Optimization And Matching Analysis Of Pinch Point Temperature Difference In Evaporator And Condenser For Organic Rankine Cycle System

Nowadays, energy shortage has become one of the greatest pressures on economicdevelopment all over the world. Modern industrialization leads to a large amount of lowtemperature waste heat, therefore, recycling this heat and coverting it to power has agreat significance. Currently, the main method to recover this low-temperature wasteheat is organic Rankin cycle (ORC) technology. Evaporator and condenser, as twoimperative parts of organic Rankin cycle system (ORCs), their performance has asignificant impact on the overall system, therefore, the pinch point temperaturedifference, an essential parameter in evaporator and condenser, needs to be encounteredseriously. Thus, in this paper, an optimization and matching analysis for PPTD inevaporator and condenser with economic factors being taken into consideration will beproposed for ORCs. The research contents are mainly as follows:①Taking R600a as working fluid, an evaporator thermo-economic model forORCs has been analyzed. Both from the perspective of exergy recovery and exergydestruction, the pinch point temperature difference (PPTD) of evaporator for low-temperature waste heat recovery has been optimized, and how relevant parametersaffect the optimal results have also been discussed. It has been found that there exists aoptimal PPTD ΔTe,optwhich can make the best system performance and ΔTe,optbased onexergo-economic principle from the viewpoint of exergy recovery (profit) is a bit largerthan that from the viewpoint of exergy destruction; ΔTe,optfrom the perspective ofexergy recovery is in the range of5–12K under the given waste heat condition in thispaper, which is closely related to economic factors；along with the growth of the heatcapacity ratio R of the wasre flue gas to working fluid, ΔTe,optrises up monotonously; bycontraries, larger evaporator preheating section heat transfer coefficient KeA, heattransfer coefficient ratio1of evaporating section to preheating section and thermalexergy price celead to smaller ΔTe,opt; whether considering flow exergy loss or notmakes very little change of the optimal PPTD of evaporator.②Taking a subcritical ORCs as research object, and R601a、R245fa，mixtureR13I1/R601a、R245fa/R601a as working fluids, as working fluid, the effects of PPTDin in evaporator ΔTe, as well as the PPTD ratio of condenser to evaporator y on theexergo-economic performance of ORCs are analyzed. Also, how some other parametersinfluence the optimal ΔTeand the optimal y, and how these two parameters interact with each other are discussed. Then, an optimal and matching analysis will be presented. Theresult shows that there exists an optimal PPTD in evaporator ΔTe,optor an optimal PPTDratio of condenser to evaporator yoptthat makes system performance best; the ΔTe,optisin the range of3–6K, which is lower than that reported in previous investigations; theyoptranges between0.8and1.5. In addition, ΔTe,optand yoptare remarkly affected byheat transfer coefficient ratio of condenser to evaporator2, the temperature of workingfluid at dew point in condenser T1a, and composition of R13I1/R601a: while thetemperature of working fluid at bubble point in evaporator T3a, mass flow rate ofexhaust flue gas mgand inlet temperature of exhaust flue gas Tgion the ΔTe,optand yoptare very slight. For comparison, three other working fluids, R601a, R245fa,R245fa/R601a are taken into account.③Taking R143a as working fluid, a transcritical ORCs is established to analyzethe influence of PPTD in in evaporator ΔTe, as well as the PPTD ratio of condenser toevaporator y on system performance and how some other parameters affect the optimalresults. In addition, the optimal results in transcritical ORCs and subcritical ORCs arecompared. It can be obtained that there exists an optimal PPTD in evaporator ΔTe,optandan optimal PPTD ratio of condenser to evaporator yoptin transcritical ORCs; paremeterssuch as heat transfer coefficient ratio of condenser to evaporator2, the outlet pressureof working fluid in turbine P1, mass flow rate of exhaust flue gas mgand inlettemperature of exhaust flue gas Tgican effect both ΔTe,optand yopt.