ELECTRICAL energy storage is compulsory in numerous

Transcription

1 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH Analysis of Supercapacitor as Second Source Based on Fuel Cell Power Generation Phatiphat Thounthong, Member, IEEE, Stéphane Raël, and Bernard Davat, Member, IEEE Abstract This paper presents the utilization of a supercapacitor as an auxiliary power source in a distributed generation system, composed of a polymer electrolyte membrane fuel cell (PEMFC) as the main energy source. The main weak point of fuel cells (FCs) is slow dynamics because one must limit the FC current slope in order to prevent fuel starvation problems, to improve its performance and lifetime. The very fast power response and high specific power of a supercapacitor can complement the slower power output of the main source to produce the compatibility and performance characteristics needed in a load. The FC and supercapacitor characteristics are clearly presented. Experimental results with small-scale devices (supercapacitor bank: 292-F, 30-V, 400-A; PEMFC: 500-W, 40-A) illustrate excellent performance during a motor drive cycle. Index Terms Converters, current control, electric vehicles, energy storage, fuel cells (FCs), supercapacitor. I. INTRODUCTION ELECTRICAL energy storage is compulsory in numerous applications: telecommunication devices (such as cell phones), stand-by power systems, hybrid vehicles, and new electric hybrid vehicles [1], [2]. Electrochemical capacitors are presently called by a number of names: supercapacitor, ultracapacitor, or double-layer capacitor; these terms are used interchangeably. The first high-power supercapacitors were developed by the Pinnacle Research Institute (PRI) for the U.S. military applications such as laser weaponry and missile guidance systems. However, only in the 19th century did supercapacitors become well known in the context of hybrid electric vehicles promoted by the Department of Energy (DOE) [3]. Fuel cell (FC) power sources are expected to be used in a growing number of applications: in portable applications, in transportation applications [4] [6], and in stationary power applications [7], [8]. In recent works, an FC/supercapacitor hybrid Manuscript received September 8, 2006; revised April 22, First published January 9, 2009; current version published February 19, This work was supported in part by a research program in cooperation with the Thai-French Innovation Institute, King Mongkut s University of Technology North Bangkok with the 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 P. Thounthong is with the Department of Teacher Training in Electrical Engineering (TE), King Mongkut s University of Technology North Bangkok (KMUTNB), Bangkok 10800, Thailand ( phtt@kmutnb.ac.th). S. Raël and B. Davat are with the Groupe de Recherche en Electrotechnique et Electronique de Nancy (GREEN), Institut National Polytechnique de Lorraine (INPL), Nancy 54510, France ( stephane.rael@ensem.inplnancy.fr; bernard.davat@ensem.inpl-nancy.fr). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TEC source [9], [10] and an FC/battery hybrid source [11], [12] have already been reported. Nonetheless, some research has only shown the simulation results in which the FC is considered as an ideal source; some have operated with a small-scale FC in which FC weak points cannot be observed such as the fuel starvation problem, especially to utilize the FC in dynamic applications. Reliability and lifetime are the most essential considerations in such power sources. Taniguchi et al. [13] clearly demonstrated that hydrogen and oxygen starvation caused severe and permanent damage to the electrocatalyst of the FC. They have recommended that fuel starvation must absolutely be avoided, even if the operation under fuel starvation is momentary, in just 1 s [13], [14]. For these reasons, the use of the supercapacitor as an auxiliary source is expected that the very fast power response and high specific power can complement the slower power output of the main source (particularly the FC generator). Various recent researches [15] [17] have documented the subject of supercapacitor technology, but without its applications, particularly FC applications. Presented here is a hybridization of the supercapacitor as an energy storage device with an FC as a main source. The next section contains a description of the FC characteristics, especially the fuel starvation problem. In Section III, the supercapacitor is presented in detail: a state of the art, a model, a converter, and current regulation. The hybrid control algorithm will be explained in Section IV. In the final section, experimental results will show the supercapacitor characteristics during operation with a high switching frequency, a constant discharging current and an FC hybrid source during a motor drive cycle. II. FUEL CELL CHARACTERISTICS A. Fuel Cell Principle FCs are electrochemical devices that directly convert the chemical energy of a fuel into electricity. Energy is released whenever a fuel (hydrogen) reacts chemically with the oxygen in air. In the case of hydrogen/oxygen FCs, which are the focus of most research activities today, the only by-product is water and heat [18] [20]. Polymer electrolyte membrane FCs (PEM- FCs) are promising power sources because of their relatively small size, lightweight, and ease to build. The FC model here is for a type of PEM, which uses the following electrochemical reaction: H O 2 H 2 O+Heat+ Electrical Energy. (1) /$ IEEE

2 248 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009 Fig. 1. Simplified diagram of the PEMFC system. The FC voltage V FC is modeled as [20], [21]: Activation loss { ( }} ){ Ohmic loss IFC + i {}}{ n V FC = E A log R m (I FC + i n ) i o Concentration loss { ( }}{ + B log 1 I ) FC + i n (2) i L where E is the reversible no loss voltage of the FC (the thermodynamic potential), I FC the delivered FC current, i o the exchange current, A the slope of the Tafel line, i L the limiting current, B the constant in the mass transfer term, i n the internal current, and R m the membrane and contact resistances. As developed earlier, the Nernst equation for the hydrogen/oxygen FC (using literature values [21] for the standardstate entropy change) can be written as: { E = (T ) T [ ln (p H 2 )+ 1 ]} 2 ln (p O 2 ) n Cell (3) where T is the cell temperature (in Kelvin), p H 2 and p O 2 are the partial pressure of hydrogen and oxygen (in bar), respectively, and n Cell is the number of cells in series. B. Fuel Cell System Fig. 1 shows a simplified diagram of the PEMFC system, which is also employed for this research. Constructed by the ZSW Company (Germany), the FC stack (500 W, 40 A, 13 V) is composed of 23 cells with area of 100 cm 2 [22]. When an FC system is operated, its fuel flows are controlled by a Fuel Cell Controller (see Fig. 1), which receives an FC current demand (reference), i FCREF, from the user (manual operation) or from the hybrid control algorithm (in case of automatic operation). The fuel flows must be adjusted to match the reactant delivery rate to the usage rate by the FC controller [11]. C. Fuel Starvation of the FC Stack For clarity about the dynamic limitation of the FC generator, Fig. 2 presents the 0.5 kw PEMFC voltage response to a current. The tests operate in two different ways: current step and current slope. It shows the drop of the voltage curve in Fig. 2(a), com- Fig. 2. FC dynamic characteristics to (a) current step (b) current slope: 4 A/s. pared with Fig. 2(b), because fuel flows (particularly the delay of air flow) have difficulties following the current step, called the fuel starvation phenomenon. This condition of operation is evidently harmful for the FC stack [13], [14]. Without any doubt, to use the FC in dynamic applications, its current or power slope must be limited, but some research work has omitted to do this. One may lack the FC information in which failure modes for FC are not well documented, and degradation causes and mechanisms are not understood. However, recent works with evidently experimental results have been based on the control of the FC current slope, for example: 4 A/s for a 0.5-kW, 12.5-V PEMFC [9], [11] and 500 W/s for a 2.5-kW, 22-V PEMFC [23]. A. State of the Art III. SUPERCAPACITOR The concept of the supercapacitor is not recent. Nevertheless, marketing only began at the end of 1970s. These devices were low-sized devices (capacitance of some farads, low specific energy), dedicated to signal applications such as memory backup [17]. Since the early 1990s, supercapacitors dedicated to highpower industrial applications (capacitance up to some thousands farads, specific energy and specific power of several Wh/kg and kw/kg, respectively) have been available. Especially, DOE supercapacitor development programs of long-term goals are specific energy >15 Wh/kg, specific power >2.0 kw/kg after 2003 [3].

3 THOUNTHONG et al.: ANALYSIS OF SUPERCAPACITOR AS SECOND SOURCE BASED ON FUEL CELL POWER GENERATION 249 Fig. 4. Simplified equivalent circuit of a supercapacitor cell including R p. Fig. 3. Specific power versus specific energy of supercapacitor, NiCd, NiMH, and Li-Ion battery technology from the SAFT company. Finally, the comparison of storage device technologies from SAFT Company is depicted in Fig. 3. Even though it is true that a battery has the largest energy density (meaning more energy is stored per unit of weight than other technologies), it is important to consider the availability of that energy. This is the traditional advantage of capacitors. With a time constant of less than 0.1 s, energy can be taken from a capacitor at a very high rate. On the contrary, the same size battery will not be able to supply the necessary energy in the same time period. Additionally, the main drawback of the batteries is a slow charging time, limited by a charging current; in contrast, the supercapacitor can be charged in a short time, depending on the availability of a high charging current (power) from the main source. The capacitor voltage v C (t) can then be found using the following classical equation: v C (t) = 1 C tn t 0 i C (t) dt. (4) For example, an SAFT supercapacitor module (583 F, 15 V, 400 A) can be charged from zero voltage (zero of charge) to the maximum voltage in 22 s at a constant current of 400 A. More advantageous, unlike batteries, supercapacitors can withstand a very large number (thousands to millions) of charge/discharge cycles without degradation [24]. B. Supercapacitor Model The supercapacitor model is very complex because of the distributed-parameter model. Many different models have been proposed for the double-layer effect [17]. Recent works [10], [17] have proposed that the reduced order model (as portrayed in Fig. 4) for a supercapacitor cell is presented because of its simplicity and its operating times on the order of a few seconds. It is comprised of three ideal circuit elements: a capacitor C Cell, a series resistor R S called the equivalent series resistance (ESR), and a parallel resistor R p. The parallel resistor R p models the leakage current found in all capacitors. This leakage current is equal to a few milliamps in a big supercapacitor. Fig. 5. Discharge profile for a supercapacitor under constant current. Many applications require the capacitors to be connected together, in series and/or parallel combinations, to form a bank with a specific voltage and capacitance rating. Normally, they are always connected in series. Capacitance variations affect the voltage distribution during cycling, and voltage distribution during sustained operation at a fixed voltage is influenced by leakage current variations. For this reason, an active voltage balancing circuit is employed to regulate the cell voltage. It is common to choose a specific voltage, and thus, calculating the required capacitance. In analyzing any application, one first needs to determine the following system variables affecting the choice of supercapacitor: 1) maximum voltage, V CMax ; 2) working (nominal) voltage, V CNom ; 3) minimum allowable voltage, V CMin ; 4) current requirement, I C, or the power requirement, P C ; 5) time of discharge, t d ; 6) capacitance per cell, C Cell ; 7) cell voltage, V Cell. Connecting many cells in series to form a bank, this does lead to an increase in total ESR and to a decrease in total capacitance. Defining n S as the number of capacitors connected in series, the maximum capacitor voltage V CMax, total ESR, and capacitance C SuperC of the capacitor bank can be estimated as: V CMax = n S V Cell ESR = n S R S C SuperC = C Cell n S. (5) The discharge profile for a supercapacitor bank under a constant current is shown in Fig. 5. A constant discharging current

4 250 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009 Fig. 6. Discharge profile for a supercapacitor under constant power. I C is particularly useful when determining the parameters of the supercapacitor. Nevertheless, Fig. 5 should not be used to consider sizing supercapacitors for constant power applications such as a general power profile (drive cycle) used in electric vehicle. Worst case scenarios from drive cycle determine size of the storage devices. For example, Mitchell et al. [25] presented that the Renault fuel cell automobile (SCENIC II, rated power of 70 kw of a PEMFC) needs a supplementary constant power (from battery) of around 30 kw for a 3 s for transient power (vehicle acceleration). Then, the discharge profile for a supercapacitor under a constant power P C is shown in Fig. 6. To estimate the minimum capacitance requirement C Min, one can write an energy equation without losses (ESR neglected) under a discharging constant power P C as: 1 2 C ( Min V 2 C Nom vsuperc 2 (t) ) = P C t (6) where v SuperC (t) is the supercapacitor terminal voltage. Then, 2P C t d C Min = VC 2 Nom V C 2. (7) Min Since the power being delivered is constant, the minimum voltage V C Min and maximum current I C Max can be determined based on the current conducting capabilities of the supercapacitor. Equations (6) and (7) can then be rewritten as: V C Min = VC 2 Nom 2P C t d I C Max = C Min P C V 2 C Nom (2P C t d /C Min ). (8) The variables V C Max and C SuperC are related by the number of cells in series. Voltage rating is important, but the capacitor will also fail if the current is too high. The assumption is that the capacitors will never be charged above the combined maximum voltage rating of all the cells. Generally, V C Min is chosen as V C Max /2, from (6), resulting in the remaining energy of 25%. In applications where high currents are drawn, the effect of the ESR has to be taken into account. The energy dissipated E loss in the ESR, as well as in the cabling, connectors, and converter, could result in an undersizing of the number of capacitors required. For this reason, one can theoretically calculate the losses Fig. 7. in the ESR as: Two-quadrant supercapacitor converter. E loss = td 0 i 2 SuperC (τ)esrdτ = P C ESRC Min ln ( VC Nom V C Min ). (9) To calculate the required capacitance C SuperC, one can rewrite (6) as: 1 2 C ( SuperC V 2 C Nom VC 2 ) Min = PC t d + E loss. (10) From (6) and (10), one obtains: C SuperC =(1+χ) C Min χ = E loss (11) P C t d where χ is the defined energy ratio. C. Supercapacitor Converter The supercapacitor bank is connected to the dc bus by means of a 2-quadrant dc/dc converter (bidirectional converter). The converter studied here is the circuit, as shown in Fig. 7 [26]. L 1 represents the inductor used for energy transfer and current filtering. The inductor size is classically defined by switching frequency and current ripple. Note here that since it is beyond the scope of this paper to discuss converter circuit topologies, a simple circuit has been selected. The supercapacitor current, which flows across the storage device, can be positive or negative, allowing energy to be transferred in both directions. The supercapacitor converter is driven by means of complementary pulses, applied on the gates of two insulated-gate bipolar transistors (IGBTs): S 1 and S 2. The gate drive commands are generated by a hysteresis controller coming from the hybrid control algorithm. It is realized in order to control the supercapacitor current. This controller is selected because of the simplicity to implement it and its fastest response to current reference [27]. IV. HYBRID CONTROL ALGORITHM Fig. 8 depicts the hybrid source control strategy [22]. It lies in using the supercapacitor bank, which is the fastest energy source

5 THOUNTHONG et al.: ANALYSIS OF SUPERCAPACITOR AS SECOND SOURCE BASED ON FUEL CELL POWER GENERATION 251 Fig. 8. FC/supercapacitor hybrid control algorithm, where v Bus is the dc bus voltage, V BusREF the dc bus voltage reference, E BusREF the dc bus energy reference, E BusMea the filtered dc bus energy, v SuperC the supercapacitor voltage, V SuperCREF the supercapacitor voltage reference, v SuperCMea the filtered dc bus voltage, v FC the FC voltage, i SuperC the supercapacitor current, i SuperCREF the supercapacitor current reference, i FC the FC current, i FCREF the FC current reference, i FCMea the filtered FC current, p SuperCREF the supercapacitor power reference, and G SL the absolute value for the FC current slope limitation. of the system, for supplying the energy required to achieve the dc bus voltage regulation, as if this device were a standard power supply. Therefore, the FC, although obviously the main energy source of the system, may be seen as the device that supplies energy to supercapacitor bank to keep them charged. Consequently, the supercapacitor converter is driven to realize a classical dc link voltage regulation, and the FC converter is driven to maintain the supercapacitor bank at a given state of charge. For reasons of safety and dynamics, these converters are primarily controlled by inner current loops. These current loops are supplied by two reference signals, i SuperCREF and i FCREF, generated by the dc link voltage regulation loop and the supercapacitor voltage regulation loop, respectively (i FCREF also supplies the fuel cell controller to adjust fuel flows to the desired current) [11]. For the dc bus voltage control loop, supercapacitor power reference p SuperCREF is generated by means of a proportional integral (PI) regulator. This signal is then divided by the supercapacitor voltage, and limited to maintain supercapacitor voltage within an interval [V C Min,V C Max ]. This results in supercapacitor current reference i SuperCREF. This reference must also be limited within an interval (maximum charging, maximum discharging), as depicted in the block SuperC Limitation Function [9]. For the supercapacitor voltage control loop, it consists of a proportional (P) controller limited in level and slope, to respect constraints associated with the FC. The reference i FCREF that drives the FC converter through the FC current loop is then kept within an interval [I FCMin,I FCRated ]. The upper value of this interval corresponds to the rated FC current, and the lower value should be zero. Moreover, slope limitation to a maximum Fig. 9. Test bench of FC/supercapacitor hybrid power source. absolute value (G SL ) of some amperes per second enables safe operation of the FC (refer to FC Current Slope Limitation, even during transient power demand). V. EXPERIMENTAL VALIDATION A. Test Bench Description To validate the supercapacitor as a second source to assist the main source, a small-scale test bench is realized as shown in Fig. 9. The dc bus studied here is 42 V (PowerNet), a new standard voltage in automotive systems [28] [31]. The SAFT supercapacitor bank (292 F, 30 V, 400 A) is composed of two supercapacitor modules connected in series

6 252 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009 Fig. 10. SAFT supercapacitor bank: 292 F, 30 V, 400 A. (see Fig. 10). A module is six cells connected in series (single cell: 3500 F, 2.5 V, 400 A, ESR = 1mΩ). One chooses: V C Max = 30 V; V C Nom =25 V; V C Min = V C Max/2 =15 V. From (7) without losses, the energy storage (discharging from V C Nom to V C Min ) is equal to 58.4 kj or 16.2 Wh. The current ripple I of the hysteresis current controller is set at 3 A. The PEMFC system is 500 W, 40 A, 13 V (see Fig. 1). The FC stack is connected to the dc bus by a classical boost converter [11]. The FC maximum (rated) current is set at 40 A with a controlled current slope of 4 A/s (absolute value), as already presented in Fig. 2. The hybrid control algorithm is implemented in the real-time card dspace DS1104, through the mathematical environment of Matlab-Simulink. Fig. 11. Supercapacitor current response to a step 0 50 A. B. Supercapacitor Characteristics to Constant Current Discharging Fig. 11 presents the transient response of supercapacitor converter interfacing between the dc bus and the supercapacitor bank. The initial voltage of the supercapacitor bank is 30 V. The current reference i SuperCREF is Ch2 and the measured current i SuperC is Ch4. One can observe that the hysteresis controller functions well with a current ripple of 3 A and the high dynamic response of the supercapacitor auxiliary source (from 0 to 50 A in 0.4 ms). Fig. 12 illustrates the constant discharging current with a current step from 0 to 50 A, and vice versa. The initial voltage of the supercapacitor bank is 30 V. It can be observed that the initial voltage of the storage device instantaneously dropped when stepping current because of ESR effect, which can be calculated as: ESR = 30 V 28 V 50 A = 18 V A =40mΩ. (12) This value is over the theoretical value of 12 mω (12 1mΩ). After studying with SAFT engineers who designed the prototype supercapacitor cells, we may conclude that the aged cells may Fig. 12. Supercapacitor current response to a step 0 50 A, and vice versa. lead to increasing oxide connection and total ESR, but we are still waiting for demonstrations of this. From (4), one can also calculate the total capacitance as, C SuperC = 50 A 60 s = 300 F. (13) 28 V 18 V This value is closed to the theoretical value of 292 F. Moreover, one may investigate that during a constant discharging current, the supercapacitor voltage waveform is virtually linear; then the reduced order model presented in Fig. 4 is sufficient for the supercapacitor impedance model with a small leakage current. From (9) to (11), if we know the constant discharging power profile of the system, we can now estimate energy losses in ESR and the useful energy provided to the dc bus. Unquestionably, from these tests, the fast response of the supercapacitor power

7 THOUNTHONG et al.: ANALYSIS OF SUPERCAPACITOR AS SECOND SOURCE BASED ON FUEL CELL POWER GENERATION 253 Fig. 13. Hybrid source response during motor drive cycle.

8 254 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009 source can be operated with the FC main generator in order to improve the slow dynamics of the FC system. C. Performances of Fuel Cell/Supercapacitor Hybrid Source The experimental tests shown later 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. Fig. 13 presents waveforms obtained during the motor drive cycle: the dc bus and FC voltages; FC, supercapacitor, and load (motor) powers; motor speed; FC and supercapacitor currents; and supercapacitor state-of-charge (voltage). The initial state is no-load power and the storage device full-of-charge, V SuperC =25V; as a result, zero for both the FC and supercapacitor currents. At t =10s, the motor speed accelerates to the final speed of 1000 r/min; synchronously the final FC current increases with a limited slope of 4 A/s to a rated current of 40 A. Thus, the supercapacitor, which supplies most of the power required during motor acceleration, remains in a discharge state after the motor start. The final supercapacitor current is 8 A because the steady-state load power (approximately 0.6 kw) is greater than the FC rated power (0.5 kw), and the peak load power is about 1 kw, which is about two times that of the FC rated power. After that, at t =40s, the motor speed decelerates to stop with a peak load power of about 0.5 kw. The supercapacitor is deeply charged, demonstrating three phases. First, the supercapacitor recovers the energy supplied to the dc bus by the FC (0.5 kw) and the motor. Second, the supercapacitor is charged only by the FC. Third, the supercapacitor is nearly fullof-charge, then reducing the charging current. After that, both the FC and supercapacitor currents reduce to zero when V SuperC reaches V C Nom of 25 V. Fortunately, only small perturbations on the dc bus voltage waveform can be seen, which is of major importance in using supercapacitors to improve the dynamic performance of the whole system. VI. CONCLUSION The main objective of this paper is to present the supercapacitor characteristics for energy storage applications, particularly for future FC power generations. The slow dynamics of an FC generator can be ameliorated by using a very fast power response and the high specific power of the supercapacitor source. During motor starts/stops or other significant steps in load, the supercapacitor provides the balance of energy needed during the momentary load transition period, and also, absorbs excess energy from regenerative braking. Experimental results (with small-scale devices of a supercapacitor bank and a PEMFC) authenticate the excellent performance of the whole system during a motor drive cycle. ACKNOWLEDGMENT The authors would like to thank Dr. I. 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9 THOUNTHONG et al.: ANALYSIS OF SUPERCAPACITOR AS SECOND SOURCE BASED ON FUEL CELL POWER GENERATION 255 [24] J. R. Miller and D. A. Evans, Performance characteristics of high reliability double layer capacitor components, in Proc. 35th IEEE Int. Power Sources Symp., Cherry Hill, NJ, Jan , 1992, pp [25] W. Mitchell, B. J. Bowers, C. Garnier, and F. Boudjemaa, Dynamic behavior of gasoline fuel cell electric vehicles, J. Power Sources,vol.154, pp , Mar [26] A. Khaligh, Realization of parasitics in stability of dc dc converters loaded by constant power loads in advanced multiconverter automotive systems, IEEE Trans. Ind. Electron., vol.55,no.6,pp ,Jun [27] O. Wasynczuk, S. D. Sudhoff, T. D. Tran, D. H. Clayton, and H. J. Hegner, A voltage control strategy for current-regulated PWM inverters, IEEE Trans. Power Electron., vol. 11, no. 1, pp. 7 15, Jan [28] B. Fahimi, A. Emadi, and R. B. Sepe, Jr., Switched reluctance machinebased starter/alternator for more electric cars, IEEE Trans. Energy Convers., vol. 19, no. 1, pp , Mar [29] J. Garnier, A. De Bernardinis, M. C. Péera, D. Hissel, D. Candusso, J. M. Kauffmann, and G. Coquery, Study of a PEFC power generator modular architecture based on a multi-stack association, J. Power Sources, vol. 156, pp , May [30] A. Emadi, Y. J. Lee, and K. Rajashekara, Power electronics and motor drives in electric, hybrid electric, and plug-in hybrid electric vehicles, IEEE Trans. Ind. Electron., vol. 55, no. 6, pp , Jun [31] T. A. Keim, Systems for 42 V mass-market automobiles, J. Power Sources, vol. 127, pp , Mar Phatiphat Thounthong (M 09) received the B.S. and M.E. degrees from King Mongkut s Institute of Technology North Bangkok (KMITNB), Bangkok, Thailand, in 1996 and 2001, respectively, and the Ph.D. degree from the Institut National Polytechnique de Lorraine (INPL), Nancy-Lorraine, France, in 2005, all in electrical engineering. 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 an Assistant Professor. His current research interests include power electronics, electric drives, and electrical devices (fuel cell, batteries, and supercapacitor). batteries, and fuel cells. Stéphane Raël received the M.E. degree in electrical engineering from the Ecole Nationale Supérieure des Ingénieurs 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 Since 1998, he has been with the Institut National Polytechnique de Lorraine, Nancy, France, where he was earlier an Assistant Professor, and currently, a Professor. His current research interests include power electronic components, supercapacitors, Bernard Davat (M 89) received the Engineer degree from Ecole Nationale Supérieure 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 the 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 the Institut National Polytechnique de Lorraine, Nancy, France. His current research interests include power electronics, drives, and new electrical devices (fuel cell and supercapacitor).