A Study on the COP of CO2 Air Conditioning System with Minichannel Evaporator Using

Mechanics, Materials Science & Engineering, May 2017

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A Study on the COP of CO2 Air Conditioning System with Minichannel Evaporator Using Subcooling Process26 Thanhtrung Dang1,a, Chihiep Le2,b, Tronghieu Nguyen1,c, Minhhung Doan1,d 1

Department of Thermal Engineering, HCMC University of Technology and Education, Vietnam

2

Department of Heat and Refrigeration Engineering, HCMC University of Technology, Vietnam

a

trungdang@hcmute.edu.vn

b

lechihiep@gmail.com

c

hieunt@hcmute.edu.vn

d

hungdm@hcmute.edu.vn DOI 10.2412/mmse.46.29.103 provided by Seo4U.link

Keywords: air conditioning, CO2 refrigerant, subcooling, minichannel, evaporator, heat transfer.

ABSTRACT. Experimental studies on a CO 2 air conditioning system with minichannel evaporator using subcooling process are presented in this paper. Without subcooling process, the COP obtained for this case is nearly 1.59; it is lower than that of conventional air conditioning systems. But, with subcooling process, the COP strongly increases as the gas cooler outlet temperature is lower than 30 C which confirms the need of subcooling CO 2 used in air conditioners. In subcooling process, the COP of 4.97 was achieved for the gas cooler pressure of 77bar and the evaporating temperature of 15 C, it is higher than those obtained by other published results. It is suggested that the CO 2 air conditioning system should be operated corresponding to the case where the gas cooler pressure ranges from 74-77bar and the evaporating temperature ranges from 10-15 C in transcritical mode for high effectiveness and safety.

Introduction. Scientists working in air conditioning engineering have been interested in problems such as environmentally friendly refrigerants and high effectiveness heat exchangers. In these fields, CO2 is considered as a good candidate in order to replace HCFCs and compact heat exchangers would be used widely in the future. Regarding to CO2 and compact heat exchangers, an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO2 in a horizontal tube was investigated by Zhao and Bansal [1]. Due to the large surface tension, the boiling heat transfer coefficient of CO2 was found to be much lower at low temperatures but it increased with vapor quality (until dryout). However, this study was only reviewed for horizontal tube. Baheta el at.

[2] simulated performance of transcritical carbon dioxide refrigeration cycle by using EXCEL program. In this study, the highest Coefficient of Performace (COP) was 3.24 at 10MPa gas cooler pressure. The results indicated that COP increases as rising the evaporative temperature. Using numerical simulation, Cheng and Thome [3] studied on cooling of microprocessors using flow boiling of CO2 in a micro-evaporator. Based on the analysis and comparison, CO2 appeared to be a promising coolant for microprocessors at low operating temperatures but also presented a great technological challenge like other new cooling technologies. However, the investigations in [2, 3] did not experimentally perform.

A comprehensive review of flow boiling heat transfer and two-phase flow of CO2 covers both macrochannel tests and micro-channel investigations was presented Thome and Ribatski [4]. The results showed that CO2 gives higher heat transfer coefficients than those of conventional refrigerants.

Ducoulombier et al. [5] studied carbon dioxide two-phase flow pressure drops in a single horizontal -NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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stainless-steel micro-tube having the inner diameter of 0.529 mm. The apparent viscosity of the twophase mixture was larger than the liquid viscosity at low vapor qualities, namely at the lowest temperatures. Cheng et al. [6, 7] updated flow pattern map for CO2 evaporation inside tubes. The updated map was applicable for a wider range of conditions: tube diameters from 0.6 to 10 mm, mass velocities from 50 to 1500 kg/m2s, heat fluxes from 1.8 to 46 kW/m2, and saturation temperatures from 28 to +25 oC. The new CO2 two-phase flow pressure drop model predicted the CO2 pressure drop was better than the former methods. Boiling heat transfer of carbon dioxide inside a small-sized microfin tube was investigated by Dang et al. [8]. The experimental results indicated that heat flux has a significant effect on the heat transfer coefficient and the coefficient does not always increase with mass flux. In addition, the experimental results also shown that using microfin tubes may considerably increase the overall heat transfer performance.

Numerical analysis use the finite volume method on a microchannel evaporator for CO2 airconditioning systems was fulfilled by Yun et al. [9]. The performance of the microchannel evaporator for CO2 systems can be improved by varying the refrigerant flow rate to each slab and changing fin space to increase the two-phase region in the microchannel. Design optimisation of CO2 gas cooler/condenser in a refrigeration system was done Ge et al. [10]. In this study, the design optimisation of the heat exchanger dealed with different structure designs, controls and system integration at different operating conditions in order to significantly enhance the performance in a CO2 refrigeration system. As a result, the effect of heat exchanger sizes on system performance can 2

transcritical mechanical compression ejector cooling cycle. The hybrid cooling cycle is a combination of a CO2 transcritical mechanical compression refrigeration machine (MCRM) powered by electricity, and an ejector cooling machine (ECM) driven by heat rejected from the CO2 cooling cycle. Refrigerants R245ca, R601b (neopentane) and R717 (ammonia) are investigated as the working fluids of ECM in the present study. In this study, using the ejector cooling cycle for subcooling the CO2 gas after gas cooler allows increasing the efficiency of the CO2 transcritical cooling cycle up to 25-30% depending on the refrigerant type of the ejector cooling cycle. However, the investigations in [11] were done by theoretical methods only.

Kuang et al. [12] studied a semi-empirical correlation of gas cooling heat transfer of supercritical carbon dioxide in microchannels. Based on the experimental data, a new semi-empirical correlation was developed to predict the gas cooling heat transfer coefficient of supercritical CO2 in microchannels, within an error of 15% for most (91%) of the presented experimental data that were obtained in an 11-port microchannel tube with an internal diameter of 0.79 mm and with a pressure range of 8 to 10 MPa and mass flux range of 300 to 1200 kg/m2s. Haida et al. [13] studied numerical investigation of an R744 liquid ejector. However, authors only mentioned mass entrainment ratio, mixer length, diffuser angle, suction mass flow rate, and the motive mass; they did not mention COP of air conditioning system.

A review by Dario et al. [14] summarized the two-phase flow distribution in parallel channels with macro and micro hydraulic diameters. The investigation allowed us to identify the main geometrical and operating conditions which influence the two-phase flow distribution in parallel channels. Dang et al. [15], [16], [17] investigated the heat transfer and pressure drop phenomena of the microchannel and minichannel heat exchangers, both numerically and experimentally. At the same average velocity of water in the channels used in this study, the effectiveness obtained from the microchannel heat exchanger was 1.2 to 1.53 times of that obtained from the minichannel heat exchanger. Moreover, influences of gravity to heat transfer and pressure drop behaviors of the microchannel heat exchanger were presented by variation of the physical inclinations of the microchannel heat exchanger system used for experiments. However, in [15], [16], [17], the pure water was the working fluid; they did not study CO2 in these studies.

Yu et al. [18] studied the two-phase flows in microchannels. The results showed that two phase flows have many advantages in heat and mass transfer compared to single-phase flows in microchannels.

The heat transfer characteristics of R410A in microchannels were measured by Yun et al. [19]. In this MMSE Journal. Open Access www.mmse.xyz

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study, the boiling heat transfer coefficients of R410A in microchannels were much higher than single tubes at similar test conditions. Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger was studied by Kim et al. [20]. However, this study dealt with the refrigerant R134a, not CO2, as the working fluid. Hernando et al. [21] studied the pressure drop, heat transfer rate, and overall heat transfer coefficient in a single-phase micro-heat exchanger, both experimentally and analytically. A review of boiling heat transfer enhancement on micro/nanostructured surfaces was made by Kim et al. [22]. In this paper, the state-of-art of several researches on boiling enhancement surfaces was reviewed. However, experimental investigations on CO2 air conditioning system with compact heat exchangers as well as subcooled process did not show clearly.

From literature reviews above, there are no more experimental studies on CO2 air conditioning system with compact heat exchangers. In addition, they did not indicate thermodynamic parameters of CO2 air conditioning cycle as well as subcooled process clearly. Therefore, it is essential to investigate CO2 air conditioning system experimentally. In this study, an aluminum minichannel evaporator was used to get thermodynamic parameters of the air conditioning cycle and compared with conventional evaporator. In addition, the cycle will be discussed with transcritical mode.

Methodology Experimental setup. The experimental test loop for CO2 air conditioning system is shown in Fig.1.

This cycle has four main components: a CO2 compressor, a cooler, a thermal expansion valve, and an evaporator. CO2 playing the role of refrigerant enters the compressor in superheated vapor state and then it is compressed to a state at pressure and temperature higher than those at critical point. The superheated vapor is then routed through a cooler.

a.

Without subcooling coil

b.

With subcooling coil

Fig. 1. Schematic of the test loop for CO2 air conditioning system.

With subcooling process, the cooler outlet refrigerant enters the cold room to continuously reject heat (as shown in Fig. 1b). Then the cooled refrigerant continues to move to an expansion valve. The pressure is dropped dramatically when the gas runs through an expansion valve. At the outlet of expansion valve, as usual the refrigerant becomes wet saturated vapor at lower temperature. The wet saturated vapor is then sent to the tubes of the evaporator where it refrigerates the enclosed space. A fan blows the warm air in the enclosed space across the tubes, so the warm air (the room-temperature air) is cooled. Meanwhile, the liquid part of the wet saturated vapor is also heated to evaporate by the MMSE Journal. Open Access www.mmse.xyz

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warm air. To complete the refrigeration cycle, the refrigerant vapor from the evaporator with saturated vapor state is superheated and is routed back into the compressor. The temperature sensors and pressure gauges were installed to get thermodynamic parameters at main points of this system. A photo for this test loop is shown in Fig. 2.

Aluminum minichannels were used to manufacture for the evaporator, as shown in Fig. 3. The refrigerant runs in the channels with four passes (29 channels in the total). Each minichannel is rectangular in shape, with the width of 1.6 mm and the depth of 1.2 mm. The total heat transfer area and the outside volume of this evaporator are 2.5 m2 and 2.97 dm3, respectively. The desgin cooling capacity for this minichannel evaporator is 2700 W. For the cooler, the copper tubes were used in the study. The cooler and evaporator were tested with the hydraulic testing method. The former and the latter did not tear or deform at the pressure of 150 bars and 90 bar, respectively. Accuracies and ranges of testing apparatus are listed in Tab. 1 and equipments used for the experiments are listed as follows: Table 1. Accuracies and ranges of testing apparatuses.

Testing apparatus

Accuracy

Range

Thermocouples

0.1 C

0 100 C

Thermal camera

2%

-

Infrared thermometer

1 C of reading

Pressure gauge

1 FS

Clamp meter

1.5 % rdg

0

200 A

Anemometer

3%

0

45 m/s

- Thermocouples, T-types - Thermostat, EW 181 H, made by Ewelly - Infrared thermometer, AT 430L2, made by APECH - Infrared thermometer, Raynger@ST, made by Raytek - Thermal camera, Fluke Ti9, made by Fluke, USA - Pressure gauge, made by Pro

Instrument

- Anemometer, AVM-03, made by Prova - Clamp meter, Kyoritsu 2017, made by Kyoritsu

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- 32

400 C

0 100 kgf/cm2

Mechanics, Materials Science & Engineering, May 2017

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Evaporator

Cold room

Cooler

Compressor

Fig. 2. A photo of the CO2 air conditioning system.

Fig. 3. Dimensions of the minichannel evaporator.

Governing equations. To analyze the thermodynamic properties of the tested CO2 air conditioning system, the governing equations were given below: The heat transfer rate for condensation was calculated as (1)

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The power input was determined using (2) The heat transfer rate for subcooling was calculated as (3)

The isenthalpic process was presented by (4)

The heat transfer rate for evaporation was calculated as (5)

Finally the COP of the cycle was quantified by (6)

where m c is the mass flow rate of carbon dioxide.

Results and discussion The air conditioning without subcooling.

The CO2 air conditioning system without subcooling was tested more than 200 times in order to collect data for transcritical mode. The period for each time getting data is 30 minutes. The results obtained from this experiment are very stable. Table 2 shows the thermodynamic parameters of the he cooler and evaporator, the pressures at the outlets of these heat exchangers are lower than those obtained from the inlet ones. Therefore, isothermal and isobaric processes in this cycle are quasi with theory. The data were drawn on the p-h diagram of CO2.

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Table 2. Thermodynamic parameters of the CO2 cycle without subcooling. p1

t1

p2

t2

p3

t3

p4

t4

(bar)

( C)

(bar)

( C)

(bar)

( C)

(bar)

( C)

36.0

20.2

87

85.8

86.5

39.2

37

5.1

36.0

20.1

87

86.0

86.5

38.8

37

5.2

36.5

20.2

87

85.7

86.5

39.3

37

5.0

36.0

20.0

87

86.1

86.0

39.4

37

4.9

36.0

20.2

87

85.9

86.5

39.1

37

5.0

The experimental points of the cycle on the p-h diagram are shown in Fig. 4. With this case, the mass flow rate of refrigerant is 76.7 kg/h. Based on the equations from (1) to (6), the COP obtained for this case is 1.59 with the cooling capacity of 2100 W. This COP value is lower than conventional air conditioning systems. From Tab. 2 and Fig. 4, it is observed that the gas cooler outlet temperature was high, the temperature difference between gas and air was also high, leading to the vapor quality was high and the vaporization latent heat was small. Therefore, the actual cooling capacity was smaller than the design cooling capacity of the cycle. As a result, this caused the high superheat for the system. All reasons cased low COP for this system. However, the COP of this cycle is in good agreement with the COP in [2] at the gas cooler pressure of 87 bar and the gas cooler outlet temperature of 39 C.

Fig. 4. Experimental points of the cycle on p-h diagram without subcooling.

A conventional evaporator (using for an absorbed power of 750 W) was installed in this cycle. The conventional evaporator was made from copper tubes, having the design cooling capacity of 2700 W and the outside volume of 11.05 dm3. The conventional evaporator was tested hydraulic and it did not tear or deform at the pressure of 90 bar. An experimental comparison between the minichannel evaporator and conventional evaporator is listed in Table 3. At the same operating conditions, the results obtained from the conventional evaporator are the same with those obtained from the MMSE Journal. Open Access www.mmse.xyz

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minichannel evaporator. However, the outside volume of the minichannel evaporator is smaller than the conventional evaporator (only equals 0.27 times). The results indicate that the minichannel evaporator is suitable in the CO2 air conditioning system with high heat transfer rate and high allowable pressure (as the same nominal thickness).

Table 3. Comparison between minichannel and conventional evaporators. p1

t1

p2

t2

p3

t3

p4

t4

(bar)

( C)

(bar)

( C)

(bar)

( C)

Minichannel evaporator

36.5

20.2

87

85.7

86.5

39.3

37

5.0

Conventional evaporator

36.7

20.1

87

86.0

86.5

38.9

37

5.1

(bar) ( C)

The air conditioning with subcooling With subcooling, the minichannel evaporator was also used in this test loop. The CO2 air conditioning system was tested more than 100 times to collect data. The period for each time getting data was 30 minutes. The transcritical mode was also done in this study. The results obtained from this experiment are also stable. The experimental results for thermodynamic parameters of the cycle with subcooling are listed in Tab. 4. The data were drawn on the p-h diagram of CO2, as shown in Fig. 5. The experimental results indicated that the pressure drop of the heat exchangers in this case is higher than that obtained from the case of without subcooling. It may be some refrigerant lubricant or dirty enters the two heat exchangers.

Fig. 5. Experimental points of the cycle on p-h diagram with subcooling.

Figure 5 shows the experimental points of the cycle on the p-h diagram for five times getting data.

The mass flow rate of refrigerant is 84.4 kg/h in this case. Based on the equations from (1) to (6), the COP obtained for this case is 4.39, with the cooling capacity of 3890 W. The superheat in this case is also 3 C, it is smaller than that obtained from the without subcooling case (15 C). A photo of minichannel evaporator using thermal camera is shown in Fig. 6 for the evaporating temperature of MMSE Journal. Open Access www.mmse.xyz

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9.2 C. The figure showed that the profile temperature of outside evaporator is the same (around 11 C). The results are in good agreement with evaporating theory.

Table 4. Thermodynamic parameters of the CO2 cycle with subcooling. p1

t1

p2

t2

p3

t3

p4

t4

(bar)

( C)

(bar)

( C)

(bar)

( C)

( C)

(bar)

( C)

45

9.2

77

55

77

32.2

29.4

47

13.5

45

9.4

77

55

77

32.1

29.2

47

13.5

45

9.3

77

55

77

32.1

28.9

47

13.3

45

8.9

77

55

77

32.4

29.1

47

13.5

45

9.2

77

55

77

32.2

29.2

47

13.2

Fig. 6. A picture of minichannel evaporator using thermal camera.

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Fig. 7. COP vs. gas cooler outlet temperature for the gas cooler pressure of 77 bar.

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Fig. 8. COP vs. gas cooler outlet temperature for the gas cooler pressure of 87 bar.

Figure 7 shows correlations of COP, cooling capacity, and gas cooler outlet temperature. The results obtained for the gas cooler pressure of 77 bar and evaporating temperature of 15 C. The parameters obtained by adjusting the expansion device, the subcooling coil, the flow rate of gas and air, and the compressor. It is observed that the COP strongly increases as the gas cooler outlet temperature is less than 30 C. This is one of special properties of CO2 when it is used in air conditioning. This thing also indicated an importance of subcooling for the CO2 air conditioner. The experimental results for COP obtained from the present study are higher than those obtained from [2], [11]. In this study, the COP of 4.97 was achieved for the gas cooler pressure of 77 bar, the evaporating temperature of 15 C, the super heat of 15 C, and the gas cooler outlet temperature of 25 C. Figure 8 shows experimental correlation of the COP and the gas cooler outlet temperature for the gas cooler pressure of 87 bar and evaporating temperature of 5 C. The COP of 3.14 was achieved for the gas cooler pressure of 87 bar, the evaporating temperature of 5 C, the super heat of 15 C, and the gas cooler outlet temperature of 25 C. From Figs. 7 and 8, it is suggested that the CO2 air conditioning system should be operated for the gas cooler pressure of 74-77 bar and the evaporating temperature of 10-15 C in transcritical mode for high effectiveness and safety. However, to get high COP, the cycle has to add a subcooling coil. Compared with commercial catalogues, the COP of this cycle is higher than the COP of air conditioners using HCFC refrigerant (about 3.1 3.5). From experimental results above, they are needed and important for studying and developing the CO2 air conditioning systems.

Summary. Experimental studies on a CO2 air conditioning system with minichannel evaporator were done. In these studies, the minichannel evaporator was also added in order to compare with conventional one. The experimental results obtained from the two heat exchangers are nearly the same when testing in the same conditions. However, the size of minichannel evaporator is lower than that of conventional one.

Without subcooling process, the COP obtained for this case is 1.59 with the cooling capacity of 2100 W. This COP value is lower than the conventional air conditioning system.

With subcooling process, the COP strongly increases as the gas cooler outlet temperature is less than 30 C. This important feature should be considered carefully when designing CO2 air conditioning system working in transcritical region. In this case, the COP of 4.97 was achieved for the gas cooler

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pressure of 77 bar, the evaporating temperature of 15 C, the super heat of 15 C, and the gas cooler outlet temperature of 25 C. The experimental results for COP obtained from the present study are higher than those obtained from several literature reviews. It is suggested that the CO2 air conditioning system should be operated for the gas cooler pressure of 74-77 bar and the evaporating temperature of 10-15 C in transcritical mode for high effectiveness and safety. However, to get high COP, the cycle has to add a subcooling coil.

Acknowledgment The supports of this work by the project No. 35/2015/HD-SKHCN (sponsored by HCMC Department of Science and Technology, Vietnam) are deeply appreciated.

Nomenclature power input heat transfer rate mass flow rate h enthalpy p pressure t temperature Subscripts. c carbon dioxide 1 exit of evaporator 2 exit of compressor 3 exit of gas cooler exit of subcooler 4 exit of throttling valve.

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