New exergy analysis of a regenerative closed Brayton cycle Mohammad Mahdi Naserian, Said Farahat ⇑ , Faramarz Sarhaddi Department of Mechanical Engineering, University of Sistan and Baluchestan, Zahedan, Iran article info Article history: Received 10 August 2016 Received in revised form 14 November 2016 Accepted 10 December 2016 Keywords: Power Regenerative Brayton cycle Exergy Efﬁciency Optimization abstract In this study, the optimal performance of a regenerative closed Brayton cycle is sought through power maximization. Optimization is performed on the output power as the objective function using genetic algorithm. In order to take into account the time and the size constraints in current problem, the dimen- sionless mass-ﬂow parameter is used. The inﬂuence of the unavoidable exergy destruction due to ﬁnite- time constraint is taken into account by developing the deﬁnition of heat exergy. Finally, the improved deﬁnitions are proposed for heat exergy, and the second law efﬁciency. Moreover, the new deﬁnitions will be compared with the conventional ones. For example, at a speciﬁed dimensionless mass-ﬂow parameter, exergy overestimation in conventional deﬁnition, causes about 31% lower estimation of the second law efﬁciency. These results could be expected to be utilized in future solar thermal Brayton cycle assessment and optimization. Ó 2016 Elsevier Ltd. All rights reserved. 0. Introduction The underlying functional point of cyclic heat engines is maxi- mum power state. The operational point in maximum power and reversible performance possess the same importance. The efﬁ- ciency of heat engines are restricted by Carnot efﬁciency. This efﬁ- ciency is obtainable in the reversible case. Practically, all thermodynamic processes take place in ﬁnite-size components during ﬁnite-time, which leads to irreversibility (exergy destruc- tion). Accordingly, while Carnot cycle gives upper bound for ther- mal efﬁciency, it cannot be a comparison standard for real heat engines. Analysis techniques have been developed in various stud- ies to consider the internal and/or external irreversibility in heat engines. Curzon and Ahlborn [1] studied the effect of external irre- versibility, which accounted for irreversibilities in the heat- exchange processes between the power cycle and its heat sources, on Carnot cycle’s output power and thermal efﬁciency. This system was entitled endoreversible due to internal reversibility of cycle. In their research, thermal efﬁciency at the maximum power state was expressed in the form of Eq. (1). T L and T H are temperatures at cold and hot heat exchangers, respectively. g CA ¼ 1  ﬃﬃﬃﬃﬃ T L T H s ð1Þ Bejan [2] showed that the degree of thermodynamic imperfec- tion of power plants could be estimated based on a very simple model that considers only the sources of heat transfer irreversibil- ities. In a separate study, Bejan [3] investigated the optimal alloca- tion of heat exchange equipment. His study showed that the power output of various power plant conﬁgurations could be maximized by properly dividing the ﬁxed inventory of heat exchange equip- ment among the heat transfer components of each plant. Wu [4] established a comparison between endoreversible Carnot cycle and the same system with both internal and external irreversibil- ity. It was shown that the internal irreversibility reduces power and efﬁciency. Gordon [5] analyzed heat engines considering ﬁnite rate heat transfer and ﬁnite-capacity thermal reservoirs. He showed that the efﬁciency at maximum power depends on the thermal reser- voir temperatures, and other system variables such as reservoir capacity or working ﬂuid speciﬁc heats. Heat engines operate in ﬁnite time; therefore, the realistic study of their optimal performance is feasible through the concept of ﬁnite-time thermodynamics [6]. This method was applied to the optimization of regenerative endoreversible Brayton cycle with ﬁnite thermal capacitance rates in heat reservoirs [8]. In the under- taken research, application of regenerators led to decrease in the maximum power and thermal efﬁciency. Their study showed the regenerative heat-transfer rate was positive for low temperature ratios and negative for high temperature ratios. Further analyses were performed on regenerative and irreversible models of Bray- ton heat engines [9,10]. http://dx.doi.org/10.1016/j.enconman.2016.12.020 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: farahat@hamoon.usb.ac.ir (S. Farahat). Energy Conversion and Management 134 (2017) 116–124 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman