatively pre-set as 35%. Figure 3 presents the power generation of TF-SF system against the wellhead pressure and separating temperature. As the figure shows, system power output is less sensitive to the wellhead pressure than to the separating temperature. When the wellhead pressure is lower than 650 kPa, system power capacity keeps increasing as the separating temperature gradually increases towards wellhead temperature. This means that, under low wellhead pressure conditions with lower temperature but higher vapor quality, the best power generation occurs when the vapor and liquid is separated directly after being discharged from wellhead, and no flash/total-flow process is actually needed. In this way, the best solution for high power performance is to change the system to a traditional steam system in which the vapor separated from wellhead is introduced to steam turbine directly. However, as the wellhead pressure increases above 650 kPa, the optimal separating temperature occurs since the power increases first and then decreases, indicating the TF-SF system's superior to the direct steam system under these conditions in terms of power generation. As wellhead pressure increases from 650 kPa to 940 kPa, the optimal separating temperature slightly increases from 162.3 \({}^{\circ}\)C to 177.2 \({}^{\circ}\)C, and the optimal power output decreases from 851.1 kW to 812.4 kW. Overall, the maximum power capacity of the TF-SF system among the LA-8 wellhead conditions is 851.1 kW when the average total-flow turbine efficiency is only pre-set as 35%. The tendency of system power capacity in Figure 3 can be illustrated better by separating the power generation of the total-flow expansion part (TF) and the single-flash and expansion part (SF), as shown in Figure 4. Basically, power generation of the SF part significantly outweigh that of the TF part when the separating temperature is higher than 120 \({}^{\circ}\)C, meaning that the single flash part dominates the power capacity under given conditions. However, the power generation advantage of the single flash part over the total flow part shows a decreasing trend as wellhead pressure adds up. The reason is that higher wellhead pressure and temperature causes higher TF turbine inlet enthalpy, even though the wellhead vapor quality and mass flowrate are reduced. Consequently, the power generation of TF part increases along with the wellhead pressure increment. However, the lower vapor quality and flowrate of the heat source would reduce the SFS turbine inlet enthalpy, causing a reduction in SF power generation. When the separating temperature becomes lower than 120 \({}^{\circ}\)C, power capacity of the TF part overtakes the SF part, but the total power generation of the system would become lower than 629.6 kW. The analysis above indicates that, under LA-8 wellhead conditions, a high separating temperature is recommended to achieve high power generation. Figure 4: Distribution of TF and SF perspective power generation. Figure 3: Distribution of system power generation against wellhead pressure and separating temperature. As designed, a condensation process is integrated with the TF-SF system to get freshwater as extra benefits to help address the local water scarcity problem. The production of freshwater generation against wellhead conditions and system parameters are presented in Figure 5. A lower wellhead pressure and lower separating temperature lead to a higher freshwater generation.