1077-2618/11/$26.00©2011 IEEE

Development of zero-emission vehicles

BY TAEHYUNG K IM ,OLEG VODYAKHO,& JEF FERSON YANG

IN THIS ARTICLE, THE

structure and control of a fuel

cell hybrid electric scooter for

mass production are presented.

The development of a zero-emission scooter

can improve air quality and protect the envi-

ronment, especially in Asia and Europe,

where more than 25 million scooters are

produced annually. The generic advan-

tages of low-pressure hydrogen storage

of the fuel-cell-powered scooter are ex-

plained, and the control strategies of the

hybrid scooter are presented. Additionally, experimental verifications and

discussions are presented based on a prototype system controller.

Zero-Emission Vehicles

The development of an internal combustion engine (ICE) is one of the greatest achieve-

ments in modern history. Vehicles based on the ICE have made huge contributions to the

growth of modern society, facilitating mobility in everyday life. However, the growing

number of vehicles has caused serious air pollution, global warming, and depletion of

petroleum resources. The global community faces many challenges to meet the world’s

increasing energy needs while decreasing air pollution [1].

Digital Object Identifier 10.1109/MIAS.2010.939811

Date of publication: 21 January 2011

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In Asia and parts of Europe, a major source of pollutantemission comes from the exhaust of gasoline scooters. Thescooter is a major form of transportation in Asia (such as inChina, Taiwan, and Japan) and has a global annual produc-tion of more than 25 million vehicles. Therefore, the devel-opment of zero-emission scooters is of paramount importancein improving air quality and protecting the environment ofAsia, and in turn, the global community [2].

The fuel cell, first discovered in 1839, is used to gener-ate electricity by reversing the electrolysis of water [3]. Theenvironmental concerns and needs of new energy sourceshave spurred the investigation and development of fuel-cell-powered vehicles.

The fuel cell hybrid electric scooter has an inherentadvantage, compared with the fuel-cell-powered passengercars, by using a low-pressure metal-hydride hydrogen stor-age (MHHS). The MHHS scooter system meets safety, easeof distribution, and user-convenience requirements of thehydrogen infrastructure. Currently, the low-pressure MHHSis the most suitable hydrogen-storage technology for smallvehicles such as scooters.

The development of zero-emission fuel-cell-basedhybrid electric scooter will make a serious contributionin the effort to protect the environment and increaseenergy independence.

Development of Fuel Cell Hybrid ScootersAsia Pacific Fuel Cell Technology, Ltd. (APFCT) hasdeveloped a proton exchange membrane (PEM) fuel-cell-based zero-emission hybrid electric scooter. The PEMfuel cell has the inherent attributes of low temperature,rapid startup, and high power density. Integration withthe safety and convenience of the low-pressure MHHS

forms the fuel-cell-based scooter. The specifications of thedeveloped fuel cell scooter (APFCT’s ZES IV.5), which is effi-cient and mass producible with zero emission, are presentedin Table 1.

APFCT, following the initial proof of a concept fuelcell scooter, ZES I, successfully completed in April 2000,has developed a total of eight fuel cell scooter prototypes[2]. The newest version’s scooter control (ZES V, which isunder development) from the electric side is presented inthis article.

The low-pressure MHHS is ideally suited for an on-board vehicle and mobile generator. APFCT has completedthe development and testing of a low-pressure MHHScanister system and has demonstrated the validity of ahydrogen infrastructure model based on the distributionand exchange of canisters [2]. As briefly alluded earlier, thehydrogen supply to the fuel cell is stored in canisters. Ahydrogen-depleted metal-hydride canister can be easilyrefilled with hydrogen. In addition, the material is com-mercially readily available. Figure 1 shows the ZES IV.5fuel cell scooter and MHHS canister.

For fuel-cell-powered products with MHHS, thehydrogen distribution infrastructure does not present aproblem, as it employs an existing gasoline distributioninfrastructure. For fuel cell scooters, hydrogen can bemade even more convenient and readily available thangasoline, because all existing scooter shops and retailers,indeed even convenient stores, can become hydrogencanister stations. When the fuel is running out, a scooterrider would stop at any gasoline station, scooter shop, orconvenience store, exchanging empty canisters for fullycharged ones for a fee. To the rider, the process can be eas-ier and quicker than filling up with gasoline. Table 2presents the specifications of MHHS.

System Configuration and ControlThe main structure of the fuel cell hybrid electric scooteris illustrated in Figure 2. The main components of theelectrical system are

1) a fuel cell controller2) a motor driver (MD)3) a battery manager (BM)4) a boost converter (BC)5) an ultracapacitor manager (UM).For startup, the rider puts the key switch (main on/off

switch) in the on position. Then BC, BM, UM, and MDmodules are sequentially activated. Next, the hybrid

TABLE 1. SPECIFICATIONS OF ZES IV.5.

Dimensions 1,576 3 440 3 1,087 mm Motor Type BLDC Motor

Weight 113 kg Transmission Timing belt

Nominal voltage 24 V Max. speed 50 km/h

Fuel Hydrogen Climb 10 km/h at 6�

Fuel consumption 1.13 g hydrogen/km Refuel Canister exchange

Range (1) 86.6 km at 30 km/h Range (2) 62.1 km at city mode

Rated output current 60 A Max. output current 100 A (for 30 s)

Rated power 1,300 W Peak power 2,100 W

1(a) (b)

APFCT’s ZES IV.5 (a) fuel cell scooter and (b) MHHS canister.(Photo courtesy of APFCT.)26

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system is ready to drive a brushless dc(BLDC) motor, waiting for the throt-tle command. When the rider turnsoff the main switch, the motor drive isshut off. Subsequently, the UM, BM,and BC are shut off sequentially. Also,the battery is completely discon-nected from any loads to minimizedraining of the battery charge whenthe scooter is not in use. For light-to-medium loads, the fuel cell willprovide all the power to the MDand the system-parasitic loads. Whenthe load increases and the power limitof the fuel cell is reached, the bus volt-age will decrease down to the batteryvoltage. Then the battery will assist. Ifthe load increases further and the battery power limit isreached, the power delivered to theMD is reduced. The soft-ware flow chart of the system on/off procedure is illustratedin Figure 3.

The BC module delivers a regulated constant voltageto the bus. When the fuel cell voltage drops considerablyresulting from an increased load, the BC module balancesthe power supplied by the fuel cell and battery. In normaloperation, for a prototype scooter, the BC boosts the fuelcell voltage up to 55 V on the main bus. If the MD loadincreases more than the fuel cell power limit, the mainbus voltage starts to fall and eventually sinks to the bat-tery voltage. The battery adds current to the main bus tosupply the MD. The BC monitors the fuel cell voltageand computes the current limit to restrict the input power.The BC module configuration is shown in Figure 4. Since

the dc bus voltage is quite low, an isolated BC structure isnot used.

In a normal operation, the BC boosts the fuel cell volt-age up to 55 V. If the motor load increases more thanthe fuel cell power limit, the main bus voltage startsto fall and eventually falls off the battery voltage. Then,the battery adds current to the main bus to supply themotor drive.

When the switch is on, the inductor voltage, VLb of theBC is (ignore the switch voltage drop)

VLb ¼ Vfc (on): (1)

When the switch is off (ignore the diode voltage drop),

VLb ¼ Vfc � Vbus (off ): (2)

Fuel Cell Hybrid Scooter

Fuel Cell andController

dc/dcConverter

Fuel Cell andSupport System

Fuel CellController

BoostConverter

dc/acInverter

VoltageTemperature

Temperature

Current

Battery Management

Li-Ion Battery

Charging andDischarging Circuit

Relay

Battery Manager

Charging andDischarging

Circuit

UltracapacitorManagement

CurrentVoltage

Ultracapacitor Manager

BLDCMotor

Gear

Motor Driver

2System configuration of the fuel cell hybrid scooter.

TABLE 2. SPECIFICATIONS OF MHHS.

Construction Aluminum alloy extruded body

Dimension (mm) Diameter 76.2, Length 365

Safety mechanism Overpressure relief valve protection

Storage material Metal hydride, AB5 alloy

Charge condition full 1 MPa, 30 min, 10–20 �C

Charge condition 90% full 1 MPa, 10 min, 10–20 �C

Discharge 0–16 slpm, 1 atm

Hydrogen content 500–560 SL

Total weight 4.5 kg

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Introducing the duty ratio D, the steady-state dc busvoltage is given as

Vbus ¼Vfc

(1� D): (3)

The BC monitors the fuel cell voltage and sets the cur-rent limit. The BC switching frequency is 20 kHz. Thecontrols consist of an outer-loop proportional-integral

(PI) voltage regulator and an inner-loop PI current regulator.

The software flow chart of the BCmodule is presented in Figure 5.When the main switch is turned onfrom the off position, the BC is acti-vated. The pulsewidth modulation(PWM) duty ratio is initially limitedat no-load situation (when the motorflag is zero) to avoid the unstableoperation because of the discontinu-ous current conduction mode.

Figure 6 illustrates the battery cell(lithium-ion) voltage characteristics.In addition, Table 3 shows the bat-tery cell specifications. The BM mod-ule’s circuit configuration is depictedin Figure 7.

The battery charging current isregulated based on a PI controller.However, for the scooter, the discharg-ing current is not regulated from thebattery dc/dc converter side for controlsimplicity and cost savings. Instead,the load dc current is regulated andlimited not to surpass the battery’smaximum discharging current toprotect the battery (also, the fuel cellcurrent and ultracapacitor currentare regulated).

Under a steep-hill condition, atmaximummotor torque (in a possiblefull throttle situation), the fuel cell,battery, and ultracapacitor (if it wascharged) supply power together. Ifthe ultracapacitor is fully discharged,

the battery should provide the majority of the powerdemand, since the fuel cell has a current transition limita-tion. The motor drive maximum current is limited basedon the worst-case battery discharging scenario to protectthe battery.

When the load increases, the dc bus voltage willdecrease if the fuel cell and ultracapacitor (if it was charged)cannot supply enough current. Once the dc bus voltagebecomes lower than the battery voltage, the battery will

discharge, supplying current togetherwith the fuel cell and ultracapacitor(if it was charged). If the dc bus volt-age is lower than 42 V, the battery isdisconnected through the relay toprotect, and then the motor current(load current) limit is reduced ac-cording to the fuel cell voltage.

When the switch is on, during thecharging, the inductor voltage, VLbm,shown in Figure 7 is

VLbm ¼ Vbus � Vbat: (4)

When the switch is off,

VLbm ¼ �Vbat: (5)

Start

Read Main On/OffSwitch

On?

Motor_Stop_Flag = 0(Reset)

Run BC Module

Run BM Module

Run CAN, RS232Functions

End

Yes

Loop

No

Run UM Module

Run MD Module

Boost_Converter_Flag = 0Battery_Flag = 0

UM_Converter_Flag = 0Motor_Flag = 0

Relay Off

3Flow chart of the system manager module.

Current Sensor

Vfc

Ifc

VbusFuel CellSystem

Lb

VoltageSensor

BusCapacitor

PWM

dc/dc Boost Converter

dc Bus +

dc Bus –

4BC hardware configurations.

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Introducing the duty ratio D, the steady-state voltageequation is given as

D(Vbus � Vbat) ¼ (1� D)Vbat: (6)

Therefore, the battery voltage is

Vbat ¼ DVbus: (7)

If the battery voltage falls below 42 V, the relay of the bat-tery-discharging side, as shown in Figure 7, is openedto protect the battery from deep discharging.

When the dc bus voltage rises above 42 V, the relay canbe closed again. For a 16-cell lithium-ion battery, the dcbus voltage is regulated at 55 V, to charge the battery.When the battery voltage reaches 53 V (almost full chargefor the 16-cell battery), the charge function shuts off.

The MD module controls the motor current with a 20-kHz frequency. The current command is given from thethrottle through an analog-to-digital (A/D) converter.

In the MD module, the regenerative braking operationis implemented. When the driver pushes the brake pedal,the motor restores the energy into the ultracapacitor. If thebattery is being charged, some amount of regenerativeenergy will also be stored into the battery.

During regenerative braking, a dc bus voltage regulatorcontrols the motor current to prevent voltage rising becauseof the reverse current, which charges the ultracapacitorand battery.

The UM module captures the regenerative energy fromthe MD during the braking operation and sends back thecharged energy during motoring operation. Therefore, theultracapacitor voltage will be maintained at around zeroafter sufficient discharging time during the motoring

Start

Main On/OffSwitch Is On?

Boost_Converter_Flag = 1

Fuel cellCurrent > OC_Limit?

Boost_Converter_Flag = 0

Boost_Converter_Flag= 1?

Control Bus Voltage

Control BC Current

Motor_Flag = 1?

Limit Duty Ratio

Generate PWM Pulses

No PWM

End

No

Yes

Yes

Yes

Yes

No

No

Loop

No

5Software flow chart for the BC.

9080706050403020100

2.5 2.7 2.9 3.1 3.3 3.5Battery Voltage (V/Cell)

Sta

te o

f Cha

rge

(%)

100

6Single battery cell (lithium-ion) voltage characteristic.

TABLE 3. BATTERY CELL SPECIFICATIONS.

Nominal voltage 3.3 V

Charging voltage 3.65 V

Cutoff discharge voltage 2.10 V

Max. continuousdischarge current

80 A

Max. 18-s peak current 100 A

Max. charge current 30 A

Energy density 85 Wh/kg

Nominal capacity 10,000 mAh

Lbm

Relay

+ –

BusCapacitor

PWM

Lithium-IonBattery

Battery Charging/Discharging Circuit

dcBus +

dcBus –

7BM hardware configuration.

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operation and will be ready to capture the regenerativebraking energy. When regeneration from the MD occurs,the dc bus voltage will be pumped up and attempt toexceed a commanded voltage. At this point, the UM willdraw current from the main bus and charge the ultracapa-citor. If the regeneration energy is large and the ultracapa-citor is fully charged or the regenerative energy flow rate isgreater than the UM charging rate, the bus voltage will riseabove a maximum limit. Then the voltage controller of theregenerative braking will limit the bus voltage by decreas-ing the motor current (regenerative braking current) com-mand. Figure 8 shows the UMmodule structure.

Experimental Results and DiscussionsThe system control is evaluated and verified based on aprototype controller board and a power electronics boarddeveloped at the University of Michigan-Dearborn (shownin Figure 9). The prototype test-bed consists of MOSFETswitches (100 V, 180 A), power diodes, inductors, buscapacitors, relay, sensor-interfacing circuits, Texas Instru-ments TMS320F2812 DSP, and current-protection circuits(Xilinx CPLD is used). The test bed is oversized to havevarious load tests as a prototype test bench.When the mainswitch is turned on, the BC module function immediatelystarts checking the level of input (fuel cell) voltage. ThePWM duty ratio of the BC is initially limited because ofthe discontinuous current conduction mode in the no-loadsituation. Once the traction load is applied, the limitedduty ratio, given initially, is released.

Figure 10 illustrates the dc bus voltage, which is theoutput of the BC module when the main switch is on. Theinitial maximum peak voltage at the transition is observedaround 85 V and is soon regulated at 55 V.

The BC module is started first when the main switch ison, and then the BM module functions once the bus volt-age is under regulation. As shown in Figure 10, the batterystarts charging after the dc bus voltage is under regulation.

Figure 11 presents the BC module function when themotor load is applied. The motor is accelerated and regu-lated at 3,000 r/min (for this testing, a PI-speed controlleris implemented in the MD software structure only fortesting purposes). The bottom waveform is the BC inputpower from the fuel cell. A power limit (in this case 500W/s) is applied to protect the fuel cell stack from heavy dis-charging. The UM module is controlled based on twoMOSFET switches. A dead band is set to prevent short cir-cuit through the switches.

The regenerative braking module implemented has acurrent controller (PI controller) to limit the regenerative

Li-Ion BatteryUltracapacitor

System Controller BLDC Motor9

Prototype hybrid system controller.

VoltageSensor

LUcap

PWM

Ultracapacitor Charging/Discharging Circuit

dcBus +

dcBus –

VBusPWM

CurrentSensor

IUcap

Ultra-capacitor

8UM configuration.

1

0 V

1) CH1 (60 V, 500 ms)2) CH2 (10 V, 500 ms)

2

10The BC function when the main switch is on. Upper: dc busvoltage [60 V/div]; lower: battery-charging current (ibat)[60 V, 10 A/div; 500 ms/div].

11

1

0 r/min

2

1) CH1 (2,000 r/min, 500 ms)2) CH2 (1,200 W, 500 ms)

Upper: motor speed; lower: input power [2,000 r/min,1,200 W/div; 500 ms/div].

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braking current and a voltage controller to limit thebus voltage.

Figure 12 illustrates the regenerative braking action(transient behavior). The dc bus voltage was regulated at55 V initially, before a braking action.

At the braking, the motor current is reversed to restorethe power to the ultracapacitor. The bus voltage commandfor the regenerative braking is set a little higher than thenormal bus voltage command to limit the bus voltage, pre-venting dangerous voltage jumping due to a fully chargedultracapacitor and battery.

As shown in Figure 12, the dc bus voltage is increasedimmediately after the braking action because of the larger busvoltage command. The motor speed is initially 4,000 r/minand decreases to zero during braking.

The regenerative braking action is shut off when thespeed is lower than a certain threshold number (400 r/minfor the testing). At low speed, the back electromotive forceis very small and, hence, unable to generate meaningfulpower. The mechanical brake is used below 400 r/min.

Figure 13 presents a system behavior during accelerationand braking. The motor speed command is initially zero,increases to 4,000 r/min, and then a braking command isgiven to stop the motor. The dc bus voltage is under regula-tion [Figure 13(a)] during the motoring mode.

For regenerative braking, the dc bus voltage increasesup to 60 V for a short period of time and then returns to55 V after braking. Even though the ultracapacitor is fullycharged, due to a possible long regenerative braking, forexample at a downward slope, the bus voltage will notincrease much more than 60 V because of the voltage con-troller in the regenerative braking module.

The ultracapacitor current waveform [Figure 13(c)]shows the charging/discharging behavior. During braking,the ultracapacitor charging current is increased to chargethe ultracapacitor and goes negative when the braking iscompleted to discharge.

ConclusionsIn this article, the controls of a fuel cell hybrid electricscooter for mass production are presented. The low-pres-sure hydrogen storage, which is readily available, is anadvantage to drivers, from the safety point of view. Theultracapacitor is used to capture the regenerative brakingpower for faster response and to protect the battery. Theexperimental results well verify the control structures ofthe fuel cell hybrid electric scooter.

References[1] M. Ehsani, Y. Gao, S. E. Gay, and A. Emadi, Modern Electric, Hybrid

Electric and Fuel Cell Vehicles. Boca Raton, FL: CRC, 2005.[2] Available: www.apfct.com[3] J. Larminie and A. Dicks, Fuel Cell Systems Explained, 2nd ed. Hobo-

ken, NJ: Wiley, 2003.[4] H.-J. Chiu and L.-W. Lin, “A bidirectional dc-dc converter for fuel cell

electric vehicle driving system,” IEEE Trans. Power Electron., vol. 21,no. 4, pp. 950–958, July 2006.

[5] B. Lin, “Conceptual design and modeling of a fuel cell scooter for urbanAsia,” J. Power Sources, vol. 86, no. 1-2, pp. 202–213, Mar. 2000.

[6] A. Emadi, K. Rajashekara, S. S. Williamson, and S. M. Lukic,“Topological overview of hybrid electric and fuel cell vehicular powersystem architectures and configurations,” IEEE Trans. Veh. Technol., vol. 54,no. 3, pp. 763–770, May 2005.

[7] M. Uzunoglu and M. S. Alam, “Modeling and analysis of an FC/UC hybridvehicular power system using a novel-wavelet-based load sharing algo-rithm,” IEEE Trans. Energy Conv., vol. 23, no. 1, pp. 263–272, Mar. 2008.

[8] P. Thounthong, V. Chunkag, P. Sethakul, B. Davat, and M. Hinaje,“Comparative study of fuel cell vehicle hybridization with battery orsupercapacitor storage device,” IEEE Trans. Veh. Technol., vol. 58, no. 8,pp. 3892–3904, Nov. 2009.

Taehyung Kim (taehyung@umich.edu) is with the Universityof Michigan-Dearborn in Dearborn, Michigan. Oleg Vodyakhois with Florida State University in Florida. Jefferson Yang iswith Asia Pacific Fuel Cell Technologies in Chunan, Taiwan.Kim and Vodyakho are Members of the IEEE. This article firstappeared as “Control of a Fuel Cell Hybrid Electric Scooter” atthe IEEE Energy Conversion Congress and Expo.

12

4,000 r/min

55 V

3

1) CH1 (500 r/min, 100 ms)2) CH2 (5 V, 100 ms)3) CH3 (50 A, 100 ms)

Upper: motor speed [500 r/min/div]; middle: dc bus voltage[5 V/div]; lower: motor phase current (phase A) [50 A/div;100 ms/div].

13

2

1

30 A

1) CH1 (20 V, 2 s)2) CH2 (100 A, 2 s)

3) CH3 (20 A, 2 s)4) CH4 (5,000 r/min, 2 s)

(a)

(b)

(c)

(d)

From top to bottom: (a) dc bus voltage [20 V/div]; (b) motorcurrent [100 A/div]; (c) ultracapacitor current [20 A/div]; (d)motor speed (0fi 4,000 r/min fi 0) [5,000 r/min/div.; 2 s/div].

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