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MPPT controller for an interleaved boost dc–dc converter used in fuel cell electric vehicles
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e1 0
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MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles N. Benyahia a, H. Denoun a, A. Badji a, M. Zaouia a, T. Rekioua b,*, N. Benamrouche a, D. Rekioua b a
Electrical Engineering Advanced Technology Laboratory (LATAGE), Universite´ Mouloud Mammeri de Tizi Ouzou, Algeria b Laboratoire LTII, Universite de Bejaia, Bejaia, Algeria
article info
abstract
Article history:
Fuel cell electric vehicle (FCEV) has recently attracted increasing research interest. This
Received 12 February 2014
paper investigates the performances of MPPT-FC generators supplying electric vehicle
Received in revised form
power train through an interleaved boost DC/DC converter (IBC). The accent is made on
20 March 2014
forcing the FC generator to operate at its maximum power point by using perturb and
Accepted 22 March 2014
observe (P&O) algorithm integrated to the IBC control. However, the MPPT-FC control
Available online xxx
creates rapid changes in the power output from the fuel cell, which cause serious life shortening, severe cell degradation, and decrease the system efficiency. To overcome these
Keywords:
shortcomings, the control of air generation system was designed to improve the power
Interleaved converter
quality and to prevent fuel starvation phenomenon during rapid power transitions. The
Fuel cell
work involves the modeling and the simulation of the fuel cell power train in the vehicular
Electric vehicle
application using the experimental data obtained in previous works. The experimental part
Supercapacitor
of the proposed FCEV is based on a low-cost, low-power consumption microcontroller, which controls the IBC and performs the MPPT-FC operation. A microcontroller is used to measure the FC output power and to change the duty ratio of the IBC control signals. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The transportation industry is highly dependent on the use of fossil fuels. With the continuous decrease in supplies of such fossil fuels the use of alternative fuels are more than necessary. Fuel cells are power sources that have a relatively high energy density and use a renewable fuel as its primary energy source, that is, the hydrogen. Therefore, they are often considered as an ideal candidate for a zero emission vehicular applications due to their low operating temperature which
makes them well suited for personal vehicle applications [1,2]. There is a great variety of fuel cells leading to different ways in classifying them. Indeed, the common approach is to classify them according to their electrolytes. The proton exchange membrane fuel cells (PEMFC) present attractive proprieties for vehicle applications for the following reasons: (1) lower operation temperature, thus they can be rapidly turned on and off; (2) lower operation pressure, hence greater safety; (3) they can be easily set into mode system; (4) they have lower emission ratio and higher conversion ratio.
* Corresponding author. E-mail addresses: [email protected], [email protected] (T. Rekioua). http://dx.doi.org/10.1016/j.ijhydene.2014.03.185 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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Fuel Cell Electric Vehicles (FCEVs) are proposed to replace conventional vehicles in the near future. In this paper, the PEMFC is used as the main energy system (MES), and a supercapacitor or a battery is used as rechargeable energy storage system (RESS). MES provides extended driving range, and RESS provides good acceleration and regenerative braking. DCeDC converters can be used to interface these elements in the electric power train by boosting or chopping the voltage levels. Due to the automotive constraints, the power converter structure has to be reliable, lightweight, small volume, with high efficiency, low electromagnetic interference and low current/voltage ripples. Furthermore, FCEVs are facing many challenges, such as low cost, low fuel consumption, low catalyst loss and carbon-support corrosion, and a long life span. A classical boost dcedc converter is reaching its limitations to meet these challenges simultaneously. Despite its extensive use in FC applications due to its capability to operate in the current control mode in a continuous conduction mode (CCM) and thus improving the FC efficiency [3e6], the high current ripples flowing out of the fuel cell cause many disagreements such as increasing the fuel consumption, accelerating the catalyst loss and the carbon-support corrosion, shortening the lifetime, and incurring a nuisance tripping in overload situations [7]. Interleaved boost dcedc converter (IBC) is a promising candidate to meet these challenges. Indeed; interleaving techniques provide high power capability, modularity and they improve reliability [8]. The IBC is considered as a good solution due to its merits, such as high efficiency, ripple reductions and small filter components. Extensive research work has been carried out on IBCs and is available in the literature. In order to obtain optimal parameter values of the input inductors and the output capacitors, an analytical method has been used in Ref. [9] in which design considerations included the leg number, the switching frequency, the output loads, and the dynamic response. An equivalent circuit model with equivalent series resistors for a multi-leg boost converter was also presented. An optimal selection methodology for the leg number for an IBC based on the analysis of the input current ripples according to continuous conduction mode (CCM) and discontinuous conduction mode (DCM) has been also proposed in Ref. [10]. A digital resistive current control method for an interleaved DC/DC converter has been
presented in Ref. [11], this control method offers the possibility to use the converter operating in CCM and in DCM. The IBC proposed is implemented using a DSP-based controller. It offers a fast-response and improves the stability of the overall system. However, the DSP-based control unit increases the implementation cost of the system. Moreover, due to the need for regenerative power and the high variation of power demands in vehicles, fuel cells are generally used in conjunction with a reversible power sources. These sources consist of storage devices such as batteries or ultra-capacitors. Investigating these issues has already begun, but most of the research has focused on batteries. In fact, a PEMFC and battery hybrid system for tramway applications has been presented in Ref. [12]. The used management strategy was based on supplying the power requirements of the tramway forcing the fuel cell to work around its maximum efficiency. In addition, a hybrid system combining a 2 kW PEMFC stack and a leadeacid battery pack was developed for a lightweight cruising vehicle [13] and the design of a vehicle powered by a 1.2 kW PEM fuel cell to charge a lead acid battery pack has been described in Ref. [14]. The results of these studies show that such a proposed hybrid system is a good solution for the dynamic power supply requested by road vehicles. However, such systems are mode adapted for drive cycles with a low number of starts and stops like those encountered in highway driving modes. For urban operating modes involving a high number of starts and stops, the lifetime of the battery will be drastically decreased compared to the case of highway driving modes. In this paper, ultra capacitors are used instead of batteries to ensure the power reversibility in the drive train of the FCEV (Fig. 1). The option of using ultra capacitors based auxiliary devices is a viable solution to support the operation of the FCEV in order to ensure a fast response to any load power transient. Compared to batteries, supercapacitors have one or two orders of magnitude higher specific powers, and much longer lifetime. They are capable of performing for several millions of cycles [15]. They are virtually free of maintenance. Their high rated currents enable fast discharges and fast charges. Their quite low specific energy, compared to batteries, is in most cases; the factor that determines the feasibility of increasing power density and satisfying reliability requirements of the FCEV. This component is characterized in a previous work [16].
Fig. 1 e Fuel cell electric vehicle topology. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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IBC analysis
ripples across them are small compared to their dc voltages. In this analysis it is also assumed that all semiconductor components are ideal, i.e., they represent zero impedances in the on state and infinite impedances in the off state. The analysis of the input current waveforms in CCM is given for three cases as a function of the duty ratio value. Case A: The converter can be in this case only if the duty ratio is lower than 1/3 (a < 1/3). As depicted in (Fig. 2(aec)), only one of the inductor currents increases during the period [0, aT] because only one switch is turned on. As all the switches are turned off during [aT, T/3], the inductor currents and the input currents decrease. During the period [0, aT], the inductor Lfc1 starts to charge, while Lfc2 and Lfc3 continue to discharge. The rate of change of ifc can be expressed as:
It is assumed that the resistances of the inductors are negligible and the filter capacitor is large enough that the voltage
difc vfc vfc vbus vfc vbus ¼ þ þ dt L L L
The basic objective of this paper is the development of an inexpensive control of an IBC which performs the MPPTPEMFC operation for an Electric Vehicle application. A detailed modeling of the converter topology has been presented. After that, the design of a low cost control circuit based on a microcontroller has been provided. The control circuit described uses a two 8 bits microcontrollers and a shift register. To validate this study a PEMFC real time simulator was developed and implemented using a dSpace1103 card [5] and an experimental set up was build.
System modeling and analysis
(1)
Fig. 2 e Different cases of the three interleaved boost dcedc converter: a, b) ideal waveforms for 0
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Input current ripples (A)
2.5
difc vbus ¼ ð3 3aÞ dt L
1 Leg
2
2 Legs
The input current ripple of the interleaved boost converter is expressed as shown in
3 Legs
1.5
8 vbus > ð1 3aÞaT; 0 a T=3 > > L > > > > < vbus 1 ð2 3aÞ a T; T=3 a 3T=3 Difc ¼ 3 L > > > > > > v 2 bus > : ð3 3aÞ a T; 2T=3 a T 3 L
1
0.5
0
0
0.1
0.2
0.3
0.4 0.5 0.6 Duty ratio
0.7
0.8
0.9
vbus Difc a ¼ 1; a ¼ 1; a ¼ 5 ¼ 12Lf 6 2 6
(2)
Case B: The converter can be in this case only if the duty ratio is between (T/3
(3)
difc vbus ¼ ð2 3aÞ dt L
(4)
difc vfc vfc vfc ¼ þ þ dt L L L
(5)
480 460
Net power (W)
λ =12
λ =14
λ =10 λ =8
400 380 360 340 320 300
vbusmax 12Difcmax f
(9)
Where the dc bus voltage is the maximum voltage vbus ¼ Vbusmax. The current ripple Difcmax is given as a design criterion. Kit should be noted that the inductance (9) is 25% of the inductance of two legs interleaved boost dcedc converter for the same current ripple Difcmax and the same switching frequency f. That means the inductor volume is 25% of that of the conventional converter. Fig. 3 shows the current ripples versus duty ratio for an ordinary boost converter (one leg), two legs converter and a three legs converter.
Fuel cell modeling
Case C: The converter can be in this case only if the duty ratio is between (2T/3
420
(8)
From (8) one finds the inductance L L
difc vbus ¼ ð1 3aÞ dt L
(7)
The maximum current ripple is defined as
1
Fig. 3 e Input current ripple variations according to the duty ratio.
440
(6)
7
6 5 4 Oxygen excess ratio
3
2.36 2
1
Fig. 4 e Net power variation versus oxygen excess ratio. Solid line 80 C and dash line 0 C.
0
PEMFCs are a promising technology for vehicle applications thanks to their higher efficiency, low emissions and direct production of electricity [17]. A PEMFC is a device converting the chemical energy into electrical energy. Its input is the load current flowing in the PEMFC stack and the output is the voltage provided by the PEMFC. The output voltage of a single cell can be defined as fellows [18]. vfc ¼ nfc vcell ¼ nfc ðENernst EAct ECon EOhm Þ
(10)
Where, ENernst (Nernst voltage) is the thermodynamic potential of the cell representing its reversible voltage. The Nernst voltage is calculated starting from a modified version of Nernst equation with an extra term to take into account changes in temperature with respect to a new standard temperature [14]; Eact is the activation voltage drop which is the amount of voltage lost in driving the reaction. This voltage drop is described by the general form of Tafel’s equation; Econ is the concentration voltage drop, which is the voltage lost when the concentration of the reactant at the electrode is diminished according to Fick’s first law and Faraday’s law [15]; Eohm is the ohmic voltage drop, which is the amount of voltage lost due to the resistance to electron flow in the electrodes and the resistance to ion flow in the polymer membrane. It represents the conducting resistance between the membrane and electrodes and the resistance of
Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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of the membrane conductivity and cell temperature is defined as: tm Rohm ¼ (15) sm Where tm and sm are the thickness of the membrane and the membrane conductivity respectively, the membrane conductivity is defined as: 350 1:155 Tfc (16) sm ¼ ð0:005139lm 0:00326Þe
Fig. 5 e Flowchart of the MPFC-MPP algorithm.
electrodes [13]. Each term of Eq. (10) can be calculated by the following equations: ENernst ¼ 1:229 8:5e4 Tfc 298:15 þ 4:308e5 ln pH2
1 (11) þ ln PO2 2 Eact ¼
RT ln ifc I0 ¼ Tfc a þ b ln ifc azF
(12)
ifc Econ ¼ 0:016 ln 1 25
(13)
Eohm ¼ ifc Rohm
(14)
Where Rohm is the internal electrical resistance, it is a function
Where lm is the membrane water content, it is considered as a parameter in this paper. The value of lm varies between 0 and 14, which is equivalent to the humidity of 0% and 100%. Under supersaturated conditions, however, the maximum possible value of lm can be as high as 23. In addition, lm can also be influenced by the membrane preparation procedure, the relative humidity of the feed gas, the oxygen and hydrogen excess ratios, and the age of the membrane. The dynamic gas transport model describes the variation of the partial pressure of the reactants during the FC operation. The following balance equation describes the amount of hydrogen and oxygen in the input and the output and that reacted in the FC: dpca RTfc ¼ Wca;in Wca;out Wca;rec dt Vca
(17)
Where, the oxygen consumption rate Wca,rec, the inlet cathode mass flow rate Wca,in and the outlet cathode mass flow rate Wca,out are given respectively by: nfc ifc 4F
(18)
1 Win 1 þ 6atm
(19)
Wca;rec ¼ MO2 Wca;in ¼ With,
Fig. 6 e Electrical circuit of the three paralleled phases interleaved boost dcedc converter. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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Fig. 7 e Supercapacitor simplified model. (a) Bloc diagram model (b) RC circuit model (c) Experimental results of supercapacitor charge and discharge mode compared with simulation results.
6atm ¼
Mv fatm psat ðTatm Þ $ Ma patm fatm psat ðTatm Þ
(20)
Win ¼ kca;in psm pca
(21)
Wca;out ¼ kca;out pca prm
(22)
Where, Mv and Ma are the molar mass of vapor and air respectively, fatm is the relative humidity at ambient conditions, 6atm, kca,in and kca,out are the humidity ratio, the cathode inlet orifice constant and the cathode outlet orifice constant respectively. The angular speed ucp verifies the following differential equation: ducp 1 ¼ scm scp Jcp dt
(23)
Where, Jcp is the compressor motor inertia. scm and scp denote the compressor motor torque and the load torque required to drive the compressor respectively.
kt hcm vcm kv ucp Rcm # " g Cp Tatm psm g1 1 Wcp scp ¼ hcp ucp patm
scm ¼
(24) (25)
Where Rcm, kt and kv are motor constants, hcp is the compressor efficiency, hcm is the motor mechanical efficiency, Cp is the specific heat capacity of air, vcm is the compressor motor voltage and Wcp is the compressor mass flow rate. The air pressure in the supply manifold is given by the following differential equation: RTcp dpsm ¼ Wcp Win dt Ma Vsm
(26)
The total net FC power, Pnet, can be defined as: Pnet ¼ Pfc Pcm
(27)
With
Fig. 8 e Complete IBC-FCEV power system Simulink model. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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30
Vbus
Current (A)
Voltage (V)
60 42
Vfc 20 0
5 Time (s)
Current (A)
0
5 Time (s)
10
6.0325
6.033
6.0325
6.033
22.33
10
22.2 6.032
0
7.6
io ifcf
-10 0
2.8 2.6 2.4 2.2 2 1.8
isc 5 Time (s)
7.25 6.032
10
500 Pnet (W)
Oxygen excess ratio
ifc123
ifc
10 0
10
20
20
0
5 Time (s)
10
450 400 0
5 Time (s)
10
Fig. 9 e Simulation results of the IBC-FCEV model.
Pfc ¼ vfc ifc Pcm ¼
kt vcm kv ucp Rcm
(28) (29)
Where Pfc, is the FC power, which is the product of FC voltage, vfc, and FC current, ifc, and Pcm is the power consumed by the compressor. One of the problems in the PEMFC generation systems is that the amount of electric power by the PEMFC is always changing with water membrane content. Fig. 4 shows that the values of FC net power increases as Tfc and as water membrane lm content increases. An MPPT control strategy, which has fast response characteristics and is able to make good use of the electric power generated in any water membrane content, is needed to solve the aforementioned problems. The most commonly used MPPT algorithm is the Perturb and Observe (P&O), due to its ease of implementation in its basic form (Fig. 5). In this
algorithm, the duty cycle (a) of the converter is changed by applying small and constant changes (c) as disturbance thereby changing the system operating point. Furthermore, the PEMFC proper operation is achieved by controlling those auxiliaries, in particular the air supply system. The air consumption system is about 20% of the power supplied by the fuel cell, which reduces the effective capacity of the PEMFC. This is why; the air system control is an important challenge for the development of the fuel cell electric vehicles. Indeed, maintaining an adequate level of the oxygen partial pressure in the cathode during rapid variations of fuel cell current in MPPT operation mode is important to avoid a degradation of the membrane and a decrease in the system efficiency. The oxygen excess ratio is defined as a ratio between the mass flow rate of oxygen WO2,ca,e entering the cathode and the mass flow rate WO2,ca,c of oxygen consumed in the reaction at the cathode: lO2 ¼
WO2 ;ca;e WO2 ;ca;c
(30)
Fig. 10 e Experimental set up. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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sr1 sr2 sr3
Control signal
Shift process 74LS164P
MPPT Implementation 16F877A
Clock signal
Clock signal generation 16F84A
Fig. 11 e Circuit principle of shifting process.
The research work realized on this topic indicates that the level of the oxygen partial pressure in the cathode can be successfully controlled by a feed-forward control on the compressor motor voltage. In Ref. [19], a single control device acting on the compressor motor voltage based on the current measurement of the fuel cell was used. A static or a dynamic function that correlates the steady-state value between the control input, vcm, and the fuel cell current, ifc, can be used in the feed-forward path. However, these control methods suffer from the sensitivity to modeling errors and the variation of ambient conditions. To improve the system robustness, a simple proportional integral (PI) controller is used for controlling the oxygen excess ratio. Its optimal values are determined and a feedback control is added (Fig. 6).
Supercapacitor modeling Due to the need for regenerative power and the high variation of power demands in the vehicles, fuel cells are generally used in conjunction with a reversible power sources. These sources consist of a storage device such as batteries or supercapacitors. The supercapacitor based auxiliary devices is a viable solution to support the operation of the PEMFC in order to ensure a fast response to any electric power transient. Compared to batteries, supercapacitors have one or two orders of magnitude higher specific powers, and much longer lifetime.
Fig. 12 e Control signals generated.
Fig. 13 e Gate signals.
The proposed studies of supercapacitor modeling in the literature can be divided into two groups using equivalent circuits. In the first category of models, a non-linear equivalent capacitance is taken into account [20,21]. While in the second category of models, long range phenomena observed during supercapacitor relaxation are taken into account through the introduction of fractional differentiation in modeling [22]. This model is known by its accuracy. However, its implementation in software environments requires consuming a very long time. The supercapacitor model presented in this paper is based on a simplified (RC) model more suitable for applications where the energy stored in the capacitor is of a primary importance [20,21]. This model consists of a non-linear capacitance C(vsc0), an equivalent series resistance ESR and a rated capacitance C0 (Fig. 7). The model parameters are identified using the constant current tests method described in Ref. [22].
Simulation and experimental results Simulation results Fig. 8 shows the complete IBC-FCEV power system Simulink model. The core of the FC model block is a state-space block
Fig. 14 e Inductor currents and PEMFC emulator current.
Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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that implements the FC dynamic model developed in Section Fuel cell modeling. The IBC converter is controlled to provide the FC maximal power point (MPP) and the super-capacitor provides the extra power which represents difference of power between the load demand and the fuel cell supply. The simulation parameters are summarized as follows: Pveh ¼ 500 W, C ¼ 220 mF, fs ¼ 5 kHz, Lfc1 ¼ Lfc1 ¼ Lfc1 ¼ 9 mH. The simulation results are shown in Fig. 9, the DC bus voltage is kept constant at the value of 42 V; the effect of MPPT control on the fuel cell voltage is also shown. The fuel cell delivers a maximum power and the supercapacitor provides the instantaneous power. Despite the rapid variation of the current delivered by the fuel cell, the oxygen excess ratio in the cathode is kept constant. It should be noted that the current ripple supplied by the fuel cell is reduced by three times through the use of interleaved converter.
Experimental results To evaluate the performances of the proposed MPPT-PEMFC, an experimental set up of a PEMFC emulator associated with an IBC converter has been designed and implemented. The 16F877A microcontroller is used for the MPPT-PEMFC implementation. The system is built as shown in Fig. 10. Measuring the voltage and the current of the PEMFC emulator is necessary to search the MPP. This acquisition is assured by voltage and current (LEM) sensors. The semiconductor devices used are the IRFP360 power MOSFET and a BY329 rapid diode. The shifting process is realized by using a shift register (74LS164P) and a (16F84A) microcontroller for generating the clock signals needed for the shifting process. The clock signals generated by the (16F84A) is used to create a shift of the PWM signals generated by the (16F877A). The control signals (sr1, sr2 and sr3) are taken on pins 1, 4 and 8 of the (74LS164P) respictively (Fig. 11). Fig. 12 shows the clock signal and the control signals supplied by the 74LS164P. The experimental results of the inductor currents and the gate signals waveforms are shown in Figs. 13 and 14. The experimental results are in agreement with those obtained with theoretically simulation.
Conclusion In this paper, an interleaved dc/dc boost converter with MPPT for fuel cell electric vehicle is proposed. Fuel cell MPPT technique is implemented using microcontroller components. The MPPT technique is implemented to get the MPP from the fuel cell. Simulation and experimental result illustrates that the IBC solves the problems of high ripple currents. The MPPT implementation can be easily extended to other interleaved converters.
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[20] Zubieta L, Bonert R. Characterization of double layer capacitor (DLCs) for power electronics application. IEEE Trans Ind Appl 2000;36:199e205. [21] Belhachemi F, Rae¨l S, Davat B. Supercapacitors electrical behaviour for power electronics application. In: Proc. 9th international conference and exhibition on power
electronics and motion control (EPE-PEMC), 5e7 September; 2000 [Kosice, Slovakia]. [22] Lajnef W, Vinassa JeM, Briat O, Azzopardi S, Woirgard E. Characterization methods and modeling of ultracapacitors for used as peak power sources. J Power Sources 2007;168:553e60.
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Available online at www.sciencedirect.com
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MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles N. Benyahia a, H. Denoun a, A. Badji a, M. Zaouia a, T. Rekioua b,*, N. Benamrouche a, D. Rekioua b a
Electrical Engineering Advanced Technology Laboratory (LATAGE), Universite´ Mouloud Mammeri de Tizi Ouzou, Algeria b Laboratoire LTII, Universite de Bejaia, Bejaia, Algeria
article info
abstract
Article history:
Fuel cell electric vehicle (FCEV) has recently attracted increasing research interest. This
Received 12 February 2014
paper investigates the performances of MPPT-FC generators supplying electric vehicle
Received in revised form
power train through an interleaved boost DC/DC converter (IBC). The accent is made on
20 March 2014
forcing the FC generator to operate at its maximum power point by using perturb and
Accepted 22 March 2014
observe (P&O) algorithm integrated to the IBC control. However, the MPPT-FC control
Available online xxx
creates rapid changes in the power output from the fuel cell, which cause serious life shortening, severe cell degradation, and decrease the system efficiency. To overcome these
Keywords:
shortcomings, the control of air generation system was designed to improve the power
Interleaved converter
quality and to prevent fuel starvation phenomenon during rapid power transitions. The
Fuel cell
work involves the modeling and the simulation of the fuel cell power train in the vehicular
Electric vehicle
application using the experimental data obtained in previous works. The experimental part
Supercapacitor
of the proposed FCEV is based on a low-cost, low-power consumption microcontroller, which controls the IBC and performs the MPPT-FC operation. A microcontroller is used to measure the FC output power and to change the duty ratio of the IBC control signals. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The transportation industry is highly dependent on the use of fossil fuels. With the continuous decrease in supplies of such fossil fuels the use of alternative fuels are more than necessary. Fuel cells are power sources that have a relatively high energy density and use a renewable fuel as its primary energy source, that is, the hydrogen. Therefore, they are often considered as an ideal candidate for a zero emission vehicular applications due to their low operating temperature which
makes them well suited for personal vehicle applications [1,2]. There is a great variety of fuel cells leading to different ways in classifying them. Indeed, the common approach is to classify them according to their electrolytes. The proton exchange membrane fuel cells (PEMFC) present attractive proprieties for vehicle applications for the following reasons: (1) lower operation temperature, thus they can be rapidly turned on and off; (2) lower operation pressure, hence greater safety; (3) they can be easily set into mode system; (4) they have lower emission ratio and higher conversion ratio.
* Corresponding author. E-mail addresses: [email protected], [email protected] (T. Rekioua). http://dx.doi.org/10.1016/j.ijhydene.2014.03.185 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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Fuel Cell Electric Vehicles (FCEVs) are proposed to replace conventional vehicles in the near future. In this paper, the PEMFC is used as the main energy system (MES), and a supercapacitor or a battery is used as rechargeable energy storage system (RESS). MES provides extended driving range, and RESS provides good acceleration and regenerative braking. DCeDC converters can be used to interface these elements in the electric power train by boosting or chopping the voltage levels. Due to the automotive constraints, the power converter structure has to be reliable, lightweight, small volume, with high efficiency, low electromagnetic interference and low current/voltage ripples. Furthermore, FCEVs are facing many challenges, such as low cost, low fuel consumption, low catalyst loss and carbon-support corrosion, and a long life span. A classical boost dcedc converter is reaching its limitations to meet these challenges simultaneously. Despite its extensive use in FC applications due to its capability to operate in the current control mode in a continuous conduction mode (CCM) and thus improving the FC efficiency [3e6], the high current ripples flowing out of the fuel cell cause many disagreements such as increasing the fuel consumption, accelerating the catalyst loss and the carbon-support corrosion, shortening the lifetime, and incurring a nuisance tripping in overload situations [7]. Interleaved boost dcedc converter (IBC) is a promising candidate to meet these challenges. Indeed; interleaving techniques provide high power capability, modularity and they improve reliability [8]. The IBC is considered as a good solution due to its merits, such as high efficiency, ripple reductions and small filter components. Extensive research work has been carried out on IBCs and is available in the literature. In order to obtain optimal parameter values of the input inductors and the output capacitors, an analytical method has been used in Ref. [9] in which design considerations included the leg number, the switching frequency, the output loads, and the dynamic response. An equivalent circuit model with equivalent series resistors for a multi-leg boost converter was also presented. An optimal selection methodology for the leg number for an IBC based on the analysis of the input current ripples according to continuous conduction mode (CCM) and discontinuous conduction mode (DCM) has been also proposed in Ref. [10]. A digital resistive current control method for an interleaved DC/DC converter has been
presented in Ref. [11], this control method offers the possibility to use the converter operating in CCM and in DCM. The IBC proposed is implemented using a DSP-based controller. It offers a fast-response and improves the stability of the overall system. However, the DSP-based control unit increases the implementation cost of the system. Moreover, due to the need for regenerative power and the high variation of power demands in vehicles, fuel cells are generally used in conjunction with a reversible power sources. These sources consist of storage devices such as batteries or ultra-capacitors. Investigating these issues has already begun, but most of the research has focused on batteries. In fact, a PEMFC and battery hybrid system for tramway applications has been presented in Ref. [12]. The used management strategy was based on supplying the power requirements of the tramway forcing the fuel cell to work around its maximum efficiency. In addition, a hybrid system combining a 2 kW PEMFC stack and a leadeacid battery pack was developed for a lightweight cruising vehicle [13] and the design of a vehicle powered by a 1.2 kW PEM fuel cell to charge a lead acid battery pack has been described in Ref. [14]. The results of these studies show that such a proposed hybrid system is a good solution for the dynamic power supply requested by road vehicles. However, such systems are mode adapted for drive cycles with a low number of starts and stops like those encountered in highway driving modes. For urban operating modes involving a high number of starts and stops, the lifetime of the battery will be drastically decreased compared to the case of highway driving modes. In this paper, ultra capacitors are used instead of batteries to ensure the power reversibility in the drive train of the FCEV (Fig. 1). The option of using ultra capacitors based auxiliary devices is a viable solution to support the operation of the FCEV in order to ensure a fast response to any load power transient. Compared to batteries, supercapacitors have one or two orders of magnitude higher specific powers, and much longer lifetime. They are capable of performing for several millions of cycles [15]. They are virtually free of maintenance. Their high rated currents enable fast discharges and fast charges. Their quite low specific energy, compared to batteries, is in most cases; the factor that determines the feasibility of increasing power density and satisfying reliability requirements of the FCEV. This component is characterized in a previous work [16].
Fig. 1 e Fuel cell electric vehicle topology. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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IBC analysis
ripples across them are small compared to their dc voltages. In this analysis it is also assumed that all semiconductor components are ideal, i.e., they represent zero impedances in the on state and infinite impedances in the off state. The analysis of the input current waveforms in CCM is given for three cases as a function of the duty ratio value. Case A: The converter can be in this case only if the duty ratio is lower than 1/3 (a < 1/3). As depicted in (Fig. 2(aec)), only one of the inductor currents increases during the period [0, aT] because only one switch is turned on. As all the switches are turned off during [aT, T/3], the inductor currents and the input currents decrease. During the period [0, aT], the inductor Lfc1 starts to charge, while Lfc2 and Lfc3 continue to discharge. The rate of change of ifc can be expressed as:
It is assumed that the resistances of the inductors are negligible and the filter capacitor is large enough that the voltage
difc vfc vfc vbus vfc vbus ¼ þ þ dt L L L
The basic objective of this paper is the development of an inexpensive control of an IBC which performs the MPPTPEMFC operation for an Electric Vehicle application. A detailed modeling of the converter topology has been presented. After that, the design of a low cost control circuit based on a microcontroller has been provided. The control circuit described uses a two 8 bits microcontrollers and a shift register. To validate this study a PEMFC real time simulator was developed and implemented using a dSpace1103 card [5] and an experimental set up was build.
System modeling and analysis
(1)
Fig. 2 e Different cases of the three interleaved boost dcedc converter: a, b) ideal waveforms for 0
4
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Input current ripples (A)
2.5
difc vbus ¼ ð3 3aÞ dt L
1 Leg
2
2 Legs
The input current ripple of the interleaved boost converter is expressed as shown in
3 Legs
1.5
8 vbus > ð1 3aÞaT; 0 a T=3 > > L > > > > < vbus 1 ð2 3aÞ a T; T=3 a 3T=3 Difc ¼ 3 L > > > > > > v 2 bus > : ð3 3aÞ a T; 2T=3 a T 3 L
1
0.5
0
0
0.1
0.2
0.3
0.4 0.5 0.6 Duty ratio
0.7
0.8
0.9
vbus Difc a ¼ 1; a ¼ 1; a ¼ 5 ¼ 12Lf 6 2 6
(2)
Case B: The converter can be in this case only if the duty ratio is between (T/3
(3)
difc vbus ¼ ð2 3aÞ dt L
(4)
difc vfc vfc vfc ¼ þ þ dt L L L
(5)
480 460
Net power (W)
λ =12
λ =14
λ =10 λ =8
400 380 360 340 320 300
vbusmax 12Difcmax f
(9)
Where the dc bus voltage is the maximum voltage vbus ¼ Vbusmax. The current ripple Difcmax is given as a design criterion. Kit should be noted that the inductance (9) is 25% of the inductance of two legs interleaved boost dcedc converter for the same current ripple Difcmax and the same switching frequency f. That means the inductor volume is 25% of that of the conventional converter. Fig. 3 shows the current ripples versus duty ratio for an ordinary boost converter (one leg), two legs converter and a three legs converter.
Fuel cell modeling
Case C: The converter can be in this case only if the duty ratio is between (2T/3
420
(8)
From (8) one finds the inductance L L
difc vbus ¼ ð1 3aÞ dt L
(7)
The maximum current ripple is defined as
1
Fig. 3 e Input current ripple variations according to the duty ratio.
440
(6)
7
6 5 4 Oxygen excess ratio
3
2.36 2
1
Fig. 4 e Net power variation versus oxygen excess ratio. Solid line 80 C and dash line 0 C.
0
PEMFCs are a promising technology for vehicle applications thanks to their higher efficiency, low emissions and direct production of electricity [17]. A PEMFC is a device converting the chemical energy into electrical energy. Its input is the load current flowing in the PEMFC stack and the output is the voltage provided by the PEMFC. The output voltage of a single cell can be defined as fellows [18]. vfc ¼ nfc vcell ¼ nfc ðENernst EAct ECon EOhm Þ
(10)
Where, ENernst (Nernst voltage) is the thermodynamic potential of the cell representing its reversible voltage. The Nernst voltage is calculated starting from a modified version of Nernst equation with an extra term to take into account changes in temperature with respect to a new standard temperature [14]; Eact is the activation voltage drop which is the amount of voltage lost in driving the reaction. This voltage drop is described by the general form of Tafel’s equation; Econ is the concentration voltage drop, which is the voltage lost when the concentration of the reactant at the electrode is diminished according to Fick’s first law and Faraday’s law [15]; Eohm is the ohmic voltage drop, which is the amount of voltage lost due to the resistance to electron flow in the electrodes and the resistance to ion flow in the polymer membrane. It represents the conducting resistance between the membrane and electrodes and the resistance of
Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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5
of the membrane conductivity and cell temperature is defined as: tm Rohm ¼ (15) sm Where tm and sm are the thickness of the membrane and the membrane conductivity respectively, the membrane conductivity is defined as: 350 1:155 Tfc (16) sm ¼ ð0:005139lm 0:00326Þe
Fig. 5 e Flowchart of the MPFC-MPP algorithm.
electrodes [13]. Each term of Eq. (10) can be calculated by the following equations: ENernst ¼ 1:229 8:5e4 Tfc 298:15 þ 4:308e5 ln pH2
1 (11) þ ln PO2 2 Eact ¼
RT ln ifc I0 ¼ Tfc a þ b ln ifc azF
(12)
ifc Econ ¼ 0:016 ln 1 25
(13)
Eohm ¼ ifc Rohm
(14)
Where Rohm is the internal electrical resistance, it is a function
Where lm is the membrane water content, it is considered as a parameter in this paper. The value of lm varies between 0 and 14, which is equivalent to the humidity of 0% and 100%. Under supersaturated conditions, however, the maximum possible value of lm can be as high as 23. In addition, lm can also be influenced by the membrane preparation procedure, the relative humidity of the feed gas, the oxygen and hydrogen excess ratios, and the age of the membrane. The dynamic gas transport model describes the variation of the partial pressure of the reactants during the FC operation. The following balance equation describes the amount of hydrogen and oxygen in the input and the output and that reacted in the FC: dpca RTfc ¼ Wca;in Wca;out Wca;rec dt Vca
(17)
Where, the oxygen consumption rate Wca,rec, the inlet cathode mass flow rate Wca,in and the outlet cathode mass flow rate Wca,out are given respectively by: nfc ifc 4F
(18)
1 Win 1 þ 6atm
(19)
Wca;rec ¼ MO2 Wca;in ¼ With,
Fig. 6 e Electrical circuit of the three paralleled phases interleaved boost dcedc converter. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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Fig. 7 e Supercapacitor simplified model. (a) Bloc diagram model (b) RC circuit model (c) Experimental results of supercapacitor charge and discharge mode compared with simulation results.
6atm ¼
Mv fatm psat ðTatm Þ $ Ma patm fatm psat ðTatm Þ
(20)
Win ¼ kca;in psm pca
(21)
Wca;out ¼ kca;out pca prm
(22)
Where, Mv and Ma are the molar mass of vapor and air respectively, fatm is the relative humidity at ambient conditions, 6atm, kca,in and kca,out are the humidity ratio, the cathode inlet orifice constant and the cathode outlet orifice constant respectively. The angular speed ucp verifies the following differential equation: ducp 1 ¼ scm scp Jcp dt
(23)
Where, Jcp is the compressor motor inertia. scm and scp denote the compressor motor torque and the load torque required to drive the compressor respectively.
kt hcm vcm kv ucp Rcm # " g Cp Tatm psm g1 1 Wcp scp ¼ hcp ucp patm
scm ¼
(24) (25)
Where Rcm, kt and kv are motor constants, hcp is the compressor efficiency, hcm is the motor mechanical efficiency, Cp is the specific heat capacity of air, vcm is the compressor motor voltage and Wcp is the compressor mass flow rate. The air pressure in the supply manifold is given by the following differential equation: RTcp dpsm ¼ Wcp Win dt Ma Vsm
(26)
The total net FC power, Pnet, can be defined as: Pnet ¼ Pfc Pcm
(27)
With
Fig. 8 e Complete IBC-FCEV power system Simulink model. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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30
Vbus
Current (A)
Voltage (V)
60 42
Vfc 20 0
5 Time (s)
Current (A)
0
5 Time (s)
10
6.0325
6.033
6.0325
6.033
22.33
10
22.2 6.032
0
7.6
io ifcf
-10 0
2.8 2.6 2.4 2.2 2 1.8
isc 5 Time (s)
7.25 6.032
10
500 Pnet (W)
Oxygen excess ratio
ifc123
ifc
10 0
10
20
20
0
5 Time (s)
10
450 400 0
5 Time (s)
10
Fig. 9 e Simulation results of the IBC-FCEV model.
Pfc ¼ vfc ifc Pcm ¼
kt vcm kv ucp Rcm
(28) (29)
Where Pfc, is the FC power, which is the product of FC voltage, vfc, and FC current, ifc, and Pcm is the power consumed by the compressor. One of the problems in the PEMFC generation systems is that the amount of electric power by the PEMFC is always changing with water membrane content. Fig. 4 shows that the values of FC net power increases as Tfc and as water membrane lm content increases. An MPPT control strategy, which has fast response characteristics and is able to make good use of the electric power generated in any water membrane content, is needed to solve the aforementioned problems. The most commonly used MPPT algorithm is the Perturb and Observe (P&O), due to its ease of implementation in its basic form (Fig. 5). In this
algorithm, the duty cycle (a) of the converter is changed by applying small and constant changes (c) as disturbance thereby changing the system operating point. Furthermore, the PEMFC proper operation is achieved by controlling those auxiliaries, in particular the air supply system. The air consumption system is about 20% of the power supplied by the fuel cell, which reduces the effective capacity of the PEMFC. This is why; the air system control is an important challenge for the development of the fuel cell electric vehicles. Indeed, maintaining an adequate level of the oxygen partial pressure in the cathode during rapid variations of fuel cell current in MPPT operation mode is important to avoid a degradation of the membrane and a decrease in the system efficiency. The oxygen excess ratio is defined as a ratio between the mass flow rate of oxygen WO2,ca,e entering the cathode and the mass flow rate WO2,ca,c of oxygen consumed in the reaction at the cathode: lO2 ¼
WO2 ;ca;e WO2 ;ca;c
(30)
Fig. 10 e Experimental set up. Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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sr1 sr2 sr3
Control signal
Shift process 74LS164P
MPPT Implementation 16F877A
Clock signal
Clock signal generation 16F84A
Fig. 11 e Circuit principle of shifting process.
The research work realized on this topic indicates that the level of the oxygen partial pressure in the cathode can be successfully controlled by a feed-forward control on the compressor motor voltage. In Ref. [19], a single control device acting on the compressor motor voltage based on the current measurement of the fuel cell was used. A static or a dynamic function that correlates the steady-state value between the control input, vcm, and the fuel cell current, ifc, can be used in the feed-forward path. However, these control methods suffer from the sensitivity to modeling errors and the variation of ambient conditions. To improve the system robustness, a simple proportional integral (PI) controller is used for controlling the oxygen excess ratio. Its optimal values are determined and a feedback control is added (Fig. 6).
Supercapacitor modeling Due to the need for regenerative power and the high variation of power demands in the vehicles, fuel cells are generally used in conjunction with a reversible power sources. These sources consist of a storage device such as batteries or supercapacitors. The supercapacitor based auxiliary devices is a viable solution to support the operation of the PEMFC in order to ensure a fast response to any electric power transient. Compared to batteries, supercapacitors have one or two orders of magnitude higher specific powers, and much longer lifetime.
Fig. 12 e Control signals generated.
Fig. 13 e Gate signals.
The proposed studies of supercapacitor modeling in the literature can be divided into two groups using equivalent circuits. In the first category of models, a non-linear equivalent capacitance is taken into account [20,21]. While in the second category of models, long range phenomena observed during supercapacitor relaxation are taken into account through the introduction of fractional differentiation in modeling [22]. This model is known by its accuracy. However, its implementation in software environments requires consuming a very long time. The supercapacitor model presented in this paper is based on a simplified (RC) model more suitable for applications where the energy stored in the capacitor is of a primary importance [20,21]. This model consists of a non-linear capacitance C(vsc0), an equivalent series resistance ESR and a rated capacitance C0 (Fig. 7). The model parameters are identified using the constant current tests method described in Ref. [22].
Simulation and experimental results Simulation results Fig. 8 shows the complete IBC-FCEV power system Simulink model. The core of the FC model block is a state-space block
Fig. 14 e Inductor currents and PEMFC emulator current.
Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e1 0
that implements the FC dynamic model developed in Section Fuel cell modeling. The IBC converter is controlled to provide the FC maximal power point (MPP) and the super-capacitor provides the extra power which represents difference of power between the load demand and the fuel cell supply. The simulation parameters are summarized as follows: Pveh ¼ 500 W, C ¼ 220 mF, fs ¼ 5 kHz, Lfc1 ¼ Lfc1 ¼ Lfc1 ¼ 9 mH. The simulation results are shown in Fig. 9, the DC bus voltage is kept constant at the value of 42 V; the effect of MPPT control on the fuel cell voltage is also shown. The fuel cell delivers a maximum power and the supercapacitor provides the instantaneous power. Despite the rapid variation of the current delivered by the fuel cell, the oxygen excess ratio in the cathode is kept constant. It should be noted that the current ripple supplied by the fuel cell is reduced by three times through the use of interleaved converter.
Experimental results To evaluate the performances of the proposed MPPT-PEMFC, an experimental set up of a PEMFC emulator associated with an IBC converter has been designed and implemented. The 16F877A microcontroller is used for the MPPT-PEMFC implementation. The system is built as shown in Fig. 10. Measuring the voltage and the current of the PEMFC emulator is necessary to search the MPP. This acquisition is assured by voltage and current (LEM) sensors. The semiconductor devices used are the IRFP360 power MOSFET and a BY329 rapid diode. The shifting process is realized by using a shift register (74LS164P) and a (16F84A) microcontroller for generating the clock signals needed for the shifting process. The clock signals generated by the (16F84A) is used to create a shift of the PWM signals generated by the (16F877A). The control signals (sr1, sr2 and sr3) are taken on pins 1, 4 and 8 of the (74LS164P) respictively (Fig. 11). Fig. 12 shows the clock signal and the control signals supplied by the 74LS164P. The experimental results of the inductor currents and the gate signals waveforms are shown in Figs. 13 and 14. The experimental results are in agreement with those obtained with theoretically simulation.
Conclusion In this paper, an interleaved dc/dc boost converter with MPPT for fuel cell electric vehicle is proposed. Fuel cell MPPT technique is implemented using microcontroller components. The MPPT technique is implemented to get the MPP from the fuel cell. Simulation and experimental result illustrates that the IBC solves the problems of high ripple currents. The MPPT implementation can be easily extended to other interleaved converters.
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Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185
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Please cite this article in press as: Benyahia N, et al., MPPT controller for an interleaved boost dcedc converter used in fuel cell electric vehicles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.185