january/february 2008 IEEE power & energy magazine 691540-7977/08/$25.00©2008 IEEE

E© EYEWIRE

ENERGY AND ENVIRONMENT CRISES ARE FAST BECOMING THE BIGGESTproblems around the world so, as a consequence, new renewable and clean energy powersources must be considered. One of the prevalent alternative sources of electric power is the fuelcell, discovered by Sir William Grove in 1839. One expects that fuel cell power generation sys-tems will be used in a growing number of areas: in portable applications, in transportation appli-cations, and in stationary power applications, for which fuel cell systems can provide bothpower and heat with cogeneration efficiencies as high as 80%. Numerous recent works havealready highlighted the possibility of using the fuel cell in distributed power generation systems.

The fuel cell utilizes the chemical energy of hydrogen (H2) and oxygen (O2) to generateelectricity without pollution, as shown in Figure 1. The byproducts are simply pure water andheat. There are several types of fuel cells, which are characterized by the employed electrolyte.One of the most promising is the small, lightweight, and relatively easy to build polymer elec-trolyte membrane fuel cell (PEMFC), first used by NASA in the 1960s as part of the Geminispace program.

A single cell voltage of the fuel cell is given by Gibb’s free energy ΔG and is equal to 1.23 V.This theoretical value is never reached even at no-load. For the rated current, the voltage of anelementary cell is about 0.6–0.7 V. Then a fuel cell is always an assembly of elementary cellsthat constitute a stack as shown in Figure 2.

Digital Object Identifier 10.1109/MPE.2007.911814

Fuel Cell SystemA fuel cell stack requires fuel, oxidant,and coolant in order to operate. The com-position, pressure, and flow rate of each ofthese streams must be regulated. In addi-tion, the gases must be humidified and thecoolant temperature must be controlled.To achieve this, the fuel cell stack must besurrounded by a fuel system, fuel deliverysystem, air system, stack cooling system,and humidification system.

Once operating, the output power gen-erated by the fuel cells must be condi-tioned and absorbed by a load. Suitablealarms must shut down the process ifunsafe operating conditions occur and acell voltage monitoring system must mon-itor fuel cell stack performance. Thesefunctions are performed by electrical con-

trol systems. As an example, Figure 3 shows a simplified dia-gram of the PEM fuel cell system.

When a fuel cell operates, its fuel (hydrogen and oxygen)flows are controlled by a “fuel cell controller,” which receivescurrent demand. This current demand is the fuel cell current ref-erence iFCREF (see Figure 3) coming from the energy manage-ment controller. The fuel flows must be adjusted to match thereactant delivery rate to the usage rate by the fuel cell controller.

Fuel Cell Dynamic LimitationIt is widely accepted that one of the main weak points of thefuel cell is its time constants dominated by temperature andfuel delivery system (pumps, valves, and in some cases, ahydrogen reformer). As a result, fast load demand will cause

a high voltage drop in a shorttime, recognized as “fuelstarvation phenomena.”

For clarity, Figure 4presents the 0.5-kW PEMfuel cell voltage response toa current profile. The testsoperate in two differentways: current step and cur-rent slope. It shows the dropof voltage curve in Figure4(a), compared with Figure4(b), which implies thatfuel supply and deliveredelectrical current do notcoincide. Fuel flows (partic-ularly the delay of air flow)have difficulties followingthe current step. This condi-tion of operation is evident-ly hazardous for the fuelcell stack.

70 IEEE power & energy magazine january/february 2008

figure 1. Fuel cell principle discovered by Sir William Grove.

figure 2. PEMFC stack (500 W, 40 A) composed of 23cells of 100 cm2 developed by ZSW Company (Germany).

figure 3. Simplified diagram of the PEM fuel cell system. vFC and iFC are the fuel cellvoltage and current.

O2 H2

ElectronFlow

Dilute AcidElectrolyte

Pt Electrodes

The Electrolysis of Water

O2H2

ElectronFlow

Dilute AcidElectrolyte

Pt ElectrodesThe Electrolysis of Water

Is Reversible.

A

Hydrogenfrom Bottle

− +

Flux Controller

ElectricHeater

Heat ExchangerExcess

PressureController

Air fromCompressor

Humidifier

Fuel CellController

iFCREF (t)VFC (t )

PEMFC Stack500 W, 23 Cells

Condenser

Excess

iFC (t )

january/february 2008 IEEE power & energy magazine

Reliability and lifetime are the most essential considera-tions in such power sources. The hydrogen and oxygen star-vations cause severe and permanent damage to theelectro-catalyst of the fuel cell. The fuel starvation must beabsolutely avoided even if the operation under fuel starvationis momentary just within one second.

It is therefore recommended, when utilizing a fuel cell, toemploy a power loop or a current loop in order to preventoverloads and fault conditions and to associate it with, at least,a fast auxiliary power source to improve the dynamic per-

formances of the whole system. Moreover, one can takeadvantage of this fast auxiliary power source to achieve anactual hybrid source in order to disassociate mean power siz-ing from peak transient power sizing; the aim being a reduc-tion in volume and weight, and in the case of fuel cells used asmain energy source, the possibility of regenerative braking.

SupercapacitorsRecent progress in supercapacitor technology has principallybeen applied in computer memory backup systems, but with the

71

figure 4. Fuel cell dynamic characteristics to (a) current step and (b) current slope: 4 A/s

figure 5. Principle of operation of a supercapacitor.

Ch1: FC Voltage

Ch2: FC Current

Ch3: HydrogenFlow

Ch4: Air FlowTime: 4 s/DivTime: 1 s/Div

Ch4: Air Flow

Ch3: Hydrogen Flow

Ch1: FC Voltage [4 V/div]

[20 Liter/Min/Div]

[2 Liter/Min/Div]

Ch2: FC Current [10 A/div]

5 A 5 A

4 A/s

40 A40 A

Fuel Starvation Phenomena

Tek Stopped Single Seq 1 Acqs Tek Stopped 1 Acqs28 Oct 05 16:56:06 28 Oct 05 17:01:48

2

3

4(a) (b)

1

2

3

4

1

Activated Carbon Electrode−Ions +Ions

OrganicElectrolyte

Power Supply Load

C =

εo the Permittivity of Free Spaceεr the Relative Permittivity of the DielectricA the Plate Surface Aread the Plate Separation

εoεrA

d

DischargingCharging

latest increases in capacitor energy storage levels, higher powerapplications [especially uninterruptible power supply (UPS) andhybrid vehicle] have become practicable. Electrochemical capac-itors are presently called by a number of names: supercapacitor,ultracapacitor, or electrochemical double-layer capacitor. Theseterms are used interchangeably, and they refer to a capacitor thatstores electrical energy in the interface that lies between solidelectrodes and an electrolyte, as delineated in Figure 5.

The double-layer capacitor phenomenon was discoveredby Helmholtz, one of the greatest natural scientists, who

mathematically formulated the first main theorem of thermo-dynamics, in the 1800s. To maximize the capacitance C, thearea A must be maximized and d minimized. Nowadays, theequivalent plate separation distance at the double layer con-sists of a few electrolyte molecule diameters of about 10−10

m. This plate separation distance is impossible in a conven-tional capacitor due to traditional dielectric breakdown, con-sisting of ionization followed by spark discharge. Terminalvoltage of the supercapacitor is limited though, due to disso-ciation of the electrolyte. This limits the maximum voltage

72 IEEE power & energy magazine january/february 2008

figure 7. Concept of the actively controlled hybrid source for a distributed generation system. pLoad, pFC, and pSuperCare the load, fuel cell, and supercapacitor powers, respectively; vBus and vSuperC are the dc bus and supercapacitor volt-ages; iFC and iSuperC are the fuel cell and supercapacitor currents.

figure 6. SAFT supercapacitor: a single cell, a module in six-series, and a bank in 108 cells in series.

Fuel CellFuel CellConverteriFC

PFC

PLoadvBus

42 V dc Bus

SuperCBank

SuperCConverter

ControlSignal

ControlSignal

+

−

+

− +

PowerConverter

TractionMotor

iSuperC

PSuperC

vBusvSuperC

+−

Energy Management Controller

A SAFT Supercapacitor(3,500 F, 400 A, 2.5 V)

SAFT Supercapacitor Module:Six Cells in Series

SAFTSuperC Bank:

33 F, 270 V

january/february 2008 IEEE power & energy magazine 73

(2.5–3 V) of a supercapacitor cell used in this experiment to2.5 V. Electrode area in the supercapacitor is maximized byuse of activated carbon with an effective surface area up to3,000 m2/g of material. The large surface area combined withthe high capacitance per unit area yields the very large capac-itance seen in the supercapacitor.

The first high-power supercapacitors were developed bythe Pinnacle Research Institute (PRI) forU.S. military applications such as laserweaponry and missile guidance systems.However, only in the 20th century didsupercapacitors become well known in thecontext of hybrid electric vehicles promotedby the Department of Energy (DOE) undera supercapacitor development program.

The supercapacitors are true capacitorsin that energy is stored via electrostaticcharges on opposing surfaces, and theycan withstand a very large number (thou-sands to millions) of charge/dischargecycles without degradation. They are alsosimilar to batteries in many respects,including the use of liquid electrolytes andthe practice of configuring various sizecells into arrays to meet the power, energy,and voltage requirements of a wide rangeof applications.

Supercapacitors with ratings from a fewfarads to a thousand farads have been man-ufactured by Panasonic, Montena, Ness,ELNA, Maxwell Technologies, EPCOS,and SAFT, Inc. As an example, Figure 6illustrates the photographs of the recentsupercapacitor prototype bySAFT Company (France):

✔ a cell (3,500 F, 2.5 V, 400 A)✔ a module (583 F, 15 V,

400 A)✔ a bank (33 F, 270 V, 400 A).These capacitors are not yet

available commercially; nonethe-less, prototype specifications havebeen made available.

Supercapacitor Versus BatteryOnly one-half the energy at thepeak power from the battery is inthe form of electrical energy tothe load, and the other one-half isdissipated within the battery asheat in the equivalent seriesresistance. This is to say that theefficiency of batteries is around50%. For supercapacitors, the

peak power is usually for a 95% efficient discharge in whichonly 5% of the energy from the device is dissipated as heat inthe equivalent series resistance (ESR). For a correspondinghigh-efficiency discharge, batteries would have a much lowerpower capability.

Additionally, the main disadvantage of the batteries is aslow charging time, limited by a charging current [known as

figure 8. Power profile of a hybrid power source.

figure 9. Hybrid system test bench.

PLoad = PFC + PSuperC

Load

Pow

erF

uel C

ell

Pow

erS

uper

CP

ower

Minimum Power

Average Power

Maximum Power

t10

0

0

t2 t3 t4

t5

t

t

t

SAFT Supercapacitor Bank: 292 F

Motor andGenerator Load

Control Panel

Fuel CellConverter

dSPACEInterfacing Card

the battery state-of-charge (SOC)]; in contrast, the superca-pacitor may be charged in a short time depending on a highcharging current (power) available from the main source. Forexample, a SAFT supercapacitor module (583 F, 15 V, 400 A)may be charged from zero voltage (zero-of-charge) to themaximum voltage (maximum-of-charge) within 22 s at aconstant current of 400 A.

How Fast Can Supercapacitor Energy and Battery Energy Be Used?Supercapacitors fit between traditional capacitors and batteriesin terms of time constant, specific energy, and specific power.Even though it is true that a battery has the largest specificenergy (meaning more energy is stored per weight than othertechnologies), it is important to consider the availability ofthat energy. For example, how fast can it be used? This is thetraditional advantage of capacitors. With a time constant ofless than 0.1 s, energy can be taken from a capacitor at a veryhigh rate. On the contrary, the same size battery will not beable to supply the necessary energy in the same time period.

Concept of Fuel Cell/SupercapacitorHybrid Power SourceIt is expected that the very fast power response and high spe-cific power of supercapacitors can be used to complement theslower power output of the main source (particularly the fuel

cell generator) to produce the compatibility and performancecharacteristics needed to be load-compatible. The superca-pacitors will be used to improve the load-following character-istics of a power source by providing a more robust powerresponse to changes in system loading.

Figure 7 illustrates the functional block diagram of thehybrid control structure showing all of the major compo-

nents, which include fuel cell source, fuel cellconverter, supercapacitor storage device, superca-pacitor converter, and energy management con-troller. The energy management controlleroperates to handle the energy balance (voltageconversion) between main source, auxiliarysource, and load (as presented in Figure 8) bycontrolling both the main and auxiliary convert-ers. The load is capable of providing regenerativebraking that can be used to charge the auxiliarysource. The system operates primarily on mainsource power and draws over- or under-energyfrom the supplementary source only for peak ortransient energy requirements, such as vehicleacceleration (a high load step). This structure iscalled a series hybrid architecture that is com-monly employed for all-electric hybrids.

As the fuel cell is not current-reversible, astep-up (boost) converter is selected to adapt thelow dc voltage delivered by the fuel cell to the 42V dc bus (new standard voltage for automotivesystem, “PowerNet”). Supercapacitors are con-

nected to the 42-V dc bus by means of a two-quadrant dc/dcconverter (bidirectional converter). Supercapacitor current,which flows across the storage device, can be positive ornegative, allowing energy to be transferred in both directions.

To manage energy flows, one may then define three oper-ating modes (or states):

1) charge mode, in which the main source supplies energyto the auxiliary source and/or to the load (refer to Fig-ure 8: t2 − t4),

2) discharge mode, in which both the main source and theauxiliary source supply energy to the load (refer to Fig-ure 8: t1 − t2),

3) recovery mode, in which the load supplies energy to thestorage device (regenerative braking) (refer to Figure 8:t4 − t5).

The main objective of the control is to regulate the dc busvoltage (energy balance in the dc bus). Taking into accountthe fuel cell dynamics, the fuel cell is only operating in nearly

74 IEEE power & energy magazine january/february 2008

figure 10. Supercapacitor current response to a step 0 A to 50 A.

Ch4: iSuperC

Ch2: iSuperCREF

50 A

0 A

Tek Run Hi Res 1 Acqs 15 Sep 05 12:09:12

Ch2↓Ch4

10.0 A10.0 A

M 100 μs 500 kS/sA Ch2 \ −9.6 Y

Ω 2.0 μs/pt

4

The fuel cell utilizes the chemical energy of hydrogen and oxygen to generate electricity without pollution.

january/february 2008 IEEE power & energy magazine

steady-state conditions, and the supercapacitors are function-ing during transient energy delivery or transient energyrecovery with the following constraints:

✔ fuel cell current slope must be limited to a maximumabsolute value (for example: 4 A/s) in order to guaranteematching the reactant delivery rate and the usage rate

✔ fuel cell current must bekept within an interval (ratedvalue, minimum value, orzero)

✔ supercapacitive storagedevice voltage must be keptwithin an interval (minimumvalue, maximum value).Normally, the systemattempts to reach the nomi-nal (operating) voltage,called full-of-charge.

Performance of HybridPower SourcesTo examine the hybrid source per-formance, a small-scale test benchis realized in the laboratory, asdepicted in Figure 9. The PEMfuel cell system (Figure 2 and Fig-ure 3) was achieved by the ZSWCompany. It is supplied using purehydrogen from bottles under pres-sure and with clean and dry airfrom a compressor. The superca-pacitive storage device (292 F) isobtained by means of twelve SAFTsupercapacitors (capacitance:3,500 F, rated voltage: 2.5 V, ratedcurrent: 400 A, low frequencyequivalent series resistance: 0.8m�) connected in series. Maxi-mum supercapacitor voltage is then30 V. One chooses the supercapaci-tor operating voltage of 25 V andminimum voltage of 15 V (classi-cally 50% of the maximu voltage).Accordingly, the energy storagewithout losses (discharging fromthe operating voltage to the mini-mum voltage) is equal to 58.4 kJ.

The energy management controller has been implementedin the real-time card dSPACE DS1104, through the mathe-matical environment of Matlab-Simulink, with a samplingfrequency of 25 kHz. Moreover, ControlDesk softwareenables change to the parameters of the control. It is alsoused for driving the load.

75

figure 11. Fuel cell/supercapacitor hybrid source response during drive cycle.

42

352821

1470

1.0

0.5

0.0

−0.5

−1.01,000

800

600

400

200

0

40

20

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−20

−4025.5

25.0

24.5

24.0

23.5

23.00 10 20 30 40 50 60 70 80 90 100

Vol

tage

[V]

Pow

er [k

W]

Mot

or S

peed

[rpm

]C

urre

nt [A

]S

uper

C V

olta

ge [V

]

DC Bus

SuperC

Fuel Cell

Fuel CellLoad (Motor)

SuperC

Fuel Cell

Time [s]

The fuel cell stack must be surrounded by a fuel system, fuel delivery system, air system, stack cooling system, and humidification system.

High Dynamics of Supercapacitor SourceFigure 10 presents the transient response during supercapaci-tor discharging. The supercapacitor converter interfacesbetween the 42 V dc bus and the supercapacitor bank. Theinitial voltage of supercapacitor bank is 30 V. The superca-pacitor current reference iSuperCREF is Ch2 and the measuredcurrent iSuperC is Ch4. One can observe the high dynamicresponse of the supercapacitor source from 0 to 50 A in 0.4ms. Unquestionably, the fast response of supercapacitorpower source can function with the fuel cell main generatorto improve the slow dynamics of the whole system.

Drive CycleThe experimental tests shown below were carried out by con-necting the 42-V dc bus to an active load composed of a two-quadrant converter, loaded by a dc motor coupled with a dcgenerator. The motor functions with the cascade current-speed control method. A hysteresis and P controller areselected for the motor current and speed loops, respectively,with a current limitation at ±60 A.

The test, as illustrated in Figure 11, presents waveformsobtained at motor start to 1,000 rpm and stop. The motorstarts at t = 10 s, to the final speed of 1,000 rpm, and thefuel cell power increases to its rated value of around 500 W.The peak load power is about 1 kW, which is two times thefuel cell rated power. Thus, the storage device, which sup-plies most of the power required during motor acceleration,remains in discharge state after motor start. In fact, the finalsupercapacitor current is 8 A because the steady-state loadpower (about 600 W) is greater than the fuel cell ratedpower (500 W).

After that, the motor speed, at t = 40 s, deceleratesfrom 1,000 rpm to stop; consequently, the storage deviceis deeply discharged, demonstrating three phases: first,during t = 40 s to 44 s, it recovers the power supplied tothe dc bus by the fuel cell and the motor regenerativebreaking (the peak power of about 0.5 kW); then it recov-ers the power supplied only by the fuel cell. From t = 44 sto 48 s, this power is constant, limited to the fuel cell ratedpower. After t = 48 s, the fuel cell power decreases due tothe supercapacitor voltage regulation. During the two firstphases, the fuel cell power is at a rated value of 500 W. Inthe third phase, it decreases down to zero. Excellently,only small perturbations on the dc bus voltage waveformcan be seen, which is of major importance for the energymanagement controller.

Finally, fuel cells are good energy sources to providereliable power at steady state and supercapacitor energystorage devices can advance the load following character-istics of a fuel cell by providing a stronger powerresponse to changes in system loading. During motorstarts/stops or other considerable steps in load, the super-capacitors provide the balance of energy needed duringthe temporary load transition periods and also absorbexcess energy from the generator source (motor braking).Adding supercapacitor energy storage to distributedpower systems improves power quality and efficiency andreduces capital expenses by allowing the systems to besized more closely to the steady-state power requirementsrather than over-sizing the main generator to meet tran-sient loading requirements.

For Further ReadingW. Friede, S. Raël, and B. Davat, “Mathematical model andcharacterization of the transient behavior of a PEM fuel cell,”IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1234–1241,Sept. 2004.

P. Thounthong, S. Raël, and B. Davat, “Test of a PEM fuelcell with low voltage static converter,” J. Power Sources, vol.153, pp. 145–150, Jan. 2006.

R.M. Nelms, D.R. Cahela, and B.J. Tatarchuk, “Modelingdouble-layer capacitor behavior using ladder circuits,” IEEETrans. Aerosp. Electron. Syst., vol. 39, pp. 430–438, Apr.2003.

P. Thounthong, S. Raël, and B. Davat, “Utilizing fuel celland supercapacitors for automotive hybrid electrical system,”in Proc. 2005 IEEE Applied Power Electronics Conf.(APEC), Mar. 2005, pp. 90–96.

P. Thounthong, S. Raël, and B. Davat, “Control strategyof fuel cell/supercapacitors hybrid power sources for electricvehicle,” J. Power Sources, vol. 158, pp. 806–814, July2006.

J.W. Dixon and M. Ortúzar, “Ultracapacitors + DC-DCconverters in regenerative braking system,” IEEE Aerosp.Electron. Syst. Mag., vol. 17, pp. 16–21, Aug. 2002.

BiographiesPhatiphat Thounthong is a lecturer at King Mongkut’sInstitute of Technology North Bangkok, Thailand.

Bernard Davat is a professor at the Institut National Poly-technique de Lorraine, Nancy, France.

Stéphane Raël is an assistant professor at the InstitutNational Polytechnique de Lorraine.

76 IEEE power & energy magazine january/february 2008

It is widely accepted that one of the main weak points of the fuel cell is its time constants dominated by temperature and fuel delivery system.

p&e