Control Algorithm of Fuel Cell and Batteries for Distributed Generation ...

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Control Algorithm of Fuel Cell and Batteries for Distributed Generation System Phatiphat Thounthong, St´ephane Ra¨el, and Bernard Davat, Member, IEEE

Abstract—This paper intends to propose a novel control algorithm for utilizing a polymer electrolyte membrane fuel cell (PEMFC) as a main power source and batteries as a complementary source, for hybrid power sources for distributed generation system, particularly for future electric vehicle applications. The control, which takes into account the slow dynamics of a fuel cell (FC) in order to avoid fuel (hydrogen and air) starvation problems, is obviously simpler than state machines used for hybrid source control. The control strategy lies in using an FC for supplying energy to battery and load at the dc bus. The structure is an FC current, battery current, and battery state-of-charge (SOC) cascade control. To validate the proposed principle, a hardware system is realized by analogical circuits for the FC current loop and numerical calculation (dSPACE) for the battery current and SOC loops. Experimental results with small-scale devices (a 500 W PEM FC and 33 Ah, 48 V lead-acid battery bank) illustrate the excellent control scheme during motor drive cycles. Index Terms—Batteries, cascade control, converters, current control, electric vehicles, energy storage, fuel cells (FCs).

I. INTRODUCTION UEL CELL (FC), known as a high specific energy source of the present times, is one of the well-known alternative sources of electric power generation in the context of decreasing oil resources and hazardous CO2 emissions [1], [2]. There are many types of FCs characterized by their electrolytes. One of the most promising ones to be utilized in electric vehicle applications is the polymer electrolyte membrane FC (PEMFC) because of its relatively small size, lightweight nature, and ease of construction [3], [4]. According to recent works on a 1.2-kW PEMFC (Ballard System Power, Inc.) [5], and a 0.5-kW PEMFC (Zentrum f¨r Sonnenenergie und Wasserstoff-Forschung, Germany (ZSW, Inc.)) [6], one of the main weak points of the FC is the fact that its time constants are dominated by temperature and fuel delivery system (pumps, valves, and in some cases, a hydrogen reformer). As a result, fast load demand will cause a high volt-

F

Manuscript received February 6, 2006; revised July 21, 2006. This work was supported in part by a research program in cooperation with the Thai-French Innovation Centre with Institut National Polytechnique de Lorraine under the “Franco-Thai on higher education and research joint project” and in part by the French National Center for Scientific Research (CNRS) and the Nancy Research Group in Electrical Engineering (GREEN: UMR 7037). Paper no. TEC-000442006. P. Thounthong is with King Mongkut’s Institute of Technology North Bangkok (KMITNB), Bangkok 10800, Thailand (e-mail: [email protected]; phatiphat. [email protected]). S. Ra¨el and B. Davat are with Institut National Polytechnique de Lorraine (INPL), Nancy 54510, France (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TEC.2006.888028

Fig. 1. FC dynamic characteristics to (a) current step (b) current slope: 4 A·s−1 .

age drop in a short time, which is recognized as fuel starvation phenomena [7], [8]. Fig. 1 presents the 0.5-kW PEMFC voltage response to a current profile obtained by means of a pulse width modulator (PWM) boost converter operated by a PID current corrector, which will be presented later in this paper. The tests operate in two different ways: current step and current slope. It shows the drop of voltage curve in Fig. 1(a), compared to Fig. 1(b), because fuel flows (particularly the delay of air flow) have difficulties following the current step. This condition of operation is evidently harmful for the FC stack [9], [10]. Thus, to utilize the FC in dynamic applications, its current or power slope must be limited, for example, 4 A·s−1 for a 0.5 kW, 12.5 V PEMFC [11]; a 2.5 kW·s−1 for a 40 kW, 70 V PEMFC [12]; and 500 W·s−1 for a 2.5 kW, 22 V PEMFC [13]. To employ a PEMFC as the main source in an electric vehicle called FC vehicles (FCVs), the electrical system must have at least an auxiliary power source to improve the system performance when electrical loads at a dc bus demand high power in a short time (for example, vehicle acceleration and deceleration). Therefore, many recent works have already presented how to use batteries as a secondary source to cogenerate high power in a short time and also to compensate warm-up time of an FC system [14]–[16]. One can also take advantage of this auxiliary

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Fig. 3.

FC converter.

Fig. 4.

Analogical PID controller [27].

Fig. 2. Structure of FC/battery hybrid power source (p F C , v F C and iF C are the FC power, voltage, and current. Note that it has been assumed that there are no losses in FC converter. p L , iL , and p B a t are the load power, current, and the battery power, respectively.

source to achieve an actual hybrid source to dissociate a mean power sizing from peak transient power and to be proficient of regenerative braking [17], [18]. The main objective of the present paper is to propose a new control algorithm of the FC/battery hybrid power source for electric vehicle applications. The control strategy lies in using the FC to supply energy to load and to keep the battery charged with respect to the FC dynamics. This is easier than the common state definitions, which are used for hybrid source regulation and freedom from chattering problems. To validate the proposed principle, the hardware system is realized by analogical circuits for an FC current loop and numerical calculation (dSPACE) for loops of the battery current and the state-of-charge (SOC). Experimental results obtained with a small-scale device test bench based on a 40 A, 12.5 V PEM FC connected to dc link (48 V, 500 W) by a classical boost converter, lead-acid batteries (33 Ah, 12 V) connected in four-series, and a dc motor coupled with a dc generator as an electrical load connected to dc link by two-quadrant converter present the system performances. II. FC/BATTERY HYBRID POWER SOURCES A. Structure of FC/Battery Hybrid Power Source FC voltage vFC is the highest when no current is flowing and drops with increasing current because of activation overvoltage and ohmic resistance losses in the membrane. At rated current, vFC drops to around half of the no-load voltage [19]–[21]. Nevertheless, battery voltage, for example, in a lead-acid battery, is nearly constant and virtually independent from discharge current and drops sharply when almost fully discharged [22]–[24]. Many previous works with FC/battery hybrid source have operated by connecting batteries directly to a dc link [for example, dc link voltage (battery voltage) of the Honda Hybrid Insight is 144 V] and interfacing FC to dc link by step-up unidirectional converter because vFC is normally lower than dc bus voltage (Fig. 2). Note that a resistive brake (RB ) is a protection device to prevent overvoltage at the dc bus. When an FC operates, its fuel (hydrogen and oxygen) flows are controlled by a “fuel cell processor,” which receives current demand. This current demand is the FC current reference iFCREF coming from the hybrid control algorithm detailed here-

after. The fuel flows must be adjusted to match the reactant delivery rate to the usage rate by the FC processor. For this reason, the inner FC current control loop is obligatory and the hybrid control algorithm demands energy from the FC to dc link by generating iFCREF [25], which is sent to the FC processor synchronously (Fig. 2). One can take advantage of the safety and high dynamic characteristics of this loop as well; thus, it must be realized by analogical circuits to function at high bandwidth. The studied hardware system is a dc bus: 48 V, 500 W; ZSW PEMFC [26]: 500 W, 40 A, around 12.5 V; lead-acid battery module: 48 V, 33 Ah. B. FC Converter [6], [25] A classical boost converter is selected for an FC converter (Fig. 3). It is composed of a high-frequency inductor L1 (ferrite core: 72 µH), a total dc bus capacitor CBus = C1 + C2 (0.702 F), a diode D1 , and a main switch S1 . Note that switch S2 is a shutdown device to prevent the FC stack from short circuit in case of an accidental destruction of S1 , or a faulty operation of the regulator. The FC converter is driven, through MOSFET S1 gate signal, by means of a PWM for average current control in continuous conduction mode, in order to obtain a constant switching frequency of 25 kHz for the FC current. The used PWM generator is a high-speed PWM generator UC28025B (Texas Instruments, Inc.). Moreover, an analogical PID corrector is chosen for the FC current controller (Fig. 4). The open-loop transfer function (OL) of an FC current regulation can be expressed as follows [11]:
˜iFCM ea (s)

˜iFCREF (s)
OL 

Analogical PID controller

PW M

˜ ˜i F C (s)/ d(s)

filter

         Gi (Tz s+1) (TCi s+1)(TCd s+1) 1 GFC = GC TCi s VP ( ωs )2 + ω2ζ s+1 TFC s+1 n

n

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Fig. 5.

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Fig. 6.

Battery SOC control loop.

Fig. 7.

Charging battery algorithm.

Open loop bode diagram of ˜iF C M e a (s)/˜iF C R E F (s).

with 

Gi = Tz =

IF C (1−D ) VB u s CB u s (1−D )I F C



 ω = (1−D ) 2 n L1 CB u s and  ζ = R L 1 C B 2u s ω n (1−D )

(1)

2

where D is the nominal duty cycle (chosen at 0.7) of the PWMFC converter, d˜ is the duty cycle variations, VBus is the nominal dc bus voltage (selected at 42 V, new standard automotive voltage “PowerNet”), IFC is the nominal FC current (40 A), ˜iFC is the FC current variations, and RL 1 is the total series resistance (47.5mΩ) of L1 , wiring, and FC. The gain GFC and time constant TFC are 8 V/40 A and 1 ms, respectively. The derivative time constant TCd is chosen in order to compensate the filter pole −1/TFC associated with the FC current measurement filter. The gain GC (0.02) and integral time constant TCi (0.38 ms) are set to obtain a phase margin of 55◦ at a crossover frequency of 1500 rad/s (Fig. 5). C. Proposed Control Algorithm To manage energy exchanges between the dc link, the main source, and the storage device, one may define three operating modes (or states): 1) Charge mode: In this, the main source supplies energy to the storage device and to the load. 2) Discharge mode: Here, both the main source and the storage device supply energy to the load. 3) Recovery mode: In this, the load supplies energy to the storage device. This method has already been investigated earlier, for example, in [15], where an unregulated voltage FC/battery hybrid source was investigated, or in [28], where a regulated voltage FC/supercapacitor hybrid source was studied. The problem of such a control strategy is well known. The definition of system states (used state machine) implies control algorithm permutations, which may lead to a phenomenon of chattering when the system is operating near a border between two states. The proposed control scheme hereafter is not based on a state definition, so that it presents no chattering problem. Rather, one

takes advantage of a battery bank, which is directly connected to a dc link for supplying transient energy demand and peak loads required during motor acceleration and deceleration, as if this device is a standard power supply. In addition, the FC, as a slow dynamic device, functions to supply energy to a battery bank in order to keep it charged, although it is evidently the main power energy source of the system. The proposed control strategy is a cascade control structure composed of three loops. The outer loop is the battery SOC controller that links the battery SOC to the battery charging reference current iBatREF . The middle loop controls the battery charging current and links iBatREF to the FC current reference IFCREF . The inner loop is the FC current control, already explained in the previous section. 1) Battery SOC Control Loop: The proposed control structure for charging the battery is presented in Fig. 6. The familiar battery SOC estimation is defined as [13]  t 1 iBat (τ ) dτ (2) SOC(t) = SOC0 + QBat t 0 where SOC0 is the known battery SOC (in percentage) at the time t0 and QBat is the rated capacity (ampere-hour). The simple method to charge the battery is constant current (maximum current IBatm ax is set around QBat /2– QBat /5; for an Li-ion battery, it can be set at IBatm ax = QBat ) when SOC is far from the SOC reference value SOCREF [29] and reduced current when SOC is near SOCREF and zero when SOC is equal to SOCREF (Fig. 7). More importantly, in automotive applications, battery monitoring is compulsory in order to replace an aged battery [30],

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Fig. 9.

Fig. 8.

Battery current limitation to dc bus voltage.

If variations are introduced and load power is considered as disturbance of the system, (5) becomes

[31]. In particular, the potential capacity QBat is dependent on the depth of discharge, discharge rate, cell temperature, charging regime, dwell time at low and high SOC, battery maintenance procedures, current ripple, and amount and frequency of overcharge [32], [33]. It is beyond the scope of this paper to observe the potential capacity of the battery. It is assumed that QBat is constant. Additionally, in a real system of applications, SOC0 can be retained in a storage device. According to this SOC algorithm, a proportional (P)controller is enough to generate a battery charging current iBatChar for the battery current control loop. And the charging current must be limited at IBatm ax . The P-controller gain (GSOC ) can be sized as GSOC =

IBatm ax ∆SOC

(3)

where ∆SOC is the defined band of battery SOC. To avoid overvoltage at the dc link in case of an erroneous SOC estimation or regenerative braking, the dc bus voltage must be monitored to limit charging current. The battery current limitation function (Fig. 8) consists of limiting the reference iBatREF versus the dc bus voltage as [25]

VBusm ax − vBus (t) iBatREF (t) = iBatChar (t) min 1, ∆VBus (4) where VBusm ax is the defined maximum dc bus voltage and ∆VBus is the defined voltage band. 2) Battery Current Control Loop: The battery current control loop receives iBatREF from an SOC regulation loop as illustrated in Fig. 9. A P controller is sufficient to generate the FC current reference iFCREF . It must be limited in level, within an interval maximum IFCm ax (corresponding to a rated current of the FC) and minimum IFCm in (set to 0 A) and limited in slope to a maximum absolute value GSL (in amperes per second), which enables the safe operation of the FC in order to respect constraints associated with the FC, as far as the proportional gain GiBat is high enough to introduce only a small static error. To obtain the OL associated with the battery current control loop, one may write power conservation (without losses) as vFC (t)iFC (t) = vBus (t)iBat (t) + pL (t).

Battery current control loop.

(5)

˜iFC (t) = VBus ˜iBat (t) VFC

(6)

where VBus and VFC are the nominal dc bus and FC voltage, respectively. Therefore, the OL may be written with a gain depending on the operating point as P controller
   ˜iBatM ea (s)

= GiBat
˜iBatREF (s) OL

˜i B a t (s) /˜i F C (s)

   VFC VBus



filter

  GBat . TBat s + 1

(7)

A first-order low-pass filter is used for the battery current measurement in order to reduce ripple current coming from the switching frequency of the FC and electric load (motor drive) converters. D. Conclusion of Proposed Control Algorithm The control of the whole system is based on the SOC of the battery, whatever the load current is. r If SOC is lower than SOCREF, the battery charging current reference is positive and an FC current is necessary to charge the battery. r If SOC is higher than SOCREF, the battery charging current reference is equal to zero and the FC current reference is reduced to zero. As a consequence, a transient in the load modifies the FC current when the battery SOC becomes lower than SOCREF. In any case if SOC is higher than SOCREF, the FC current reference is equal to zero. For transient conditions, as FC current dynamics have been intentionally reduced, the battery supplies all load variations. III. EXPERIMENTAL VALIDATION A. Test Bench Description The small-scale test bench is presented in Fig. 10. The storage device is obtained by means of four aged lead-acid batteries 7.78 Ah (33 Ah at name plate), 12 V connected in series. The PEMFC system (Fig. 11) was constructed by the ZSW company. It is composed of 23 cells of 100 cm2 in series. It is supplied with pure hydrogen from bottles under pressure and with clean, dry air from a compressor [26].

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Fig. 10.

Hybrid source test bench.

Fig. 11.

Simplified ZSW fuel cell system [17]. TABLE I BATTERY SOC REGULATION LOOP PARAMETERS

Fig. 12. TABLE II BATTERY CURRENT REGULATION LOOP PARAMETERS

Hybrid source response during starting motor to 600 r/min.

The battery current control loop, which generates iFCREF , has been implemented in the real time card dSPACE DS1104, through the mathematical environment of Matlab–Simulink, with a sampling frequency of 25 kHz. The ControlDesk software enables changes in the parameters of the control loops. It is employed to maneuver a dc motor drive as well.

B. Control Description The measurements of the FC current iFC , the battery current iBat , and the dc link voltage vBus are realized by means of zero-flux Hall effect sensors. The parameters associated with the battery SOC regulation loop can be seen in Table I. The parameters associated with the battery current control loop are detailed in Table II. The FC current absolute slope limitation (GSL ) is set to 4 A·s−1 . This value has been experimentally determined as the highest current slope of this FC system, where no fuel starvation occurs [9], [11], [25].

C. Test Bench Results The experimental tests shown below were carried out by connecting the dc link to an active load composed of a two-quadrant converter, loaded by a dc motor coupled with a dc generator. A motor functions with the cascade current–speed control method. A hysteresis and PI controller are selected for the motor current and speed loops, respectively, with a current limitation at ±60 A. Figs. 12 and 13 present waveforms obtained at the motor start: the dc bus and FC voltages, motor speed, FC, battery and

THOUNTHONG et al.: CONTROL ALGORITHM OF FUEL CELL AND BATTERIES

Fig. 13.

Hybrid source response during starting motor to 1 500 r/min.

load (motor) powers, FC, battery and motor currents and battery SOC. The initial state is zero for both the FC and battery currents and, 100% for the battery SOC. In Fig. 12, the final motor speed is 600 r/min, so that the final FC current is less than IFCRated . One can observe the following: r The battery supplies most of the power required during motor acceleration. r The peak load power required during the motor start is about 320 W and the steady state load power is about 300 W, entirely supplied by the FC. r The FC current increases with a limited slope, up to a level lower than 40 A. r Simultaneously, the battery current, after a sharp increase during motor acceleration, decreases slowly to zero. r The final battery SOC is lower than 100%, because of a small static error introduced by a P corrector of the battery SOC control loop.

153

Fig. 14.

Hybrid source response during braking motor from 600 r/min.

In the case of Fig. 13, the final motor speed is 1 500 r/min, such that the final FC current is IFCRated . Thus, the battery, which once again supplies most of the power required during motor acceleration, remains in a discharge state after the motor start. The final battery current is −10 A, because the steady-state load power (approximately 800 W) is greater than the FC rated power (500 W). Note that the FC current increases up to 40 A in 10 s (one can calculate, here, the slope limitation of 4 A·s−1 ) and the peak load power is about 1.7 kW, which is around three times that of the FC rated power. Fig. 14 presents waveforms obtained when reducing the motor speed to stop from 600 r/min. The initial state is zero for the battery current and nearly 100% for the battery SOC. The total load power for driving the motor is supplied solely by the FC. The final state is zero for the FC, battery and motor currents and 100% for the battery SOC. This shows that the battery, first, recovers the power supplied to the dc link by the FC and by

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current is nearly constant at around 22 A, to charge the battery at a maximum battery current (IBatm ax ) around 6 A. After that, both the FC and battery current will reduce to zero when SOC will reach SOCREF. IV. CONCLUSION The main objective of this paper is to propose a new control scheme for a hybrid power source supplied by a battery bank associated with a PEMFC, in order to manage the energy transfer from an FC to the dc link in high dynamic applications, especially for future electric vehicles. The important point here is to show the slow dynamics of the FC system because of mechanical delays. Hence, the control principle presents how to avoid the fast transition of the FC current, in order to prevent fuel starvation problems by controlling the FC current slope, and then, reducing mechanical stresses in the FC system (fuel pressure, water pressure in tubes, and stack). Experimental results, with small-scale devices of a 500 W, 40-A PEMFC and a 48 V, 33 Ah battery bank, authenticate the excellent performances of the proposed control algorithm during motor drive cycles. ACKNOWLEDGMENT The authors would like to thank Mr. I. Sadli for operating the FC system during experimentations. REFERENCES

Fig. 15.

Hybrid source response during braking motor from 1500 r/min.

the motor (a motor current is negative, known as a regenerative braking energy), and then, the battery is slightly charged by the FC up to 100%. The FC current immediately decreases with a limited slope, and in a second phase (the end of charging of the battery), slowly decreases to zero. The peak load power during motor brake is about −100 W, automatically recovered by the battery due to the connection at the directly dc link. Fig. 15 illustrates waveforms obtained when reducing the motor speed to stop from 1 500 r/min with a peak load power of about –700 W. The battery is more deeply charged than in the previous case, demonstrating the three phases. First, the battery recovers the power supplied to the dc link by the FC and the motor. Second, the battery recovers the reduced power supplied to the dc link by only the FC. Third, the battery is charged at a constant current by the FC. During the first and second phases, the FC current reduces from a rated current 40 A with a constant slope 4 A·s−1 . In the third phase, the FC

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Phatiphat Thounthong received the B.S. and M.E. degrees in electrical engineering from King Mongkut’s Institute of Technology North Bangkok (KMITNB), Bangkok, Thailand, in 1996 and 2001, respectively, and the Ph.D. degree in electrical engineering from Institut National Polytechnique de Lorraine (INPL), Nancy, France, in 2005. From 1997 to 1998, he was an Electrical Engineer with E.R. Metal Works Ltd. (EKARAT Group), Thailand. From 1998 to 2002, he was an Assistant Lecturer at KMITNB where he is currently a Lecturer. His current research interests include power electronics, electric drives, and electrical devices (fuel cell, batteries and supercapacitor).

St´ephane Ra¨el received the M.E. degree in electrical engineering from the Ecole Nationale Sup´erieure des Ing´enieurs Electriciens de Grenoble (ENSIEG), Grenoble, France, in 1992, and the Ph.D. degree in electrical engineering from the Institut National Polytechnique de Grenoble (INPG), Grenoble, in 1996. Since 1998, he has been an Assistant Professor at the Institut National Polytechnique de Lorraine (INPL), Nancy, France. His research interests include power electronic components, supercapacitors, batteries, and fuel cells.

Bernard Davat (M’89) received the Engineer degree from Ecole Nationale Sup´erieure d’Electrotechnique, d’Electronique, d’ Informatique, d’Hydraulique et des Telecommunications (ENSEEIHT), Toulouse, France, in 1975, and the Ph.D. and Docteur d’Etat degrees in elecrical engineering from Institut National Polytechnique de Toulouse (INPT), Toulouse, in 1978 and 1984, respectively. From 1980 to 1988, he was a Researcher at French National Center for Scientific Research (CNRS), Laboratoire d’Electrotechnique et d’Electronique Industrielle (LEEI). Since 1988, he has been a Professor at Institut National Polytechnique de Lorraine (INPL), Nancy, France. He is the Author of Power Semiconductor Converters. His current research interests include power electronics, drives, and new electrical devices (fuel cell and supercapacitor).