supercapacitor hybrid power source for fuel cell hybrid electric vehicles

supercapacitor hybrid power source for fuel cell hybrid electric vehicles

Accepted Manuscript Novel fuel cell/battery/supercapacitor hybrid power source for fuel cell hybrid electric vehicles Hassan Fathabadi PII: S0360-54...

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Accepted Manuscript Novel fuel cell/battery/supercapacitor hybrid power source for fuel cell hybrid electric vehicles

Hassan Fathabadi PII:

S0360-5442(17)31812-1

DOI:

10.1016/j.energy.2017.10.107

Reference:

EGY 11751

To appear in:

Energy

Received Date:

27 May 2017

Revised Date:

21 October 2017

Accepted Date:

23 October 2017

Please cite this article as: Hassan Fathabadi, Novel fuel cell/battery/supercapacitor hybrid power source for fuel cell hybrid electric vehicles, Energy (2017), doi: 10.1016/j.energy.2017.10.107

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Highlights  Novel PEMFC/battery/SC hybrid power source proposed to be utilized in FCHEVs  Higher efficiency (96.2%) compared to the state-of-the-art power sources of FCHEVs  Highly accurate DC-link voltage regulation  Providing higher speed (161km/h) compared to the state-of-the-art power sources  Providing 0-100 km/h acceleration in 12.2 sec and significant cruising range (545km)

ACCEPTED MANUSCRIPT

Novel fuel cell/battery/supercapacitor hybrid power source for fuel cell hybrid electric vehicles Hassan Fathabadi School of Electrical and Computer Engineering, National Technical University of Athens (NTUA), Athens, Greece. Email: [email protected]

Abstract A fuel cell hybrid electric vehicle (FCHEV) is more advantageous compared to a gasoline-powered internal combustion engine based vehicle or a traditional hybrid electric vehicle (HEV) because of using only one electric motor instead of an internal combustion engine or an electric motor in combination with an internal combustion engine. This study proposes a novel fuel cell (FC)/battery/ supercapacitor (SC) hybrid power source to be utilized in FCHEVs. The power source includes a 90 kW proton exchange membrane fuel cell (PEMFC) stack used as the main power source and a 19.2 kW Lithium (Li)-ion battery together with a 600 F SC bank used as the auxiliary energy storage devices. A prototype of the FC/battery/SC hybrid power source has been constructed, and experimental verifications are presented that explicitly substantiate having a power efficiency of 96.2% around the rated power, highly accurate DC-link voltage regulation and producing an appropriate three-phase stator current for the traction motor by using PWM technique are the main contributions of this work. Providing a maximum speed of 161 km/h, 0-100 km/h acceleration in 12.2 sec and a cruising range of 545 km are the other advantages. The proposed FC/battery/SC hybrid power source is also compared to the state of the art of all kinds of power sources used in FCHEVs and reported in the literature that clearly demonstrates its better performance such as higher speed and acceleration.

Keywords Fuel cell hybrid electric vehicle; fuel cell; Lithium- ion battery; supercapacitor; power source. 1

ACCEPTED MANUSCRIPT Nomenclature C

Capacitance occurring between the anode and cathode of the fuel cell (F).

Cstack

Total capacitance of fuel cell stack (F).

C1

Parasitic capacitance of the N-MOSFET switch of the converter connected to the PEMFC stack (F).

C2

Secondary-side serial capacitor of the converter connected to the PEMFC stack (F).

C dc

DC-link capacitor (F).

C fc

Input capacitor of the converter connected to the PEMFC stack (F).

Cout

Output capacitor of the converter connected to the PEMFC stack (F).

D

Diffusion constant ( m 2 s 1 ).

D1 & D2

Diodes of the converter connected to the PEMFC stack.

Dbat char

Duty cycle of the switching pulse supplied to the converter connected to the Li-ion battery in charging mode.

Dbat disc

Duty cycle of the switching pulse supplied to the converter connected to the Li-ion battery in discharging mode.

Dscchar

Duty cycle of the switching pulse supplied to the converter connected to the SC bank in charging mode.

Dsc disc

Duty cycle of the switching pulse supplied to the converter connected to the SC bank in discharging mode.

D fc

Duty cycle of the converter connected to the PEMFC stack.

fs

Constant switching frequency of the converter connected to the PEMFC stack (Hz).

F

Faraday’s constant (96485.3 C mol1 ).

2

ACCEPTED MANUSCRIPT i (t )

Fuel cell current (A).

i0

Exchange current (A).

iL

Limiting current (A).

I bat

Li-ion battery output current (A).

I sc

SC bank output current (A).

I fc

PEMFC stack output current (A).

I load

Load current supplied to the three-phase inverter and traction motor (A).

Llk1

Primary-side leakage inductor of the transformer of the converter connected to the PEMFC stack (H).

Llk 2

Secondary-side leakage inductor of the transformer of the converter connected to the PEMFC stack (H). Equivalent magnetizing inductor of the transformer of the converter connected to the

Lm

PEMFC stack (H). N2

N1

Turns ratio of the transformer of the converter connected to the PEMFC stack.

n

Number of electrons involved in the fuel cell reaction.

N cell

Fuel cells number in the FC stack.

Pbat char

Charging power of the Li-ion battery (W).

Pbat disc

Discharging power of the Li-ion battery (W).

Pfc

PEMFC stack output power (W).

Pscchar

Charging power of the SC bank (W).

Psc disc

Discharging power of the SC bank (W).

3

ACCEPTED MANUSCRIPT

Pload

Total electric power supplied to the three-phase inverter and traction motor (W).

R

Gas constant (8.314 J mol1K 1 ).

Ract

Activation resistance associated with  act (  ).

Rconc

Concentration resistance of the fuel cell associated with  conc (  ).

Rbat esr

Equivalent series resistance (ESR) of the Li-ion battery (  ).

Resr

ESR of the DC-link capacitor (  ).

Rsc esr

ESR of the SC bank (  ).

Ri

Internal resistance of the fuel cell (  ).

Rin

Input resistance of the converter connected to the PEMFC stack (  ).

RL

Equivalent load resistance observed from the output terminal of the converter connected to the PEMFC stack (  ).

Rload

Load resistance connected to the fuel cell (  ).

R Lbat

Resistance of the inductance Lbat of the bidirectional boost-buck converter connected to the Li-ion battery (  ).

RLsc

Resistance of the inductance Lsc of the bidirectional boost-buck converter connected to the supercapacitor bank (  ).

Rohm

Ohmic resistance of the fuel cell associated with  ohm (  ).

S fc

N-MOSFET switch used in the converter connected to the PEMFC stack.

ti

Transference number.

T

Fuel cell temperature (K).

Ts

Switching period of the converter connected to the PEMFC stack (sec).

4

ACCEPTED MANUSCRIPT

Tsc

Switching period of the control signal supplied to the converter connected to the SC bank (sec).

Vbat

Li-ion battery output voltage (V).

Vdc

DC-link voltage (V).

V fc (t )

FC stack output voltage (V).

Voc (t )  E cell Open-circuit voltage of the fuel cell (V). Vout (t )

Fuel cell voltage under load (V).

Vsc

SC bank output voltage (V).



Activity coefficient; 0    1 .

 conc

Concentration polarization of the fuel cell (V).

 act

Activation polarization of the fuel cell (V).

 ohm

Ohmic polarization of the fuel cell (V).



Equivalent conductance of reacting ion ( m 2 Ω 1 equiv 1 or m 2 Ω 1 mol1 ).

1. Introduction Nowadays, environmental problems and economic considerations cause an upward trend in developing electric vehicles (EVs) rather than the vehicles with internal combustion engines [1]. In particular, a traditional HEV consists of an internal combustion engine used as the main power source and an auxiliary energy storage device with the capability of storing energy such as a battery. The auxiliary energy storage device is mainly used to extend the cruising range of the vehicle, to provide the extra energy needed whenever the vehicle accelerates, and to store the regenerative energy produced during braking. A FCHEV is a type of HEV that utilizes a FC stack as the main power source combined with a SC and/or battery used as the auxiliary energy storage device to power the vehicle’s traction motor 5

ACCEPTED MANUSCRIPT which is an electric motor, not an internal combustion engine [2]. A FCHEV is more advantageous compared to a traditional HEV or an internal combustion engine based vehicle because as mentioned it uses an electric motor instead of a gasoline-powered internal combustion engine, so it not only satisfies the environmental issues but also is more efficient [3-4]. It is reminded that even a plug-in hybrid electric vehicle (PHEV) uses an internal combustion engine to extend its cruising range [5], and to produce the electric power needed to be supplied to the vehicle’s electric motor when the level of the vehicle’s battery becomes low and gets to a predetermined state of charge (SOC) [6]. A FC stack produces electric power through the chemical reaction basically occurs in the presence of hydrogen, oxygen and an electrolyte. Compared to an internal combustion engine based power source, relatively higher efficiency [7], lower pollution, the usage of clean energy resources with lower price such as methanol [8], and being appropriate for various industrial applications [9] such as distributed power generations [10] and vehicular systems [11-12] are some benefits of utilizing FC stacks. Among the different FC systems available in the market, the PEMFC technology is more appropriate to be utilized in vehicles because of higher density in electric power production along with lower heat generation causing a lower temperature which is a necessity in a vehicle equipped with a FC stack [13-14]. The first drawback of utilizing a FC stack in a vehicle is that, the FC stack cannot provide appropriate responses to sudden variations in the load demand of the vehicle [15-17]. For instance, the FC stack cannot efficiently respond to the sudden upward and downward powers needed respectively during accelerating and decelerating, or the considerable initial electric power required to start up the vehicle [18-19]. The second drawback is that the FC stack cannot store the regenerative power produced during decelerating and braking, so an extra energy storage device such as a rechargeable battery or SC bank is also needed [20-21]. The two above-mentioned drawbacks demonstrate that an additional device with a suitable storage capacity and high-speed dynamic response should be utilized as an auxiliary energy storage device along with the FC stack.

6

ACCEPTED MANUSCRIPT Because of the advantages of FCHEVs explained in detail, this study focuses on this type of HEV, and proposes a novel FC/battery/SC hybrid power source to be utilized in FCHEVs. The power source includes a 90 kW PEMFC stack used as the main power source and a 19.2 kW Li-ion battery together with a 600 F SC bank used as the auxiliary energy storage devices. A prototype of the FC/battery/SC hybrid power source has been built, and experimental verifications are presented that demonstrate having a power efficiency of more than 96% around the rated power, highly accurate DC-link voltage regulation, and producing an appropriate three-phase current using pulse-width modulation (PWM) technique which is supplied to the traction motor are some contributions of this work. The power source presented in this study is also compared to the state-of-the-art power sources used in FCHEVs that demonstrates the better capability of the proposed power source. The rest of this paper is organized as follows. The behavior of a FC stack in response to sudden variations in load demand is presented in Section 2. The proposed FC/battery/SC hybrid power source is designed and implemented in Section 3. Details about the constructed power source, experimental verifications and comparing the proposed power source to the state-of-the-art power sources used in FCHEVs are given in Section 4. Finally, the paper is concluded in Section 5.

2. Behavior of a FC stack in response to sudden variations in load demand Fig. 1 shows a simple schematic diagram of a typical FCHEV in which a PEMFC stack has been used as the main power source to supply the power needs of the FCHEV, in particular, its traction motor which is an electric motor. Electric power is produced in a PEMFC via a set of the chemical reactions which can be expressed and summarized as [22]:

At Anode :

2H 2  4H   4e 

At Cathode : O 2  4H   4e   2H 2 O

(1)

 Over all : 2H 2  O 2  2H 2 O

7

ACCEPTED MANUSCRIPT At steady state, the output voltage of a FC is expressed as [23]:

Vout (t )  E cell   conc   act   ohm  Ri i (t )

(2)

where Ri represents the internal resistance of the fuel cell consisting of the resistances of the electrolyte, anode, cathode and any diaphragms available between the anode and cathode that the electrons pass through them. Concentration polarization  conc is caused by changing in the concentration gradients at the surface of the two electrodes resulting from the continuous chemical reactions, and activation polarization  act is due to the slowness of the individual chemical reactions performed at the two electrodes. Ohmic polarization  ohm shows the voltage drop caused by the change in specific conductivity resulted from the electro-chemical reactions occurring inside the fuel cell. To formulize the three mentioned polarizations, the activation polarization  act of a fuel cell is expressed using Tofel equation as [24]:

 act 

RT RT ln(i0 )  2.303 ln[i (t )]  nF  nF

(3)

The concentration polarization  conc of the same fuel cell can be formulized as:

 conc 

RT  i L  i (t )  ln   n F  iL 

(4)

Similarly, the ohmic polarization  ohm of the fuel cell is obtained as:

 ohm 

 i  i (t )  nFD ln  L   (1  t i )  i L 

(5)

Using linearization at a given fuel cell current such as i (t )  in and by defining the three resistances

Ract (T ) , Rconc (T ) and Rohm (T ) , Eqs. (3)-(5) are rewritten as:

   conc  conc i (t )  Rconc (T ) i (t ) Eq. i i(6)  Substituting in in Eq. (2) results that:   act  i (t )  Ract (T ) i (t )  act   i  i in     ohm i (t )  Rohm (T ) i (t )  ohm  i i i n 

(6)

8

ACCEPTED MANUSCRIPT Vout (t )  E cell  Rconc (T ) i (t )  Ract (T ) i (t )  Rohm (T ) i (t )  Ri i (t )

(7)

Noting Eq. (7) and adding the effect of the electric field appearing between the two electrodes of the FC (cathode and anode), a FC can be modeled as the electric circuit shown in Fig. 2, where the capacitor C with a large capacity at the level of 1-9 Farad represents the electric capacity resulted from the electric field occurring between the cathode and anode. The model is in fact a first-order circuit with the time constant of  oc  ( Rconc  Ract  Rohm ) C . As shown in Fig. 3, under loading condition, the electric power produced by the FC is supplied to a load ( Rload ), so the time constant is expressed as:



( Rconc  Ract  Rohm )( Ri  Rload ) C Rconc  Ract  Rohm  Rload  Ri

(8)

Eq. (8) demonstrates that a time delay of 2.2  appears when the FC responds to sudden variations in load demand. In industrial applications, a FC stack with the required rated power and voltage is constructed by connecting in series an appropriate number ( N cell ) of FCs, so under steady state condition, the output voltage of a FC stack is found by using Eq. (7) as:

V fc (t )  N cell Vout (t )  N cell E cell  N cell Rconc (T ) i (t )  N cell Ract (T ) i (t )  N cell Rohm (T ) i (t )  N cell Ri i (t )

(9)

On the one hand, Eq. (1) substantiates that the electric power production in a PEMFC stack is resulted from a set of the low-speed chemical reactions. On the other hand, the time constant (  ) of a FC given in Eq. (8), and hence, the associated time delay ( 2.2  ) occurring in the response of a FC to sudden variations in load demand explicitly demonstrates that a FC stack is actually a power source with a low-speed dynamic response. To provide experimental evidence, a PEMFC stack H-1000 manufactured by Horizon company has been used [25], the dynamic response of the stack obtained experimentally by varying the resistive load connected to the stack is shown in Fig. 4. The dynamic response simulated by using the proposed model depicted in Fig. 2 is also shown as blue curve in Fig. 9

ACCEPTED MANUSCRIPT 4. The model parameters obtained from Eq. (6) at the stack operating point ( in  35 A , T=300 K) and the technical specification of the H-1000 PEM fuel cell stack reported in its user manual [25] are presented in Table 1. The experimental and simulation results shown in Fig. 4 explicitly verify that a FC stack really has a low-speed dynamic response. This point demonstrates that a PEMFC stack cannot respond to sudden variations in the load demand of an EV such as sudden upward and downward powers needed during transient states such as acceleration and deceleration, or the considerable initial electric power required to start up the EV. As another important point, it is reminded that a FC stack cannot store the regenerative power produced during decelerating and braking, so an additional device such as a rechargeable battery or a SC bank with appropriate capacity is also needed. It can be summarized that an additional device with high-speed dynamic response is also needed to be utilized as an auxiliary power source in a FCHEV to assist the FC stack in providing effective responses to different instant load demands and also saving the regenerative power.

3. Implementation of the FC/battery/SC hybrid power source proposed for FCHEVs The configuration of the FC/battery/SC hybrid power source proposed to be utilized in FCHEVs is shown in Fig. 5. It is composed of a PEMFC stack used as the main power source, a Li-ion battery together with a SC bank used as the auxiliary energy storage devices, a unidirectional DC/DC boost converter connected to the PEMFC stack, two similar bidirectional DC/DC boost-buck converters connected to the battery and SC bank, a three-phase bidirectional PWM DC/AC inverter connected the traction motor which is a three-phase induction motor, in practice, a three-phase permanent magnet synchronous motor (PMSM), and a power control unit. The DC-link voltage is continually regulated to a specific constant value by the power control unit. Fig. 6 shows the electric circuit of the unidirectional DC/DC boost converter connected to the PEMFC stack. The average power efficiency of the converter is 98% and its gain is given as [26]:

10

ACCEPTED MANUSCRIPT

Vdc n  V fc 1  D fc

(10)

The DC-link voltage can be obtained from Eq. (10) as:

Vdc  (

n ) V fc 1  D fc

(11)

Eq. (11) demonstrates that when the PEMFC stack output voltage varies over time the DC-link voltage can be continuously regulated to a specific value by changing the duty cycle D fc . When D fc is changed by the power control unit to regulate the DC-link voltage, the available output power of the PEMFC stack also varies in accordance with the following power equation: Pfc  V fc I fc 

V fc 2 Rin



V fc 2 (1  D fc ) 2 n2

(12) RL

where Rin and RL introduced in Nomenclature section are shown in Figs. 5-6. As shown in Fig. 5, the PEMFC stack output current and voltage, and hence, the stack output power is continuously measured by the power control unit. The electric circuit of the bidirectional DC/DC boost-buck converter with the average power efficiency of 90% connected to the SC bank is shown in Fig. 7. The discharging power of the SC bank is given as:

Pscdisc  Vsc I sc

(13)

The SC bank output voltage and current ( Vsc and I sc ), and hence, the discharging and charging powers of the SC bank are continuously measured by the power control unit as shown in Fig. 5. Noting Fig. 7 demonstrates that in discharging mode, the converter operates as a boost converter, and the discharging power of the SC bank is expressed as:

11

ACCEPTED MANUSCRIPT

Psc disc

 1  ) Vsc  Vdc (  1  Dsc disc 1  1   ( ) Vsc  0.9  1  Dsc disc 0.001  Resr    

     

(14)

where Dsc disc is the duty cycle of the control signal supplied to the gate of the insulated gate bipolar transistor (IGBT) Q1 (SC discharge switch) as shown in Fig. 7. It is deduced from Eq. (14) that the discharging power can be regulated to a required rated power by varying the duty cycle Dsc disc . In charging mode, the direction of the SC bank current becomes reverse, the converter operates as a buck converter, and the charging power is found as:

 Dsc char Vdc  Vsc Psc char  Dsc char Vdc   0.001  Rsc esr  R L sc 

   

(15)

where Dscchar is the duty cycle of the switching pulse supplied to the gate of the IGBT Q2 (SC charge switch) as shown in Fig. 7. Thus, the charging power is regulated to a required power rate by varying the duty cycle Dscchar . The discharging power of the Li-ion battery is obtained as:

Pbat disc  Vbat I bat

(16)

Similar to SC bank, a bidirectional DC/DC boost-buck converter has been also connected to the Li-ion battery, so similar to the SC bank, the battery discharging power is expressed as:

Pbat disc

 1  ) Vbat  Vdc (  1  Dbat disc 1  1   ( ) Vbat  0.9  1  Dbat disc 0.001  Resr    

     

(17)

Similarly, the battery charging power is given as:

 Dbat char Vdc  Vbat Pbat char  Dbat char Vdc   0.001  Rbat esr  R L bat 

   

(18)

12

ACCEPTED MANUSCRIPT

As shown in Fig. 5, the power control unit also measures the DC-link voltage ( Vdc ) and load current (

I load ), and then computes the total electric power ( Pload ) supplied to the three-phase bidirectional PWM DC/AC inverter connected to the three-phase traction motor as:

Pload  Vdc I load

(19)

The power control in the FC/battery/SC hybrid power source proposed for FCHEVs is performed by the power control unit as below: Case 1 (charging mode): If 0.98 Pfc  Pload , then the power control sets the Li-ion battery and SC bank in charging mode by activating the switching pulses with the duty cycles Dbat char and Dscchar as shown in Figs. 5 and 7. In this case, the power balance in the hybrid power source is expressed as: 0.98 Pfc  Pload 

1 Psc char 0.9

(20)

So, the electric power consumed to charge the SC bank is given as:



Psc char  0.9 0.98 Pfc  Pload



(21)

It is derived from comparing Eq. (10) with Eq. (16) that:

 Dsc char Vdc  Vsc Dsc char Vdc   0.001  Rsc esr  R L sc 

   0.9 0.98 Pfc  Pload  





(22)

Eq. (22) explicitly demonstrates that in this case (charging mode), the power control unit measures Pfc and Pload , and then regulates the charging power of the SC bank to the required amount specified in Eq. (21) by varying Dscchar . When the charging current of the SC bank reaches below 0.5 A, the SC bank becomes fully charged. Then, the control unit stops charging the SC bank, and sets the Li-ion battery in charging mode, so the power balance is expressed as: 0.98 Pfc  Pload 

1 Pbat char 0.9

(23)

The electric power required to charge the battery is obtained as: 13

ACCEPTED MANUSCRIPT



Pbat char  0.9 0.98 Pfc  Pload



(24)

It is deduced from substituting Eq. (18) in Eq. (24) that:

 Dbat char Vdc  Vbat Dbat char Vdc   0.001  Rbat esr  R L bat 

   0.9 0.98 Pfc  Pload  





(25)

Similar to the SC bank, Eq. (25) demonstrates that the charging power of the battery is regulated to the amount demanded in Eq. (24) by varying Dbat char . Case 2 (discharging mode): If 0.98 Pfc  Pload , then the power control unit sets the SC bank in discharging mode by activating the duty cycle Ddisc as shown in Fig. 5 to provide the additional electric power needed. In this case, the power balance in the hybrid power source is expressed as:

0.98 Pfc  0.9 Psc disc  Pload

(26)

In this case, the amount of the supplementary electric power that should be provided by discharging the SC bank is found from Eq. (26) as: Psc disc 



1 Pload  0.98 Pfc 0.9



(27)

By replacing Psc disc from Eq. (14) in Eq. (27), it is found that: 1  ) Vsc  Vdc ( 1  Dsc disc 1  ( ) Vsc  1  Dsc disc 0.001  Resr  

    P  0.98 P fc  load  

14

(28)

ACCEPTED MANUSCRIPT

It is deduced from Eq. (28) that in this case (discharging mode), the power control unit measures Pfc and Pload , and then regulates the discharging power of the SC bank to the amount demanded in Eq. (27) by varying Dsc disc . When the discharging current of the SC bank reaches below 0.5 A, the SC bank becomes fully discharged, the control unit stops discharging the SC bank, and sets the Li-ion battery in discharging mode, so the power balance is expressed as:

0.98 Pfc  0.9 Pbat disc  Pload

(29)

The supplementary electric power that should be provided by discharging the battery is found from Eq. (29) as: Pbat disc 



1 Pload  0.98 Pfc 0.9



(30)

By substituting Pbat disc from Eq. (17) in Eq. (30), it is found that: 1  ) Vbat  Vdc ( 1  Dbat disc 1  ( ) Vbat  1  Dbat disc 0.001  Resr  

    P  0.98 P fc  load  

(31)

It is deduced from Eq. (31) that the power control unit measures Pfc and Pload , and regulates the discharging power of the Li-ion battery to the required amount specified in Eq. (30) by varying Dbat disc . The electric circuit of the proposed three-phase bidirectional PWM six-switch DC/AC inverter connected the traction motor is shown in Fig. 8. It comprises the six IGBTs that convert the DC-link voltage into a three-phase three-level PWM AC voltage supplied to the stator of the traction motor which is in practice a three-phase PMSM. Since the traction motor acts as a three-phase inductive load, the current delivered to the stator is the integral of the three-level PWM AC voltage, and so the current waveform is close to a sinusoidal form. As shown in Fig. 8, each IGBT itself includes an emitter-tocollector connected diode, the six diodes operate as a three-phase rectifier to convert the three-phase 15

ACCEPTED MANUSCRIPT AC voltage resulted from the regenerative power produced by the traction motor during decelerating and braking into the DC-link voltage which is used to charge the SC bank and battery. Thus, the proposed three-phase PWM six-switch DC/AC inverter is a bidirectional inverter.

4. Construction of the FC/battery/SC hybrid power source and experimental verifications The detailed specifications of all the components used to construct the FC/battery/SC hybrid power source proposed to be utilized in FCHEVs are listed in Table 2. As reported in Table 2, an 80 kW three-phase PMSM used as the traction motor, a 90 kW PEMFC stack, a 19.2 kW Li-ion battery and a SC bank with the rated capacitance of 600 F composed of the four parallel-connected blocks that each block itself consists of 20 series-connected SC cells ESHSR-3000C0-002R7A5T have been utilized. The waveforms of the line voltages ( V AB and VBC ) supplied to the PMSM and the DC-link voltage are shown in Fig. 9. The waveforms of the line voltages show that they are the three-level AC voltages with correct magnitude and phase produced using PWM technique by the three-phase bidirectional DC/AC inverter. This point explicitly verifies the correct operation of the three-phase bidirectional PWM six-switch DC/AC inverter connected to the traction motor. Similarly, the waveform of the DC-link voltage explicitly demonstrates that the DC-link voltage is exactly regulated to the appointed value (400 V) by the unidirectional DC/DC boost converter connected to the PEMFC stack in accordance with Eq. (10). The waveforms of the currents ( I A and I B ) supplied to the stator of the PMSM are shown in Fig. 10. The periodic switching pulse with the switching frequency of 25 kHz and the duty cycle D fc determined according to Eq. (10) to regulate the DClink voltage along with the regulated DC-link voltage is also shown in Fig. 11. As mentioned, the periodic switching pulse is supplied to the switch S fc of the unidirectional DC/DC boost converter connected to the PEMFC stack. The electric power supplied to the traction motor (output power), and the total of the electric power provided by the PEMFC stack power production, discharging the 16

ACCEPTED MANUSCRIPT SC bank and Li-ion battery (input power) were measured point by point. Then, the power efficiency of the proposed FC/battery/SC hybrid power source was computed point by point as: Power efficiency 

Output power 100 Input power

(32)

The power efficiency curve resulted from the point by point computing is shown in Fig. 12. The following points which are the main contributions of this study are deduced from the experimental results shown in Figs. 9-12: ● The power efficiency curve explicitly demonstrates that the proposed FC/battery/SC hybrid power source provides a power efficiency of 96.2% around the rated power. ● The DC-link voltage is high accurately regulated to the appointed value (400 V). ● The traction motor is well supplied by the sinusoidal currents resulted from the three-level AC voltages produced by using PWM technique. The other benefit of the FC/battery/SC hybrid power source presented in this study is its flexibility, so that, it can be also utilized as a FC/battery or FC/SC power source. In detail, on the request, the power control unit disables the switching pulses with the duty cycles Dscchar and Dsc disc supplied to the converter connected to the SC bank, so the SC bank is isolated from the system, and the system changes into a FC/battery power source. Similarly, the proposed power source operates as a FC/SC hybrid power source by disabling the switching pulses with the duty cycles Dbat char and Dbat disc supplied to the converter connected to the Li-ion battery. Thus, the power source can be utilized in the three modes; FC/battery/SC, FC/battery and FC/SC. The technical parameters of the power source in the three modes are summarized and compared in Table 3. The comparison demonstrates that utilizing the SC bank mainly improves the acceleration of the vehicle, while the Li-ion battery provides longer cruising range, so that, a FCHEV with the weight of 1880 kg equipped with the proposed FC/battery/SC hybrid power source has a maximum speed of 161 km/h, 0-100 km/h acceleration in 12.2 sec, and a cruising range of 435 km on one 17

ACCEPTED MANUSCRIPT tank of hydrogen with the fuel capacity/tank pressure of 5.4 kg/5000 psi and 110 km on the 19.2 kWh Li-ion battery (totally, 545 km). As another attempt to prove the contributions of this work, a comparison between the proposed FC/battery/SC hybrid power source and the state of the art of all kinds of power sources reported in the literature to be used in FCHEVs is performed in Table 4 [15], [27-63]. It is worthwhile to note that while the majority of the other works are only simulated models, the comparison explicitly demonstrates the excellent performance of the proposed FC/battery/SC hybrid power source such as higher speed and acceleration.

5. Conclusion In this paper, a novel FC/battery/SC hybrid power source was proposed to be used in FCHEVs. A 90 kW PEMFC stack, a 600 F SC bank and a 19.2 kW Li-ion battery were used to construct a prototype of the FC/battery/SC hybrid power source. The experimental results demonstrated that the power efficiency of 96.2% around the rated power, highly accurate DC-link voltage regulation and providing a suitable three-phase current by using PWM technique that is supplied to the stator of the traction motor are the main contributions of this work. For a FCHEV with the weight of 1880 kg, the proposed power source provides a total cruising range of 545 km on one tank of hydrogen with the fuel capacity/tank pressure of 5.4 kg/5000 psi, a maximum speed of 161 km/h and 0-100 km/h acceleration in 12.2 sec. The proposed hybrid power source was also compared to the state-of-the-art power sources used in FCHEVs and reported in the literature that clearly substantiated its better parameters such as higher speed and acceleration, while most of the other works were simulated models. References [1] H. Fathabadi, Utilization of electric vehicles and renewable energy sources used as distributed generators for improving characteristics of electric power distribution systems, Energy 90 (2015) 1100-1110. 18

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[41] A. Benrabeh, F. Khoucha, O. Herizi, M. Benbouzid, A. Kheloui, FC/battery power management for electric vehicle based interleaved DC-DC boost converter topology, Proceedings of 15th European Conference on Power Electronics and Applications (EPE) IEEE (2013) 1-9. [42] M. Uzunoglu, M. Alam, Modeling and analysis of an FC/UC hybrid vehicular power system using a novel-wavelet-based load sharing algorithm, IEEE Transactions on Energy Conversion 23 (2008) 263-272. [43] P. Rodatz, G. Paganelli, A. Sciarretta, L. Guzzella, Optimal power management of an experimental fuel cell/supercapacitor-powered hybrid vehicle, Control Engineering Practice 13 (2005) 41-53. [44] M. Kisacikoglu, M. Uzunoglu, M. Alam, Load sharing using fuzzy logic control in a fuel cell/ultracapacitor hybrid vehicle, International Journal of Hydrogen Energy 34 (2009) 1497-1507. [45] J.S. Martínez, D. Hissel, M.-C. Péra, M. Amiet, Practical control structure and energy management of a testbed hybrid electric vehicle, IEEE Transactions on Vehicular Technology 60 (2011) 4139-4152. [46] Q. Li, W. Chen, Y. Li, S. Liu, J. Huang, Energy management strategy for fuel cell/battery/ultracapacitor hybrid vehicle based on fuzzy logic, International Journal of Electrical Power & Energy Systems 43 (1) (2012) 514-525. [47] D. Gao, Z. Jin, Q. Lu, Energy management strategy based on fuzzy logic for a fuel cell hybrid bus, Journal of Power Sources 185 (2008) 311–317. [48] M. Hannan, F. Azidin, A. Mohamed, Multi-sources model and control algorithm of an energy management system for light electric vehicles, Energy Conversion and Management 62 (2012) 123130. 23

ACCEPTED MANUSCRIPT [49] P. García, J.P. Torreglosa, L.M. Fernández, F. Jurado, Control strategies for high-power electric vehicles powered by hydrogen fuel cell, battery and supercapacitor, Expert Systems with Applications 40 (12) (2013) 4791-4804. [50] Z. Yu, D. Zinger, A. Bose, An innovative optimal power allocation strategy for fuel cell, battery and supercapacitor hybrid electric vehicle, Journal of Power Sources 196 (2011) 2351-2359. [51] X. Liu, D. Diallo, C. Marchand, Design methodology of fuel cell electric vehicle power system, Proceedings of 18th IEEE International Conference on Electrical Machines (ICEM) (2008) 1-6. [52] A. Melero-Pérez, W. Gao, I.J. Fernández-Lozano, Fuzzy logic energy management strategy for Fuel Cell/Ultracapacitor/Battery hybrid vehicle with multiple-input DC/DC converter, Proceedings of Vehicle Power and Propulsion Conference (VPPC'09 IEEE) (2009) 199-206.

24

ACCEPTED MANUSCRIPT

Fig.1. Schematic diagram of a typical FCHEV.

25

ACCEPTED MANUSCRIPT

Fig. 2. Electric circuit representing the behavior of a FC.

Fig. 3. Electric circuit representing the behavior of a FC under loading condition.

26

ACCEPTED MANUSCRIPT

Fig. 4. Dynamic response of a PEMFC stack H-1000.

27

ACCEPTED MANUSCRIPT

Fig. 5. Configuration of the FC/battery/SC hybrid power source proposed to be utilized in FCHEVs. 28

ACCEPTED MANUSCRIPT

Fig. 6. Unidirectional DC/DC boost converter connected to the PEMFC stack.

Fig. 7. Bidirectional DC/DC boost-buck converter connected to the SC bank.

29

ACCEPTED MANUSCRIPT

Fig. 8. Three-phase bidirectional PWM six-switch DC/AC inverter.

30

ACCEPTED MANUSCRIPT

Fig. 9. Experimental results: The waveforms of the line voltages ( V AB and VBC ) supplied to the PMSM, and the regulated DC-link voltage.

31

ACCEPTED MANUSCRIPT

Fig. 10. Experimental results: The waveforms of the currents ( I A and I B ) supplied to the stator of the PMSM.

32

ACCEPTED MANUSCRIPT

Fig. 11. Experimental results: The periodic switching pulse supplied to the switch S fc of the DC/DC boost converter connected to the PEMFC stack, and the regulated DC-link voltage.

33

ACCEPTED MANUSCRIPT

Fig. 12. Experimental results: Point by point measured power efficiency of the FC/battery/SC hybrid power source.

34

ACCEPTED MANUSCRIPT

Table 1. Technical specification of the PEM fuel cell stack H-1000 [25] and model parameters obtained from Eq. (6) at the stack operating point ( in  35 A ) and T=300 K. Technical specification of the PEM fuel cell stack H-1000 48 N cell

Model parameters obtained from Eq. (6) at the stack operating point ( in  35 A ) and T=300 K

E0 (V)

46

Stack efficiency

40% @ 28.8 V

Rconc ()

0.0374

Rated power (W)

1000

Ract ()

0.1092

Operating point

28.8 V @ 35 A

Rohm ()

0.0637

Max. stack temperature ( C  )

65

Ri ()

0.1224

Over current shut down (A)

42

C (F)

4.8623

H 2 pressure (bar)

0.45-0.55

Cstack (F)

0.1013

Flow rate at max. output power (lit/min)

13

35

ACCEPTED MANUSCRIPT Table 2. Technical specification of the components used in the constructed FC/battery/SC hybrid power source. Traction motor Product name

DC/DC boost converter connected to PEMFC stack

C fc ( μ F )-Aluminum

LSRPM 200 L

electrolytic capacitor/400 V Converter switching frequency: f s (kHz)

470

Made by

Leroy-Somer Co.

Type

3-phase, permanent magnet synchronous motor (PMSM)

electrolytic capacitor/600 V

220

Nominal line voltage (V)

400

DC-link voltage Vdc (V)

400

Rated power (kW)

80

C2 ( μ F )-Premium metallized

Rated torque (Nm)

170

Rated current (A)

157

Speed range (rpm)

0-4500

Efficiency (%)

95.7

MOSFET switch S fc

IRFPS40N60K

Maximum torque/Rated torque Magnet material

1.4

Diodes: D1 - D2

15ETH06S

Cout ( μ F )-Aluminum

polypropylene capacitor/600 V C1 ( n F )-Parasitic capacitance of IRFPS40N60K Type of transformer T

N2

NdFeB

Maximum current/Rated current Moment of inertia ( kg.m 2 ) Weight (kg)

N1

25

22 15.6 Pulse

11 10  13

PEMFC stack

1.5

Product name

HD-90

0.15 145

Made by

Hydrogenics Co.

Peak efficiency

55%

Rated power (kW)

90

Supercapacitor cell Product name

ESHSR-3000C0002R7A5T

Maximum power (kW)

99

Made by

Nesscap Co.

Stack operating pressure (kPa)

<120

Rated voltage (V)

2.7

Operating current (A)

0-500

Surge voltage (V)

2.85

Operating voltage (V)

180-360

Rated capacitance (F)

3000

Average capacitance tolerance (%)

5

Maximum continuous current (A)

148

DC/DC converter connected to the supercapacitor bank Type Bidirectional boost-buck IGBT switches: SW, Q1-Q2 STGY40NC60VD

6

36

ACCEPTED MANUSCRIPT

Maximum leakage current (mA)

5.2

Average ESR ( mΩ )

0.14

Maximum stored energy (Wh) Maximum specific energy (Wh/kg)

3.03

Parallel-connected blocks

4

5.67

20

Usable specific power (kW/kg) Nominal weight (g)

6.28

Series-connected supercapacitor cells in each block Rated capacitance (F)

600

535

Rated voltage (V)

54

Maximum continuous current (A) Maximum stored energy (Wh)

592 242.4

Average ESR ( Rsc esr ( mΩ ))

0.7

RLsc ( mΩ ) Supercapacitor bank

Three-phase bidirectional PWM six-switch DC/AC inverter IGBT switches: Q9-Q14

13.3

STGY40NC60VD

5

C dc ( μ F )-Aluminum

220

electrolytic capacitor/600 V DC/DC converter connected to the Li-ion battery bank Type IGBT switches: SW, Q1Q2

R Lbat ( mΩ )

Battery bank: Sixteen 12 V/100 Ah Li-ion batteries Rated voltage (V) 48

Bidirectional boost-buck STGY40NC60VD

Current capacity (Ah)

400

Capacity (kWh)

19.2

Series-connected batteries

4

Parallel-connected sets

4

Average ESR ( Rbat esr ( mΩ ))

2.9

8 10.8

Table 3. Comparison between the technical parameters of the hybrid power source in different modes. Mode of the power source

Analysis type

FC/Battery/SC FC/SC FC/Battery

Experiment Experiment Experiment

Cruising range (km) 545 435 530

Max. speed ( km/h) 161 158 155

37

0-100 km/h Max. Fuel Tank Vehicle acceleration efficiency capacity Pressure weight (sec) (%) (kg) (psi) (kg) 12.2 96.2 5.4 5000 1880 12.2 96.2 5.4 5000 1880 13.4 96.1 5.4 5000 1880

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Table 4. Comparison between the proposed FC/battery/SC hybrid power source and the state-of-the-art power sources used in FCHEVs and reported in the literature. Power source type

DC-link Max. speed Analysis type voltage (V) ( km/h)

PEMFC/battery/SC Experiment PEMFC/Battery Simulation& experiment PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation & experiment PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/Battery Simulation PEMFC/SC Simulation PEMFC/SC Simulation PEMFC/SC Simulation PEMFC/SC Simulation PEMFC/Battery/SC Experiment PEMFC/Battery/SC Simulation PEMFC/Battery/SC Experiment PEMFC/Battery/SC Simulation PEMFC/Battery/SC Simulation & experiment PEMFC/Battery/SC Simulation PEMFC/Battery/SC Simulation PEMFC/Battery/SC Simulation & experiment

545 400

161 –

0-100 km/h acceleration (sec) 12.2 –

– – 300 450

– 108 80 90

– – – –

62 30 40 40

[28] [29] [30] [31]

– – – – 60 – – 400 – – 400 188 340 206 560 – 400 120 750

60 88.5 – 120 – 129 – 50 144 120 120 91 120 91 20 160 23.4 47 50

– – – – – – – – – – 12.5 – – – – – – – –

130 75 – 80 2 80 2 8 4 60 48 60 60 40 40 40 120 3 400

[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [15] [42] [43] [44] [45] [46] [47] [48] [49]

300 300 400

128 120 –

– – –

100 58 38.5

[50] [51] [52]

38

Max. power (kW)

Reference

99 30

This work [27]