Research On Selection And Thermoeconomic Analysis Of Zeotropic Mixtures For Organic Rankine Cycle

Increasing concerns over energy shortage and environment pollution inspire us to explore new technologies for utilization of the large amount of low/medium temperature thermal energy in industry and new energy.Because of the simple structure,high reliability,low maintenance cost,as well as more available working fluids,organic Rankine cycle(ORC)has been widely applied in utilization of low/medium temperature thermal energy.The system performance can be improved by using zeotropic mixtures as working fluids because of the temperature glide during the phase change process.Thus,the ORC system using zeotropic mixtures has been paid more and more attentions.Considering the heat source properties,in this paper,we divide the heat sources into the open heat source(without outlet temperature limitation)and the closed heat source(with outlet temperature limitation).For the two type heat sources,the match mechanism between mixture and heat source/sink is studied and the corresponding selection criteria of mixture working fluid are proposed based on the thermodynamic and thermoeconomic analysis of the ORC system.In the analysis,the system exergy efficiency is used as the optimization objective to represent thermodynamic performance,and the levelized energy cost is set as the thermoeconomic evaluation index.For the open heat source,the improvement of temperature match in the evaporator exhibits more significant influence on the cycle performance than that in the condenser.Thus,the match of the system with heat source shoud be firstly satified during the selection of the mixtures.In addition,the contribution of the temperature glide in the condenser to the improvement of the cycle performance mainly relies on combine effects of the increase in the heat source utilization,the decrease in the condenser exergy loss and the increase in the evaporator exergy loss.And the influence of the former two is greater than the latter.The proposed selection criteria for zeotropic mixtures consist of two correlations which describe the optimal temperature match in the evaporator and condenser,respectively.The prediction method of condensation temperature glide is also proposed.Therefore,the optimal mixture can be directly determined according to their thermophysical properties without massive thermodynamic calculation.Finally,results of thermoeconomic analysis show that the selected optimal mixtures based on the proposed criteria also exhibits good thermoeconomic performance.Compared with 'dry' and 'isentropic'mixtures,the 'wet' mixtures have relatively lower thermodynamic performance but higher thermoecomic performance.For the closed heat source,the better cycle performance can be achieved under the heat source outlet temperature varying from 343.15K to 363.15K.Thus,if possible,the heat source ouelet temperature should be adjusted to approach to this temperature range.And under large and small heat source temperature drop,it is different for the variation trends of system exergy efficiency with the working fluid critical temperature.With the increase of critical temperature,it decreases monotonically for the larger heat source temperature drop while it first increases and then decreases slightly under the smaller heat source temperature drop.This phenomenon is attributed to the change trend transformation of evaporator exergy loss.Accordingly,the heat source temperature drop can be divided into the larege temperature drop region,transition region and small temperature drop region.For the large heat source temperature drop,a better temperature match between the working fluid and heat source can be achieved in the preheating section,while for the small heat source temperature drop,it appears in the evaporation section.Furthermore,the critical temperature range of optimal working fluid is proposed for heat source with inlet temperature of 473.15K and below.Finally,based on the thermoeconomic analysis,a superior thermodynamic and themoeconomic performance can be achieved at the same time for the small heat source temperature drop.Under the large heat source temperature drop,the selection of working fluid requires a trade-off between thermodynamic and thermoeconomic performance.