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ScienceDirect

Procedía CIRP 26 (2015) 482 - 485

12th Global Conference on Sustainable Manufacturing

Performance investigation of transcritical carbon dioxide refrigeration cycle

Aklilu Tesfamichael Bahetaa*, Suhaimi Hassana, Allya Radzihan B Reduana, and Abraham D.

Woldeyohannesb

aUniversiti Teknologi PETRONAS, Department of Mechanical Engineering, Bandar Seri Iskandar,31750,Tronoh, Perak, Malaysia hCaledonian College of Engineering, PO Box 2322, CPO Seeb 111, Sultanate of Oman

* Corresponding author. Tel.: +6053687690; fax: +6053656461. E-mail address: aklilu.baheta@petroans.com.my

Abstract

CO2 has low critical pressure and temperature. This gives an opportunity CO2 cycles to work in a transcritical nature where heat rejection and absorption are done at supercritical and subcritical conditions, respectively. However, this characteristic posed some performance issues for CO2 refrigeration cycle such as the pressure and temperature of CO2 becomes independent of one another above the critical point thus specifying the operating conditions would be tough. It is also important to identify the optimum cooler pressure and control it; in order to get high cycle coefficient of performance (COP). Thus, the objective of this paper is to investigate the performance of a transcritical CO2 compression refrigeration cycle for different parameters and evaluate its COP. To achieve that, a refrigeration cycle was modeled using thermodynamic concepts. Then, the model was simulated for various parameters that were manipulated to investigate the cycle performance. Maintaining other operating parameters constant the highest COP was 3.24 at 10MPa gas cooler pressure. It was also observed that the cycle is suitable for air-condition application than refrigeration cycle, as COP increases when the evaporator temperature increases. Simulations were conducted using EXCEL developed program. The results can be used in the design of CO2 refrigeration cycle.

© 2015Publishedby Elsevier B.V.This isanopenaccess article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Assembly Technology and Factory Management/Technische Universität Berlin.

Keywords: CO2; transcritical cycle; refrigeration; optimum pressure; COP; supercritical

1. Introduction

CO2 has low critical pressure and temperature which are 7.36 MPa and 31.1oC, respectively. The low critical temperature causes the heat rejection process to occur above the critical point and heat absorption process to happen below the critical point. Figure 1 represents a p-h phase diagram of CO2 transcritical refrigeration cycle. Heat is rejected at supercritical pressure and the fluid will exist in the superheated region. Existence of optimum heat rejection pressure gives maximum COP. During heat rejection process, the refrigerant experiences large temperature glide. One of the challenges of this cycle is, due to the high pressure level there is a need to control the pressure. One method is to adopt dynamic pressure control [1]. This pressure influences the highest COP value the cycle can produce [2]. Thus, having the ability to control high-side pressure will provide optimum COP. However, practically this pressure changes as it is influenced by various operating parameters of the cycle. With this respect, McEnaney [3], investigated CO2 for mobile air conditioning application and

his studies showed that maximum COP was obtained as a function of various operating parameters. Kim et al. [4] reviewed many research works and explained this cycle COP is optimum at a specific operating parameters combination. Sarkar [5] explained that maximum COP occurred at specific gas cooler pressure which in turn is affected by evaporator temperature (T1), gas cooler exit temperature (T3), and components' efficiency. Moreover, Perez-Garcia et al. and Xue et al. [6, 7], supports that compressor inlet temperature (T1) influences the COP and added another variable which is compressor efficiency.

Thus, in order to obtain the maximum value of COP, the popt for the system must be achieved and controlled. Since the pressure is not constant and influenced by other working parameters, the relationship between the parameters and its influence on the system COP must be understood. With this understanding only the parameters that significantly affect the refrigeration cycle of COP could be controlled and CO2 refrigeration system COP can be improved. Thus, the objective of the paper is to understand operating parameter changes on each and the subsequent devices and on the cycle

2212-8271 © 2015 Published by Elsevier B.V. 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 Assembly Technology and Factory Management/Technische Universität Berlin. doi : 10.1016/j .procir.2015.02.084

performance. In order to do that, a model was developed using thermodynamic concepts. Then, the model was simulated for various parameters that were manipulated to investigate the cycle performance. Simulations were conducted using EXCEL developed program.

Fig.1. CO2 vapor compression cycle on a p-h diagram

Nomenclature

W power

Q heat transfer rate

m mass flow rate

h enthalpy

p pressure

T temperature

cp specific heat

popt gas cooler optimum pressure

x quality

nis,c isentropic efficiency of compressor

Subscripts

R Refrigerant

1 exit of evaporator

2 exit of compressor

3 exit of gas cooler

4 exit of throttling valve

W1-2a = mR (h2a — h1)

The fluid flows into the gas cooler where heat rejection is done. In here the refrigerant will experience large temperature glide and exits the gas cooler at slightly higher than the coolant temperature. In the gas cooler, the heat rejection process occurs at constant pressure. The heat rejection in this component can be quantified using:

QQ3-2a = mR (h2a — h3)

The value of h2a is influenced by the value of compressor efficiency. For COP calculation, actual compressor exit enthalpy (h2a) was used. And it was calculated by Eq. (4).

h2a = h1 —

h2 — hi Vis, c

Then, the refrigerant enters into the throttling device where it was expanded and experienced isenthalpic process. The enthalpies of the refrigerant both at gas cooler exit and evaporator inlet are equal as represented by Eq. (5).

h3 = h4

Enthalpy at point three is a function of both gas cooler pressure and exit temperature. Whereas, the enthalpy at point four is a function of the evaporator pressure and the quality at the expansion valve exit. If T3 and cooler exit pressure is known, then the enthalpy is obtained from CO2 property tables. When x4 was used as the input parameter, the value was obtained by using Eq. (6) at the given evaporator pressure.

h 4 = h4 f + x4 h fg 4

Finally the coefficient of performance (COP) of the cycle was calculated as

COP = -

Refrigeration effect Compressor power input

2. Methodology

2.1. Modeling Components of the System

Each process that represent the transcritical CO2 refrigeration cycle was identified. With some assumptions each component process was modeled thermodynamically. Inside the evaporator, the refrigerant absorbs heat from the refrigerated space and the amount of heat absorbed is evaluated as

Q4-1 = mR(hi -h4) (1)

Once the refrigerant exits the evaporator, it flows into the compressor where it is compressed to superheated state. Compressor is power consuming device and the power input used to compressor the fluid is given as

Once each process is represented mathematically they are integrated by simulation model which was developed in Microsoft Excel. For the study of effect of each parameter and to evaluate the COP of the cycle practical operating parameters were used. Gas cooler pressure and its exit temperature were varied from 8 to 13 MPa and 35 to 50oC, respectively. The evaporator exit temperature was varied from -15 to 15oC, whereas its pressure was maintained at 4 MPa. The compressor efficiency was assumed to be 100%.

2.2. Varying the Cycle Parameters

At this stage, various input parameters were manipulated and analyzed to understand their influence on the COP. First, only one parameter was varied to see its effect on COP and then two parameters were manipulated to see the influence of their relationship on the cycle COP.

3. Results and Discussion

Figure 2 shows the COP versus gas cooler pressure (p2). For this simulation, the input parameters that were maintained are pi = 4 MPa, T3 = 40°C, r|is c = 100% and p2 was varied. The graph shows as p2 increases initially the COP increases, reach maximum and reduces. At this given conditions the optimum pressure is i0 MPa and the corresponding highest COP is 3.24. Increasing the pressure, increases the COP initially however the added capacity no longer able to compensate compressor additional work thus the COP value decreases. The initial COP shows that the as the p value close to the critical pressure the COP is below one and hence to improve COP value, p2 should not be too close to the p cr«

3.50 3.00 2.50

Pu 2.00

u 1.50 1.00 0.50 0.00

8 9 10 11 12 13 Gas cooler pressure, p2 (MPa)

Fig.2. Variation of COP with respect to gas cooler pressure

Figure 3 shows the variation of COP, at 4 MPa and 10 MPa evaporator and gas cooler pressures, respectively, for different CO2 gas cooler exit temperatures. Almost linear relationship is observed between the COP and T3. The highest value of COP was 3.82 at 35oC which is the heat sink temperature. The smaller the refrigerant temperature leaving the gas cooler, the bigger will be the COP, but this is limited by the heat sink temperature. At temperature more than 50oC, the COP value is less than one.

Fig.3. Variation of COP versus gas cooler exit temperature

The effect of the evaporator temperature was investigated while other parameters are maintained constant. Here, the constant parameters were p2 = 10 MPa, T3 = 40oC and at r|is,c

= 100%. It can be seen in Figure 4 as T1 increases the cycle COP value increases. However, the temperature of the evaporator is determined by the space to be cooled. This result also showed that CO2 refrigeration cycle is suitable for air conditioning purpose than for refrigeration application.

Fig.4. Variation of COP versus evaporator temperature

Effect of two parameters change on COP was investigated. Fig. 5, shows the COP variation against the gas cooler pressure for different gas cooler exit temperatures. At a given gas cooler pressure the smaller the gas cooler temperature, the higher will be the COP. For the pressure range analyzed maximum COP was observed at 35oC and 40oC gas cooler exit temperatures at unique pressure (popt). However, at T3 45oC and above the COP initially increases and then becomes flat. Here it can be deduced that T3 has a significant effect on the popt. Moreover, it would take higher pressure for the system to achieve the highest COP as the gas cooler exit temperature increases. Negative value of COP was also observed at 50oC and 8 MPa which shows that the cycle has failed to provide refrigeration or evaporator becomes condenser. However, at 35oC, the value suddenly increased to 3.23. This was due to the effect of enthalpy value at h4. At higher T3, the value was bigger compared to enthalpy at 35oC, thus enthalpy difference at refrigerating capacity was smaller (even negative) as the temperature increases. Thus appropriate gas cooler pressure must be used for a given gas cooler exit temperature.

Fig.5. Variation of COP versus gas cooler pressure for different gas cooler exit temperatures

In Fig. 6 the COP value is plotted against gas cooler pressure (p2) for different evaporator temperatures

maintaining other parameters constant. At a given gas cooler pressure as the evaporator temperature increases the COP increases. Apart from that, by varying the p2, maximum COP is observed at popt and this more distinct at higher T1 especially at 0oC and above. This figure also shows that the maximum COP happened almost at the same optimum pressure. Hence, the effect of evaporator temperature on the optimum gas cooler pressure for maximum COP is not significant compare to the gas cooler exit temperature which is shown in Fig. 6.

Model of CO2 transcritical refrigeration cycle was developed thermodynamically. The model was used to investigate the effect of the various parameters on the cycle COP and to identify the combined effect for optimum COP. The following were drawn from the investigation.

Transcritical CO2 refrigeration cycle has specific gas cooler pressure (popt) that gives maximum COP. This pressure is not constant and varies when the rest of the cycle operating parameters change. Moreover, gas cooler exit temperature and evaporator temperature have significant effect on the cycle pressure that gives maximum COP. In general, higher evaporator and smaller gas cooler exit temperatures would give better cycle COP. The best

combinations of these parameters can be obtained by analyzing the cycle for the given parameters. It was also observed that the cycle is suitable for air-condition application than refrigeration cycle, as COP increases when the evaporator temperature increases. Based on these outcomes, it is hoped that a better understanding of controlling CO2 transcritical refrigeration cycle COP can be achieved. Apart from that, with the identification of the parameters that affect the COP significantly, it is hoped that future design of CO2 refrigeration cycle can be improved.

Acknowledgments

The authors wish to thank Universiti Teknologi PETRONAS for providing financial support to publish the paper.

References

[1] Nordic Chemical Group (NKG), "The transcritical CO2 Cycle Fact sheet 2.2.4," Natural Refrigerants for new Application, Copenhagen K: Denmark, AIP 2009:426.

[2] Sarkar, J., Bhattacharyya, S., and Ram Gopal, M., Optimization of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications, International Journal of Refrigeration, 2004; 27: 830-838.

[3] McEnaney, R.P., Yin, J. M., Bullard, C.W., and Hrnjak, P.S., An Investigation of Control-Related Issues in Transcritical R744 and Subcritical 134a Mobile Air Conditioning Systems, 1999.

[4] Kim, M-H., Pettersen, J. and Bullard, C-W., Fundamental process and system design issues in CO2 vapor compression systems, Progress in Energy and Combustion Sciences, 2004; 30: 119-174.

[5] Sarkar, J., Review On Cycle Modifications of Transcritical CO2 Refrigeration and Heat Pump Systems, Journal of Advanced Research in Mechanical Engineering, 2010; 1: 22-29.

[6] Perez-Garcia, V., Belman-Flores, J-M., Navarro-Esbri, J. and Rubio-Maya, C., Comparative study of transcritical vapor compression configurations using CO2 as refrigeration mode base on simulation, Applied Thermal Engineering, ., 2012; 51: 1038-1046.

[7] Xue, J., Koyama, S., and Kuwahara, K. (Eds.), Proceedings from 2010 International Symposium on Next-generation Air Conditioning and Refrigeration Technology: Performance Prediction of A R744 Transcritical Cycle for Air Conditioning. Tokyo: Japan, 2010.

Gas cooler pressure, p2 (MPa)

Fig.6.Variations of COP vs Gas Cooler Pressure for different evaporator temperatures

4. Conclusion