Research on injection characteristics and volumetric matching characteristics of CO2 two-stage vapor compression heat pumps

doi:10.11949/0438-1157.20250694

Abstract

Abstract:

The application of CO₂ two-stage compression heat pump technology with inter-stage injection significantly enhances the low-temperature performance of air-source heat pump systems. During actual operation, CO₂ two-stage compression heat pumps with interstage injection often suffer from excessively high exhaust temperatures and compressor burnout due to imperfect system control strategies. This study aims to optimize the control strategy of CO₂ two-stage compression heat pump systems to increase their operational reliability and practical viability. The research investigates the injection ratio (R_inj) and volume ratio (R_v), focusing on their effects on discharge temperature (T_dis,H), coefficient of performance (COP), and other performance metrics across different operating conditions. A simulation model of a CO₂ transcritical two-stage compression heat pump with inter-stage injection was developed using Dymola software to analyze the system's injection and capacity matching behavior. Subsequently, the effects of R_inj and R_v on COP and heating capacity (Q_H) were assessed using response surface methodology and analysis of variance. The results indicate that as R_inj increases, both COP and Q_H initially rise rapidly and then decline gradually, while T_dis,H initially decrease slowly, followed by a more rapid decline. At an evaporating temperature (Tₑ) of -20℃, within the rapid COP growth region, increasing R_inj from 0 to optimal injection ratio (R_inj,opt) enhances COP by 18.4% and Q_H by 29.1%, with T_dis,H decreasing by 3℃ for every 0.05 increment in R_inj. Beyond R_inj,opt, as R_injapproaches maximum injection ratio (R_inj,max), COP and Q_H decrease by 2.6% and 2.3%, respectively, while T_dis,H decreases by 7℃ per 0.05 increment in R_inj. These injection characteristics are attributed to the dynamic response of flow and heat transfer dynamics, as well as variations in intermediate pressure (Pₘ) with changing injection ratios. Furthermore, sensitivity analysis reveals that R_inj has a greater influence on COP, while R_v has a more significant effect on Q_H.

Key words: heat pump, two-stage compression, carbon dioxide, dynamic coupled, intermediate pressure

摘要：

具有级间喷射特征的CO₂双级压缩热泵技术在实际设备运行时常因系统控制策略不完善，造成排气温度过高导致压缩机烧毁。为此，以喷射比（R_inj）与容量比（R_v）为研究对象，针对变工况时R_inj与R_v对高压级压缩机排气温度（T_dis,H）、COP等参数的影响程度开展研究。首先，利用Dymola软件构建具有级间喷射特性的CO₂跨临界双级压缩热泵仿真模型，探究系统的级间喷射特性和容量匹配特性。其次，基于响应面法和方差分析，研究R_inj和R_v对COP及制热量（Q_H）的影响特性。结果表明：随着R_inj的增大，COP、Q_H呈现先快速上升后缓慢下降的趋势，T_dis,H随着R_inj增大呈现先缓慢下降后快速下降的趋势。通过分析数据发现造成喷射特性的原因可归结于系统内部流动与换热特性的变化及中间压力（P_m）随R_inj变化的动态响应过程。

关键词: 热泵, 双级压缩, 二氧化碳, 动态耦合, 中间压力

CLC Number:

TK 01

Aruna, Hao ZHANG, Shuai SHA, Xu JIN, Zhongyan LIU, Wei SU, Jiapeng ZHANG, Zheng QIU. Research on injection characteristics and volumetric matching characteristics of CO₂ two-stage vapor compression heat pumps[J]. CIESC Journal, 2025, 76(12): 6658-6668.

阿如娜, 张浩, 沙帅, 金旭, 刘忠彦, 苏伟, 张家鹏, 邱政. CO₂双级压缩热泵喷射特性及容量匹配特性研究[J]. 化工学报, 2025, 76(12): 6658-6668.

Figures/Tables 14

Fig.1 Schematic and enthalpy diagrams of a transcritical CO₂ system featuring interstage injection

Fig.2 Photograph of the comprehensive experimental setup for the transcritical CO2 two-stage heat pump system

Table 1 Structural specifications of system component

部件	参数
压缩机	活塞式压缩机；排气量:低压级压缩机为4.67 m³/h@50 Hz；高压级压缩机为2.39 m³/h@50 Hz
蒸发器	板式换热器；厚度为91.4 mm;长度为616 mm;宽度为189 mm;质量为34.97 kg;板片数为36
中间换热器	板式换热器；厚度为41 mm;长度为187 mm;宽度为77 mm;质量为2.5 kg;板片数为30
回热器	板式换热器；厚度为34.8 mm;长度为524 mm;宽度为108 mm;质量为9.9 kg;板片数为10
节流阀	超高压控制用电动阀(鹭宫)；口径：节流阀1为2.0 mm,节流阀2为2.4 mm;流经系数：节流阀1为2.0,节流阀2为2.4

Fig.3 Comparison between experimental and simulated results reported by Ref.[26]

Fig.4 Variation of COP and QH in the OTHS sytem as a function of Rinj

Fig.5 Variation trends of system parameters in the OTHS system with respect to Rinj

Fig. 6 Variation trends of parameters including Pm in the OTHS system with respect to Rinj

Fig.7 Variation in the inlet state point of the high-pressure compressor

Fig.8 Enthalpy diagram corresponding to various Rinj

Fig.9 Influence of volume ratio on COP and QH

Table 2 Parameter values used in sensitivity analysis

No.	R_inj	R_v	COP	Q_H
1	0.15	1.2	2.44	11.04
2	0.2	1.4	2.55	13.07
3	0.3	1.6	2.72	15.60
4	0.4	1.8	2.68	16.67
5	0.5	2.0	2.57	17.28
6	0.6	2.0	2.52	16.97

Table 3 Variance analysis

项	SS		DF	MS		F值		P值
项	COP	Q_H	COP和Q_H	COP	Q_H	COP	Q_H	COP	Q_H
R_inj	14.23	3.01	1.0	14.23	3.01	316.70	454.69	<0.001	<0.001
R_v	9.27	42.79	1.0	9.27	42.79	206.31	6461.39	<0.001	<0.001
R_vR_inj	5.67	0.98	1.0	5.67	0.98	126.15	147.44	<0.001	<0.001
$R i n j 2$	16.37	2.73	1.0	16.37	2.73	364.28	412.92	<0.001	<0.001
$R v 2$	2.47	0.19	1.0	2.47	0.19	55.17	29.15	<0.001	<0.001
residual	1.97	0.29	44.0	0.045	0.006	—	—	—	—

Table 3 Variance analysis

项	SS		DF	MS		F值		P值
项	COP	Q_H	COP和Q_H	COP	Q_H	COP	Q_H	COP	Q_H
R_inj	14.23	3.01	1.0	14.23	3.01	316.70	454.69	<0.001	<0.001
R_v	9.27	42.79	1.0	9.27	42.79	206.31	6461.39	<0.001	<0.001
R_vR_inj	5.67	0.98	1.0	5.67	0.98	126.15	147.44	<0.001	<0.001
$R i n j 2$	16.37	2.73	1.0	16.37	2.73	364.28	412.92	<0.001	<0.001
$R v 2$	2.47	0.19	1.0	2.47	0.19	55.17	29.15	<0.001	<0.001
residual	1.97	0.29	44.0	0.045	0.006	—	—	—	—

Fig.10 Influence of Rinj and Rv on COP

Fig.11 Influence of Rinj and Rv on QH

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Research on injection characteristics and volumetric matching characteristics of CO₂ two-stage vapor compression heat pumps

CO₂双级压缩热泵喷射特性及容量匹配特性研究

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