EXERGY ANALYSIS OF THE COUPLING OF TWO CO2 HEAT PUMP CYCLES
Exergy analysis of the coupling of two CO2 heat pump cycles
Keywords:Exergy destruction, Structural optimization, Internal heat exchanger
The study is looking for the optimal configuration of an air-to-water heat pump capable of heating water in a flow-return system. Carbon dioxide is used as a working agent. In the optimization strategy, exergy analysis is considered. Using exergy analysis, the magnitude and the location of any functional or constructive malfunction can be revealed. First, a standard one-stage heat pump system is considered. Due to the high exergy destruction in the throttling valve, the efficiency of the standard system is improved by coupling it with an auxiliary one. The coupling process is undertaken in an internal subcooler-evaporator-heat exchanger. By subcooling the CO2 before entering the throttling valve of the standard cycle, the exergy destruction associated with this process diminishes. To increase the efficiency of the globally coupled system, the heat transferred outside in the gas cooler of the auxiliary heat pump is used in the water heating process. The energetic and exergetic efficiencies of the coupled system increased by 19 % and 18.3 %, respectively, compared to the standard heat pump cycle.
(1) ***Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases and repealing Regulation (EC) No 842/2006
(2) H. Bin, W.U. Di, L.W. Wang, R.Z. Wang, Exergy analysis of R1234ze (Z) as high temperature heat pump working fluid with multi-stage compression, Energy 11 (4), pp. 493-502 (2017).
(3) Y. Alptug, K. Ali, K. Irfan, Exergy analysis of R1234yf and R1234ze as R134a replacements in a two evaporator vapor compression refrigeration system, International Journal of Refrigeration 60, pp 26-37 (2015).
(4) Z. Yingbai, W. Zhichao, C. Kuikui, Z. Xuedong, The exergy analysis of gas cooler in CO2 heat pump system, Procedia Environmental Sciences 11, pp. 1555-1560 (2011).
(5) L. Schengchun, L. Zheng, D. Baomin, Z. Zhifeng, S. Hailong, M. Song, Z. Sun, Energy, economic and environmental analysis of air source transcritical CO2 heat pump system for residential heating in China, Applied Thermal Engineering 148, pp. 1425-1439 (2019).
(6) Z. Jian-Fei, Q. Yan, W. Chi-Chuan, Review on CO2 heat pump water heater for residential use in Japan Renewable and Sustainable Energy Reviews 50, pp. 1383-1391 (2015).
(7) Q. Xiang, L. Huadong, M. Xiangrui, W. Xinli, Z. Linghua, Y. Lingxiao, A study on the compressor frequency and optimal discharge pressure of the critical CO2 heat pump system, International Journal of Refrigeration 99, pp. 101-113 (2019).
(8) B. Evangelos, T. Christor, A comparative study of CO2 refrigeration systems, Energy Conversion and Management X, 1, pp. 100002 (2019).
(9) B. Changhyun, H. Jaehyeok, J. Jongho, C. Honghyun, K. Yongchan, Performance characteristics of a two-stage CO2 heat pump water heater adopting a subcooler vapor injection cycle at various operating conditions, Energy 77, pp. 570-578 (2014).
(10) B. Tao, Y. Jianlin, Y. Gang, Advanced exergy analyzes of an ejector expansion transcritical CO2 refrigeration system, Energy Conversion, and Management 126, pp. 850-861 (2016).
(11) S. Taleghani Taslimi, M. Sorin, S. Poncet, H. Nesreddine, Performance investigation of a two-phase transcritical CO2 ejector heat pump system, Energy Conversion and Management 185, pp. 442-454 (2019).
(12) S. Bhattacharyya, A. Mukhopadhyay, A. Kumar, R.K. Khurana, J. Sarkar, Optimization of a CO2 -C3H8 cascade system for refrigerator and heating, International Journal of Refrigeration 28, pp. 1284-1292 (2005).
(13) C. Feng, C. Cui, X. Wei, C. Yin, M. Li, X. Wang, The experimental investigation on a novel transcritical CO2 heat pump combined system for space heating, International Journal of Refrigeration 106, pp539-548 (2019).
(14) K.J. Chua, S.K. Chou, W.M. Yang, Advances in heat pump systems: A review, Applied Energy 87, pp 3611-3624 (2010).
(15) BO Bolaji and Z Huan, Ozone depletion and global warming: Case for the use of natural refrigerant - A review, Renew. Sustain. Energy Rev. 18, pp. 49-54 (2013)
(16) *** https://climalife.dehon.com/
(19) V. Radcenco, S. Porneala, A, Dobrovicescu, Procese in instalatiile frigorifice, (in Romanian) Editura Didactica si Petagogica , Bucuresti (1983).
(20) ***https://www2.le.ac.uk/offices/itservices/ithelp/my-computer/programs /ess-10-2 Engineering Equation Solver (vs. 10.2 – 2017).