Performance assessment of CO2 supermarket refrigeration system in different climate zones of China

Energy Conversion and Management 208 (2020) 112572

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Performance assessment of CO2 supermarket refrigeration system in different climate zones of China ⁎

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Zhili Suna, , Jiamei Lia, Youcai Liangb, , Huan Suna, Shengchun Liua, Lijie Yanga, Caiyun Wanga, Baomin Daia a Tianjin Key Laboratory of Refrigeration Technology, Tianjin University of Commerce, China Refrigeration Engineering Research Center of Ministry of Education of P.R.C, Tianjin Refrigeration Engineering Technology Center, China b Systems, Power & Energy Research Division, School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 Seasonal energy efficiency ratio Supermarket refrigeration Partial cascade refrigeration system Parallel compressor

In this paper, a R744 Partial Cascaded Two-stage Compression Refrigeration System (R744-PC-TCS) was proposed and evaluated in terms of coefficient of performance (COP) and seasonal energy efficiency ratio (SEER) when it operated in five typical climate representative cities in China. R134a and CO2 are used as refrigerants in this study. The results show that the SEER of the supermarket refrigeration system in the severe cold zone and the mild zone is higher than that of the other climate zones. The performance of the R134a Two-stage Compression Refrigeration System (R134a-TCS) performs better than the R744 Two-stage Compression Refrigeration System (R744-TCS). However, both COP and SEER of the CO2 system can be improved significantly by adding a parallel compressor, as well as using the single-stage vapor compression partial cascade cycle for subcooling under the practical working conditions (such as R744-PC-TCS). The greatest improvement of R744PC-TCS can be achieved in the areas with poor climatic conditions, in which COP increases by 48.9% at an ambient temperature of 39 °C, and SEER increases by the maximum of 21.5% in hot summer and warm winter zones, compared with that of R744-TCS. Compared with R134a system, the R744-PC-TCS basically achieves the comparable or even higher energy efficiency. It can be concluded that the proposed R744-PC-TCS has a great potential to replace the existing R134a refrigeration system for the supermarket refrigeration application.

1. Introduction In recent years, issues such as environmental safety and energy supply have attracted widespread attention [1–3]. To solve the increasingly serious environmental problems and energy crisis, searching and developing green renewable energy and improving energy efficiency is an urgent need for both governments and scientists [4,5]. It shows that building energy consumption accounts for about 20.1% of Chinese total energy consumption and building energy consumption per unit area is about 3–4 times that of the developed countries [6]. Heating, Ventilation & air conditioning (HVAC) is the largest part of all energy consumption [7,8]. And the implementation for building energy efficiency has been evidenced by various efforts, strategies and actions in China. China Energy Label Management Measures issued in 2016, which is to provide the necessary information for the purchase decisions of consumers, stipulates that energy-consuming products should be labeled indicating the key performance indicators such as energy efficiency. It

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plays a role in strengthening energy management, promoting the development of energy-saving technology and improving energy efficiency simultaneously. However, refrigeration equipment participating in this energysaving measure is limited to small household refrigerating applications, such as refrigerators, air conditioners, etc. For large refrigeration system, such as the supermarket refrigeration system in this paper, more attention should be paid to the energy efficiency evaluation standards and the optimization for energy-saving because of its longer working time and greater energy consumption. Supermarket refrigeration includes the refrigeration applications mainly in display cabinets and freezers of supermarkets, cold storages and cold chains. It is mainly used to process, transport and store fish, fruits, vegetables and beverages, which is characterized by long cooling time, large refrigeration demand and high energy consumption [9]. Since the energy shortage and environmental problems have been increasingly serious, the supermarket refrigeration system is constantly being improved, and the refrigerants are also being replaced [10–12].

Corresponding authors. E-mail addresses: [email protected] (Z. Sun), [email protected] (Y. Liang).

https://doi.org/10.1016/j.enconman.2020.112572 Received 1 December 2019; Received in revised form 31 January 2020; Accepted 1 February 2020 0196-8904/ © 2020 Elsevier Ltd. All rights reserved.

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Nomenclature

Subscript

T h m Q W COP SEER k

a Ambient comp Compressor evap Evaporator i Ideal cycle j J-the temperature or bin of the bins distribution accounted s Isentropic ref Refrigerant gc Gas cooler out Outlet PCS Partial cascaded system MT Medium temperature LT Low temperature 0 Freezer environment 1,2,3,4,5,5′,6,7,8,9,10,11,12,13,14 State points

Temperature, °C Enthalpy, kJ/kg Mass flow rate, kg/s Cooling capacity, kW Power consumption, kW Coefficient of performance Seasonal energy efficiency ratio Conversion factor

Acronyms SCZ CZ HSCWZ HSWWZ MZ MT LT TCS PCS PC-TCS PAC

Severe Cold Zone Cold Zone Hot Summer and Cold Winter Zone Hot Summer and Warm Winter zone Mild Zone Medium temperature Low temperature Two-stage Compression Refrigeration System Partial cascaded system Partial Cascaded Two-stage Compression Refrigeration System Parallel compressor

Greek symbols ε ηs η △

Compression ratio Isentropic efficiency of compressor Thermodynamic perfection Difference

the amendment [15,16]. And the CO2-based refrigeration system will be commercially deployable, which is safer and more environmentally friendly [17]. The applications of CO2 in the supermarket refrigeration system mainly include: (1) As the secondary refrigerant in the indirect refrigeration system; (2) As refrigerant in the low temperature stage of cascade configurations; (3) As the main refrigerant in two-stage compression refrigeration system [18]. As for (1) and (2), the state of CO2 is

At present, Hydrochlorofluorocarbons (HCFCs) and Hydrofluorocarbons (HFCs) are two most common types of refrigerants used in supermarket refrigeration systems [13]. However, according to the Kigali Amendment, the replacement process of high-GWP refrigerants has been accelerating due to its serious impact on the environmental issues [14]. Therefore the use of mixed working fluids and natural working fluids with low GWP is consistent with the objective of Table 1 Improvements of CO2 refrigeration systems with different solutions. Reference

System

Reference system

Investigation typology

Operating conditions

COP increment

[24]

single-stage cycle with dedicated mechanical subcooler

single-stage cycle

Experimental

6.9% to 30.3%

[25]

single-stage cycle with thermoelectric subcooler

single-stage cycle

Theoretical

[26]

single-stage cycle with parallel compression

single-stage cycle

Experimental

[27]

single-stage cycle with expander

single-stage cycle

Theoretical

[28]

single-stage cycle with ejector

single-stage cycle

Experimental

[29]

double-stage cycle with ejector

double-stage cycle

Theoretical

[30]

booster with integrated mechanical subcooling

booster with parallel compression

Theoretical

booster with R290 dedicated mechanical subcooling

booster with parallel compression

two-stage transcritical CO2 refrigeration cycle with two ejectors double-stage cycle with internal exchange double-stage cycle with suction line heat exchanger double-stage cycle with internal exchange and suction line heat exchanger two-stage compression refrigeration system with parallel compression and solar absorption partial cascade refrigeration system

conventional ejector refrigeration cycle double-stage cycle double-stage cycle double-stage cycle

T0 = −10 °C Tgc,out = 24 °C, 30 °C, 40 °C T0 = −10 °C Ta = 30˚C, 35˚C T0 = −15 °C Tgc,out = 22 °C to 40 °C T0 = 5 °C Tgc,out = 40 °C T0 = −10.2 °C Ta = 33.5 °C TMT = −10 °C TLT = −30 °C Ta = 5 °C to 35 °C TMT = −6˚ °C TLT = −32 °C Ta = 0 °C to 40 °C TMT = −6 °C TLT = −32 °C Ta = 5 °C to 40 °C T0 = −10 °C to 20 °C Tgc,out = 34 °C to 50 °C T0 = 2.7 °C Tgc,in = 30 °C

TMT = −10 °C TLT = −28 °C Ta = Ambient temperature

32.60%

[31] [32]

[33]

CO2 two-stage compression refrigeration system CO2 two-stage compression refrigeration system with parallel compression

2

Theoretical Experimental, Theoretical

Theoretical

24% max 30% to 65% 28% 1.83 to 2.64 More than 10%

8.4% max

16.5% max

80% max 22.1% max 10.5% max 25.0% max

12.27%

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their products by evaluating their performance under the rated condition. Unlike other refrigeration equipment, the supermarket refrigeration system is preferred to be evaluated in terms of seasonal applicability rather than COP at certain condition due to its operation through the whole year. In 1979, the US Department of Energy (DOE) proposed the Seasonal Energy Efficiency Ratio (SEER) to measure the energy efficiency level of refrigeration equipment throughout the operating seasons. Subsequently, the European Union, Japan and some other countries have also enacted relevant standards and regulations by means of SEER. SEER has a high reference value as a measure of the seasonal energy efficiency in supermarket refrigeration systems [35]. Based on the calculation method of seasonal energy efficiency ratio proposed by EN14825 [36], this paper will analyze the seasonal energy efficiency ratio of different supermarket-refrigeration systems under the practical working conditions.

always subcritical without being affected by ambient temperature. And for (3), using CO2 as the only refrigerant in the system, which offers advantage of elimination of the environmental problem caused by refrigerant leakage. However the system has to be operated at transcritical state. Carlos Amaris et al. [19] theoretically compared conventional R744 two-stage compression refrigeration system, R744 twostage compression refrigeration system with parallel compressor and a basic R717/R744 cascade system. Two R744 configurations have better performance than R717/R744 cascade systems at ambient temperatures below 2 °C and 14 °C respectively; Polzot Alessio et al. [20] compared the R134a/CO2 cascade refrigeration system with the R404A heat pump for heating purpose in the supermarket and the R744 two-stage compression refrigeration system with heat recovery. The results showed that the latter will promote energy saving by 3.6% to 6.5%; Torrella et al. [21] found that CO2 in a transcritical state will cause a negative impact and the efficiency of refrigeration system will decrease greatly; Saman Khalilzadeh et al. [22] used CO2 as refrigerant of high temperature cycle in cascade refrigeration cycle, in which a subcritical CO2 is adopted for energy utilization. The COP of the proposed system obtained is 4.233, which is 5.74 times greater than the conventional cascade refrigeration cycle. Jakub Bodys et al. [23] evaluated the application of multi-ejector module for a carbon dioxide supermarket refrigeration system and analyzed the performance of the injector experimentally and theoretically. In addition, numerous investigations had been conducted to optimize CO2 refrigeration system and improve the efficiency. The studies and conclusions in recent years are summarized in Table 1. The previous literature reflects that the investigation of CO2 refrigeration systems receives significant attention and the most important enhancement has been presented to reduce the work consumption and to improve the COP. The improvement of the COP in the transcritical CO2 refrigeration cycle is a critical issue because it is less effective than the subcritical cycle using conventional refrigerants [34], such as R134a. As can be seen from the above references, the subcooling is an important technology to improve the performance of CO2 refrigeration system, where the dedicated mechanical subcooling will achieve a more significant improvement. From the investigation on analysis of the application of CO2 in supermarket refrigeration system, it can be easily found that the current studies generally focused on the different types of CO2 systems and performance comparison. Limited study can be found to evaluate and compare the potential of CO2 with conventional refrigerants such as R134a. Furthermore, the related studies merely take the COP into consideration, however, it is a one-side view to evaluate the supermarket refrigeration system. In this paper, a two-stage compression supermarket refrigeration system with two evaporators and a liquid receiver is taken as a basic system. It is compared with another twostage compression supermarket refrigeration system with a dedicated partial cascade and parallel compressor since it is an effective way to enhance the efficiency of the CO2-based system. The CO2 at the gas cooler outlet is subcooled by a dedicated single stage vapor compression refrigeration system through a cooling-evaporator equipped after the gas cooler. The CO2 refrigeration systems with and without the structural optimization is compared with the systems using R134a. For a more comprehensive evaluation of the system performance, SEER, thermodynamic perfection and total annual power consumption of the systems were adopted besides COP. It is supposed that the optimized CO2 system is of great potential to replace the existing R134a refrigeration system for supermarket refrigeration with comparable efficiency.

2.1. Simplified SEER calculation Under the proper functioning conditions of the refrigeration system, the variation of ambient temperature will directly affect the condensing temperature, which will affect the power consumption of the refrigeration compressor and the coefficient of performance (COP). Thus, the COP under a certain condition is insufficient to describe the fullyear performance of the refrigeration system. According to the EN 14825 [36], the SEER under the cooling condition is used to evaluate the seasonal assessment of supermarket refrigeration system. The SEER calculation method in EN 14825 is the ratio of the cooling demand over the whole year to the energy consumption [37]: x

SEER =

∑ j = 1 hjQj x

Qj

∑ j = 1 hj COP

j

(1)

where x is the number of bins between the lowest and highest ambient temperatures during the operating time of the refrigeration system in a reference area; hj is the time (number of hours) when the temperature is Tj during the reference year; Qj is the overall cooling demand of the refrigeration system when ambient temperature equals to Tj and COPj is the coefficient of performance. Equation (1) shows that SEER is a function of three parameters hj, Qj and COPj. The supermarket refrigeration system is equipped with a medium temperature cabinet for cooling purpose (evaporation temperature is −5 °C to + 5 °C) and a low temperature cabinet for freezing purpose (evaporation temperature is −25 °C to −15 °C). The large supermarket is selected as the reference, which has the consistent cooling capacity, and the assumptions about Qj will be made in Section 4.1; hj is related to the climate characteristics of the reference area. Chinese cities can be categorized into five climate zones according to their different climate characteristics, which will be specifically introduced in Section 2.2; COPj is influenced by the structure of the refrigeration system and the operating conditions. The calculation of the COP in supermarket refrigeration system will be discussed in Section 4.2. 2.2. Five major climate zones of China Climate change has a great impact on the building energy consumption [38], which influences the performance of supermarket refrigeration system in this subject. And the rational climate division is essential to identify climate characteristics and impacts in different regions. Policies and standards for building energy efficiency are usually based on climate zones in most countries. Therefore, climate region of building plays a significant role in improving building energy efficiency and sustainable development. The climate zone is usually classified according to the climate data of the historical reference period of the region. The existing climate division of buildings in China is mainly based on Standard of climatic

2. Evaluation standard With the diversification of the supermarket-refrigeration systems, it is essential to develop a simple figure to make comparison between the different solutions. To this extent, manufacturers could clearly qualify 3

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regionalization for architecture (GB 50178-93) [39] and Code for thermal design of civil building (GB 50176-93) [40]. The average daily temperatures of ≤5 °C and ≥25 °C are both used as an auxiliary indicator. The average temperatures of the coldest month (January) and the hottest month (July) are used as the main indicator to divide the country into 5 zones, namely severe cold zone (SCZ), cold zone (CZ), hot summer zone and cold winter zone (HSCWZ), hot summer warm winter zone (HSWWZ) and mild zone (MZ) [41]. Specific classification criteria and representative cities are shown in Table 2. In order to compare and analyze the COP and SEER during the whole operating time of five major climate zones in China, Qiqihar, Tianjin, Wuhan, Guangzhou, Guiyang has been elected as the typical climate representative city of SCZ, CZ, HSCWZ, HSWWZ and MZ respectively. And acquiesce in that the climatic characteristics of typical cities can represent the overall climate characteristics of the climate zone, as shown in Fig. 1.

3. System description In the traditional two-stage compression supermarket refrigeration system, there is also a medium temperature (MT) cooling cabinet and a low temperature (LT) freezing cabinet. The principle is illustrated in Fig. 2a, while Fig. 2b shows the respective p-h diagram (using R134a as the refrigerant). In this system, the refrigerant that exiting from LT evaporator is compressed in LT compressor and then mixes with the refrigerant exiting from MT evaporator. Then it enters in MT compressor and reaches the high pressure of the system. The refrigerant leaving the condenser is throttled into the intermediate pressure through a high-pressure expansion valve. And the outgoing refrigerant enters a liquid receiver, in which the vapor and liquid are separated. The vapor refrigerant named flash gas enters in the flash gas valve to be throttled in the medium pressure and mixed with the refrigerant that enters the MT compressors. Simultaneously, the liquid refrigerant expands in the MT and LT expansion valves before flowing into the MT and LT evaporators. To improve the performance of the new system using CO2 as refrigerant on the basis of the above traditional system, a parallel compressor is added to the CO2 system to directly compress the flash vapor in the liquid receiver, and mixes with the refrigerant coming out of the MT compressor before entering gas cooler. At the same time, a coolingevaporator is added after the gas cooler to exchange heat with the partial cascade cycle, in which the cooling-evaporator acts as an evaporator. And R152a is employed as the refrigerant of partial cascade cycle, which is a low GWP refrigerant used in refrigeration systems with high energy efficiency. The charge amount should not be too large due to its flammability, thus it is suitable for partial cascade cycle [42]. R152a evaporates in the cooling-evaporator to cool down CO2 exiting from gas cooler. In Fig. 3, the state point 5 switch to state point 5′ through subcooling, the temperature drops down and the amount of flash gas is reduced, which could improve the system efficiency. The degree of subcooling in the main cycle follows the principle of

Fig. 1. Typical cities in the five major climate zones.

maximizing COP to select the optimal value. Section 5 discusses the effect of ambient temperature on the optimal degree of subcooling. Fig. 3a illustrates the schematic of CO2 partial cascaded two-stage compression refrigeration system. Since the CO2 critical temperature is 31.2 °C, the cycle is subcritical when operating under the low ambient temperature, while it is transcritical when the gas cooler outlet temperature is greater than the critical temperature. The respective p-h diagram of subcritical and transcritical operations are shown in details in Fig. 3b and Fig. 3c.

4. Modeling details The climate data used in the simulation is referenced to the threenormal simplified model [43]. As shown in Fig. 4, the temperature-hour curves can be obtained base on the selecting the daily temperature of each typical city in 2018. The temperature difference between the annual maximum and minimum temperature in SCZ is great, and the overall temperature is lower than that in the other zones, showing that the winter is long and cold, and the other seasons are much shorter. The annual temperature fluctuation in CZ is less than that in SCZ, and the winter is of short duration. The HSCWZ exhibits characteristics of climate that the summer is very hot while the winter is very cold. The annual temperature difference between the highest temperature and the lowest temperature in HSWWZ is the smallest, since it is warm in winter and hot in summer. The MZ has the most comfortable climate, exhibiting a mild climate of warm winter and cool summer.

Table 2 Standards of climate zoning and representative cities.

Severe cold Cold Hot summer and cold winter Hot summer warm winter Mild

Main indicator

Auxiliary indicator

Typical cities

TAC ≤ −10 °C TAC = −10 ~ 0 °C TAC = 0 ~ 10 °C TAH = 25 ~ 30 °C TAC ≥ 10 °C TAH = 25 ~ 29 °C TAC = 0 ~ 13 °C TAH = 18 ~ 25 °C

D5 ≥ 145 D5 = 90 ~ 145 D5 = 0 ~ 90 D25 = 40 ~ 110 D25 = 100 ~ 200

Harbin, Qiqihar, Shenyang, Lhasa Beijing, Tianjin, Xi'an, Zhengzhou, Shijiazhuang, Yan'an Shanghai, Wuhan, Nanchang, Chengdu, Hangzhou

D5 = 0 ~ 90

Guiyang, Xishuangbanna

Fuzhou, Guangzhou, Haikou

TAC is the average temperature of the coldest month; TAH is the average temperature of the hottest month; D5 is the number of days that the average temperature is below 5 °C; D25 is the number of days that the average temperature is above 25 °C. 4

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Fig. 2. The traditional two-stage supermarket compression refrigeration system (TCR) schematic layout (b) the p-h diagram.

Fig. 3. CO2 partial cascaded two-stage compression refrigeration system (R744-PC-TCS) (a) schematic layout (b) subcritical p-h diagram (c) transcritical p-h diagram. 5

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Table 4 System working condition. Parameters

Values

Minimum condensation temperature (°C) MT evaporation temperature (°C) LT evaporation temperature (°C) MT cooling load (kW) LT cooling load (kW) Superheating (useful) at MT/LT evaporator (°C) CO2 Gas cooler approach temperature difference (°C) R134a Condenser approach temperature difference (°C) Cooling-evaporator approach temperature difference (°C) Subcooling at CO2 gas cooler (°C) Subcooling at R134a condenser (°C) Superheating and subcooling at PCS evaporator (°C)

10 −10 −28 200 35 5 5 10 10 Optimum 0 0

COP [46]. The COP and SEER simulation results are analyzed based on the optimal degree of subcooling. Based on the above assumptions, Table 4 lists the specific operating conditions of the system.

Fig. 4. The temperature-hour curve of the typical city.

4.1. Boundary conditions and assumptions

4.2. Thermodynamic model

(1) In order to ensure the quality of the frozen foods, the air temperature in the cooling and freezing cabinet should be kept −3 °C ~ 15 °C and −18 °C ~ −10 °C normally, so it is assumed that the evaporating temperatures in MT evaporator and the LT evaporator are −10 °C and −28 °C respectively. The degree of effective superheat after evaporators in the main cycle is 5 K [44], while it is saturated in the partial cascaded cycle. (2) The cooling demand in a supermarket is assumed to be constant, and the cooling demand for MT and LT is 200 kW and 35 kW [45]. (3) The pinch point temperature difference in cooling-evaporator is 10 K, and the temperature difference with air in R134a or R152a condenser is 10 K, while it is 5 K in CO2 gas cooler. (4) For CO2 refrigeration system, when the ambient temperature is higher than 26.2 °C, the corresponding condensing temperature is higher than the critical temperature (31.2 °C). In that case, the condensation process is beyond the two-phase zone and becomes a gas cooling process. And the system is operating in a transcritical condition, while it is still subcritical when the ambient temperature is low (below 26.2 °C). (5) Taking into account the practical operation, the electric heating unit is assumed to be turned on for thermal compensation, which aims to avoid the problems such as frost and oil circulation of the condenser or gas cooler cause by the low system temperature, thus to ensure the operation safety of compressors. According to the compressor operating curve, it is assumed in this paper that when the temperature is lower than 10 °C, the electric heating is turned on to maintain the safe operating temperature of the compressor at 20 °C. This parameter can be obtained from the specific compressor parameters in the practical application. Therefore, in the CO2 refrigeration system, there are three different operating states in different ambient temperature ranges. The temperature and pressure of the CO2 system gas cooler outlet in each temperature range are summarized in Table 3. (6) For the main cycle, when the subcooling of the gas cooler is increasing, there is an optimal degree of subcooling to maximize the

In this section, the main equations about the thermodynamics model are presented, and the thermal calculations of the two systems are based on the combination of the following formulas. The subscripts in the formulas refer to the numbers in Fig. 3. The refrigerant flow rate mref in the cycle can be calculated according to the designed cooling capacity and the enthalpy difference Δhevap in evaporator inlet and outlet, as in Equation (2).

Q = mref Δhevap

(2)

The power consumption of each compressor is related to the refrigerant flow rate mref and inlet and outlet enthalpy difference Δhevap through the compressor and the entropy efficiency ηs, ref of the compressor, which can be calculated by Equation (3).

W = mref Δhcomp/ ηs, ref

(3)

The isentropic efficiency ηs, ref of the compressor is affected by the compressor pressure ratio ε . When the condensation temperature increases with the increase of external temperature, the compressor outlet pressure increases correspondingly, resulting in an increase in ε and a decrease in ηs, ref . For CO2 compressors and Freon compressors, it can be defined by using Equation (4) [47] and Equation (5) [48]:

ηs, CO2 = 0.815 + 0.022ε − 0.0014ε 2 + 0.0001ε 3

(4)

ηs, R = 0.083955 − 0.01026ε − 0.00097ε 2

(5)

The enthalpy of the state point 3 in Fig. 3 can be calculated using the energy balance of Equation (6):

m3·h3 = m10 ·h10 + m2 ·h2

(6)

When the refrigerant enters the liquid receiver for gas-liquid separation, the mass flow rates of the saturated vapor liquid are calculated by Equation (7) and Equation (8), respectively:

m7 = m6·(1 − x 6)

(7)

m8 = m 6 · x 6

(8)

Table 3 CO2 system gas cooler outlet temperature and pressure at different ambient temperatures. Ambient temperature range

Temperature of gas cooler outlet

Pressure of gas cooler outlet

Ta ⩽ 10°C

10 °C

Saturated pressure of 10 °C

10° C< Ta ⩽ 26. 2°C

Ta + 5°C

Saturated pressure of Tgc, out

Ta > 26. 2°C

Ta + 5°C

Optimized

6

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difference in the gas cooler, the COP of the R744-TCS is slightly higher than that of the R134a-TCS when the ambient temperature ranges between 3 °C and 10 °C, and they are equal at the ambient temperature of 10 °C. When the ambient temperature is greater than 10 °C, the COP of R744-TCS decreases rapidly, resulting in a bigger difference of the COP between R744-TCS and R134a-TCS. At a maximum ambient temperature of 39 °C, the COP of R134a-TCS is 51.6% higher than that of R744TCS. In the R744-PC-TCS with parallel compression and partial cascaded cycle, the COP has been significantly improved due to the structural optimizations. The higher ambient temperature, the sharper increase in the COP. Comparing with the R744-TCS, the COP of R744-PC-TCS increases by 48.8% at an ambient temperature of 39 °C. When the ambient temperature ranges between 3 °C and 22 °C, R744-PC-TCS has a greater COP than R134a-TCS. When the ambient temperature is 5 °C, the COP of R744-PC-TCS is 13.6% higher than that of R134a-TCS, while they are the same at 22 °C. Then, when the ambient temperature rises, these two systems exhibit comparable coefficient of performance. The above results show that under these conditions, CO2 can replace R134a with comparable system performance coefficient or even higher efficiency. Similarly, the total power consumption of compressor in Fig. 6 shows the same variation trend. Auxiliary equipment optimizes the performance of system, resulting in a drastic reduction in total system power consumption. However, the energy power consumption of each system is similar when the ambient temperature is lower than 15 °C. As the ambient temperature is gradually increased, the total power consumption of R744-TCS increases rapidly, and is 50.9% higher than that of R134a-TCS at the ambient temperature of 39 °C. In the simulation, the subcooling degree of the R744-PC-TCS main cycle is simulated by the optimal value output mode. The corresponding optimal subcooling degree are selected in responding to the changing ambient temperature, as shown in Fig. 7. Then the calculation of combining the partial cascade cycle and the main cycle are performed. The simulation results show that a higher ambient temperature will increase the maximum optimal subcooling degree, thus lead to a decrease of evaporation temperature of PCS. In that case, the power consumption of the compressor increases correspondingly, resulting in the decrease of the total COP, which agrees well with the trend in Fig. 6. The optimization of the subcooling is beneficial to the frequency conversion setting in practical applications, so that the system operates under the optimal conditions and realize the improvement of systems. The percentage difference of the COP and energy consumption between R744-PC-TCS and R134a-TCS under different ambient

The energy balance in the cooling-evaporator is given as: (9)

m5·(h5 − h5′) = m1′·(h1′ − h 4′)

As a direct parameter to measure the performance of a refrigeration system, COP reflects the ratio of the effective cooling capacity generated by the refrigeration system to the power consumption of the various parts of the system. In this paper, the two types of supermarket two-stage compression refrigeration systems are both equipped with two evaporators with different evaporation temperatures, which provide cooling capacity for MT and LT cabinets. Each part of the compressor in the system has its own working objects. Therefore, the total COP of the system should consider both COPMT of MT part and COPLT of LT part respectively, and introduce the conversion factor according to the mass flow relationship into the calculation of total COP. The calculation of the thermodynamic perfection (η) should also be calculated in this way. In addition, the thermodynamic perfection of the PCS should be considered comprehensively. The COP and thermodynamic perfection calculations for the traditional Two-stage Compression Refrigeration System (TCS) and the Partial Cascaded Two-stage Compression Refrigeration System (PC-TCS) are listed in Table 5: kMT is MT conversion coefficient defined by the Equation (10), while kLT is LT conversion coefficient calculated by the Equation (11).

mMT mMT + mLT

(10)

mLT = mMT + mLT

(11)

kMT =

kLT

The overall flow diagram of the CO2 system energy efficiency calculations is presented in Fig. 5. 5. Results and discussion 5.1. Influence of ambient temperature on system thermal performance Fig. 6 shows the calculation results of the COP and the total power consumption of R134a Two-stage Compression Refrigeration System (R134a-TCS), R744 Two-stage Compression Refrigeration System (R744-TCS) and R744 Partial Cascaded Two-stage Compression Refrigeration System (R744-PC-TCS). As the ambient temperature rises, the system condensing temperature and pressure increase, leading to a higher compressor pressure ratio. This will result in a lower compressor efficiency, and increases compressor power consumption and energy loss in various parts of the system, and ultimately cause the COP of three systems to decrease. Due to the smaller pinch point temperature Table 5 Calculations of COP and thermodynamic perfection. TCS

PC-TCS

COPMT

QMT kMT ·WMT

COPMT =

COPLT

QLT kLT ·WMT + WLT

COPLT =

COPPCS

——

COP COPi, MT

COP = kMT ·COPMT + kLT ·COPLT

COPi, LT

T0, LT Ta − T0, LT

COPi, PCS

——

ηMT

COPMT COPi, MT COPLT COPi, LT

ηLT

COPPCS =

QMT kMT ·(WMT + WPAC + WPCS ) QLT kLT ·(WMT + WPAC + WPCS ) + WLT QPCS WPSC

T0, MT Ta − T0, MT

ηPCS

——

η

kMT ·ηMT + kLT ·ηLT

T0, PCS Ta − T0, PCS

COPPCS COPi, PCS

η = (kMT ·ηMT + kLT ·ηLT )·(1 −

7

h5 − h5′ ) h14 − h5′

+ ηPCS ·

h5 − h5′ h14 − h5′

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Fig. 5. Energy efficiency calculations flow diagram of the CO2 system.

R134a-TCS. Fig. 9 shows the trend of system thermodynamic perfection with external temperature. When the outdoor temperature is lower than 10 °C, the system needs to be compensated by electric heating. In this case, the thermodynamic perfection of the three systems is on a high level. In the case of non-electric heating compensation (ambient temperature is higher than 10 °C), the system thermodynamic perfection increases first and then decreases. And the peak of 34.5% appears at 32 °C ambient temperature for R134a-TCS, 27.7% for R744-TCS at 19 °C ambient temperature and 30.6% for R744-PC-TCS at 18 °C ambient temperature. It shows that the retrofitting of system makes the

temperatures is given in Fig. 8. It is evident that R744-PC-TCS has the higher COP and lower energy consumption than R134a-TCS at ambient temperature ranging from 3 °C to 22 °C. And compared with R134aTCS, R744-PC-TCS has a higher COP and lower power consumption at the ambient temperature of 5 °C. The COP increases by 13.6%, and the total system power consumption decreases by 10.5%. When the ambient temperature is higher than 22 °C, the difference between the two systems is maintained at about 5.0%, which increases first and then decreases with the raise of ambient temperature. At an ambient temperature of 39 °C, the difference of 1.9% can be found in terms of both COP and total system power consumption between R744-PC-TCS and 8

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Fig. 6. Ambient temperature versus COP and total energy consumption for three refrigeration systems.

Fig. 9. Ambient temperature versus thermodynamic perfection of three systems.

5.2. Seasonal performance change analysis From the analysis in the previous section, it can be seen that the ambient temperature has a great influence on the power consumption, COP and thermodynamic perfection of the system. The outdoor temperature is closely relative to both the seasons and the climate of the zones where the system works at. In different climate zones the temperature in the same season is also quite different. This section will take the representative cities of Chinese five major climate zones into consideration, and analyze the temperature changes during 24-hour and the system seasonal performance in four seasons. Fig. 10 shows the variation of temperature during 24-hour in the typical climate days of five representative cities—Guangzhou, Wuhan, Qiqihar, Guiyang, and Tianjin. And the (a), (b), (c) and (d) plots that of each season respectively. The temperature of Qiqihar in SCZ is lower than that of the other zones. The annual minimum temperature can reach −30 °C, and the summer temperature can reach above 30 °C, but the summer in SCZ is shorter. Guangzhou, located in HSWWZ, has a higher temperature than the other zones in the four seasons, and the temperature difference between day and night is relatively small. The temperature difference in Guiyang represented MZ between day and night is very small. And the temperature in spring, autumn and winter mostly ranges between 10 °C and 25 °C. Even in summer, there is no extreme weather of high temperature. The climate is the most comfortable and pleasant. In Tianjin, the temperature difference between day and night is relatively large, which has cold winter. And the extremely hot weather often occurs in summer in CZ. The temperature difference between day and night in Wuhan, which represents HSCWZ, is relatively small. But the summer is long, and the temperature is generally higher than that of the milder region, while in the other seasons it is lower than that in the milder regions, showing the characteristics of obvious seasons. Fig. 11 shows the trend of 24-hour COP in the typical climate days of Guangzhou, the representative city in HSWWZ, where (a) is spring, (b) is summer, (c) is autumn, and (d) is winter. It can be seen that the COP of the R744-PC-TCS is higher than that of the R744-TCS due to structural optimization in the four figures. Since the ambient temperature in spring is normally around 20 °C, the COP of R744-PC-TCS is slightly higher than R134a-TCS excepting 12:00–17:00, the outdoor temperature is higher during this period. In summer, the temperature in Guangzhou fluctuated less during the day, and the system is more stable. However, the COP of R744-PC-TCS is slightly lower than that of R134a-TCS due to the ambient temperature above 22 °C throughout the day, but it has been still greatly improved compared with R744-TCS. As for autumn in Guangzhou, during the morning 2:00–6:00, the

Fig. 7. Ambient temperature versus optimal subcooling temperature for PCTCS.

Fig. 8. COP and energy consumption difference for R744-PC-TCS compared to R134a-TCS in different ambient temperatures.

actual CO2 cycle closer to the ideal inverse Carnot cycle. However, the increase of thermodynamic perfection is less than that of COP in Fig. 6 since the PCS is considered. The thermodynamic perfection of R744-PCTCS is still lower than that of R134a-TCS at high ambient temperature. Even so, it still has a greater improvement than that of R744-TCS, increasing by 30% when the ambient temperature is 39 °C.

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Fig. 10. Ambient temperature of typical cities for each hour of the day (a) Spring (b) Summer (c) Autumn (d) Winter.

following by that in autumn and summer. In spring, the temperature in Wuhan is around 10 °C. At this time, the COP of R134a-TCS is much lower than that of R744-PC-TCS, it is even lower than that of R744-TCS during 18:00–12:00. As the temperature in summer becomes higher, the COP of the three systems decreased significantly. Similar to Guangzhou, the COP changed smoothly throughout the day and the COP of R744-PC-TCS is slightly lower than that of R134a-TCS. Autumn is the season with the greatest temperature fluctuation in Wuhan. The temperature difference between day and night is great, causing the obvious COP fluctuation. During 11:00–17:00 in autumn, the outdoor temperature is higher than 22 °C, the COP of R744-PC-TCS is lower than that of R134a-TCS, and the rest of the time it is higher than the latter. In winter, due to the relatively low temperature, the COP of both CO2 systems is higher than that of R134a-TCS, and the advantage of R744PC-TCS is prominent.

temperature is lower than 22 °C. At this period, the coefficient of performance of R744-PC-TCS is slightly higher than that of R134a-TCS, and in the other time it is lower than the latter. In winter, the temperature in Guangzhou is around 15 °C throughout the day, so the COP of R744-PC-TCS is the highest in these three systems. However, the subcooling effect of the PCS is not obvious under the low ambient temperature, the amount of flash vapor leaving the high-pressure throttle valve is reduced, resulting in less improvement in COP for R744-PC-TCSthan that in spring, summer and autumn. Fig. 12 shows the trend of 24-hour COP in the typical climate of Wuhan, represented HSCWZ, where (a) is spring, (b) is summer, (c) is autumn, and (d) is winter. Compared with Guangzhou, Wuhan is colder in winter and hotter in summer. Because of the large temperature fluctuations in the four seasons, the system working status is relatively unstable. The COP of all cases is relatively higher in spring and winter,

Fig. 11. COP changes in typical climate days in Guangzhou for each hours (a) Spring (b) Summer (c) Autumn (d) Winter. 10

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Fig. 12. COP changes in typical climate days in Wuhan for each hours (a) Spring (b) Summer (c) Autumn (d) Winter.

the figure (d), the outdoor temperature in winter is lower than −0 °C for 24 h, and the COP of all the three systems keeps constant. At this time, the COP of R744-PC-TCS is higher than that of R744-TCS but lower than that of R134a-TCS. In Figure (b), there is no fixed condensation temperature in summer. The COP of R744-PC-TCS is slightly lower than that of R134a-TCS, and is much higher than that of R744TCS. Fig. 14 shows the trend of 24-hour COP in the typical climate of Guiyang, the representative city of MZ, where (a) is spring, (b) is summer, (c) is autumn, and (d) is winter. The climatic characteristics of MZ manifest as the small temperature difference throughout the day and the whole year, as well as no extreme weather, so the systems works more stably in all seasons. It can be seen from the figures that the daily COP changed little in spring and winter. And the performance of the three systems is comparable, while the COP of R744-PC-TCS is

Fig. 13 shows the trend of 24-hour COP in the typical climate days of Qiqihar, a representative city of SCZ, where (a) is spring, (b) is summer, (c) is autumn, and (d) is winter. Qiqihar has a special climate with extremely cold winter and cooler summer, and the temperature in spring and autumn is also relative low, resulting in large fluctuations of COP in all seasons. According to the thermodynamic analysis, the condensation temperatures for R134a-TCS is fixed to be 10 °C when the outdoor temperature is lower than 0 °C, and that for CO2 system is also fixed to be 10 °C when the outdoor temperature is lower than 5 °C. In this case, COP keeps constant. For example, in Figure (a), the outdoor temperature is between 0 °C and 5 °C from 7:00 to 16:00, and the COP of R134a-TCS varies with temperature, while the COP of the two CO2 systems keeps constant. There is a similar trend in the autumn system of COP changes in Figure (c), there is a shorter time for fixing the condensation temperature because of its higher temperature than spring. In

Fig. 13. COP changes in typical climate days in Qiqihar for each hours (a) Spring (b) Summer (c) Autumn (d) Winter. 11

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Fig. 14. COP changes in typical climate days in Guiyang for each hours (a) Spring (b) Summer (c) Autumn (d) Winter.

Fig. 15. COP changes in typical climate days in Tianjin for each hours (a) Spring (b) Summer (c) Autumn (d) Winter.

TCS from 1:00 am to 5:00 am. In Figure (c), the COP of R744-PC-TCS is the highest when the autumn temperature is lower than 22 °C in the whole day, and R134a-TCS is comparable as R744-TCS. It can be concluded from the above analysis that the systems in the MZ and the HSWWZ operate stably in all seasons and have stable performance. Climatic stability is conducive to the safe operation of the refrigeration systems and minimizes the maintenance frequency. R744PC-TCS generally exhibits worse performance under the high ambient temperature in summer, while the advantages of R744-PC-TCS will be highlighted under the lower ambient temperature. In general, R744-PCTCS improves the COP of the CO2 system greatly by adding PAC and using PCS for subcooling. Therefore, it has a comparable or even slightly better performance compared with R134a-TCS, which could be proposed to replace the HFCs refrigerant completely in the supermarket refrigeration system.

relatively high. The temperature in Guiyang is higher than 22 °C in summer. Therefore, the COP of R744-PC-TCS decreases rapidly, which is a bit lower than that of R134a-TCS without large difference. In autumn, a greater fluctuation can be found for the ambient temperature. The COP of R744-PC-TCS is lower than that of R134a-TCS during 11:00–19:00, and the other time is higher than the latter. Fig. 15 shows the trend of 24-hour COP in the typical climate of Tianjin represented CZ, where (a) is spring, (b) is summer, (c) is autumn, and (d) is winter. The climate in CZ is changeable, and the temperature difference between day and night is large. Similar to the SCZ, when the outdoor temperatures is lower than 0 °C for R134a system and lower than 5 °C for CO2 system, the condensation temperatures are fixed at 10 °C. Thus, in the figures (a) and (d), the COP does not vary with the outdoor temperature. The R134a-TCS in Figure (b) shows a good performance in summer, only worse than R744-PC12

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Guangzhou, 14.1% for Guiyang and 18.1% for Wuhan. The SEER of R744-PC-TCS is basically comparable as the R134a-TCS. At the same time, it can be seen that the structural optimization is more effective for the system working in the regions with poor climatic such as HSCWZ and HSWWZ. This is mainly because the period of high ambient temperature in these areas is relatively long and there is more flash steam generated after throttling. Adding PAC for the direct compression of the flash steam can reduce the adverse effects that caused by flash steaming. Therefore, the SEER simulation results also show that the structural optimization for system increases the possibility of replacing the R134a refrigerant with CO2 in supermarket refrigeration system.

6. Conclusions In this paper, the COP and SEER are taken as the main evaluation standards while the thermodynamic perfection and the total annual power consumption of the system are used as auxiliary parameters. A comprehensive analysis was conducted to compare the performance of R134a-TCS, R744-TCS and R744-PC-TCS under the practical working conditions. Five typical representative cities, Qiqihar, Tianjin, Wuhan, Guangzhou and Guiyang are selected to represent five major climate zones (severe cold zones, cold zones, hot summer and cold winter zones, hot summer and warm winter zones and mild zones) in China. The seasonal performance of three systems in different climates is analyzed. The important conclusions are summarized below.

Fig. 16. Annual total power consumption of three systems in different zones.

(1) For the traditional two-stage compression refrigeration system, replacing R134a with CO2 will deteriorate the system performance, especially at a higher ambient temperature. When the ambient temperature is 39 °C, the COP and thermodynamic perfection of R134a-TCS are 51.6% and 52.4% higher than that of R744-TCS, respectively. (2) Adding parallel compressor and partial cascade cycle will significantly improve the performance of CO2 refrigeration systems. The COP and thermodynamic perfection of R744-PC-TCS increased by 48.8% and 30.0% respectively compared with that of R744-TCS at the ambient temperature of 39 °C. Compared with R134a-TCS, R744-PC-TCS has a comparable COP, but its thermal perfection is still a bit lower than that of R134a-TCS. R744-PC-TCS can basically replace R134a in supermarket refrigeration systems with the comparable performance. (3) Compared with R744-TCS, the SEER of the R744-PC-TCS in five typical climate representative cities increased by 11.5%, 17.7%, 21.5%, 14.1% and 18.1%, and the total annual energy consumption decreased by 8.3%, 14.3%, 17.8%, 11.4% and 16.9%, respectively, in Qiqihar, Tianjin, Guangzhou, Guiyang and Wuhan. (4) For the five major climate zones, all the three systems show superior performance and less energy consumption in SCZ, MZ, and CZ. The structural optimization of CO2 refrigeration system is more effective in HSWWZ and HSCWZ, and the energy saving effect is significant.

Fig. 17. SEER of three systems in different zones.

5.3. Annual total performance evaluation The comparison of annual total power consumption of three systems in different regions is shown in Fig. 16, which indicates that under the practical conditions, the total annual consumption in Guangzhou is the largest, followed by Wuhan, Tianjin and Guiyang. And the system in Qiqihar has the minimum total system consumption due to its cold climate. Compared with R744-TCS, R744-PC-TCS can greatly reduce the annual total power consumption, which can be decreased by 17.8% in Guangzhou, 16.9% in Wuhan, 14.3% in Tianjin, 11.4% in Guiyang and 8.3% in Qiqihar. More specifically, the structural optimization of system shows the best improvement in the hot regions, and the annual total power consumption difference between R744-PC-TCS and R134aTCS maintains within 5%. Fig. 17 illustrates the SEER calculation results for five typical climate representative cities. Qiqihar that represents the SCZ has the harsh winter and short summer. The refrigeration system works in adverse climate condition for a relatively short time, so the SEER is significantly higher than the other zones. Due to the better climatic weather conditions, the refrigeration system has a high SEER for long-term while working in the MZ represented by Guiyang and the CZ represented by Tianjin. In contrast, the refrigeration systems in HSCWZ and HSWWZ have lower SEER and less energy efficiency due to the long and hot summers. Fig. 17 shows comparison SEER between R134a-TCS, R744-TCS and R744-PC-TCS. By adding PAC and PCS, the SEER of CO2 system can be improved significantly. The improvement effect of the system in the five typical cities is 11.5% for Qiqihar, 17.7% for Tianjin, 21.5% for

Overall, it can be concluded that the proposed R744-PC-TCS has a great potential to replace the existing R134a refrigeration system for the supermarket refrigeration application.

CRediT authorship contribution statement Zhili Sun: Conceptualization, Methodology, Reviewing. Jiamei Li: Writing - Original draft preparation, Software. Youcai Liang: Writing Reviewing and Editing, Supervision. Huan Sun: Investigation, Supervision. Shengchun Liu: Data collation, Reviewing. Lijie Yang: Data collation, Software. Caiyun Wang: Software, Chart drawing. Baomin Dai: Supervision, Reviewing. 13

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Declaration of Competing Interest

Refrig 2011;34:40–9. [22] Khalilzadeh S, Hossein Nezhad A, Sarhaddi F. Reducing the power consumption of cascade refrigeration cycle by a new integrated system using solar energy. Energy Convers Manage 2019;200:112083. [23] Bodys J, Palacz M, Haida M, Smolka J, Nowak AJ, Banasiak K, et al. Full-scale multi-ejector module for a carbon dioxide supermarket refrigeration system: numerical study of performance evaluation. Energy Convers Manage 2017;138:312–26. [24] Llopis R, Nebot-Andrés L, Cabello R, Sánchez D, Catalán-Gil J. Experimental evaluation of a CO2 transcritical refrigeration plant with dedicated mechanical subcooling. Int J Refrig 2016;69:361–8. [25] Astrain D, Merino A, Catalán L, Aranguren P, Araiz M, Sánchez D, et al. Improvements in the cooling capacity and the COP of a transcritical CO2 refrigeration plant operating with a thermoelectric subcooling system. Appl Therm Eng 2019;155:110–22. [26] Chesi A, Esposito F, Ferrara G, Ferrari L. Experimental analysis of R744 parallel compression cycle. Appl Energ 2014;135:274–85. [27] Shariatzadeh OJ, Abolhassani SS, Rahmani M, Nejad MZ. Comparison of transcritical CO2 refrigeration cycle with expander and throttling valve includingexcluding internal heat exchanger Exergy and energy points of view. Appl Therm Eng 2016:779–87. [28] Santini F, Bianchi G, Battista DD, Villante C, Orlandi M. Experimental investigations on a transcritical CO2 refrigeration plant and theoretical comparison with an ejector-based one. Energy Procedia 2019;161:309–16. [29] Huang Z, Zhao H, Yu Z, Han J. Simulation and optimization of a R744 two-temperature supermarket refrigeration system with an ejector. Int J Refrig 2018:73–82. [30] Catalán-Gil J, Llopis R, Sánchez D, Nebot-Andrés L, Cabello R. Energy analysis of dedicated and integrated mechanical subcooled CO2 boosters for supermarket applications. Int J Refrig 2019;101:11–23. [31] Eskandari Manjili F, Cheraghi M. Performance of a new two-stage transcritical CO2 refrigeration cycle with two ejectors. Appl Therm Eng 2019;156:402–9. [32] Cavallini A, Cecchinato L, Corradi M, Fornasieri E, Zilio C. Two-stage transcritical carbon dioxide cycle optimisation: a theoretical and experimental analysis. Int J Refrig 2005;28:1274–83. [33] Sun Z, Wang C, Liang Y, Sun H, Liu S, Dai B. Theoretical study on a novel CO2 Twostage compression refrigeration system with parallel compression and solar absorption partial cascade refrigeration system. Energy Convers Manage 2019::112278. [34] Ommen T, Elmegaard B. Numerical model for thermoeconomic diagnosis in commercial transcritical/subcritical booster refrigeration systems. Energy Convers Manage 2012;60:161–9. [35] Wu J, Xu Z, Jiang F. Analysis and development trends of Chinese energy efficiency standards for room air conditioners. Energy Policy 2019;125:368–83. [36] Air conditioners, liquid chilling packages and heat pumps, with electrically driven compressors, for space heating and cooling – Testing and rating at part load conditions and calculation of seasonal performance. EN148252016. p. [37] Minetto S, Rossetti A, Marinetti S. Seasonal energy efficiency ratio for remote condensing units in commercial refrigeration systems. Int J Refrig 2018;85:85–96. [38] Kharseh M, Altorkmany L, Al-Khawaj M, Hassani F. Warming impact on energy use of HVAC system in buildings of different thermal qualities and in different climates. Energy Convers Manage 2014;81:106–11. [39] China Academy of Building Research. Climatic regionalization for architecture. GB 50178reg. Beijing: M. E. Sharpe; 1993.p. [40] China Academy of Building Research. Code for thermal design of civil building. GB 50176-2016. Beijing: M. E. Sharpe; 2016. p. [41] Bai L, Wang S. Definition of new thermal climate zones for building energy efficiency response to the climate change during the past decades in China. Energy 2019;170:709–19. [42] Sánchez D, Cabello R, Llopis R, Catalán-Gil J, Nebot-Andrés L. Energy assessment of an R134a refrigeration plant upgraded to an indirect system using R152a and R1234ze(E) as refrigerants. Appl Therm Eng 2018;139:121–34. [43] Lixing D, Xiuying R, Xichao D, Menglong P. The improvement of annual dry-temperature distribution model. Building Energy Environ 2008;27:12–5. (in Chinese). [44] Mitsopoulos G, Syngounas E, Tsimpoukis D, Bellos E, Tzivanidis C, Anagnostatos S. Annual performance of a supermarket refrigeration system using different configurations with CO2 refrigerant. Energy Convers Manage: X 2019;1:100006. [45] Karampour M, Sawalha S. State-of-the-art integrated CO2 refrigeration system for supermarkets A comparative analysis. Int J Refrig 2018;86:239–57. [46] Dai B, Liu S, Sun Z, Ma Y. Thermodynamic performance analysis of CO2 transcritical refrigeration cycle assisted with mechanical subcooling. Energy Procedia 2017;105:2033–8. [47] Robinson DM, Groll EA. Efficiencies of transcritical CO2 cycles with and without an expansion turbine. Int J Refrig 1998;21:577–89. [48] Dai B, Qi H, Liu S, Ma M, Zhong Z, Li H, et al. Evaluation of transcritical CO2 heat pump system integrated with mechanical subcooling by utilizing energy, exergy and economic methodologies for residential heating. Energy Convers Manage 2019:202–20.

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. Acknowledgments The authors are grateful for the support of the No. 51906178 project supplied by the National Natural Science Foundation of China, as well as 19JCTPJC51300, TD12-5048, 18JCYBJC22200 and 17JCYBJC29600 projects supplied by Tianjin Municipal Science and Technology Commission. References [1] Qyyum MA, Chaniago YD, Ali W, Qadeer K, Lee M. Coal to clean energy: energyefficient single-loop mixed-refrigerant-based schemes for the liquefaction of synthetic natural gas. J Clean Prod 2019;211:574–89. [2] Chen Y, He L, Guan Y, Lu H, Li J. Life cycle assessment of greenhouse gas emissions and water-energy optimization for shale gas supply chain planning based on multilevel approach: case study in Barnett, Marcellus, Fayetteville, and Haynesville shales. Energy Convers Manage 2017;134:382–98. [3] Lian J, Zhang Y, Ma C, Yang Y, Chaima E. A review on recent sizing methodologies of hybrid renewable energy systems. Energy Convers Manage 2019;199:112027. [4] Wang H, Lei Z, Zhang X, Zhou B, Peng J. A review of deep learning for renewable energy forecasting. Energ Convers Manage 2019;198:111799. [5] Nawaz K, Ally MR. Options for low–global-warming-potential and natural refrigerants Part 2: Performance of refrigerants and systemic irreversibilities. Int J Refrig 2019. [6] Yang X, Zhang S, Xu W. Impact of zero energy buildings on medium-to-long term building energy consumption in China. Energy Policy 2019;129:574–86. [7] Terés-Zubiaga J, Pérez-Iribarren E, González-Pino I, Sala JM. Effects of individual metering and charging of heating and domestic hot water on energy consumption of buildings in temperate climates. Energy Convers Manage 2018;171:491–506. [8] Chen S, Li N, Guan J. Research on statistical methodology to investigate energy consumption in public buildings sector in China. Energy Convers Manage 2008;49:2152–9. [9] Ge YT, Tassou SA. Control optimizations for heat recovery from CO2 refrigeration systems in supermarket. Energy Convers Manage 2014;78:245–52. [10] Kumar KS, Rajagopal K. Computational and experimental investigation of low ODP and low GWP HCFC-123 and HC-290 refrigerant mixture alternate to CFC-12. Energy Convers Manage 2007;48:3053–62. [11] Joudi KA, Al-Amir QR. Experimental assessment of residential split type air-conditioning systems using alternative refrigerants to R-22 at high ambient temperatures. Energy Convers Manage 2014;86:496–506. [12] Mwesigye A, Dworkin SB. Performance analysis and optimization of an ejector refrigeration system using alternative working fluids under critical and subcritical operation modes. Energy Convers Manage 2018;176:209–26. [13] Sawalha S, Piscopiello S, Karampour M, Manickam L, Rogstam J. Field measurements of supermarket refrigeration systems. Part II Analysis of HFC refrigeration systems and comparison to CO2 trans-critical-main. Appl Therm Eng 2017:170–82. [14] Mota-Babiloni A, Navarro-Esbrí J, Peris B, Molés F, Verdú G. Experimental evaluation of R448A as R404A lower-GWP alternative in refrigeration systems. Energy Convers Manage 2015;105:756–62. [15] Sun Z, Liang Y, Liu S, Ji W, Zang R, Liang R, et al. Comparative analysis of thermodynamic performance of a cascade refrigeration system for refrigerant couples R41/R404A and R23/R404A. Appl Energ 2016;184:19–25. [16] Sun Z, Wang Q, Xie Z, Liu S, Su D, Cui Q. Energy and exergy analysis of low GWP refrigerants in cascade refrigeration system. Energy 2019;170:1170–80. [17] Zhang Q, Luo Z, Zhao Y, Cao R. Performance assessment and multi-objective optimization of a novel transcritical CO2 trigeneration system for a low-grade heat resource. Energy Convers Manage 2019::112281. [18] Tsamos KM, Ge YT, Santosa I, Tassou SA, Bianchi G, Mylona Z. Energy analysis of alternative CO2 refrigeration system configurations for retail food applications in moderate and warm climates. Energy Convers Manage 2017;150:822–9. [19] Amaris C, Tsamos KM, Tassou SA. Analysis of an R744 typical booster configuration, an R744 parallelcompressor booster configuration and an R717R744 cascade refrigeration system for retail food applications. Part 1 Thermodynamic analysis. Energy Procedia 2019:259–67. [20] Polzot A, Agaro PD, Cortella G. Energy analysis of a transcritical CO2 supermarket refrigeration system with heat recovery. Energy Procedia 2017;111:648–57. [21] Torrella E, Sánchez D, Llopis R, Cabello R. Energetic evaluation of an internal heat exchanger in a CO2 transcritical refrigeration plant using experimental data. Int J

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