Study On Working Fluid Selection And System Performances Considering Thermal Match Between ORC And Non-isothermal Heat Source

Organic Rankine cycle(ORC)is a potential heat recovery technology which can be used in low grade industrial heat recovery for energy saving and emission reduction.In the last decade fundamental research has been carried out while the attention was given on the ORC system itself.Abundant understanding has been acquired,but it's still far from fulfilling of wide application.Several essential problems,such as how to reduce the heat transfer irreversibility between ORC and heat source,how to select appropriate working fluid considering the impact of thermal match,how to evaluate the impact of heat source on exergy distribution and arrangement between evaporator and expander,should be solved as soon as possible.As a heat-to-work convertor system which is driven by non-isothermal low temperature heat source,definitely its performance would be affected by the heat source condition and the interaction between heat source and the cycle itself.The purpose of this paper is to investigate the influence of non-isothermal heat source on the ORC system performance and working fluid selections.The ORC is driven by heat sources,and the exergy destruction in evaporator has great impact on the system performance.However,the quantitative relationship between the level of thermal match and irreversibility has not been established.Therefore the first subject of this paper is to establish such relationships,as a result giving the theoretical basis of reducing irreversibility through heat transfer process controling.Specific heat capacity of organic fluids exhibit sudden change in their critical zone,which leading to an obviously change of T-Q curve's slope near critical zone.This supplies potential for ORC system performance elevation through heat transfer process controling.The expression of integrated temperature difference(ITD)and non-dimensional exergy temperature was deduced and the relationship between heat transfer process and irreversibility was achieved.The quasi-linear relationship between ITD and irreversibility was confirmed,therefore the evaluation of irreversibility can be replaced by the evaluation of ITD.Then the model of improving thermal match by making use of variable specific heat capacity was proposed.For a heat exchanger in which inlet and outlet temperature of the heat transfer fluids were fixed,the influence of variable specific heat capacity on ITD was evaluated,and the combination of specific heat capacity that can supply the minimum irreversibility was acquired.These results could be helpful in guiding the reduction of irreversibility between ORC and heat source.The working fluid selection is still scattered.The influence of heat source parameters on working fluid selection and the impact of working fluid properties on cycle performance need to be investigated.Working fluid selection criterion for sub-and trans-critical ORC are discovered when reducing the average temperature difference between heat source and ORC.In Chapter 2 neither realistic organic fluid properties nor parameteres like pressure were involved,while in Chapter 3 the theoretical model were established based on thermodynamic laws,and the calculation were fulfilled based on real organic fluid properties.REFPROP 9.0 was adopted as the fluid database.Performances of different fluids in saturated basic ORC were calculated when the heat source temperature lied in 100~300°C.Work output and heat exchanger area were adopted as main indicators for selection,while toxicity,flammability and environmental impact were also taken into consideration.Calculation results indicated that for fluids with Tc<Tgas,in,the cycle using fluids with different Tc exhibited apparent different performances.For fluids with Tc>Tgas,in,the cycle using fluids with different Tc exhibited similar performances.The highest thermal efficiency was achieved by the fluid whose envelop was nearly tangent with the heat-release line of flue gas on the T-S chart.Such a fluid has a critical temperature 15~25K lower than the initial temperature of heat source.The ORC thermal efficiencies are randomly distributed against other fluid physical properties except Tc.Thus,the critical temperature can be the sole criterion for the fluid selection,as far as the thermal efficiency is concerned.In Chapter 4 thermal performances of several working fluids in a trans-critical ORC driven by waste heat flue gas were calculated,by means of a theoretical model in which the inlet and outlet temperature of flue gas were fixed and therefore the thermal efficiency and net work output were unified into one parameter as long as the heat absorbed was given.Influence of pinch point temperature difference(PPTD)in the evaporator and working fluid properties on the cycle performance were analyzed.Calculation results showed that for fluids with critical temperature Tc<Tgas,out,and fluids with Tc>0.88 Tgas,in,Tc has the dominant influence on thermal efficiency;for fluids with Tc between(Tgas,out,0.88 Tgas,in),fluid dryness has significant influence on cycle performance,and wet fluids outperform dry fluids apparently.Wet fluids with Tc between(Tgas,out,0.88 Tgas,in)exhibit the highest thermal efficiency.For fluids with Tc>0.88 Tgas,in,the PPTD may lies either inside the evaporator or at the outlet.Thermal performances of the cycle with PPTD inside the evaporator outperform those with PPTD at the outlet.The application of such conclusions was confirmed through sensitivity analysis for flue gas inlet and outlet temperatures.In Chapter 2,3 and 4 main attention was focused on the influence of heat source on evaporator,while the relationship between evaporator and expander were ignored.The coupling between heat source and ORC influences the irreversibility in evaporator,and the match of irreversibility between evaporator and expander influences the system performance.Recently the relationship between ITD and expander efficiency was discovered and reported.In order to investigate the influence of variable expander efficiency on ORC performance,a theoretical thermodynamic model was proposed,in which the expander efficiency was assumed to be a function of dimensionless ITD.The experimental process of an ORC using R123 as working fluid and scroll expander was simulated.Given the structure and area of the evaporator,as well as the initial temperature of thermal conductive oil and some other basic assumptions,the work output,thermal and exergy efficiency of the system were calculated.Comparison was made between calculated results achieved by the new model and by those in which the expander efficiency was assumed to be constant.The rationality of the existence of an optimal dimensionless ITD was analysed,which means that the influence of heat source on the ORC system was not only reflected in the evaporator,but also in the expander.The model in which the expander efficiency was assumed to be a function of dimensionless ITD was more reliable than those in which the expander efficiency was assumed to be constant.