Thermo-economical Optimization And Experimental Study On Organic Rankine Cycle For Low-and Medium-grade Waste Heat

Nowadays, the rising demand for energy and serious environmental pollution become the motive to improve the energy structure, saving energy and optimize energy utilization. Meanwhile, China is rich in low-grade energy such as low temperature waste heat energy, solar and geothermal energy and so on. As a mature theoretical techonology to recover the low grade waste heat, organic Rankine cycle has advantages of simple structure, high efficiency and environment friendly. The organic Rankine cycle(ORC) is the significant technology to increase the energy efficiency and improve the environment issues for China because it can effectively convert the low-grade energy into work.Based on the first and second laws of thermodynamics, the thermodynamic and economic models were built and the sensitivity of the system operating parameters on system thermal and economic performance were investigated. Using the ranking method, the preferred working fluids were selected according to the optimal economic performance. Research demonstrated that taking the economic performance into account, R11, R123 and R245 ca were the favorite working fluids; however, from view of the comprehensive consideration of net power output, thermal efficiency and exergy efficiency, R142 b and R114 were the best working fluids.From view of the thermaldynamic and economic models for BORC and RORC, the effects of five system operating parameters on the exergy efficiency and LEC were discussed and the bi-objective optimization considering maximum exergy efficiency and minimum LEC was investigated. The further tri-objective for maximum exergy efficiency, maximum net power output and minimum LEC was also analyzed. The results show that improving the thermaldynamic performance cause a deterioration of economic performance. Comparing the results between single-objective and biobjective optimization, a higher thermal efficiency and exergy efficiency must be accompanied by lower net output power and poor economic performance; meanwhile, a higher economic performance ensure higher net work output, but lower exergy efficiency and thermal efficiency. The Pareto-optimal solution of bi-objective optimization neutralize the thermaldynamic performance and economic performance, and therefore, was more suitable for engineering and selected by decision makings.Depending on the location of bubble point temperature and dew point temperature, four mixture cases were proposed and the thermo-ecnomical comparison for four mixture models was conducted. The thermo-ecnomical comparison between pure and mixture working fluids was discussed. The results showed that, compared to the pure working fluids, the mixture working fluids don’t always present better thermaldynamic performance and economic performance. Whether the mixtures exhibit better thermodynamic and economic performance than the pure working fluids depend on the operation parameters and mass fraction of mixtures. Considering the respective singleobjective optimization of maximum exergy efficiency and minimum LEC, the mixtue model, where bubble point temperature is located at the end point of phase transition and dew point temperature is located at the beginning point of phase transition, obtained the better thermaldynamic and economic performance. Comparing the Pareto-optimal solution between mixture and pure working fluids, the mixtures present better thermodynamic performance and approaching equivalent economic performance than the pure working fluids.The thermodynamic, economic and environmental models for double ORC system were built, and the six system operating parameters(evaporator temperature of top cycle, degree of superheat, pinch point temperature difference, condenser temperature, and the evaporator and condenser temperature of bottom cycle) on system performance were discussed. The Pareto-optimal solution for bi-objective optimization considering thermodynamic performance and economic performance was obtained. The relationship between thermodynamic performance, economic performance and environmental performance was analyzed. Using the TOPSIS decision making, the Pareto-optimal solutions for thermo-economic bi-objective optimization and thermo- environmental triobjective optimization were obtained.Based on the 3 k W low-temperature ORC experimental setup, the effects of mass flow, degree of superheat, expander intlet pressure and pump inlet pressure on sytem operating characteristics were measured, and the sensitivity of operating parameters on system performance was analyzed. The theoretical model was modified by correcting the isentropic and mechanical efficiency of expander and pump. The system characteristics and performance between pure and mixture working fluids were compared. The results show that mass flow has the highest sensitivity degree. The deviation between corrected model vs experimental data and theoretical model vs experimental data decreases 67-84%. The mixture working fluids show better thermal efficiency and system power generation efficiency than pure working fluids.