Abstract
Mixture working fluids can reduce effectively energy loss at heat sources and heat sinks, and therefore enhance the organic Rankine cycle (ORC) performance. The entropy and entransy dissipation analyses of a basic ORC system to recover low-grade waste heat using three mixture working fluids (R245fa/R227ea, R245fa/R152a and R245fa/pentane) have been investigated in this study. The basic ORC includes four components: an expander, a condenser, a pump and an evaporator. The heat source temperature is 120 °C while the condenser temperature is 20 °C. The effects of four operating parameters (evaporator outlet temperature, condenser temperature, pinch point temperature difference, degree of superheat), as well as the mass fraction, on entransy dissipation and entropy generation were examined. Results demonstrated that the entransy dissipation is insensitive to the mass fraction of R245fa. The entropy generation distributions at the evaporator for R245/pentane, R245fa/R152a and R245fa/R227ea are in ranges of 66–74%, 68–80% and 66–75%, respectively, with the corresponding entropy generation at the condenser ranges of 13–21%, 4–17% and 11–21%, respectively, while those at the expander for R245/pentane, R245fa/R152a and R245fa/R227ea are approaching 13%, 15% and 14%, respectively. The optimal mass fraction of R245fa for the minimum entropy generation is 0.6 using R245fa/R152a.
Highlights
With the increasing shortage of fossil energy and the escalating demand for energy, extensive attention has been paid to the renewable energy technologies
It should be noted that the thermodynamic properties of mixture working fluids were obtained from the NIST Refprop [40]
The effects of operation parameters on entransy dissipation and entropy generation were addressed in Sections 5.1 and 5.2, respectively
Summary
With the increasing shortage of fossil energy and the escalating demand for energy, extensive attention has been paid to the renewable energy technologies. Conducted a multi-objective optimization on the ORC system with a multi-objective function by the incorporation of four single-objective functions They found that the performance of mixture working fluids was not always better than that of pure working fluids, which is similar to the results obtained by Feng et al [22]. Lecompte et al [23] investigated the performance of ORC with zeotropic mixtures based on a second-law analysis They found that the evaporator accounts for the highest energy loss. Shu et al [26] conducted the thermal efficiency and exergy loss analysis of ORC system using three pure hydrocarbons and two retardants, stating that zeotropic mixtures presented better thermodynamic performance than the pure working fluids. (a) examining the effects of operating parameters on the entropy generation and entransy dissipation; and (b) investigating the effects of mass fraction on the entropy generation and entransy dissipation
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