Performance Analysis And Thermal-economic Optimization Of Subcritical Organic Rankine Cycle System With Mixture Working Fluids

The problems of energy shortage and environmental pollution have becomeincreasingly outstanding. Converting the low-grade thermal energy into electricity is anefficient way to improve energy efficiency and reduce environmental pollution. OrganicRankine cycle (ORC) system has some prominent advantages and wide applicationprospects, and it has been a key technique and hot spot in recovering low grade wasteheat.There are a large number of researches on the ORC with pure working fluids. Inrecent years, the mixture working fluids have gradually caused much attention. Thenon-isothermal phase change of the mixture provides a better thermal match betweenthe working fluid and heat source or cold source. Then the irreversibility loss in heatexchanger can be reduced and the system performance may be improved. Therefore, themixture fluid is employed as the working fluid in subcritical ORC. Aiming at thesubcritical ORC with mixture working fluid to recover waste heat of flue gas fromindustrial boiler, this thesis performs some theoretical researches from the heatexchangers to the whole system, from thermodynamic to economic performance, andfrom parameter optimization to influence factors. The main contents are as follows:(1) An exergoeconomic analysis and performance optimization of an evaporator andcondenser with the binary mixture in the ORC system has been carried out based on thefirst and second laws of thermodynamics. Results show that there exist optimal heattransfer unit number and heat capacity ratio that minimize the total cost per unit heattransfer rate. For the evaporator, the optimal heat transfer unit number increases with theincrease of the inlet temperature ratio. The optimal heat capacity ratio has the oppositevariation with the optimal heat transfer unit number. For the condenser, the optimal heattransfer unit number increases with the increase of the inlet temperature ratio anddecreases with the increase of the temperature ratio of bubble point and dew point. Thevariation of the optimal heat capacity ratio with the inlet temperature ratio andtemperature ratio of bubble point and dew point is the opposite with the optimal heattransfer unit number. In the three kinds of flow arrangements, the counter flow heatexchanger has the lowest total cost per unit heat transfer rate, and the parallel flow heatexchanger has the highest one.(2) The performance of the ORC system with pure and mixture working fluids has been examined based on the thermodynamics. The performance comparison andanalysis have also been conducted for various mixture working fluids with differentcompositions under different condensation restricted conditions. Results show that thecomposition of the mixtures has an important effect on the ORC system performancethat is related with the temperature glide of the mixture during the phase change. Whenthe condensing dew point temperature is fixed, the net power output of the ORC systemwith the mixtures is larger than that with pure working fluids, and a larger temperatureglide of the mixture during phase change causes larger net power output. When thecondensing bubble point temperature is fixed, the net power output of the ORC withpure working fluids is larger than that with the mixtures. When the condensing pressureis fixed, the net power output of the mixture working fluids is between that of theoriginal pure working fluids.(3) The electric production cost (EPC) is introduced as the evaluation criteria. Theeffects of the evaporating temperature, condensing temperature and pinch pointtemperature difference (PPTD) of the heat exchanger on the system performance areanalyzed. And these cycle parameters are optimized for the ORC system with variouscompositions and proportions of mixtures. The effects of the mass flow rate and inlettemperature of the flue gas and cooling air, isentropic efficiency of the expander andpump, heat transfer coefficient of evaporation and condensation on the optimizationresults are also discussed. It is found that the ORC systems with different working fluidspossess different optimal evaporating temperatures, while the difference of the optimalPPTD in the evaporator for different working fluids is small. The optimal PPTD in theevaporator for most working fluids is about (7-10)℃. The variation of the optimalcondensing temperature and the PPTD in the condenser is similar to the temperatureglide of working fluids during phase change. The optimal mean condensing temperaturefor most ORC systems is nearly the same, which is about40℃. Meanwhile, with theincrease of the mass flow rate and inlet temperature of the flue gas, the optimalevaporating temperature, condensing temperature and the PPTD of the evaporator allincrease, and the PPTD of the condenser has little change. With the increase of the massflow rate of cooling air, the optimal evaporating temperature and condensingtemperature decrease, the PPTD of the condenser changes little and that of theevaporator increases. The isentropic efficiency of the expander and pump has slightlyeffect on the optimal parameters. With the increase of heat transfer coefficient ofevaporation, the optimal evaporating temperature, condensing temperature and PPTD of the condenser all increase, the optimal PPTD of the evaporator decreases. The variationof optimal parameters with the heat transfer coefficient of condensation is contrary tothat with the heat transfer coefficient of evaporation.