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Transcritical Pressure Organic Rankine Cycle (ORC) Analysis Based on the Integrated-Average Temperature Difference in Evaporators

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Transcritical Pressure Organic Rankine Cycle (ORC) Analysis Based on the Integrated-Average Temperature Difference in Evaporators Applied Thermal Engineering 88 (2015) 2e13 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng Research paper Transcritical pressure Organic Rankine Cycle (ORC) analysis based on the integrated-average temperature difference in evaporators * Chao Yu, Jinliang Xu , Yasong Sun The Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy Utilizations, North China Electric Power University, Beijing 102206, PR China article info abstract Article history: Integrated-average temperature difference (DTave) was proposed to connect with exergy destruction (Ieva) Received 24 June 2014 in heat exchangers. Theoretical expressions were developed for DTave and Ieva. Based on transcritical Received in revised form pressure ORCs, evaporators were theoretically studied regarding DTave. An exact linear relationship be- 12 October 2014 tween DT and I was identified. The increased specific heats versus temperatures for organic fluid Accepted 11 November 2014 ave eva protruded its TeQ curve to decrease DT . Meanwhile, the decreased specific heats concaved its TeQ Available online 20 November 2014 ave curve to raise DTave. Organic fluid in the evaporator undergoes a protruded TeQ curve and a concaved T eQ curve, interfaced at the pseudo-critical temperature point. Elongating the specific heat increment Keywords: fi Organic Rankine Cycle section and shortening the speci c heat decrease section improved the cycle performance. Thus, the fi Integrated-average temperature difference system thermal and exergy ef ciencies were increased by increasing critical temperatures for 25 organic Exergy destruction fluids. Wet fluids had larger thermal and exergy efficiencies than dry fluids, due to the fact that wet fluids Thermal match shortened the superheated vapor flow section in condensers. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Carbon dioxide (CO2) is a non-flammability, non-toxic and non- corrosive fluid, having a low critical temperature of 31.3 C. It can be The waste heat resource is vast. The Organic Rankine Cycle used in a transcritical power cycle, which has been investigated (ORC) is one of the good solutions to recover the low grade waste recently [8,9]. Chen et al. [8] compared CO2 transcritical power heat [1]. Tchanche et al. [2] listed the operating ORC power plants in cycle with subcritical pressure R123 ORC. The flue gas had an inlet the world. The work capacity ranged from 125 kW to 6 MW. Many temperature of 150 C with a mass flow rate of 0.4 kg/s. It was authors investigated ORCs. The investigations focused on the se- shown that subcritical pressure ORC had higher thermal efficiency lection of organic fluids [3e5] and the parameter optimizations than CO2 power cycle. But CO2 power cycle increased the work [6e12]. output due to the raised heat received from the heat source. Cayer The geothermal water or flue gas drives ORCs to generate power et al. [9] investigated transcritical power cycle using CO2, ethane or electricity [5]. It is discharged to the environment after it releases and R125 as working fluids at a heat source temperature of 100 C. heat to ORC system. For such power systems, the objective The work output reached maximum values by optimizing various parameter is the maximum work or the largest utilization degree of parameters. the heat source. The temperature of heat carrier fluid is decreased Transcritical pressure ORCs were investigated in after heat is transferred to ORC. For subcritical pressure ORCs, Refs. [12,15e21]. Schuster et al. [12] explored the maximum work organic fluid undergoes an isothermal evaporating process to yield output. These authors compared system thermal efficiencies and large exergy loss, deteriorating ORC performance. Alternatively, exergy efficiencies at similar operating conditions between supercritical pressure ORCs increase organic fluid temperatures subcritical and transcritical pressure cycles. It was found that continuously during the heating process, resulting in better ther- transcritical pressure ORCs increased the system thermal effi- mal performance compared with subcritical pressure ORCs [12]. ciencies by 8% compared with subcritical pressure cycles. Zhang et al. [15] performed parameter optimizations for geothermal water driven subcritical and transcritical pressure Rankine cycles. The transcritical pressure cycle with R125 as working fluid could utilize * Corresponding author. Tel.: þ86 10 61772613. E-mail address: [email protected] (J. Xu). the geothermal water energy to a maximum degree, having better http://dx.doi.org/10.1016/j.applthermaleng.2014.11.031 1359-4311/© 2014 Elsevier Ltd. All rights reserved. C. Yu et al. / Applied Thermal Engineering 88 (2015) 2e13 3 economic and environmental performances. The hot water tem- performance was to decrease the temperature difference. The pa- peratures ranged from 80 Cto100C. Vetter et al. [16] compared per was organized as follows. Section 2 described the new subcritical and transcritical pressure Rankine cycles driven by the parameter and the cycle computation, consisting of three sub- 150 C geothermal water. The objective parameters included spe- sections. Section 2.1 shortly described the transcritical power cycle. cific net work output, thermal efficiency and heat received from the Section 2.2 described the temperature difference and Section 2.3 hot water. Transcritical pressure Rankine cycle with propane as described the computation process. Section 3 described the re- working fluid increased the net work output by 30% compared with sults and discussion, in which Section 3.1 explained why the ORC subcritical pressure Rankine cycle with isopentane as working cycle was mainly influenced by the fluid critical temperatures using fluid. The maximum work output could be reached when the crit- the integrated-average temperature difference, Section 3.2 gave the ical temperature of organic fluid was 0.8 times of the geothermal results and data for the computations with 25 organic fluids. The water temperature. Shu et al. [17] proposed a dual-loop ORC to paper gave a guide for the fluid selection to ORCs with respect to recover the heat of engine flue gas. The net work output and exergy efficiencies. efficiencies were compared between transcritical pressure Rankine cycles with R125, R134a, R218 as working fluids and subcritical 2. The integrated-average temperature difference and the pressure Rankine cycles with R124, R134a, R245fa, R600, R600a, cycle computation R1234yf as working fluids. The transcritical pressure Rankine cycle with R143a as working fluid showed the best cycle performance. 2.1. The ORC cycle Baik et al. [18] compared the work output between transcritical pressure cycle of R125 and subcritical pressure cycle of R134a, Fig. 1a shows an ORC coupled with a heat source. The flue gas R245fa and R152a. The geothermal water temperature was 100 C. was considered as the heat source, which was characterized by the The work output was larger by using the transcritical pressure cy- mass flow rate, mgas, inlet temperature, Tgas,in, and outlet temper- fi cles. Chen et al. [19] identi ed that transcritical pressure cycles ature, Tgas,out. The analysis of this study was also suitable for other could increase the system thermal efficiencies by 10e30% heat carrier fluids such as geothermal water. The flue gas trans- compared with subcritical pressure cycles. The hottest tempera- ferred heat to the ORC via an evaporator. Fig. 1b shows the Tes tures of the cycle ranged from 393 K to 473 K. Khennich et al. [20] diagram for transcritical pressure ORC. Process 1-2s is an ideal investigated effects of cycle highest pressures and temperature isentropic expansion while 1e2 is a real non-isentropic expansion. difference between heat carrier fluid and cycle highest temperature on subcritical and transcritical pressure cycles. The heat source was flue gas, which was regarded as the ideal dry air, having the tem- peratures of 100 C, 160 C and 230 C, respectively. Subcritical pressure cycle with R141b as working fluid and transcritical pres- sure cycle with R134a as working fluid were investigated for the 160 C heat source. Alternatively, both the subcritical and tran- scritical pressure cycles were studied with R141b as working fluid for the 230 C heat source. It was found that R141b had the better performance for all the cycles studied. Guo et al. [21] explored the difference between subcritical and transcritical pressure Rankine cycles. When the outlet temperature of heat carrier fluid was relatively low, the transcritical pressure cycles were better than the subcritical pressure cycles regarding thermal efficiencies, exergy efficiencies and work output. The available studies identified better thermal performance by using transcritical pressure cycles instead of subcritical pressure cycles, due to the better thermal match between heat carrier fluid such as flue gas or geothermal water and organic fluid in evaporators. However, the detailed mechanism needs further studies. One may ask what is the direction to further improve the performance of transcritical pressure cycles? What is the criterion to select suitable working fluids for transcritical pressure cycles? In fact, transcritical pressure cycles are strongly dependent on the heat transfer performance in evaporators. Baik et al. [18] proposed an “averageZ temperature difference”,which Qa was defined as 1=DTm ¼ 1=Qa 1=DTdQ,whereQa is the total heat transferred by the heat0 exchanger. When the average temperature difference approaches the pinch temperature
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