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Energy Procedia 105 (2017) 2033 - 2038

The 8th International Conference on Applied Energy - ICAE2016

Thermodynamic Performance Analysis of C02 Transcritical Refrigeration Cycle Assisted with Mechanical Subcooling

Baomin Daia, Shengchun Liua'*, Zhili Suna, Yitai Mab

aTianjinKey Laboratory ofRefrigeration Technology, Tianjin University of Commerce, Tianjin 300134, China bKeyLaboratory ofEfficient Utilization ofLow andMedium Grade Energy, MOE, Tianjin University, Tianjin 300072, China

Abstract

The thermal performance of C02 transcrtical refrigeration cycle can be improved by cooling the C02 fluid exiting the gas cooler with an assisted vapor compression refrigeration cycle (auxiliary cycle). Thus, a thermodynamic analysis is performed to study the operation characteristics of the subcooling C02 transcritical refrigeration cycle. The results indicate that a maximum COP is achieved at the corresponding optimum discharge pressure and the optimum subcooling temperature. The improvement in COP is more significant in the case of higher ambient temperature and lower evaporation temperature, and the discharge pressure and temperature can be obviously reduced. The auxiliary cycle refrigerant is screened and R717 performs with the highest COP. The C02 transcritical assisted with mechanical subcooling is recommended for the cases with higher ambient temperature and lower evaporation temperature.

© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.

Keywords: C02, transcritical cycle, mechanical subcooling, thermodynamical performance

1. Introduction

In the field of heating, ventilation, air-conditioning and refrigeration (HVAC&R), the natural refrigerant C02 becomes the focus of many researchers and manufactures, because of the advantages in environmentally-friendly, safety, and low price etc. The C02 refrigeration and air conditioning systems have been applied in many fields, including heat pump water heater, mobile air-conditioning, beverage showcase, cold storage and so on. However, the coefficient of performance (COP) of C02 refrigeration

* Corresponding author. Tel.: +86 022 26684065; fax: +8602226667502. E-mail address: liushch@tjcu.edu.cn (S. Liu); dbm@tjcu.edu.cn (B. Dai).

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.579

system is lower in comparison to the ones using traditional refrigerants. This is because the operation pressure is extremely high and the throttling irreversibility is quite large.

The latest statistical data of Shecco Publications [1, 2] show that the commercial CO2 transcritical refrigeration systems are mostly located in the lower ambient temperature areas at higher latitudes in European or North American. But the application is much less in the warm or hot regions. This is attributed to that the CO2 fluid can not be sufficiently cooled in the gas cooler in warm or hot climate, which leads a lower cooling capacity and COP. A dedicated vapor compression cycle (VCC) can be utilized to cool down the outgoing fluid from the gas cooler/condenser of the main cycle. The analytical results of Pottker and Hrnjak [3] indicate that the COP can be improved by 12% for the CO2 subcritical dedicated subcooling cycle. She et al. [4] proposed a dedicated subcooling cycle combined with an expander. Llopis et al. [5] presented a thermodynamical analysis to study the performance of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling. They demonstrated that the COP and cooling capacity are increased up to 20% and 28.8%, respectively. Though the dedicated subcooling cycle needs an auxiliary VCC, the power consumption of the auxiliary cycle is much less than that of the main cycle [6]. The analytical results of Llopis et al. [5] show that the power consumption for the dedicated subcooling cycle is below 20% ofthat in the main cycle at the optimum operation condition.

The dedicated mechanical subcooling is a more promising and practical way to improve the CO2 transcritical cycle energy efficiency, and broaden its application in warm and hot areas. However, until now a small amount of theoretical analyses were reported, and the influence of subcooling degree should be detailed conducted to intensively explore the characteristics of the cycle performances. Therefore, in this study the matching performances between the main and auxiliary cycles are studied, and the effect of subcooling degree is discussed in depth.

2. Cycle modeling

2.1. Cycle description

Throttle

, valve 2

Compressor 2

Auxiliary cycle

Throttle

valve 1

Main cycle

Compressor 1

Evaoprator

Fig. 1. Schematic of dedicated mechanical subcooling cycle Fig. 2. T-s diagram of CO2 transcritical refrigeration cycle

The schematic and T-s diagram of a CO2 refrigeration cycle are shown in Fig. 1 and 2, respectively. The cycle 1-2-3-4-1 shown in Fig. 2 is an ordinary C02 transcritical cycle, which is combined with a compressor, a gas cooler, a throttle valve, and an evaporator. In contrast, cycle l-2-3"-4"-l is called main cycle, which is combined with a dedicated subcooling cycle. Cycle l'-2'-3'-4'-l' is a vapor compression

cycle by using traditional work fluid, named auxiliary cycle or deicated subcooling cycle. It consists of a compressor, a condenser, a throttle valve and a subcooler. The main cycle and the auxiliary one are linked by the subcooler. It operates as an evaporator in the auxiliary cycle, and a subcooler for the main cycle. The evaporation process (4'-l') of the auxiliary cycle absorbs heat from the C02 fluid exiting the gas cooler. Consequently, C02 fluid is subcooled (3-3"), and the cooling capacity is enhanced.

2.2 Assumption ofthe model

The model is developed on the base ofthe following assumptions:

(1) The cycles operates under steady working condition;

(2) The pressure drop in the heat exchangers and pipe lines are neglected;

(3) The refrigerant at the outlet of the evaporator is saturated vapor, and the outgoing fluid of the condenser is at saturated liquid state;

(4) The gas cooler outlet temperature is 5 °C higher than the ambient temperature;

(5) For the auxiliary cycle, the temperature difference between condensation temperature and the ambient temperature is set as 10 °C;

(6) For the subcooler, the approach temperature (the temperature difference between the outgoing C02 and the ingoing traditional refrigerant) is selected equal to 5 °C;

(7) The mechanical and electrical efficiency ofthe compressors in main and auxiliary cycle are set as 0.9; the isentropic efficiency of C02 compressor is equivalent to 0.8 and that ofthe auxiliary cycle is 0.7.

3. Results and discussion

The GWP of R152a is 124, belonging to the category of "low-GWP" refrigerant. Moreover, it is low cost and easy available, which is characterized with excellent comprehensive performance. Thus, the cycle performance is analyzed and discussed on the base of using R152a as the auxiliary cycle refrigerant.

Fig. 3. Variation of COP Fig. 4. Performance parameters of SC cycle

Fig. 3 depicts the variation tendency of the mechanical subcooling cycle (shorted for SC) COP at evaporation temperature (7e) of 0 °C and ambient temperature (7a) being 30 °C. It can be found that the COP is not only influenced by the discharge pressure, but also related to the subcooling temperature. Generally, the COP increases dramatically first and then decreases gradually with the discharge and the subcooling temperature. In particular, a maximum COP of 2.84 is achieved at discharge pressure of 8.34 MPa and subcooling temperature of 13.9 °C, which is named as optimal discharge pressure and

subcooling temperature in this study. According to the analysis results of Kauf [7], there exists an optimal discharge pressure as the gas cooler outlet temperature is fixed. This is attributed to the interactions between the S-shape isotherm at supercritical region and the compression process. Moreover, the effect of C02 subcooling temperature on the C02 subcooling cycle can be explained by Fig. 4.

Fig. 4 shows the variation of the thermal performance of the CO2 SC cycle. The parameters studied includes the cycle COP, cooling capacity (Qevb, sc), overall power consumption (WCom, sc), main cycle compressor input work (WCom, c02), and auxiliary cycle compressor input work (WCom, aux)- It can be concluded that the CO2 cycle compressor energy consumption is a constant because the ambient temperature and the discharge pressure are both fixed. Whereas, the overall energy consumption increases rapidly with the subcooling temperature, due to the decrease in evaporation temperature of the auxiliary cycle, and a notable increase in the auxiliary cycle compressor energy consumption. The cooling capacity increases linearly with the subcooling temperature. Consequently, the cycle COP firstly increases and then decreases, and a maximum COP is achieved at the optimum subcooling temperature. Thus, the following analysis are performed based on the working condition with highest COP at the optimum discharge pressure and subcooling temperature.

-Psc,TE=S-C

-Psc,TE = - 5°C

28 32 36

Ambient temperature (°C)

24 28 32

Ambient temperature

Fig. 5. Maximum COP

Fig. 6. Optimum discharge pressure

Fig. 5 illustrates the variation of maximum COP for the base cycle and the SC cycle with ambient and evaporation temperature. As can be seen from Fig. 5 that COPbase and COPsc both decreases dramatically as the ambient temperature increases. COPbase is always higher than COPsc with the ambient temperature changing from 20 to 40 °C. The increasing rate is defined as (COPsc - COPbase)/ COPbase x 100%. At the ambient temperature of 40 °C and evaporation temperature of-15 °C, the COP increasing rate is as high as 43.8%. Whereas, it is only 6.2% at ambient temperature of 20 °C and evaporation temperature of 5 °C. It can be concluded that the energy efficiency of CO2 transcritical cycle can be significantly improved with the assistant of vapor compression cycle, especially for the cases with high ambient temperature and low evaporation temperature, such as the regions of hot climate.

Fig 6 shows the variation of optimum discharge pressure. It increases linearly with the ambient temperature. Nevertheless, the influence of evaporation temperature is not notable. Especially for the subcooling cycle, at the ambient temperature of 40 °C, the optimum pressure increases by 0.12 MPa as the evaporation temperature decreasing form 5 to -15 °C. It can also be noted that the discharge pressure of the subcooling cycle is lower than that of the base cycle, indicating that the discharge pressure can be reduced with the dedicated mechanical subcooling. Additionally, the function of pressure reduction is more effective as the ambient temperature increases and the evaporation temperature reduces. At the ambient temperature of 40 °C and evaporation temperature of-15 °C, the optimum discharge pressure of SC cycle decreases from 12.28 to 10.27 MPa in comparison with the base cycle. As a consequence, it can

also be proved that the subcooling cycle shows a better performance for the regions with high ambient temperature.

24 28 32 36

Ambient temperature (°C)

Fig. 7. Optimum subcooling temperature

160 140 ■

& 100 -

-COPsc, ii = 5°C -COPsc, Te = -5 °C -COPsc, ii = -15°C - COPbase, Te = 5°C ■ COPLse, TE = -S °C -COPbase, Te = -15 °C

24 28 32 36 4

Ambient temperature (°C)

Fig. 8. Variation of discharge temperature

rE = 0°C, Ta = 30 °C

The variation of the optimum subcooling temperature is sketched in Fig. 7. As can be seen, the optimum subcooling temperature increases with the increase of ambient temperature and reduction of evaporation temperature. When the temperature is -15 °C and the ambient temperature is above 28 °C, the optimum subcooling temperature exceeds 20 °C. Thus, a relatively high subcooling temperature is required to meet the optimum working condition. Moreover, the higher the ambient temperature is and lower ofthe evaporation temperature is, the larger ofthe optimum subcooling temperature is.

The discharge temperature of the base cycle and subcooling cycle at the optimum working condition are shown in Fig 8. As indicated in the figure, the discharge temperature shows a linear increasing tendency with the increase of ambient temperature and reduction of evaporation temperature. The discharge temperature of the subcooling cycle is lower than that of the base one. Furthermore, the advantage in discharge temperature reduction becomes more apparent as the ambient temperature increases and the evaporation temperature reduces. In particular, at ambient temperature of 40 °C and evaporation temperature of -15 °C, the discharge temperature reduces by 17.5 °C for the subcooling cycle compared with the base one. Fi& 9- C0P usin§ different auxiliary cycle refrigerants

The overall cycle performance is closely relative to the refrigerant selection ofthe auxiliary cycle. Thus, eleven refrigerants are selected to screen the suitable working fluid for the auxiliary cycle. Fig. 9 shows the overall cycle COP under optimum working condition for the base cycle and the subcooling cycle with eleven refrigerants. It indicates that the cycle using R41 as auxiliary cycle refrigerant shows the lowest COP. It lies on the fact that the critical temperature is as low as 44.1 °C, but the condensation temperature is 40 °C, so the vapor quality of the refrigerant at the outgoing of the expansion valve is relatively high. Thus, the auxiliary cycle performs with a low energy efficiency, which weakens the overall cycle COP. Whereas, minor differences are observed between the subcooling cycles using the other ten refrigerants. Furthermore, the cycle using R717 operates with the highest COP, followed by R152a and RE170.

4 Conclusions

It is an promising way to use a vapor compression cycle to further cool the CO2 exiting the gas cooler. In this study a thermodynamic analysis is performed to study the thermal performance of the CO2 transcritical cycle assisted with dedicated mechanical subcooling. The conclusions are achieved as follows:

(1) Discharge pressure and subcooling temperature are two main factors influencing the overall subcooling cycle. The subcooling cycle obtains the maximum COP at optimum discharge pressure and subcooling temperature.

(2) The dedicated mechanical subcooling cycle can efficiently improve the C02 transcritical cycle energy efficiency and reduce the discharge pressure and temperature. The COP improving rate is as high as 43.8%, and the discharge pressure and temperature decreases by 2.01 MPa and 17.5 °C.

(3) The auxiliary cycle using R717 as refrigerant performs with the highest overall COP, and that of R41 is the lowest.

(4) The method of mechanical subcooing is recommend to be applied in the case scenario with high ambient temperature and low evaporation temperature.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51006073), Tianjin Technical Envoy Program (15JCTPJC62600), and Science Research Innovation Team Project in Tianjin, China (TD12-5048).

References

[1] Shecco Publications. Guide 2014: Natural refrigerants continued growth & innovation in Europe, 2014.

[2] Shecco Publications. Guide to natural refrigerants in North America - State of the industry, 2015.

[3] G. Pottker, P. Hrnjak, Effect of the condenser subcooling on the performance of vapor compression systems, International Journal of Refrigeration, 50 (2015) 156-164.

[4] X. She, Y. Yin, X. Zhang, A proposed subcooling method for vapor compression refrigeration cycle based on expansion power recovery, International Journal of Refrigeration, 43 (2014) 50-61.

[5] R. Llopis, R. Cabello, D. Sánchez, E. Torrella, Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling, International Journal ofRefrigeration, 55 (2015) 129-141.

[6] B.A. Qureshi, S.M. Zubair, Mechanical sub-cooling vapor compression systems: Current status and future directions, International Journal of Refrigeration, 36 (2013) 2097-2110.

[7] F. Kauf, Determination of the optimum high pressure for transcritical C02-refrigeration cycles, International Journal of Thermal Sciences, 38 (1999) 325-330.

Biography

Baomin Dai received his Ph.D. and M.S. degrees from the School of Mechanical Engineering at Tianjin University, Tianjin, China, in 2015 and 2012, and received his B.S. degree from Hebei University of Technology, Tianjin, China, in 2010. He is currently lecturing in Tianjin University ofCommerce.