Performance analysis of a dual temperature heat pump based on ejector-vapor compression cycle

doi:10.1016/j.enbuild.2021.111194

Energy and Buildings

Volume 248, 1 October 2021, 111194

https://doi.org/10.1016/j.enbuild.2021.111194 Get rights and content

Highlights

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A new dual-temperature heat pump based on ejector-vapor compression cycle is introduced.
•
The NDAHP system outperforms the CDAHP system.
•
The ejector improves the mass flowrate of refrigerant in the evaporator.
•
R290 is recommended among the different refrigerants.

Abstract

The dual temperature air-source heat pump reduces energy consumption and has wide application potential. A novel dual temperature air-source heat pump (NDAHP) system based on ejector-vapor compression cycle is proposed in this study. The system produces two types of heat sources (high- and low-temperature hot water). A theoretical model describing the performance of the system is established and validated, and the performance indices of the system, including the heating coefficient of performance (COP_h), heating capacity per volume (q_v), and the second law efficiency (ƞ_II) are obtained. The above-mentioned indices of the conventional dual temperature air-source heat pump (CDAHP) system and the NDAHP system using various refrigerants (R717, R1234yf, R134a, and R290) are compared. Results show that the NDAHP outperforms the CDAHP under various operating conditions. Compared with the CDAHP, the NDAHP improves the COP_h, q_v, and ƞ_II by 20.0% to 48.9% under a typical operating condition. The R290 shows the best performance among the various refrigerants. The performance of the NDAHP system is more sensitive to the evaporating temperature than the condensing temperature. Additionally, the ƞ_II of the NADHP system is significantly affected by the high-temperature heating load ratio (LR) and increases with the increase of the LR. This study hopes to provide a foundation for further research on the NDAHP systems.

Introduction

Air-source heat pump systems based on the vapor-compression cycle have received increasing attention due to their outstanding performance in terms of energy-savings potential and convenient installation [1], [2], [3]. In practical applications, most air-source heat pumps are used to provide hot water at a temperature of 55–60°C and residential heating [4], [5], [6], [7]. The performance of an air-source heat pump system is significantly affected by the ambient temperature conditions [8], [9]. The system performance degrades significantly under a low ambient temperature condition, particularly when frost occurs [10], [11]. Therefore, most research has focused on improving the operational efficiency of air-source heat pump systems using different technologies [12], [13], [14], [15], [16].

Of these, ejector-vapor compression technology is regarded as effective because the ejector can raise the suction pressure to a higher level than the evaporation pressure. The system power consumption is then reduced, and performance is improved [17], [18], [19], [20]. Operation and control can also be simplified at an optimal level by applying an ejector [21]. Novel technologies and systems that employ an ejector are increasingly gaining attention, where the ejector can be used to enhance the efficiency of air-source heat pump systems. Wang et al. [22] introduced a novel air-source heat pump enhanced by an ejector using three refrigerants. Their results showed that the system performance improved, and the compressor discharge temperature decreased under the same given operating conditions. Li et al. [23] compared the performance of an ejector-expansion refrigeration cycle with different refrigerants. Their results showed that the coefficient of performance (COP) and volumetric heating capacity improvements of the R1234yf ejector-expansion refrigeration cycle (EERC) were greater than that of the R134a cycle. R1234yf EERC outperformed the standard refrigeration, particularly under the conditions of a higher condensation temperature and lower evaporating temperature. Zhu et al. [24] introduced a solar-assisted air-source heat pump system using a dual-nozzle ejector. Analytical results showed that the system COP and volumetric heating capacity improved notably in comparison with the conventional ejector heat pump system. Bai et al. [25] introduced a vapor injection heat pump based on the CO₂ trans-critical cycle. Thermodynamic analytical results showed that the new cycle performance was superior to the conventional cycle under certain conditions. Chen et al. [26] studied a vapor compression heat pump cycle using an ejector associated with a sub-cooler. It was determined that the system heating efficiency and capacity could be enhanced since the refrigerant mass flow rate was improved in the condenser with the application of the ejector. In addition, environmentally friendly refrigerants have also shown a significant effect on injection systems. Yilmaz and Mehmet [27] introduced a new ejector subcooling system using different refrigerants. Results showed that the best performance was obtained for R1234yf with an increment of approximately 20% in the coefficient of performance and 18% in exergy efficiency. Atmaca et al. [28] studied mixing theories on the performance of ejector expansion refrigeration cycles with environmentally friendly refrigerants. Results showed that the system using R1234yf outperformed other systems that used other refrigerants. Takleh and Zare [29] introduced a novel ejector expansion refrigeration cycle that employs a booster compressor using six different refrigerants. Results showed that R1234ze outperformed other refrigerants and the system exhibited 15.5% and 5.7% higher exergy efficiency than the conventional vapor-compression and standard ejector expansion refrigeration cycles, respectively.

As previously mentioned, air-source heat pump system performance can be enhanced significantly by adding an ejector combined with different technologies. However, air-source heat pumps typically produce a single-temperature heat source [30], [8]. Few studies have been conducted on air-source heat pump systems that can produce a dual-temperature heat source, that is, both high- and low-temperature heat sources [31], [32].

A dual temperature system has wide application and energy-saving potential based on our previous studies [33], [34], particularly for cooling systems. With respect to warm air heating systems, different air handling process modes exist, where the low- and high-temperature heat sources can be used to handle the fresh and return air loads, respectively. Otherwise, the fresh and return air are first mixed in a mixed box, and then the air is pre-heated by the low-temperature heat source and handled by the high-temperature heat source to the supply air state. Regarding the wall radiator heating system, the high-temperature heat source is supplied to the radiator to meet desired levels of thermal comfort, whereas the low-temperature heat source can be used to handle the fresh air load to meet desirable indoor air quality.

Therefore, a novel dual-temperature air-source heat pump (NDAHP) system based on an ejector and separator is proposed in this study. The main novelties of this study include two parts. First, the system produces two types of heat sources with different temperatures simultaneously (high and low temperatures of approximately 60°C and 35°C, respectively), which satisfies the dual heat source demand. Second, in the conventional ejector-compression vapor cycle, the ejector is used to improve the compressor suction pressure, then the cycle performance is improved [23]. The ejector is applied with a novel method in the NDAHP, which uses high-pressure refrigerant from the compressor (Point 3) as the motive fluid to entrain low-pressure refrigerant (Point 11) from the evaporator to a low-temperature condenser. Then the mid-grade energy is recycled via the low-temperature condenser, as Fig. 1B shows. In this way, the refrigerant mass flow rate passing through the evaporator increases. In addition, the decrease of condensing pressure in the low-temperature condenser improves the condensing heat per unit of the refrigerant. Thus, the heating performance of the dual-temperature heat pump cycle improves, and the evaporator absorbs more low-grade air source energy.

The contribution of this study is to introduce a new high-efficiency ejector-compression dual-temperature air source heat pump cycle. The effects of principal operating parameters, including condensation and evaporation temperatures, the high-low temperature heating load ratio (LR), and different refrigerants, on system performance are analyzed in this study. In addition, two air-source heat pump cycles are compared, with detailed information concerning the differences between the two systems provided in Section 2. The results provide a foundation for further research and design of dual-temperature air source heat pump cycle with the use of ejector.

Section snippets

System description

A schematic and pressure-enthalpy diagram for dual-temperature air source heat pump is introduced in this section, where two configurations are provided in Fig. 1.

The conventional dual-temperature air source heat pump (CDAHP) system includes two condensers, an evaporator, a throttle valve, and a compressor. The refrigerant at a low-temperature and low-pressure state (Point 1) is sucked in by the compressor and compressed to a high-temperature and high-pressure state (Point 2). The refrigerant

Mathematical model and simulation procedure

For evaluating the application potential of the NDAHP system, a mathematical model of the system based on mass-energy and momentum conservation is built on the following assumptions [17], [35].

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Steady-state and steady-flow conditions are assumed for all cycles;
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The vapor that flows from the evaporator is saturated, and the liquid that flows from the low-temperature condenser is also saturated;
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The throttling processes in the capillary tubes are isenthalpic;
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The flow pressure drops in the cycle are

Validation of ejector model

The ejector model is validated with the experimental data from the literature [39]. Based on Equations (1) to (14) and input the parameters $Δ P$ , P₁, s₁, P₄, s₄, $η_{sn}, η_{mn}, η_{d}$ , the states of each points and entrainment ratio $(μ)$ of the ejector can be obtained via a iterative process. The results show that a good agreement between the present ejector model and experimental data as Table 3 shows. The relative error of the entrainment ratio (µ) ranged between 0.05% and 13.9%, and the average error is

Conclusions

This study proposes a dual temperature air-source heat pump system combined with an ejector and separator. This system is designed to provide two types of heat sources with different temperatures, and the system has wide application potential. The high-temperature heat source at approximately 60°C is used for air handling units, and the low-temperature heat source at approximately 35°C is used for a floor or ceiling radiation heating system. Comparison between the CDAHP and NDAHP systems are

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The work described in this paper is supported by a General Research Grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 11208220), and the Basic Research Fund from Shenzhen Science and Technology Innovation Commission, China (Project No. JCYJ20170818095706389).

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View full text

Performance analysis of a dual temperature heat pump based on ejector-vapor compression cycle

Highlights

Abstract

Introduction

Section snippets

System description

Mathematical model and simulation procedure

Validation of ejector model

Conclusions

Declaration of Competing Interest

Acknowledgment

Two-stage air-source heat pump for residential heating and cooling applications in northern US climates

Int. J. Refrig

Review on improvement for air source heat pump units during frosting and defrosting

Appl. Energy

Advances in vapor compression air source heat pump system in cold regions: A review

Renew. Sustain. Energy Rev.

Application of an air source heat pump (ASHP) for heating in Harbin, the coldest provincial capital of China

Energy Build.

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Energy Build.

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Energy policy

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Energy Build.

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A study on the performance of the airside heat exchanger under frosting in an air source heat pump water heater/chiller unit

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