Testing of an automotive fuel cell system

International Journal of Hydrogen Energy 29 (2004) 1001 – 1007

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Testing of an automotive fuel cell system Pucheng Pei∗ , Minggao Ouyang, Qingchun Lu, Haiyan Huang, Xihao Li State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, People’s Republic of China Received 15 August 2003; received in revised form 24 November 2003; accepted 11 January 2004

Abstract This paper is to present a test platform for automotive fuel cell systems and report some test results on this platform. The test platform was developed based on a test bed of internal combustion engine with a dynamometer, the dynamometer acted as both a load and a measurement instrument. A fuel cell engine, a DC/DC converter and an induction traction drive motor with a DC/AC inverter were integrated to a system and were tested in the platform. Test results of one fuel cell system showed that the e6ciency was 41% (LHV) while 50 kW of electrical power is produced in the engine; the cell current density was 400 mA=cm2 when 0:65 V of average cell voltage is obtained in the stacks; the maximum mechanical power of the fuel cell system was 41 kW, and the best speci=c fuel consumption was 102 g=kWh. This test platform is feasible for evaluating all components of fuel cell systems, such as stacks, parasitic powers, engines, DC/DC converters and traction drive motors; and in this platform it is convenient to uncover problems of electromagnetism compatibility in the fuel cell systems before being mounted into vehicles. ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Fuel cell; Automotive fuel cell system; Test platform; Performance

1. Introduction Supported by the National High Technology Development Plan (NHTDP) program fund of China, Tsinghua University qua a head unit developed the =rst fuel cell bus of China, collaborating with 20 more other units of China. Four fuel cell engines, two DC/DC converters, two traction drive motors and others were delivered to Tsinghua University in 2002. Some of the tasks of Tsinghua University were to evaluate all of them and take a fuel cell system to mount the bus. A fuel cell engine is an integration of fuel cell stacks, air system, hydrogen system, coolant system and electric control systems. It can generate electrical power while being fueled with hydrogen and air. When a fuel cell engine is used for a vehicle, the electrical power produced by the fuel cell engine must be converted into mechanical power by

∗ Corresponding author. Tel.: +86-10-62789134; fax: +86-1062785708. E-mail address: [email protected] (P. Pei).

a motor; and a DC/DC converter is often used [1]. There are often many problems of electromagnetism compatibility (EC) among the Electric Control Unit (ECU) of engine, the DC/DC converter, the motors, the electric monitor and so on. The performances of the fuel cell system including the fuel cell engine, the DC/DC converter and the traction drive motor are more important and the EC problems need careful consideration. Many fuel cell test systems use electric loads [2,3], which can be used to test stacks or engines, but cannot be used to test the automotive fuel cell system with a drive motor; and it is impossible for them to uncover the EC problems before the system being mounted into a vehicle. Although all parts of the fuel cell system can stand the tests of EC separately, some EC problems often occur yet when they are integrated. We built a test platform based on a test bed of internal combustion engines with a traditional dynamometer. The electromagnetism conditions in the test platform were similar to that in a vehicle, because all components of the fuel cell system tested is completely the same as those to be mounted on the vehicle. Tests of the above four 50 kW fuel cell engines were carried out in this test platform, and some

0360-3199/$ 30.00 ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.01.008

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P. Pei et al. / International Journal of Hydrogen Energy 29 (2004) 1001 – 1007

test results of one fuel cell engine combined with an induction motor was reported in this paper. Problems of EC in the fuel cell system were solved; this was very necessary for developing fuel cell vehicles.

voltage sensor was 0.6% in range of 0–500 V DC, and the precision of the each current sensor was 0.4% in range of 0–1000 A. A computer data acquisition system was developed to collect all the measured data in the test platform. Fig. 2 is the visual interface. In this visual interface, there were more than thirty variables, and any variable could be chosen to show in a numeral or in a history curve. E6ciencies and powers and hydrogen utilization coe6cient could be also showed. At a steady condition, attack the button symbol of “Meas.”, then data of all variables would be averaged by a constant time, such as 90 s. And the time could be changed in the interface by pressing the button “Setup”. The measured results in mean data could be shown directly in an EXCEL =le form while the layer of “Mean Result ” was opened. All variables could be seen in the layer of “All Variables”. In the interface of “Curves” layer, each black triangle in every block could be opened and any variable could be chosen to show, four variables shown in the top right corner could be shown real-time curves in the left.

2. Test platform Fig. 1 shows the test platform for an automotive fuel cell system. Electrical power produced from the fuel cell engine Jow =rst through a DC/DC converter, then a DC/AC inverter and =nally into a motor before entering a dynamometer as mechanical power. The dynamometer acted as both a load and a measurement instrument. The maximum permitted speed of the dynamometer was 6000 rpm, and it could absorb 70 kW at 1500 rpm. A startup battery was used at the time of starting and after that it was cut oK automatically. A DC source was used when the DC/DC converter and the traction drive motor were tested separately, it could deliver 170–430 V DC. Hydrogen was fed to the fuel cell stacks through a mass Jow meter. The current and the voltage were measured in all the output ports of the stacks, the engine and the DC/DC converter. The voltage from the engine was the same as that from the stacks. The dynamometer, the hydrogen mass Jux meter, and the sensors of current, voltage, pressure, temperature and so on were all calibrated before the experiment began. The hydrogen mass Jux meter precision was 0.5% in range of 0–10 kg=h, the precision of the each

3. Test results From the measured data in this test platform, we can gain the performances of the stacks, the engine, the DC/DC converter, the electric motor and the integration of the automotive fuel cell system. Following are the test results about one automotive fuel cell system.

ENGINE HEAT EXCHANGER

DC Power Source Dynamometer

H2 Compressor

FC STACKS

Converter

Air Emission

Air H2

DC/DC

H2

Mechanical Power DC/ AC

M

DC/AC

BATTERY

m'

Φ T P

Φ T P

I

I

I•U Interface

Fig. 1. Test platform for automotive fuel cell system.

I•U

n•Mc

P. Pei et al. / International Journal of Hydrogen Energy 29 (2004) 1001 – 1007

1003

Fig. 2. Visual interface of the test system.

3.1. Speci6cations of the fuel cell system

Table 1 Overview of the fuel cell system

Table 1 provides the main speci=cations of the automotive PEM fuel cell system.

Parameter

Description

Fuel cell type Nominal operating pressure Fuel cell cooling system Cell operating temperature Hydrogen inlet humidity Air inlet humidity Number of stacks Number of cells per stack Cell active area Fuel cell stacks voltage Air compressor DC/DC output voltage Traction drive motor Fuel source

Direct-H2 PEM 0:25 MPa Water 60–80c◦ 50–100% 100% 4 160 450 cm2 200–320 V DC Screw@up to 4000 rpm 600 V DC Induction@384 V AC Gaseous hydrogen 3–13 MPa

3.2. Performances of the stacks 3.2.1. Average cell polarization curve and power curve From the measured data of stacks output current and voltage and the data of cell active area and cell number, we can gain the average cell voltage, the current density and the stacks power. Fig. 3 shows the curves of the average cell voltage and the stacks’ power to current density of the fuel cell stacks in the engine. It is shown that the current density was about 340 mA=cm2 and the stacks power was 68:1 kW at the average cell voltage of 0:70 V; and at the average cell voltage of 0:65 V, the current density was 400 mA=cm2 and the stacks power was up to 73:6 kW. 3.2.2. Fuel utilization coe:cient A fuel utilization coe6cient can be de=ned as f =

mass of fuel reacted in cell : mass of fuel input to cell

The rate of usage of hydrogen [4] is Pe kg=s H2 usage = 1:05 × 10−8 × Vc

(1)

(2)

or H2 usage = 0:037685I1 N g=h;

(3)

where Pe is the stacks power, W; Vc is the average cell voltage, V; I1 is the stacks output current, A; and N is the number of the cells in series. So, 0:037685I1 N f = 100%; (4) m where m is the mass Jow rate of hydrogen, g/h, it is the real hydrogen feed quality measured by a Jow meter, and it can be called fuel consumption. Fig. 4 shows the curves of fuel consumption and utilization vs. the stacks power. For the reason of constant periodic

100

330

60

0.90

80

300

50

0.80

60

270

40

0.70

40

240

30

210

20

180

10

0.60

Voltage/ V

1.00

20

0

100

200

300

0 500

400

Current density / mA.cm-2

Fig. 3. Average cell voltage vs. current density of the stacks.

4000

80

3000

60

2000

40

1000

20

0 0

10

20

30 40 50 Stacks power /kW

60

70

0

50

100

150 200 Current /A

250

0 300

Fig. 5. V –I plot and power curves of the engine.

0 80

Ratio of parasitic power to stack power /%

100

Fuel utilization /%

5000

150

120

60

100

50

80

40

60

30

40

20

20

10

Engine power /kW

0.50

Fuel consumption /g.h -1

Power /kW

P. Pei et al. / International Journal of Hydrogen Energy 29 (2004) 1001 – 1007

Stacks power / kW

Average cell voltage / V

1004

Fig. 4. Fuel consumption and utilization of the stacks. 0

0 0

time of fuel discharge to atmosphere from the purge circuit in the engine, the fuel utilization coe6cient was lower at low power condition. It was necessary to change the discharge periodic time according with output power of the stacks. The curve of fuel consumption vs. stacks power was linear approximately for the stacks. Formula (4) can be changed to m =

0:03768 Pe g=h: f Vc

(5)

In the stacks the product of f multiplied to Vc was about a constant of 0.69. As seen in Fig. 4 the relation between the fuel consumption and the stacks power was m = 0:0546Pe g=h for this engine.

20

40

60

80

Stack power /kW

Fig. 6. Parasitic power and engine power vs. the stack power.

3.3. Performances of the engine 3.3.1. V–I plot and power curves Fig. 5 shows the V –I plot and power curves of the engine. The maximum power of the engine was up to 54:8 kW while the average cell voltage of 0:65 V, and the engine output voltage was 210 V DC. When the engine output voltage was 225 V DC, the average cell voltage was 0:7 V, the engine output electrical power was 51:5 kW.

P. Pei et al. / International Journal of Hydrogen Energy 29 (2004) 1001 – 1007 95

1005

60 fuel consumption efficiency(LVH) efficiency(HHV)

50

85

40

80

30

75

20

Efficiency / %

Fig. 9. E6ciency map for the induction motor with a DC/AC inverter. 70

10

65

0

300

0

10

20

30

40

50

50

fuel consumption efficiency(LVH) efficiency(HHV)

60

Engine power /kW 260

40

Motor speed 1500rpm Specific fuel consumption/ (g/kW.h)

Fig. 7. Fuel economic performance of the engine.

220

30

180

20

140

10

100

Efficiency / %

Specific fuel consumption/ (g/kW.h)

90

0 0

10

20 30 System power /kW

40

50

Fig. 10. Fuel economic performance of the fuel cell system.

performance to decrease the power consumption in the air compressor. Fig. 8. DC/DC converter performance.

The diKerence between the stacks power and the engine power is the parasitic power. The parasitic power was used for the air compressor and some pumps in the engine, and the air compressor was the main consumer. Fig. 6 shows that the parasitic power was 24–36% of the stacks power. When the engine power was 50 kW, the parasitic power was 16:2 kW; and when the engine reached the maximum power of 54:8 kW, the parasitic power was 18:8 kW, about 25.5% of the stacks power. It is the key to improve the engine

3.3.2. Fuel economic performances The fuel cell engine electrical e6ciency is given by I2 V2 en =  100%; (6) m Rhf where I2 and V2 are the output current and the output voltage of the engine, A and V; Rhf is the enthalpy change from hydrogen reacting with oxygen to water, kJ/g. Rhf is 120:9 kJ=g for the lower heating value (LHV), and 142:9 kJ=g for the higher heating value (HHV) [4]. The speci=c fuel consumption of the engine is de=ned as ge; en =

1000m I2 · V2

g=kWh:

(7)

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P. Pei et al. / International Journal of Hydrogen Energy 29 (2004) 1001 – 1007

Hydrogen Energy(in LHV) (100EU)

Stack efficiency (57%)

Engine efficiency (41%) DC/DC converter efficiency(97.8%)

Stacks Power 66.2kW (57EU)

Engine Power 50kW (41EU)

Heat loss (43EU)

DC/AC inverter and motor efficiency(72%)

Parasitic power loss (16EU) Mechanical Power 35.4kW (28.9EU)

DC/DC converter loss (0.8EU) DC/AC inverter and motor loss (11.3EU)

Integration efficiency (28.9%) Note: EU------Energy Unit Fig. 11. Energy distribution in the fuel cell system (Engine power 50 kW, motor speed 1500 rpm).

It is shown that the speci=c fuel consumption is unrelated with the fuel heating value. The fuel economic performances of the engine are showed in Fig. 7, including the engine e6ciency and the speci=c fuel consumption curves. The highest e6ciency was 42.8% in LHV and 39% in HHV in a middle load operation. The lowest speci=c fuel consumption was 70 g=kWh. The engine economic performance was better in part load conditions. From this =gure, the power of 50 kW could be taken as the rated power of the engine. The e6ciency was 41.0% (LHV) and 37.3% (HHV), and the speci=c fuel consumption was about 73 g=kWh at the rated power condition. 3.4. DC/DC converter e:ciency The DC/DC converter e6ciency (co ) is the ratio of output power to input power. The input voltage was between 170 and 430 V DC, and the output voltage was 600 V DC. Fig. 8 shows the maximum e6ciency was 98% while the

input power was 60 kW in this converter. When the input power was 50 kW, the engine rated power, the converter e6ciency was 97.8%. 3.5. Performances of the traction drive induction motor with a DC/AC inverter Fig. 9 gives the e6ciency map for the traction drive induction motor with a DC/AC inverter. The e6ciency is calculated by the following formula: nMe mo = 100%; (8) 9:55I3 V3 where I3 and V3 are the input current and voltage to the inverter, A and V; n is the rotate speed of the motor, rpm; Me is the output torque from the induction motor, Nm. The variables of n and Me can be measured by the dynamometer. It can be seen in Fig. 9 that the higher e6ciency was in the higher power conditions [4]. The maximum e6ciency was 85%, but this e6ciency was only obtained for a narrow

P. Pei et al. / International Journal of Hydrogen Energy 29 (2004) 1001 – 1007

range of conditions. The e6ciency was above 80% in the large range between 50 and 80 kW of the output power. (2)

3.6. Performances of the automotive fuel cell system The speci=c fuel consumption of the integration of this automotive fuel cell system is de=ned as 9550m ge; sy = g=kWh: (9) nMe

(3) (4)

The e6ciency of the system is sy = en co mo

(10)

or sy =

nMe 100%: 9:55m Rh f

(11)

Fig. 10 shows the performances of the fuel cell system. The maximum mechanical power was 41 kW. The best speci=c fuel consumption was 102 g=kWh, and the consumption curve was a hook shape, being similar to that of a gasoline engine. The system e6ciency was higher in high power conditions, diKerent from the curve of the fuel cell engine. At the rated power of the engine, the output mechanical power of the system was 35:4 kW, and the e6ciency was 28.9% in LHV and 26.3% in HHV at the motor speed of 1500 rpm. Fig. 11 shows the energy distribution in the fuel cell system at the engine rated power of 50 kW and the motor speed of 1500 rpm. The parasitic power and the power loss in the traction drive motor were the key factors to the e6ciency of the fuel cell system for the certain fuel cell stacks. It was very important to improve the traction drive induction motor or displace by other high e6ciency motor types. 4. Conclusions An automotive fuel cell system test platform was developed in order to evaluate all components of the fuel cell systems delivered separately by several companies of China. Tests were carried out in this platform, and some test results about one fuel cell system were reported in this paper. The following conclusions were drawn from the test: (1) The fuel cell engine could produce 50 kW of electrical power and 41% (LHV) of e6ciency at rated power

(5) (6)

1007

condition. The engine could reach 70 g=kWh of the lowest speci=c fuel consumption and 42.8% (LHV) of the highest e6ciency at part load. The cell current density was 400 mA=cm2 and the stacks power was up to 73:6 kW at the average cell voltage of 0:65 V. The e6ciency of the induction motor was higher in higher power conditions. The maximum mechanical power of the fuel cell system was 41 kW, and the best speci=c fuel consumption was 102 g=kWh. At conditions of the rated power of the engine, the system output mechanical power was 35:4 kW, and the e6ciency was 28.9% (LHV) or 26.3% (HHV). In the hydrogen purge circle of a fuel cell engine, it is necessary to change the discharge periodic time according with output power of the stacks. The higher e6ciency region of a fuel cell system maybe not consistent with that of the fuel cell engine, so it is important to match the engine and the motor in the system.

Acknowledgements Postdoctoral research members Mingji Liu, Dawei Gao, Languang Lu and Ximing Cheng took parts in this work. And some members shared the test work also coming from the companies delivering the fuel cell engine, the DC/DC convert, the DC/AC inverter, the motor and others.

References [1] Ogburn M, Nelson DJ, Luttrell W, et al. Systems integration and performance issues in a fuel cell hybrid electric vehicle. Fuel cell power for transportation 2000, SAE sp-1505. 2000. p. 125–37. [2] Yang JC, Park YS, Seo SH, et al. Development of a 50 kW PAFC power generation system. J Power Sources 2002;106: 68–75. [3] Johnson R, Morgan C, Witmer D, et al. Performance of a proton exchange membrane fuel cell stack. Int J Hydrogen Energy 2001;26:879–87. [4] Lariminie J, Dicks A. Fuel cell systems explained. England, Wiley. [Reprinted January 2002].