Operating Performance Research Of Zzeotropic Mixtures System Used In Waste Heat Recovery At Medium-high Temperatures

With the rapid increase of global energy consumption and the aggravation of greenhouse effect,energy conservation and emission reduction become imminent.Waste heat,which is huge abundant,could be recovered.Thus,it is significant to energy saving and environmental protection.As an important waste heat recovery method,researches has been devoted to improving the performance of ORC system.Zeotropic mixtures are widely used in ORC systems due to their unique temperature glide characteristics,which can reduce the irreversible loss of the system.While at present,the physical properties of zeotropic mixtures are mostly derived from fitting method,which are not so accurate.Meanwhile,the phenomenon called composition shift and the dynamic response characteristic of zeotropic mixture-based system are not considered in most of current studies.Due to the above situation,this paper takes R245fa/R134 a mixture as the working fluid to carry out the following research.Firstly,the PVT properties of the mixtures are measured,and the virial equation is fitted to provide data for the establishment of the equation of state in the future.Then,according to the test platform,a composition shift model of medium and high temperature ORC system is constructed.The effects of composition shift on system performance and factors affecting composition shift are analyzed.Meanwhile,a method for predicting composition shift is proposed.Finally,after using R245 fa to run the test platform,the comparative experiment of zeotropic mixture and pure working fluid is carried out.The dynamic response characteristics of the system using mixture and pure working fluid are summarized.Based on the experimental results,the virial equation of R245fa/R134 a mixture with mass fraction of 18.88%/81.12% in the range of 353~373K,613~2216kPa was fitted.Compared with the experimental results,the average relative error of the pressure calculated by the equation is 0.55%.The average relative error of density calculation value is 0.6965%,which has certain significance to the following study.It is also found that the circulating concentration of R134 a increases due to composition shift,which results in the performance decrease of the ORC system.Take network as an example,there is a 2.5% maximum relative loss of network when considering composition shift.This phenomenon can be effectively suppressed by strengthening heat transfer and increasing charge quality,while increasing evaporation pressure would exacerbate the composition shift.The composition corresponding to the maximum composition shift can be simply predicted by temperature glide.The results also show that the speed of dynamic response for this experimental system using zeotropic mixture is generally faster than that using pure working fluid.With the increase of heat source temperature,the difference of the response speed between system using zeotropic mixture and pure working fluid becomes more obvious.