J. Cent. South Univ. (2017) 24: 2829−2841 DOI: https://doi.org/10.1007/s11771-017-3698-z Thermodynamic optimization and fluid selection of organic Rankine cycle driven by a latent heat source XU Peng(徐鹏)1, LU Jian(吕建)1, LI Tai-lu(李太禄)1, ZHU Jia-ling(朱家玲)2 1. School of Energy and Safety Engineering, Tianjin Chengjian University, Tianjin 300384, China; 2. Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, Ministry of Education (Tianjin University), Tianjin 300072, China © Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract: Organic Rankine cycle (ORC) is applicable for the heat-work conversion. Whereas, there also exist a lot issues that influence the efficiency and the cost of the system. In this work, eleven pure working fluids (as categorized into alkanes, and fluorinated alkanes) are investigated based on the first and second law of thermodynamics. The major objective is to obtain the most suitable working fluid for the latent heat source. The results show that the working fluid is an important factor of the system performance. The heat absorption of the working fluid in the evaporator is inversely proportional to the evaporating temperature, but the thermal and exergetic efficiencies are just the opposite. RC318 has the highest net power output and the lowest outlet temperature of the heat source, but its global warming potential (GWP) value is too high. The cyclohexane shows the highest thermal efficiency among the fluids investigated. Moreover, the figure of merit (FOM) of the isobutane is higher than that of other working fluids. Overall, the cyclohexane shows that the optimal comprehensive performance is more feasible for medium grade heat source in engineering applications. Key words: organic Rankine cycle; working fluids; thermodynamics; low-temperature; evaporating temperature PHEs will effectively be smaller than the real example 1 Introduction given, with a consequent reduction in cost. CLEMENTE et al [12] investigated scroll expanders derived from the Renewable energies have attracted much more compressors in the heating, ventilation and air- attention as a result of the shortage of the global energy, conditioning (HVAC) field to recover heat from an such as solar energy [1], geothermal energy [2], the internal combustion engine. WANG et al [13] proposed a waste heat [3], ocean thermal energy [4], and wind novel system combining a dual loop ORC with a energy. Among them, the heat-work conversion for low- gasoline engine. LI et al [14] investigated the low and medium-temperature heat source is the most temperature solar thermal electric generation with ORC. frequently studied. Especially, the organic Rankine cycle MAVROU et al [15] proposed a systematic sensitivity (ORC) has attracted much attention due to its simple analysis procedure explicitly considering the impacts of cycle configuration, high reliability and flexibility, and working fluid and ORC design/operating decisions on convenient maintenance in the past few decades [5–8]. the ability of the ORC to operate under conditions Whereas, the thermal efficiency is low, only 8%–12% [9], different from its nominal design settings to select but the economic benefit is poor. working fluid mixtures in view of operating variability in According to the research in recent years, in order solar organic ranking cycles (ORC). HE et al [16] to improve the utilization of the heat source, the proposed a combined ORC system utilizing exhaust optimization of the ORC system has been mostly focused waste as its heat source and liquid natural gas (LNG) as on, such as the minimization of the system irreversibility its heat sink to provide alternative power for an LNG- loss and the choice of the working fluids. MAGO et al fired vehicle and examined five working fluids at various [10] did an analysis about the exergy destruction and working conditions, and they found that R236fa has the obtained that the exergy destruction in evaporator is highest thermal efficiency. WU et al [17] presented the about 77%, much higher than other cycle configuration. performance of ORC using hot air as heat resource using ZHU et al [11] analyzed the plate heat exchangers (PHE) zeotropic mixture fluids R227ea/R245fa, Butane/R245fa in evaporator and found that by optimizing its design, the and RC318/R245fa as the working fluids. The result Foundation item: Project(51406130) supported by the National Natural Science Foundation of China Received date: 2016−02−29; Accepted date: 2016−05−17 Corresponding author: LI Tai-lu, Lecturer, PhD; Tel: +86–22–28305107; E-mail: [email protected] 2830 J. Cent. South Univ. (2017) 24: 2829–2841 indicates that better thermal performance can be such as corrosive, toxic, flammable and environmental achieved when the temperature difference of cooling harmful. So the mixed working fluids are a way to solve water is near the temperature glide of zeotropic mixture the issues. For example, some hydrocarbons have a very in the condenser. DESAI et al [18] did a thermo- good power performance, but flammable is a problem for economic analysis and selection of working fluid for hydrocarbons working fluids. Therefore, adding a certain solar ORC. proportion of the working fluid which has excellent There are also some scholars focus on the performance and flame retardant, can realize safe and components change of the ORC system for the power efficient application of the working fluids. output and economy of the system, such as the PAPADOPOULOS et al [35] presented that the selection regenerative organic Rankine cycle (RORC), the parallel of the working fluids significantly limited the double-evaporator organic Rankine cycle (PDORC), the opportunities of identifying highly performing options. two-stage series organic Rankine cycle (TSORC) and so CHEN et al [36] indicated that zeotropic mixtures had a on. MAGO et al [19], XI et al [20], PEI et al [21], higher efficiency. LAKEW et al [37] found that the FERNÁNDEZ et al [22], and ROY et al [23] analyzed power output of the R227ea is highest for the heat source RORC and found that the supercritical RORC is temperature range from 80 °C to 160 °C, and the R245fa preferable for high temperatures heat source. The internal gave the highest power output when the temperature heat exchanger is the main factor to system efficiency. LI ranges from 160 °C to 200 °C. PASETTI et al [38] et al [24] proposed and compared PDORC with the ORC presented an improved survey method for the evaluation to show that the irreversible loss of the PDORC is lower of the thermal stability of working fluids for ORCs. LI than that of the ORC. BORSUKIEWICZ-GOZDUR et al et al [39] proposed a novel combined cooling, heating [25] tried to increase the working fluid flow rate as a and power organic Rankine cycle (CCHP-ORC) system kind of method to enhance the power output of installed with heat pumps to select optimal zeotropic geothermal power plant. LI et al [26] constructed and mixtures and determined the component concentration experimentally analyzed the RORC and found that the that gives a better performance. The results showed that thermal load of the condenser and the irreversible loss R141b/R134a, R141b/R152a and R123/152a have a reduced at the same time. Moreover, the thermal higher COP and exergy efficiency than others. efficiency of the RORC is higher than that of the ORC MAVROU et al [40] investigated the performance of by 1.83%. working fluid mixtures for use in solar ORC with heat Besides the cycle parameters and the cycle storage employing flat plate collectors (FPC) configuration, the working fluid has an important and assessed the impact of heat source variability on the influence on the system performance. BADR et al [27] ORC performance for different working fluid mixtures, investigated thermodynamic and thermophysical and a mixture of neopentane-2-fluoromethoxy-2- properties of organic working fluids. HUNG et al [28] methylpropane at 70% neopentane was the most efficient used ammonia, benzene, R11, R12, R134a and R113 as in all the considered criteria simultaneously. Besides the the working fluid, respectively, and compared the mentioned above, many factors should be considered for efficiencies of ORCs with each working fluid. SALEH the working fluid choice, such as specific volume, low et al [29] gave a thermodynamic screening of 31 pure viscosity, global warming potential (GWP), ozone component working fluids for ORCs, the largest amount depletion potential (ODP), high latent heat of of heat can be transferred to a supercritical fluid and the vaporization, high thermal conductivity and a stable least to a high-boiling subcritical fluid. Based on the first operating pressure. It should be pointed out that a large and second laws of the thermodynamics, YARI [30, 31] number of researchers have focused on the heat-work investigated several dry fluids for ORC, and DESAI et al conversion for low- and medium-grade sensible heat, and [32] presented that the dry fluids were the better working few literatures have been found to discuss the working fluid for ORC utilizing low-grade heat sources. Also, the fluid selection of ORC driven by low- and medium-grade wet fluids were investigated by HUNG et al [33], latent vapor resource resources. showing that the wet fluids with very steep saturated In this work, the saturated vapor was adopted as the vapor curves in the T–s diagram for the refrigerant-series heat source, which has the latent and sensible heat at the and benzene-series fluids had a higher overall same time, and eleven kinds of working fluids was performance in energy conversion efficiencies than that chosen for comparison. Based on the first and second of dry fluids. LIU et al [34] presented that the thermal laws of thermodynamics, the parameters, such as power efficiency and the total heat recovery efficiency were output, irreversible loss, total thermal conductance, different for various working fluids.