تجزیه و تحلیل عملکرد مقرون به صرفه از سیکل رانکین آلی برای اتصال بهره برداری با درجه حرارتی کم با شرایط عملیاتی

This paper analyzed the influence of working fluids selection and operation conditions on the cost-effectiveness performance and net power output of an ORC for low grade heat utilization. A net power output model has been proposed theoretically and compared with the theoretical data calculated from thermodynamic analysis, exhibiting excellent agreements with the theoretical data. The proposed net power output model theoretically indicates that Jacob number and the ratio of evaporating temperature and heat rejected temperature play essential roles in discriminating the net power output among various working fluids at the same operation condition. For a given condensing and evaporating temperature, it can be concluded theoretically that fluid with low Jacob number will show attractive performance in an ORC. The maximum net power output is determined by the heat source rather than working fluids with a low inlet temperature of heat source. Cost-effectiveness performance analysis reveals that the maximum net power output and the best CEP cannot be achieved at the same time and compromise must be made when choosing the most suitable organic working fluids in different ORC designs.

About one-third of the heat supply in winter season was provided by natural gas boilers in Beijing (China). Unfortunately, the exhaust flue gas, with temperature of 370–570 K, is directly discharged into environment [1]. As China surveys the world, low grade heat such as geothermal, industrial waste heat and heat from low to moderate temperature solar collectors, accounts for more than one half of the total heat generated worldwide [2], [3] and [4]. In the past decades, large attempts are made to extend the market share of renewable energy sources, extensive researches have been performed on converting low temperature heat (353 < T < 573 K) into electricity. Contrasted with the conventional steam Rankine cycle [5], [6], [7] and [8], organic Rankine cycle possesses the capability to convert low grade heat sources into electricity and has become a significant issue in power engineering and the number of published papers is rapidly increasing. The investigations on the ORC system can be summarized into the following sorts: • Modification of the basic ORC for improving system efficiency. • Selection of the suitable working fluids for different heat sources. • Replacement of the pure working fluids with zeotropic mixture. • Optimization of the ORC system. Hung et al. [7], Maizza et al. [9] and [10], Liu et al. [11], Hung [12], Larjola [13], Srinivasan et al. [14] proposed and analyzed various ORCs designed for the waste heat recovery systems. Techanche et al. [15], Hung et al. [16] studied the application of ORCs in solar organic Rankine cycle systems. Drescher et al. [17] proposed a method to find suitable thermodynamic fluids for ORC in biomass power and heat plants. Saleh et al. [18] analyzed the thermodynamic performances of alkanes, fluorinated alkanes, ether and fluorinated ethers as working fluids in ORCs for geothermal power plants. Hung et al. [16] studied the system efficiency of ORC using ocean thermal energy conversion (OTEC) system as heat sources. Schuster et al. [19] investigated the efficiency optimization potential in supercritical organic Rankine cycle. In view of numerical simulations and experimental studies, Yamamoto et al. [20], Quoilin et al. [21] presented numerical simulation models of ORC and carried out experimental analysis, respectively. Wei et al. [22] and Quoilin et al. [23] proposed dynamic models of ORC using turbine and scroll expander, respectively and provided insight in transient conditions due to fluctuations of the heat parameters and load demand. Desai et al. [24] considered the process integration of ORC and reported that the basic ORC can be modified by incorporating both regeneration and turbine bleeding to improve its thermal efficiency. Kuo et al. [25] analyzed the system performance of a 50 kW ORC system subject to influence of various working fluids and proposed a dimensionless for quantitatively screening working fluid. Quoilin et al. [26] focused on the thermodynamic and economic optimization of a small scale ORC in waste heat recovery application. Meanwhile, Dai et al. [27] considered the exergy efficiency as an objective function to optimize the thermodynamic parameters of the ORC for each working fluid by means of the genetic algorithm. They found that the cycle has the best performance property with saturated vapor at the turbine inlet. Wang et al. [28] showed their interest in the working fluid selection of ORCs for engine waste heat recovery. The outcomes indicate that R11, R141b, R113 and R123 manifest slightly better thermodynamic performances than the others, while R245fa and R245ca are the most environment-friendly working fluids for engine waste heat-recovery applications. The system performance of ORC is strongly related to the working fluid. Hence it is essential to carefully select the working fluid. No single physical property can be used as the sole indicator for quantitatively screening the working fluid [25]. Hung et al. [7] showed that the major physical property of screening the working fluid includes specific heat, latent heat and slope of saturation vapor curve. As shown in Fig. 1, working fluids can be classified into dry, isentropic or wet respectively in terms of the slope of saturation curve in T-s diagram to be positive, infinite or negative [5]. We confirmed that dry and isentropic fluids exhibit more desirable for ORC in virtue of avoiding liquid droplet impingent in the turbine blades during the expansion, wet fluid is normally inappropriate for ORC system or the wet fluid should be superheated. Nevertheless, with the development of volumetric expander [29], such as scroll expander and screw expander, it is better adapted to larger capacities and shows the advantage of accepting a high liquid fraction at the inlet, allowing the design of “wet” cycles [30]. That is to say, wet fluids may also well exhibit desirable performance for ORCs without superheat. Liu et al. [11] found that the thermal efficiency is a weak function of the critical temperature. While Aljundi [8] found that the higher the critical point temperature of the fluid is, the better the cycle thermal and exergetic efficiencies will be. Tchanche et al. [31] preferred to select working fluids with high latent heat and high specific heat. Kuo et al. [25] proposed a dimensionless number (FOM) to screening the working fluid as far as thermal efficiency is concerned. Maizza et al. [9], Badr et al. [5], Chen et al. [32], Hung et al. [16] suggested that high latent heat and low liquid specific heat are preferable. However, appreciable inconsistencies are encountered. Yamamoto et al. [20] suggested that low latent heat is better because the saturated vapor at the turbine inlet would provide the best operating condition. Full-size image (32 K) Fig. 1. Three types of working fluids: dry, isentropic and wet. Figure options The abundant relevant literature mainly focused on the extensive analysis among different thermodynamic cycles and the selection of working fluids. However, appreciable inconsistencies are always encountered when screening working fluids and few people have focused this. Meanwhile, most of the analyses were conducted under certain predefined temperature conditions and only a few working fluids were employed. The best working fluids and highest effective cycle may be untenable under different operating conditions and working fluids. A net power output model was proposed to discriminate the net power output among various working fluids at the same operation condition based on thermodynamic analysis and compared with the thermodynamic data, the influence of various working fluids on the net power output and thermal efficiency was assessed theoretically. Additionally, the optimal evaporating temperature and maximum net power output have been analyzed. Attempts were made to analyze the cost-effectiveness performance for evaluating different ORCs.

This paper analyzed the effect of working fluids selection and operation conditions on the cost-effectiveness performance and net power output of an ORC for low grade heat utilization. A net power output model has been proposed theoretically. The proposed net power output model has been compared with theoretical data calculated from thermodynamic analysis and it exhibits excellent agreements with the theoretical data. The proposed net power output model theoretically indicates that Jacob number and the ratio of evaporating temperature and condensing temperature play essential roles in discriminating the net power output among various working fluids at the same operation condition. For a given condensing and evaporating temperature, it can be concluded theoretically that fluid with lower Jacob number will show attractive performance in an ORC. In other words, fluid with low specific liquid heat and high vaporization latent is preferable to use in an ORC. The maximum net power output is determined by the heat source rather than working fluids in the low inlet temperature of heat source. Cost-effectiveness performance analysis reveals that the maximum net power output and the best CEP cannot be achieved at the same time. Compromise must be made when choosing the most suitable organic working fluids in different ORC designs.