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Thermodynamic Simulation of Scroll Compressor/Expander Module in Automotive Fuel Cell Engine Zhao Yuanyang, Li Liansheng and Shu Pengcheng Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 2006 220: 571 DOI: 10.1243/09544070D14304 The online version of this article can be found at: http://pid.sagepub.com/content/220/5/571

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Thermodynamic simulation of scroll compressor/ expander module in automotive fuel cell engine Zhao Yuanyang*, Li Liansheng, and Shu Pengcheng School of Energy and Power Engineering, Xi’an Jiaotong University, Xian, People’s Republic of China The manuscript was received on 4 August 2004 and was accepted after revision for publication on 12 January 2006. DOI: 10.1243/09544070D14304

Abstract: Considering the leakage and heat transfer, the mathematical model of the scroll compressor/expander module (CEM) used in the automotive fuel cell systems is developed based on thermodynamics theory. The performance of the scroll CEM is simulated at different rotation speed, operating pressure, inlet temperature, and so on. The simulated results show that the power consumed by the scroll CEM is about 50 per cent of that of just the scroll compressor. However, for different operating pressures, the consumed power of the scroll CEM has the least value at the design point. The power consumed by the scroll CEM decreases about 4.5 per cent when the inlet temperature rises from 0 to 35 °C. The flowrate of dry air increases with the rise of the scroll CEM rotation speed. Moreover, it decreases 11.4 per cent when the scroll CEM inlet temperature changes from 0 to 35 °C. Keywords: scroll compressor, expander, fuel cell, thermodynamic

1 INTRODUCTION Automobiles consume a great deal of liquid fuel every year. At the same time, they release many pollutants to the aerosphere. The increase of global energy consumption and pollutant emissions has been forcing the automobile manufacturers to look for future power systems and alternative drive concepts to improve the quality of the urban environment and to save energy. The automobile manufacturers have presented alternative drive systems, such as electric, hybrid, or fuel cell systems. Among these systems, the proton exchange membrane (PEM) fuel cell system seems to have the greatest potential to compete with the internal combustion engine and the fuel cell is one of the most effective hydrogen energy applications [1]. The PEM fuel cell has the features of low noise, highenergy conversion ratio, good working properties, low operating temperature, short start-up time, and

* Corresponding author: National Engineering Research Centre of Fluid Machinery and Compressors, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China. email: [email protected]

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higher power density [2]. Hence, PEM fuel cells are becoming an increasingly important energy conversion technology for transportation applications (land and sea) and are attracting people’s interest all over the world. The PEM fuel cell system for the automotive engine is composed of many subsystems: fuel (hydrogen) and air (oxygen) supply, cooling, energy management, controller, electric system, and fuel cell itself. The air supply system is an important part of the PEM fuel cell systems, while the compressor/expander module (CEM) is the core of the air supply system. In PEM fuel cell systems air is normally used for the cathode of the fuel cell. In the fuel cell stack, high pressure of compressed air can result in better performance of the fuel cell, especially instantaneously [3]. However, the high pressure of compressed air in the fuel cell system will consume 20–30 per cent of the power generated by the fuel cell system itself [3]. To reduce the power consumed by the compressor, an expander is used to recover the power from gas discharged from the fuel cell and still having higher pressure. The scroll machines have been used widely in the fields of refrigeration and air conditioning, power engineering, and new energy resources technology Proc. IMechE Vol. 220 Part D: J. Automobile Engineering


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Zhao Yuanyang, Li Liansheng, and Shu Pengcheng

because of its compactness, high efficiency, low vibration and noise level, and excellent running reliability [4]. Moreover, the scroll machine has been developed for use in the automotive fuel cell engine [5, 6].

2 WORKING PRINCIPLE OF THE SCROLL CEM Figure 1 shows the CEM and PEM fuel cell system with the air supply system. The scroll CEM contains a scroll compressor, a scroll expander, and a motor. The motor and the scroll expander supply the power to the scroll compressor at the same time. In the PEM fuel cell system, any contamination including lubricating oil will result in clogging of the membrane and performance degradation of the system. Hence, some measures have to be taken to avoid oil leakage into the air stream, either by fitting sealing systems or by absolutely avoiding the use of oil or oil-containing lubricants. Because the oilinjection compressors cannot be used in the PEM fuel cell, the oil-free air compressor needs to be researched and developed. In the fuel cell engine, the water from the stack is used as the lubricant and coolant of the oil-free air compressor. The air is filtered before entering the suction chamber of the scroll compressor, and the water is injected in the process of compression. Next, compressed air with water or/and water vapour is discharged from the discharge hole of the compressor, and the water is separated from the air in the water–air separator. Finally, the compressed air is supplied to the fuel cell stack, where oxygen from air and hydrogen reacts and generates electricity (direct current) and water. The gas flowing from the fuel cell stack still has considerable pressure com-

Fig. 1 The diagram of the CEM and PEM fuel cell system

pared to the atmospheric pressure; in other words, there is still available enthalpy in the air out of the stack. Hence, it is significant to recover the enthalpy of air by the scroll expander and supply corresponding power to the compressor. Thus, the real power consumed by the scroll CEM will decrease. The working processes of the scroll compressor and the scroll expander are opposite. The sealed chamber volume of the scroll compressor decreases with the rotation of the CEM shaft. Moreover, for the scroll expander, this volume increases with the shaft rotation. Figure 2 shows the working processes of the scroll compressor and expander. The scroll compressor and expander are fixed at the two sides of the motor, respectively, and they share the same shaft with the motor.

3 SIMULATION SYSTEM OF THE SCROLL CEM 3.1 Model assumptions Selecting one working chamber of the scroll CEM as the control volume, to establish the simulation model of the scroll CEM process, the following assumptions are made: 1. In the control volume, the pressure and temperature of the gas, respectively, are assumed to be homogeneous at any one time. 2. The changes of gravitational and kinetic energy of the gas are negligible. 3. The processes of the gas or gas-water flowing through all ports are adiabatic steady flow. 4. The processes of the suction and discharge, respectively, are continued and thus the gas pulsation is neglected. 5. The wet air is saturated in the scroll CEM because there is a large quantity of water in the scroll CEM. 6. Heat transfer between the shell of the scroll CEM and the wet air is negligible.

Fig. 2 Volume change of the scroll CEM

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Thermodynamic simulation of scroll compressor/expander module

3.2 Governing equations The control volume of the scroll CEM can be simplified and shown in Fig. 3 [7]. In the control volume, the suction and discharge of the wet air, the leakage in and out, and the heat and mass transfer between air and water are taking place. Applying the first law of thermodynamics on the working fluids inside the control volume, the following equation can be obtained dQ dm dm dW d(mu) ih +∑ oh + =∑ +∑ i o dh dh dh dh dh

(1)

Here, the sign is defined as follows: the sign is ‘+’ when the energy flows into the working fluids; the sign is ‘−’ as the energy flows out of the working fluids. For the wet air, from equation (1) dh dm dV dp d(m u ) g +h g −p g −V g g =m g dh g dh g dh dh dh

(2)

Hence, the change rate of the wet air temperature can be written [8]

C A B A BD A B A B G C DH A B A B

1 qh qp dv g − g v qv qv dh dT g g g T T g= dh qp 1 qh g − qT v qT g v g g v 1 dm dQ i (h −h )+ ∑ g ∑ g V qh i dh − g (3) qp 1 qh g − qT v qT g v g g v Using the mass conservation equation, the mass change for the wet air in the control volume is dm =dm +dm +dm g i o w

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where dm is the mass leaked in the control i volume; dm is the mass leaked out of the cono trol volume; dm is the mass of the water evaporation w or condensation [9, 10]. 3.3 Description of mass flow in the scroll CEM The mass flow of the wet air through the suction port, discharge port, and the clearance are estimated in the following way. The suction process is used, whereby the working fluids fill the suction chamber at a constant suction pressure. Thus, the pressure and temperature of the working fluids are constant in the suction process and the same as that of the working fluids in the inlet. Based on the nozzle flow analysis [10], the mass of flow through the clearance and the discharge port can be calculated. The mass flowrate is determined by applying the equation of one-dimensional compressible flow in a nozzle, assuming the process to be isentropic. The equation of the mass flowrate can be shown dm A l =C dt n 1

S

A

2k p n e2/k−e(k+1)/k k−1 1 1

B

(5)

Here, parameters e and e are defined as follows cr

A B

k/(k−1) 2 e = cr k+1

(6)

p p e= 2 for 2 e cr p p 1 1 p e=e for 2 ∏e cr cr p 1 (7)

(4)

In equations (5) and (7), p is the high pressure 1 and p is the low pressure, the leakage gas flows from 2 the high-pressure p district to the low-pressure p 1 2 district. 3.4 Heat exchange model

Fig. 3 Model of the control volume D14304 © IMechE 2006

For the compressor of the scroll CEM, the air and water flow into the suction chamber at the same time. After water has thoroughly cooled by the air before it is injected into the scroll compressor, the temperature of the water can be assumed to be the same as that of the suctioned air when the temperature difference is small. In addition, as the gas is compressed, the wet air is assumed to be at the saturation state because there is a great deal of water with it. Proc. IMechE Vol. 220 Part D: J. Automobile Engineering


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When the temperature difference is great between the spray water and the inlet air, the following equation can be used to calculate the heat transfer and their final temperature before compression m c (T −T )=m c (T −T ) (8) a p 2 in w w w 2 For the expander of the scroll CEM, the suctioned gas is the air from the fuel cell stack and with high pressure. Because the gas from the fuel cell stack is with the reacted water, it can also be assumed that the wet air is at the saturation state during the process of expansion. In the working process, it is assumed that the wet air is always at the saturation state and is the same temperature as the water. Hence, the following equations can be obtained

Fig. 4 Effect of rotation speed on the power consumption

dQ=dm r+dT c m (9) w w w w dm =Ddm (10) w g where the heat transfer between air and water contains the latent and sensible heat transfer; Dd is the change of the moisture content and can be obtained from the property of the saturated wet air.

4 RESULTS AND DISCUSSION Considering the guidelines of US DOE for the 50 kW vehicles, the operating conditions used in the simulation are shown in Table 1. The simulation results are shown in Figs 4–8. The scroll CEM was tested on the traditional air compressor experiment rig. The performances of the scroll CEM were tested by varying the speed from 1000 to 4500 r/min with a 500 r/min interval. The power consumed in the compression process, the power recovered in the expansion process, and the real power consumed in the CEM vary with the rotation speed as shown in Fig. 4. It is evident that all powers increase with the rise of the scroll CEM rotation speed. The power produced by the scroll expander is a little more than 50 per cent of that

Fig. 5 Dry air and water vapour flowrates on varying rotation speed

Table 1 Operating conditions

Inlet pressure (bar) Inlet temperature (°C) Rotation speed (r/min) Pressure ratio Dry air flowrate (g/s) Water flowrate (kg/h) Expander ratio

Scroll compressor

Scroll expander

1.0 20.0 5000 3.0 95 30

2.5 80.0 5000

2.5

Fig. 6 Effect of pressure supplied to the fuel cell on power consumption



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Fig. 7 Effect of pressure supplied to the fuel cell on the discharge temperature and the water vapour flowrate in the gas supplied

Fig. 8 Effect of inlet temperature on the CEM power and dry air flowrate

consumed by the scroll compressor; therefore, the power consumed by the air supply system is reduced by about 50 per cent when using the scroll expander. The simulation results are close to the test data. Thus, the accuracy of the simulation model is testified. The air flowrate is very important for the operation of the fuel cell. On the one hand, the wet air flowing through the fuel cell provides the oxidant for the electrochemical conversion of the fuel (hydrogen). On the other hand, the airflow also carries away excess water that is produced by the reaction. If this excess water is not removed, the air ducts in the fuel cell stack are blocked, reducing the available oxygen and lowering the output power of the fuel cell system [11]. Figure 5 shows the flowrates of the dry air and water vapour supplied to the PEM fuel cell system. The flowrates of dry air and water vapour increase with the increase of the rotation speed. The D14304 © IMechE 2006

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percentage of the mean mass of water vapour in the total gas supplied to the fuel cell stack is about 5 per cent. This percentage of mass rises with the increase of the dry air flowrate. The experimental air flowrate is lower than anticipated, especially at low speed. This is because the leakage flow model used in the simulation model is ideal, but does not take into account the two-phase flow, heat transfer, and so on. The effect of all assumptions used in the leakage model caused the difference between experimental data and the simulation results. Figure 6 shows the powers for different pressures. With a higher pressure, the powers consumed by the scroll compressor and produced by the expander increase. However, the relationship between the power consumed by the scroll CEM and the pressure supplied to the fuel cell is not monotonic. At the design point (3.0 bar), the CEM-consumed power has the least value. However, in the conditions of any deviation of the design point, the CEM consumed power rises. In Fig. 6, the scroll expander-produced power is negative at the low pressure, which means that the scroll expander does not produce power but consumes power. When the operating pressure of the fuel cell is less than 3.0 bar, the working process of the scroll compressor is overcompression and the process of the scroll expander is overexpansion. On the other hand, the working processes are undercompression and underexpansion for compressor and expander, respectively, when the pressure is greater than the design point (3.0 bar). The under/over compression and expansion result in a reduction of efficiency of the scroll compressor and expander. Thus, the scroll CEM is less efficient when the operating pressure deviates from the design point. Figure 7 shows the discharge temperature of the scroll compressor and water vapour flowrate under different pressures supplied to the fuel cell. The temperature of the air discharged from the scroll compressor increases when the pressure rises. However, the water vapour flowrate decreases with rising pressure. When the water flowrate is reduced, the rise of the discharge temperature of the scroll compressor is greater. The condition of the atmosphere affects the fuel cell air supply system. Atmospheric pressure changes very little in different regions and seasons. Moreover, such changes have little effect on the fuel cell operating pressure (3.0 bar). However, the atmosphere temperature changes significantly in different regions and seasons. The scroll CEM inlet temperature is related directly to the atmosphere Proc. IMechE Vol. 220 Part D: J. Automobile Engineering


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temperature. Figure 8 shows the changes of the CEM power and dry air flowrate at different inlet temperatures of the scroll CEM. As the scroll CEM inlet temperature rises from 0 to 35° C, the power consumed by the CEM decreases by about 4.5 per cent of the power value at 0 °C. At the same time, the dry air flowrate decreases by 11.4 per cent of the value at 0 °C when the inlet temperature changes from 0 to 35 °C. This is mainly because the volume flow of the scroll CEM is almost fixed under different atmospheric temperatures but the density of the inlet air decreases with the increase of the atmospheric temperature. The reduction of the dry air flowrate has a direct effect on the fuel cell stack performance because the mass flowrate of oxygen decreases in the fuel cell stack. Here, the decrease of the scroll CEM power is mainly caused by the reduction of the dry air flowrate.

5 CONCLUSIONS Considering the leakage and heat transfer, the mathematical model of the scroll CEM used in the automotive fuel cell systems was developed based on thermodynamics theory. The performance of the scroll CEM is simulated in different conditions. The simulated results indicate that the power consumed by the scroll CEM is about 50 per cent of that consumed by the air-supplied system when only using the scroll compressor. However, for different operating pressures, the scroll CEM-consumed power has the least value at the design point (3.0 bar). The power consumed by the CEM decreases by about 4.5 per cent of the power value at 0 °C when the inlet temperature rises from 0 to 35 °C. The dry air flowrate increases with the rise of the CEM rotation speed. The water vapour flowrate decreases when the operating pressure rises. The dry air flowrate decreases by 11.4 per cent of the value at 0 °C when the CEM inlet temperature changes from 0 to 35 °C.

ACKNOWLEDGEMENTS The work described in the paper is funded by the key Research Project Fund of the Chinese Ministry of Education (grant No. 104211), National Natural Science Foundation of China (grant No. 50476054), and the Doctorate Foundation of Xi’an Jiaotong University (DFXJTU 2003-6).

REFERENCES 1 Wright, S. E. Comparison of the theoretical performance potential of fuel cells and heat engines. Renewable Energy, 2004, 29, 179–195. 2 Thirumalai, D. and White, R. E. Steady-state operation of a compressor for a proton exchange membrane fuel cell system. J. Appl. Electrochem., 2000, 30, 551–559. 3 Boettner, D. D., Paganelli, G., and Guezennec, Y. G. Proton exchange membrane fuel cell system model for automotive vehicle simulation and control. J. Energy Resour. Technol., 2002, 124, 20–27. 4 Liansheng, L. Scroll compressor, 1998 (Beijing, China Machine Press, in Chinese). 5 Transportation fuel cell power systems, 2000 annual progress report, US Department of Energy, 2000. 6 Yuanyang, Z., Liansheng, L., Jiang, S., and Pengcheng, S. Research on oil-free air scroll compressor with high speed in 30 kW fuel cell. Appl. Thermal Engng, 2003, 23, 593–603. 7 Dutta, A. K., Yanagisawa, T., and Fukuta, M. An investigation of the performance of a scroll compressor under liquid refrigerant injection. Int. J. Refrig., 2001, 24, 577–587. 8 Lee, G. H. Performance simulation of scroll compressors. Proc. IMechE, Part A, Journal of Power and Energy, 2002, 216, 169–179. 9 Chen, Y., Halm, N. P., and Groll, E. A. Mathematical modeling of scroll compressors–part I: compression process modeling. Int. J. Refrig., 2002, 25, 731–750. 10 Park, Y. C., Kim, Y., and Cho, H. Thermodynamic analysis on the performance of a variable speed scroll compressor with refrigerant injection. Int. J. Refrig., 2002, 25, 1072–1082. 11 Spiegel, R. J., Gilchrist, T., and House, D. E. Fuel cell bus operation at high altitude. Proc. IMechE, Part A, J. Power and Energy, 1999, 213, 57–68.

APPENDIX Notation A c C d m P Q r T V v W

area specific heat flow coefficient moisture content mass pressure quantity of heat exchange latent heat of vaporization temperature volume specific volume power



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e h k

pressure ratio orbiting angle specific heat ratio

Subscripts a c

air control volume

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cr d g i o s w

577

critical point discharge gas flow into the control volume flow out of the control volume suction water



Engineering Engineers, Part D: Journal of Automobile ...

Engineering Engineers, Part D: Journal of Automobile ...

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