CO_2+DME混合体系跨临界吸收式动力循环热转换机理
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摘要
本文以CO_2+DME二元体系作为吸收式动力循环的潜力工质对.引入"化学热机"概念,用子循环划分的方法将整个循环分为化学热机子循环和热机子循环.基于循环系统的操作压力和与热源的匹配程度,对比分析了CO_2+DME体系跨临界操作条件优于亚临界和超临界条件的本质原因.建立了一套典型的CO_2+DME跨临界吸收式动力循环模拟模型.基于文献报道的气液相平衡数据,选择PR方程作为物性计算模型,分别计算了循环物流的组成、流率、焓值和熵值.根据循环系统的T-s和lgp-h图,分别分析了两个子循环之间的耦合关系与能量转换.从分析结果可以看出,由于耦合了化学热机,不仅实现了对能量的梯级利用,同时进一步降低了透平出口压力,热转功过程得到强化,使得热机子循环热转功效率由14.06%提升到15.79%.最后,采用参数分析法,探索了不同吸收温度(25, 30, 35和40℃)下,化学热机子循环高压端压力对整个循环热转功效率的影响.结果表明,降低化学热机子循环运行压力是循环优化的一个方向,有助于提高对能量的二次利用率。
CO_2+DME binary system was considered to be the working fluid of absorption power cycle in this work. The "Chemical heat engine" concept was introduced, and divided the cycle into two sub-cycles: Chemical heat engine sub-cycle and Heat engine sub-cycle. Then comparatively analyzed the essential reason of trans-critical operation condition was better than sub-critical and supercritical based on the operation pressure and the degree of match the heat source. An absorption power cycle simulation program was established, and the PR equation was selected as the calculation model. Then the composition, flow rate, enthalpy value and entropy value of the cycle streams were calculated respectively. According to the T-s and lg p-h diagrams of the cycle, the coupling relationship and heat conversion mechanism of the two sub-cycles were analyzed. As the result shows, due to the coupling of Chemical heat engine sub-cycle, the heat achieved gradient utilization and the turbine outlet pressure has reduced, and the heat conversion work efficiency of Heat engine was improved from 14.06% to 15.79%. Finally, according to the parameter analysis method, the influence of the pressure to the heat conversion work efficiency for Chemical heat engine sub-cycle was explored at different absorption temperature(25, 30, 35 and 40) ℃. The results show that reduces the operation pressure of Chemical heat engine sub-cycle can increase the heat conversion work efficiency and improve energy utilization.
CO_2+DME binary system was considered to be the working fluid of absorption power cycle in this work. The "Chemical heat engine" concept was introduced, and divided the cycle into two sub-cycles: Chemical heat engine sub-cycle and Heat engine sub-cycle. Then comparatively analyzed the essential reason of trans-critical operation condition was better than sub-critical and supercritical based on the operation pressure and the degree of match the heat source. An absorption power cycle simulation program was established, and the PR equation was selected as the calculation model. Then the composition, flow rate, enthalpy value and entropy value of the cycle streams were calculated respectively. According to the T-s and lg p-h diagrams of the cycle, the coupling relationship and heat conversion mechanism of the two sub-cycles were analyzed. As the result shows, due to the coupling of Chemical heat engine sub-cycle, the heat achieved gradient utilization and the turbine outlet pressure has reduced, and the heat conversion work efficiency of Heat engine was improved from 14.06% to 15.79%. Finally, according to the parameter analysis method, the influence of the pressure to the heat conversion work efficiency for Chemical heat engine sub-cycle was explored at different absorption temperature(25, 30, 35 and 40) ℃. The results show that reduces the operation pressure of Chemical heat engine sub-cycle can increase the heat conversion work efficiency and improve energy utilization.
引文
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[2] Kalina A I. Combined Cycle and Waste Heat Recovery Power Systems Based on a Novel Thermodynamic Energy Cycle Utilizing Low-temperature Heat for Power Generation[M]. New York:American Society of Mechanical Engineers, 1983:1-5
[3] Maloney J D, Robertson R C. Thermodynamic Study of Ammonia-water Power Cycles[M]. Oak Ridge, TN:Oak Ridge National Laboratory, 1953:CF-53-8-43
[4] Kalina A I. Combined Cycle System with Novel Bottoming Cycle[J]. Journal of Engineering for Gas Turbines and Power-transactions of the ASME. 1984, 106:737-42
[5] Ibrahim O M, Klein S A. Absorption Power Cycles[J].Energy, 1996, 21:21-27
[6] CHEN Huijuan, Goswami D Y, Stefanakos E K. A Review of Thermodynamic Cycles and Working Fluids for The Conversion of Low-grade Heat[J]. Renewable and Sustainable Energy Reviews, 2010, 14:3059-3067
[7] Agrawal N, Bhattacharyya S. Parametric Study of a Capillary Tube-suction Line Heat Exchanger in a Transcritical CO_2 Heat Pump Cycle[J]. Energy Conversion and Management, 2008, 49:2979-2985
[8] Neksa P. CO_2 Heat Pump Systems[J]. International Journal of Refrigeration-revue Internationale Du Froid, 2002,25:421-427
[9] Kim MH, Pettersen J, Bullard CW. Fundamental Process and System Design Issues in CO_2 Vapor Compression Systems[J]. Progress in Energy and Combustion Science,2004, 30:119-74
[10] Kim YM, Kim CG, Favrat D. Transcritical or Supercritical CO_2 Cycles Using Both Low-and High-temperature Heat Sources[J]. Energy, 2012, 43:402-415
[11] Yamaguchi H, Zhang XR, Fujima K, Enomoto M, Sawada N. Solar Energy Powered Rankine Cycle Using Supercritical CO_2[J]. Applied Thermal Engineering, 2006, 26:2345-2354
[12] GUO Tao, WANG Huaixin, ZHANG Shengjun, Comparative Analysis of CO_2-based Transcritical Rankine Cycle and HFC245fa-based Subcritical Organic Rankine cycle(ORC)Using Low-temperature Geothermal Source[J].Science in China Series E:Technological Sciences, 2010,53:1869-1900
[13] Sarkar J. Review and Future Trends of Supercritical CO_2Rankine Cycle for Low-grade Heat Conversion[J]. Renewable and Sustainable Energy Reviews, 2015, 48:434-451
[14] ZHOU Li, HU Shanying. Study on Systems Based on Coal and Natural Gas for Producing Dimethyl Ether[J].Industrial&Engineering Chemistry Research, 2009, 48:4101-4108
[15] NIST Standard Reference Database 23. NIST Thermodynamic and Transport Properties of Refrigerants and Refrigerant Mixtures REFPROP, Version 9.1, 2010.
[16] Tsang YC, Streett WB. Vapor-Liquid Equilibrium in The System Carbon Dioxide/dimthyl Ether[J]. Journal of Chemical and Engineering Data, 1981, 26:155-159
[17] ZHENG Danxing, JING Xuye. Chemical Amplifier and Energy Utilization Principles of Heat Conversion Cycle Systems[J]. Energy, 2013, 63:180-188