Thermodynamic performance analysis of combined cooling and heating system based on combination of CO2 with the zeotropic refrigerant assisted subcooled

doi:10.11949/0438-1157.20221670

Abstract

Abstract:

In order to meet the requirements of uperization and freezing in the field of food processing, two-stage compression transcritical CO₂ cooling and heating system with dedicated mechanical subcooling using zeotropic refrigerant is proposed. The subcooling cycle that uses low global warming potential (GWP) zeotropic refrigerant makes full use of the heat of the condenser and gas cooler to produce hot water. Additionally, cooling capacity is also generated for food freezing. Setting the coefficient of performance (COP) as the objective function, the component, mass fraction, discharge pressure and subcooling degree of the system are analyzed and optimized. The results show that the system has optimal discharge pressure and subcooling degree. The large temperature glide working fluid can significantly improve the system performance and reduce the discharge pressure comparing with the small temperature glide and pure working fluid. The highest COP of 3.45 is achieved when R32/R1234ze(Z) (0.4/0.6) is utilized, which is 4.77% and 5.15% higher than that of pure R32 and R1234ze(Z), and the discharge pressure drops by 0.958 MPa and 0.910 MPa, respectively. The CO₂ cycle plays a dominant role for the overall system. The system exergy efficiency can also be improved by using zeotropic refrigerant. This study can provide a theoretical reference to widen the application of CO₂ refrigeration and heat pump technology.

Key words: zeotropic refrigerant, transcritical CO₂, mechanical subcooling, combined cooling and heating supply, large temperature glide, two-stage compression

摘要：

为满足食品加工领域对高温消毒和冷冻的双重需求，提出了基于非共沸工质辅助过冷的跨临界CO₂冷热联供系统，辅助循环采用低GWP非共沸工质，充分利用冷凝热及气体冷却器放热生产高温热水，并同时进行食品冷冻。以COP为目标函数，对混合工质的组元、组分进行分析，对系统的排气压力和过冷度的影响进行优化。结果表明系统存在最优排气压力和最优过冷度，大温度滑移工质相对小温度滑移工质和纯工质能够显著提升系统性能，降低排气压力。采用R32/R1234ze(Z)(0.4/0.6) 时COP最高为3.45，相对纯工质R32和R1234ze(Z) 分别提高4.77%和5.15%，排气压力降低0.958 MPa和0.910 MPa。CO₂循环对系统整体起主导作用。采用非共沸工质可提高系统效率。可为推广CO₂制冷热泵技术的应用提供理论参考。

关键词: 非共沸工质, 跨临界CO₂, 机械过冷, 冷热联供, 大温度滑移, 双级压缩

CLC Number:

TK 11⁺4

Baomin DAI, Qilong WANG, Shengchun LIU, Jianing ZHANG, Xinhai LI, Fandi ZONG. Thermodynamic performance analysis of combined cooling and heating system based on combination of CO₂ with the zeotropic refrigerant assisted subcooled[J]. CIESC Journal, 2023, 74(S1): 64-73.

代宝民, 王启龙, 刘圣春, 张佳宁, 李鑫海, 宗凡迪. 非共沸工质辅助过冷CO₂冷热联供系统的热力学性能分析[J]. 化工学报, 2023, 74(S1): 64-73.

Figures/Tables 13

Fig.1 Diagram of mechanical subcooling CO2 combined cooling and heating system

Fig.2 T-s diagram of mechanical subcooling CO2 combined cooling and heating system

Table 1 The physical properties， safety and environmental characteristics of refrigerant［28］

工质	物理性质				安全特性		环境特性
工质	分子量	T_b/℃	T_cr/℃	p_cr/MPa	LEL/%	安全等级	大气寿命/a	ODP	GWP
R744	44.01	-78.5	31.1	7.38	—	A1	>50	0	1
R1270	42.08	-47.7	92.4	4.66	2.0	A3	0.001	0	-20
R601	72.15	36.1	196.6	3.37		A3		0	4
R290	44.10	-42.1	96.7	4.25	2.1	A3	0.041	0	约20
R161	48.06	-37.6	102.2	5.09	3.8	—	0.21	0	12
R32	52.02	-51.7	78.1	5.78	14.1	A2	4.9	0	675
R1234ze(Z)	114.04	9.0	153.6	3.97	—	—	—	—	<6
R600	58.12	-0.49	152.0	3.8	1.85	A3	—	0	0.1
R1234yf	114.04	-29.5	94.7	3.38	6.2	A2L	0.029	0	<4.4
R152a	66.05	-24.0	113.3	4.52	4.8	A2	1.4	0	124

Fig.3 Variation of temperature glide with the mass fraction of the first component

Fig.4 COP variation with discharge pressure and subcooling degree

Fig.5 Variation of system performance parameters with subcooling degree

Fig.6 Variation of COP with the mass fraction of the first component

Fig.7 Effect of mass fraction on the temperature matching

Fig.8 System optimum subcooling degree corresponding to different working fluid pairs

Fig.9 System optimum discharge pressure corresponding to different working fluid pairs

Fig.10 Variation of compressor power consumption ratio with mass fraction of the first component

Fig.11 System COP variation at different working conditions

Fig.12 System exergy loss and exergy efficiency with mass fraction of the first component

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Thermodynamic performance analysis of combined cooling and heating system based on combination of CO₂ with the zeotropic refrigerant assisted subcooled

非共沸工质辅助过冷CO₂冷热联供系统的热力学性能分析

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References 30

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