Elsevier

Applied Energy

Volume 185, Part 2, 1 January 2017, Pages 2074-2084
Applied Energy

Theoretical analysis of ejector refrigeration system performance under overall modes

https://doi.org/10.1016/j.apenergy.2016.01.103Get rights and content

Highlights

  • Real gas theoretical model is used to get ejector performance at critical/sub-critical modes.

  • The model has a better accuracy against the experiment results compared to ideal gas model.

  • The overall performances of two refrigerants are analyzed based on the parameter analysis.

Abstract

The ejector refrigeration integrated in the air-conditioning system is a promising technology, because it could be driven by the low grade energy. In the present study, a theoretical calculation based on the real gas property is put forward to estimate the ejector refrigeration system performance under overall modes (critical/sub-critical modes). The experimental data from literature are applied to validate the proposed model. The findings show that the proposed model has higher accuracy compared to the model using the ideal gas law, especially when the ejector operates at sub-critical mode. Then, the performances of the ejector refrigeration circle using different refrigerants are analyzed. R290 and R134a are selected as typical refrigerants by considering the aspects of COP, environmental impact, safety and economy. Finally, the ejector refrigeration performance is investigated under variable operation conditions with R290 and R134a as refrigerants. The results show that the R290 ejector circle has higher COP under critical mode and could operate at low evaporator temperature. However, the performance would decrease rapidly at high condenser temperature. The performance of R134a ejector circle is the opposite, with relatively lower COP, and higher COP at high condenser temperature compared to R290.

Introduction

Due to the society development and greenhouse effect, the air-conditioning and refrigeration requirements grow rapidly, and demand more fossil fuel consumption. However, the main issue in the engineering fields is the high energy conversion efficient and the low emissions [1]. Therefore, it’s highly desirable to use the green energy technique or the widely available low-grade heat energy from the industrial field to replace the fossil fuel [2], [3]. The ejector can be actuated by the low-grade thermal energy, considered to be an economically feasible and environmentally-friendly technology. Hence, the ejector integrated in the refrigeration system is considered to be a promising technology [4].

Ejectors are not new technology and have been already used in the industrial fields for more than 100 years. As the core part of the ejector refrigeration system, the information about the ejector design and performance is of great importance. Although the experimental method is considered as the best way to obtain data, it’s not economical because it will cost so much labor power and material resources. Therefore, the theoretical method is usually the priority selection for the ejector performance evaluation and parameter optimization based on its advantages, such as simplicity and accuracy.

Some theoretical models were proposed to present ejector performance according to the fluid dynamics theory. The original model about a simple air ejector was proposed by Keenan and Neumann [5]. The models named as constant area mixing (CAM) theory and constant pressure mixing (CPM) theory were first put forward by their research group [6]. Generally speaking, it is widely believed that the CPM ejectors are more widely used, because of the superior performance compared to the CAM ejectors [7]. Therefore, many works have been carried out to give further understanding about the CPM ejectors. Munday and Bagster [8] found that the ejector had three operation modes, and an “effective area” was assumed to deal with the performance at critical mode. Huang et al. [9] inherited their model and further developed the concept of “effective area”. Their model was validated with a good deal of experimental data, which was used as the comparison object by the later theoretical models [10], [11], [12], [13]. Recently, Zhu et al. [10] proposed the concept of “shock circle” model to replace the “effective area”, they assumed that the pressure still remains constant, but the velocity obeys exponential distribution in the shock circle section. Besides, several researchers introduced the Prandtl–Meyer fan flow [11] and Fanno flow [12] to represent the actual flow inside the ejector. Those models focused on the entrainment ratio at critical mode. Then, a theoretical model was proposed by Chen et al. [13] to handle with the ejector capability under overall modes, and its effectiveness was validated against the experimental data from the literature.

In brief, the above mathematical methods assumed that the working fluids obey the ideal gas law, and the working fluids used in these models are always R141b or air which could be treated as the ideal gas, so they could accurately predict the entrainment ratio. However, alternative environment-friendly refrigerants become research hotspot in the ejector refrigeration system [14]. Tashtoush et al. [15] analyzed the ejector performance under different working fluids (R290, R717, etc.), and they thought R134a was more safe and suitable. However, their model assumed that all of the working fluids obey ideal gas law. As we know, the refrigerants used in the ejector refrigeration system, such as Freon gas (R11, R22, R142b), inorganic compounds (R717), usually deviate far from the ideal gas law. If the working fluid is still treated as the ideal gas, the calculation will deviate far from the realistic performance of ejector. To authors’ knowledge, several researchers tried to build up a theoretical model while the working fluid obeys the real gas property. For example, Zhu and Li [16] put forward a theoretical method using real gas property. R141b, R11 and steam were selected as representative fluids. Cardemil and Colle [17] also carried out their model for real gas property, and the effectiveness is validated against the experimental data of R141b, steam and CO2 ejectors. Chen et al. [18] paid special consideration on the superheat of the refrigerants, and found the minimum superheat for the wet, dry and isentropic fluids to avoid the droplet formation. Li et al. [19] investigated the performance of R1234yf ejector refrigeration cycle theoretically. However, these models are always applied to obtain the ejector performance at critical mode.

As mentioned above, the ejector performance at critical mode has been widely studied theoretically. However, most of them are based on ideal gas law, and the theoretical model represents the ejector performance information at sub-critical mode is really rare. In domestic air-conditioning system, the users’ demands are always fluctuating at different time and seasons. It means that the ejector refrigeration system often works at the sub-critical mode. Therefore, the performance information at sub-critical model is also crucial in the industrial fields. In the present study, a theoretical model would be established to present the ejector capability at critical/sub-critical modes based on real gas property. Then the ejector performance for different working fluids would be analyzed, the operation parameter for the optimum conditions and the performance under different operation modes would be investigated.

Section snippets

Theoretical model

The ejector refrigeration system is a promising technology because of the characteristics of environmental protection and energy saving. Fig. 1 is the system diagram of ejector cooling refrigeration circle. Besides ejector, the system contains three heat exchangers: generator, evaporator and condenser, as well as pump and expansion valve. The generator could absorb heat from low-grade heat energy, and then the stream is heated in the generator and becomes the primary flow. The induced flow

Model validation

As mentioned above, the theoretical model based on the real gas property has been built up. To validate its accuracy, some experimental results from literature are employed. The experimental data of R141b ejector from Huang et al. [9] is applied to verify the accuracy of theoretical model at critical mode. The experimental results of R11 ejector in Aphornratana et al. [20] are used to validate its ability at critical/sub-critical mode operations.

Results and discussions

As stated above, the information about the ejector design and performance is of great importance when the ejector is applied in industrial fields. Generally, the performance of ejector refrigeration system is strongly influenced by the refrigerant type. Therefore, it’s really crucial to select a suitable refrigerant, which could be used to obtain high COP, and environmentally-friendly, safe and economic. The refrigerants used in the refrigeration system could be divided into two types based on

Conclusions

A theoretical model using real gas property is proposed to predict ejector performance under overall modes (critical/sub-critical mode) based on CPM theory. The main contribution is that the present model could offer the performance of ejector refrigeration system at sub-critical mode based on real gas law. Firstly, the experimental results of Huang et al. [9] with R141b are applied to validate the effectiveness of the model at critical mode, while the results from Aphornratana et al. [20]

Acknowledgements

Authors express thanks to Prof. He Maogang and Asso. Prof. Liu Yingwen at the School of Energy and Power Engineering, Xi’an Jiaotong University, China, for their valuable contribution on model improvement .This research was supported by National Natural Science Foundation of China (No. 51476128, No. 51506167) and Postdoctoral Science Foundation of China (No. 2014M552441).

Cited by (119)

View all citing articles on Scopus
View full text