## 6 Conclusions  
This study investigated the thermodynamic, economic, and sustainability feasibility of a solar-driven ORC system for micro-cogeneration application in buildings. A thermodynamic model was developed for simulating an ORC both with and without a regenerator and with zeotropic working fluid mixtures. Twelve zeotropic mixtures with various compositions of R245 fa/R152a, R245 fa/propane, R245 fa/hexane, R245 fa/heptane, butane/propane, butane/hexane, butane/heptane, pentane/hexane, isobutane/propane, isobutane/hexane, isobutane/heptane, and isopentane/hexane were evaluated and compared to identify the best combinations of mixtures for deriving high system cycle efficiency. The study also examined the effects of the volume ratio, the evaporation, and condensation temperature glides of the zeotropic mixtures on the ORC thermodynamic performance. Following a detailed analysis of each mixture, R245 fa/propane and butane/propane are selected for parametric study to examine the effects of operating parameters on the system's efficiency and sustainability index. Appropriate operating conditions selected to simulate available waste heat from industrial processes or solar water heaters where possible and municipal water supply for cooling the working fluid. All fluid mixtures with the exception of mixtures with heptane and hexane compositions would have the potential to be implemented with a solar energy system or waste heat for the heating source. Most of the working  
Figure 26: Comparison of sustainability index for different mixtures. Figure 25: Comparison of predicted (**a**) savings in petroleum; (**b**) reduction in CO\({}_{2}\) emissions for different mixtures. fluid mixtures exhibit potential for power generation at a residential scale. Each working fluid could be cooled by existing water sources from the local municipality or ground water. The thermodynamic performance results disclosed that for zeotropic mixtures, there is an optimal composition range within which binary mixtures tend to achieve higher system efficiency than the component pure fluids. In addition, a substantial increase of cycle efficiencies can be obtained with a regenerative ORC, with cycle efficiencies in the range of 2.3-9.8% and 8.1-18.5% for ORC without and with regeneration, respectively. The maximum net power output of the ORC system using mixtures corresponded to an optimal mixture that provided the best temperature glides but not always the highest temperature glides. Using zeotropic mixtures could expand the limitations of using WFs for low-temperature solar organic Rankine cycles. Results also revealed that butane/propane and R245 fa/propane have the optimal outputs at operating condition with hot source temperature of 85 \({}^{\circ}\)C, where R245 fa/propane (0.4/0.6) has net work output of 7.86 kW followed by butane/propane (0.8/0.2) with the net work output of 5.7 kW. With parametric study on these fluids, it was found that the optimal conditions for these mixtures were at the hot source temperature 95 \({}^{\circ}\)C and cold temperature 20 \({}^{\circ}\)C with mixture compositions R245 fa/Propane (0.6/0.4) and butane/propane (0.8/0.2) and with cycle efficiencies of 9.59% and 7.49%, respectively. In the subsequent part of this study, a detailed economic analysis and a sensitivity analysis of the solar ORC systems with mixtures R245 fa/R152a (0.9/0.1), R245 fa/propane (0.4/0.6), and butane/propane (0.8/0.2), were performed to evaluate the selling price of produced electricity and to assess the influence in the NPV of the solar ORC system. Results disclosed that using a R245 fa/R152a (0.9/0.1) mixture was the most economical mixture out of three in terms of selling price of electricity, about 0.15 USD/kWh, with a payback period of 14 years. While the payback period for the system is relatively high, which is nearly equal to life cycle of the system, the selling price is competitive to the average residential price of electricity.