IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERINGIEEJ Trans 2014; 9: 548–554Published online in Wiley Online Library (wileyonlinelibrary.com). DOI:10.1002/tee.22004

Paper

Online Energy Management Strategy of a Hybrid FuelCell/Battery/Ultracapacitor Vehicular Power System

Toufik Aziba, Non-member

Cherif Larouci, Non-member

Ahmed Chaibet, Non-member

Moussa Boukhnifer, Non-member

A hybrid power system based on a fuel cell (FC) and an energy storage system appears to be very promising for satisfying thehigh energy and high power requirements of automotive applications in which the power demand is impulsive rather than constant.This paper deals with the use of a hybrid energy storage system with the battery (BAT)/ultracapacitor (UC) as ancillary powersource in FC electric vehicles. The energy management strategy (EMS) is one of the most important issues for the efficiencyand performance of such systems. The designed EMS uses a splitting method, allowing a natural frequency decomposition ofthe power demands. It takes into account the slow dynamics of FC and the state of charge of the UC and BAT.A simulation is conducted using MATLAB/SIMULINK software in order to verify the effectiveness of the proposed controlstrategy. It confirms the performance of the control method and also demonstrates the robustness and stability of the controlstrategy with good tracking response (transient performance), low overshoot, zero steady-state error, and control flexibility duringa power demand cycle. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Keywords: hybrid power sources, energy management strategy, fuel cell electric vehicle, hybrid energy storage system

Received 27 February 2013; Revised 3 October 2013

1. Introduction

In the last years, research results have demonstrated that theuse of energy storage devices provides additional advantages inimproving the energy efficiency, power quality, stability, andreliability of power supply sources. In the case of the fuelcell electric vehicle (FCEV), which is considered to be themost promising alternative to replace fossil fuels in the longrun, the FC represents a power source with nearly unlimitedenergy (limited only by the hydrogen tank size) and almost zeropollution. However, this technology has some drawbacks linkedto its inherent characteristics, such as high cost, poor transientperformance (low dynamics), and inability to allow bidirectionalpower flow (no regenerative energy recovery during braking),which need to be overcome [1–4]. Hybridization of an FC with anenergy storage system (ESS) can be a solution to overcome thesedrawbacks.

While good progress is being made in FC technology, thereis an immediate need for efficient design of hybridization of thevehicle power train. Benefits from such designs include capturingregenerative braking energy (improved fuel economy), loweringthe cost, providing optimization possibility, and mitigating thestress on the FC stack by shifting some portion of dynamic powerdemand (transient conditions) to a second power source, thusimproving the FC’s life and efficiency [5–9].

Nowadays, these ESSs can be either mechanical (as in aflywheel) or electrical [as in a battery or ultracapacitor (UC)]energy storage devices. Almost all existing hybrid power sys-tems of FCEVs mainly use electrical storage devices and arecomposed of a fuel cell/battery hybrid (FC/BAT) [8,10,11], or a

a Correspondence to: Toufik Azib. E-mail: toufik.azib@estaca.fr

Control and Systems Laboratory, ESTACA Engineering School, 34 RueVictor Hugo, 92300 Levallois-Perret, France

fuel cell/ultracapacitor hybrid (FC/UC) [8–11]. But, unfortunately,none of them satisfies perfectly the requirements of the automotiveapplications: an ESS must have a high power density in order towithstand fast power variations (transient conditions: accelerationand braking), and at the same time it must have a high energy den-sity to give autonomy to the vehicle and ensure maximum energyrecovery during the braking. For that reason, it is necessary to asso-ciate more than one storage technology, creating a hybrid energystorage system (HESS). Many research works have been devotedto the use of these components in different applications: e.g. hybridelectric vehicle [8,10,12–14], remote area power supply [15,16].In this paper, we investigate an HESS based on the associationof a BAT, as long-term storage device, and a UC, as a short-term storage device. The association BAT and UC permits takingadvantage of the characteristics of both devices, thereby obtaininghigh energy density, high power density, high life-cycle, and highefficiency.

An energy management strategy (EMS) is one of the mostimportant issues for the efficiency and performance of such asystem. The objective of this paper is to develop an efficient EMSthat integrates three components together (FC, BAT, UC), whichallows achieving power requirements, maintaining acceptable stateof charge (SOC) of UC and BAT, respecting slow dynamic ofthe FC, and also ensuring safe and durable operation of theglobal system. The proposed strategy is based on frequencydecomposition of the power demand cycle using cascaded loops:UC supplies the high band of the load power frequency spectrum,BAT ensures the middle frequencies, and FC provides lowfrequencies, thereby contributing to the long-term autonomy andimproving the component reliability (life cycle).

Simulation results are presented to confirm the effective systembehavior and to verify the performance of the control method.Our focus is on the stability of the system and on the energyflux management. The obtained results prove the validity of

© 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

ENERGY MANAGEMENT STRATEGY FOR FUEL CELL POWER SYSTEM

- Traction System -

CBUS

L FC

L BAT

DC/DC

DC/DC

- FC -

- BAT -

LUC

- UC -

DC/DC

Fig. 1. Parallel structure of the hybrid FC/BAT/UC power system

the proposed approach and show the good performance of theelaborated control method for various load power demands. Italso demonstrates the robustness and stability of the controlstrategy with good tracking response (transient performance), lowovershoot, and zero steady-state error.

2. Hybrid Power System Description

2.1. System description Many hybrid FC system con-figurations have been studied and documented [17–20]. They canbe classified into three main categories: series, parallel, and cas-caded architectures. It has also been proven that the parallel struc-ture is the most advantageous one [18–20]. Many advantages canbe stressed: fewer component constraints, simple energy manage-ment, and good reliability.

The parallel structure considered in our application is presentedin Fig. 1. It consists in associating a static converter with itscontrol device to every energy source. The converter dedicatedto the FC is unidirectional in power and works in the step-upconverter mode (‘boost’). The converters connected to the UC andBAT work in the ‘boost mode’ when these supplies deliver a partof the power demand, and operate in step-down converter mode(‘buck’) when the UC and BAT get back the energy of recovering.Thus, static converters (‘choppers’) and their control devices areused to coordinate the energy sources to meet the requirement ofthe electric load. Therefore, given a certain load power demandPLoad(t), it can be supplied partially from the FC system PFC(t),the rest of power being supplied by the ESS PUC(t) and PBAT(t).The power balance in the DC bus must be fulfilled at all times:

PLoad(t) = ηFC·PFC(t) + ηBAT·PBAT(t) + ηSC·PSC(t)∀t (1)

where ηFC, ηBAT, and ηUC are the efficiency of the powerconverters connected to the different sources. We assume that theconverter efficiencies are known and fixed at 90%.

2.2. Fuel Cell, Ultracapacitor, and Battery ModelingThis section shows the models of the different sources that have

been used during simulations. The complexity of the dynamicmodels has been chosen according to the investigated phenomena.

2.2.1. Fuel cell (FC) An FC system consists of many cellsconnected in series to provide the desired output terminal voltageand current, and exhibits a nonlinear I–V characteristic [21,22].Furthermore, the FC system is a complex device with many auxil-iary components. Hence, a significant part of the electrical powergenerated is used internally. Inside, the FC system supplies theseauxiliary components, which means that the real generated stackcurrent is greater than the output current iFC. More precisely, inlow dynamic conditions, the FC characteristics can be consid-ered as a voltage source with ohmic, kinetic, and mass transfer

VUC

iUC

R1

C1 C2 C3

R3R2

Fig. 2. RC transmission line model

resistances. The relationship between the FC voltage vFC and theoutput current iFC is given by the following equations:

vFC = N (ECel·R·jStack − A · ln(jStack + jl)−m · exp(n·jStack)) (2)

jStack = IStack

ACel, and here IStack = α+(1+β)iFC + γ (iFC)2 (3)

where ACel is area of each cell, N is the stack cell number, ECel

is the reversible cell voltage, R is the membrane area specificresistance, A is the Tafel coefficient, IStack is the stack current, mand n are the two coefficients of the mass transfer equation, andα, β, and γ and are the coefficients of the second-order modelapproximating IStack as function of the output current iFC [21,22].

ACell = 130 cm2, N = 120, ECell = 1.071 V, A = 0.03 V, m =2.11 × 10−5 V, n = 8 × 10−3 cm2 mA−1, α = 0.029, β = 0.971,γ = −8 × 10−4.

2.2.2. Ultracapacitor (UC) Most of the UC models pre-sented in the literature consider a finite ladder RC network whichidentifies several branches in parallel, each one with own character-istic time constants, that do not have a linear behavior due to thedependence of the capacitances on the internal voltage. VariousUC models can be found in the literature, especially for hybridsystems [10,11,23]. Classically, a theoretical UC model uses atransmission line where the voltage (VUC) depends on distributedcapacitance [23]. From this theory and in order to focus on theEMS, a three-branch (RC) transmission line model is considered,each one with its own characteristic time constant (Fig. 2).

The impedance model can be expressed in the following form:

Z (s) = a0 + a1s + a2s2 + a3s3

b0 + b1s + b2s2 + b3s3(4)

This model allows to us take into account the UC’s nonlinearbehavior during charging and discharging and both dynamic andlong-time behaviors. Thus, the RUC and CUC parameters values areestimated from the test results [23]: a3 = 5.74 × 105, a2 = 3.28 ×105, a1 = 1826, a0 = 1, b3 = 8.45 × 108, b2 = 5.26 × 106, b1 =3410, and b3 = 0. which gives

R1 ≈ 0.3ESRCel, R2 ≈ 0.2ESRCel, R3 ≈ 0.3ESRCel

C1 ≈ 0.4CCel, C2 ≈ 0.44CCel, C3 ≈ 0.16CCel

with

C = NP

NSCCel and ESR = NS

NPESRCel (5)

These coefficients are based on an UC with the followingparameters: VCel = 2.5 V, CCel = 2600 F, ESRCel = 0.4 m�.

2.2.3. Battery (BAT) A propulsion lithium-ion battery sys-tem consists of serially connected battery cells. The battery systemis relatively simple, compared to the FC system which has sub-stantial number of auxiliary components and requires a controllerto supply gas.

A battery pack can be scaled simply according to its numberof cells and the cell capacitance. It can be modeled over a largerange of operating conditions by a model including an ideal voltagesource UOC to define the battery open-circuit voltage, internalresistances, and equivalent capacitances. The internal resistancesinclude the ohmic resistance and polarization resistance. The

549 IEEJ Trans 9: 548–554 (2014)

T. AZIB ET AL.

VBATCTr

R0

RP

UOC

U

iBAT

Fig. 3. Battery equivalent circuit model

0 20 40 60 100 120 140 160

0

10

20

30

40

50 ECE

V [km/h]

0 20 40 60 80 100 120 140 160 180

-5000

0

5000

10000

t [s]

P [W]

Fig. 4. ECE-15 driving cycle

equivalent capacitance is used to describe the transient responseduring charging and discharging (Fig. 3).⎧⎨

⎩UBAT = UOC − U − ROiBAT

dUdt = − U

RPCTr+ iBAT

CTr

(6)

The model parameters are estimated using a battery test benchpresented in detail in Ref. [24]: RO = 0.072�, RP = 0.021�,CTr = 1214 F.

2.2.4. Power demand cycle To evaluate the dynamicresponse of a developed methodology and making suggestionsthrough the development in the overall structure, standard drivecycles can be considered. As an example, European light dutyvehicles have to satisfy the New European Driving Cycle (NEDC)which represents the typical usage of a car in Europe. The NEDC(Fig. 4) consists of repeated urban cycles (called ECE-15 drivingcycle) and an extra-urban driving cycle or EUDC [1,25].

On the power demand value, we can notice sudden powerchanges each time the driver requires a speed change (powerdemand is impulsive rather than constant). On the ECE-15 cycle,the car’s average power demand is only 1.12 kW whereas the peakpower is roughly 10 kW, which means a ratio (PMax/PAverage) of9. For those reasons, our control strategy will be tested with asevere profile consisting of a power rise hedge.

3. Controller Design of Energy ManagementStrategy

The main motivation for introducing hybridization in FC sys-tems is to solve two important problems in FC control:

(i) the dynamics of FC are relatively slow, mainly because ofthe dynamic of the air compressor;

(ii) the ESS can satisfy the transient mode and recover thebraking energy, thereby saving primary energy.

Therefore, a proper EMS is important to realize these objectives.Moreover, the control strategy must work in real time (online)to distribute the power between the power sources based onthe system conditions (unpredictable load profile). There havebeen many papers on control strategies of hybrid system energymanagement. The most widely acknowledged real-time controlstrategies are based on the evolution of the state of the system [26],in which separate control algorithms are proposed to control the

LF HF

Power demand cycle

MF

f(Hz)

P(W)

Power Splitting Strategy

LF PFC

MF PBAT

MF PUC

FilteringFrequency (fFS)

fFS1 fFS2

Fig. 5. Bandwidth splitting technique of the EMS

system when the operating mode changes because of load powervariation. Therefore, to move from one operating mode to another,it is necessary to switch from one control algorithm to another.This can result in a high demand of the instantaneous current ofthe main source (FC), with the risk of damaging the system. Otherstudies use artificial intelligence techniques [14,27–29], such asfuzzy and neural network controllers. They are based essentially onempirical adjustment, where the performance depends strongly onthe designer’s expertise. In another schemes, algorithms based onpassivity and flat systems are used [12,30]. They are characterizedby a high complexity (large a number of variables and interactions)and a good knowledge of the system parameters, with potentialparameter identification problems. That can engender additionalcosts and decrease system reliability. Lastly, EMS based on thefrequency decoupling technique (cascaded loop) or frequencysplitting using the voltage regulation of the DC bus is used. Ituses simple controllers such as proportional integrator (PI) aswell as sliding mode (SM) [11,25]. They are well adapted to thesupervision strategy specifications and present good performancein terms of energy management.

In this context, online EMS has been proposed and successfullyevaluated using a simple frequency decoupling technique strategywith the energetic macroscopic representation for only the FC/UChybrid system [31]. In this paper, a full hybrid FC/BAT/UC sys-tem is considered. In addition, a dedicated EMS is developed toimprove the HESS performance. The proposed control strategy isbased on a power splitting frequency approach with two frequencysplittings, which are the degrees of freedom of this technique. ThisEMS fulfills the fast energy demands of the load and respects theintegrity of each source. More precisely, the main idea is to assignthe load power demands to the appropriate source, as shown inFig. 5.

As a result, the EMS leads naturally to a cascaded control loop,as depicted in Fig. 6, with the following:

3.1. Inner control (local control) A closed-loop cur-rent controller can be adopted for the local control strategy ofeach converter in the FC/BAT/UC system. It is tuned to drive thecurrent of each source. Indeed, monitoring these currents is essen-tial in order to protect the choppers (as well as the sources FC,BAT, and UC) against breaking overcurrents or overvoltages. Eachchopper is modeled according to its average behavior and can beanalyzed with the following set of equations:⎧⎨

⎩Lk

dikdt = Vk − (1 − dk )VBUS

CBUSdVBUS

dt = (1 − dk )ik − iLoad

(7)

where k represents FC, BAT, or UC, and dk is the duty cycle.Hence, using the Laplace transform, (7) becomes

ik (s) = 1

Lk ·s Vk (s) − [1 − dk (s)]VBUS = 1

Lk ·s Vk (s) − Dk (s)VBUS

(8)

with Dk (s) = 1 − dk (s)where Vk acts as a low-frequency perturbation.VBUS and Vk are measured variables.

550 IEEJ Trans 9: 548–554 (2014)

ENERGY MANAGEMENT STRATEGY FOR FUEL CELL POWER SYSTEM

- CBUS -

- UC -- FC -DC/ DCDC/ DC

iFC

PI

iFCref iBATiUCref

Low-passFilter

VBUSrefVBUS

PI

PI

VBATref

PI

iFC

VFC VUC

iUC

VBUS PLoad

PFC PUC

Traction System

Energy Management Strategy EMS

- BAT -

VBAT

iBAT

PBAT

iUC

PI

VUC

PI

VBATVUCref

iBATref

Low-passFilter

Inner Control (local control)

iLoad

iLoadEST

iLoadEST

LF HF MF

Δ iBATΔ iUC

SOC Compensation fFS1

fFS2

DC/ DC

Fig. 6. Block diagram of the EMS

Then, the closed-loop transfer function of the system can bededuced in (9), where ωnk is the natural frequency of the closedloop, mk is the damping ratio, and τk is the time constant of itszero. This first loop is implemented with a PI controller withKpk the proportional gain of the regulator and ωIk its integralgain.

HBFk(s) = ik (s)

Dk (s)

VBUSLk ·s

Kk s+ωks

s + VBUSLk ·s

Kk s+ωks

= 1 + τk s

1 + 2mks

ωk+

(s

ωk

)2 (9)

ωnk =√

VBUS

Lkωk , mk =

Kk

√VBUS

Lk

2√

ωk, and τk = Kk

ωk(10)

The current loop dynamics is tuned to get a closed-loop timeresponse of about five to ten times the switching periods, andthe desired damping ratio is settled at mk = 1. The PI correctorparameters are given in (11) with VBUS = 410 V, LFC = 150 μH,LBAT = LUC = 300 μH, and fnk = 1 kHz:

ωIk = (ωnk )2

Ak, KPk = 2mk ·ωnk

Ak, and Ak = VBUS

Lk(11)

Moreover, to prevent current overshoot, the zero of the closedloop should be compensated by a low-pass filter, with a timeconstant of about

τk = KPk

ωIk(12)

The current loops references (iPaCref, iBATref, and iUCref) aregiven by the EMS, as detailed in the following paragraph.However, a current saturation (IMin < ik < IMax) based on dynamiccurrent limits and an anti-windup compensator are used to limit

these currents and to ensure the protection of the sources. Inconsequence, each source current set point is restricted as follows:⎧⎨

⎩0 ≤ iFC min ≤ iFC ref ≤ iFC max

iBAT min = −iBAT max ≤ iBAT ref ≤ iBAT max

iUC min = −iUC max ≤ iUC ref ≤ iUC max

(13)

3.2. Energy management strategy The EMS concernsthe power demand identification that affects the system accordingto the power splitting technique and SOC compensations. It allowscurrent reference synthesis for each source (iFCref, iBATref, andiUCref).

In the case of an unknown load, each load power variationmodifies the DC bus voltage. Hence its measurement is essentialfor the supervisor in order to estimate the power demand.Consequently, the DC voltage loop has to monitor the bus voltageVBUS(t) and allow the generation of the load current estimated(iLoadEST(t) ≈ iLoad(t)), which represents the power demand imagesince VBUS(t) is constant (Fig. 6). This controller can also be builtas a proportional integrator (PI) and is designed following a similarstrategy to the current loop. So, the closed-loop transfer functionof the system can be deduced as a second-order transfer functionwhere

ωnBUS =√

ωI BUS

CBUS, mBUS = KPBUS

2√

ωI BUSCBUSand τBUS = KPBUS

ωI BUS

(14)

To respect dynamic decoupling, the voltage DC bus loop mustpresent a time response ten times higher than the current loop,and the desired damping ratio is settled to mBUS = 1. So, the PIcorrector parameters are given in (14), with CBUS = 14 mF andfnBUS = 100 Hz.

ωI BUS = CBUS (ωnBUS)2 and KPBUS = 2mBUSCBUSωnBUS (15)

Once the power demand is identified, the proposed EMS shouldbe integrated. To implement this strategy, two splitting frequenciesof the load power requirement are used. The tuning parameter isthe filtering frequency (fFS), which is evaluated according to the FCand BAT requirements. In the first case, it is set to fFS1 = 50 mHz(LF for FC) and the filter is implemented in a second-order low-bandwidth filter delivering the FC reference current (iFCref), whilethe difference gives the HESS reference current (iBATref + iUCref).

This filter implemented is a Butterworth second-order low-pass filter with a 50-mHz cut-off frequency, which presents asufficient attenuation of −10 dB (≈70% current attenuation) atf ≈ 2 × fC = 100 mHz. The equation representing a second-orderButterworth filter is given as

H (jω) = 1√1 + ε2

(ω

ωC

)2×n= 1√

1 + 0.50(

ωωC

)2×2(16)

where n represents the filter order, ω is equal to 2π f , ε is themaximum pass band gain (attenuation of 1 dB), and ωC is equalto 2π fC (fC is cut-off frequency).

This reference (HESS reference) is then used in the secondfilter to split between BAT and UC delivering the BAT referencecurrent (iBATref), while the difference gives the UC referencecurrent (iUCref). Thus, the splitting system uses the same filter (asecond-order low-bandwidth) with frequency filtering set to fFS2 =200 mHz to respect the BAT limits (MF for BAT). Obviously, thisstrategy does not consider losses, which can never be perfectlyevaluated. Consequently, the UC and BAT SOC will graduallydrift. As the FC is the unique primary source, a corrective term(�iBAT, �iUC) is added through a corrector which monitors theUC voltage and BAT voltage, thanks to the compensation loops.In fact, these correctors are tuned to ensure a very low dynamic

551 IEEJ Trans 9: 548–554 (2014)

T. AZIB ET AL.

response compared to the filter response and designed following asimilar strategy.

The SC and BAT must be able to recover and supply theirenergies; for reasons of efficiency, the optimal voltage is set to0.75 and 0.70 VNom for UC and BAT, respectively.

With this EMS, we point out that the structure is a very simplearchitecture using classical controllers and can satisfy the mainobjective of this control: i.e.

– to ensure the load requirements even during fluctuations and toguarantee a continuity of operation and a safe functioning;

– to respect the limits of sources (slow dynamics of FC, SOC, ofHESS);

– to correctly manage the energy of system by sharing out thepower demand of the system into three parts depending on thefrequency. The low-frequency part is supplied by the FC, themiddle-frequency part is absorbed/supplied by the BAT, andthe high-frequency part is given by the UC.

4. Simulation Results and Discussion

4.1. Results analysis The proposed control strategy wasevaluated by extensive simulation on the hybrid systems shownin Fig. 6, using MATLAB software, and Simulink and SimPower-Systems Toolboxes. Table I gives the data used for the simulation.

Many load profiles were tried in the simulation scheme in orderto test the global system, either in linear mode or in transient mode.The load cycle shown in Fig. 7(a) consists of many rising andfalling edges. This severe profile is typical of a vehicle’s powerdemand in an urban setting [25]. At first, the investigation willbe focused on the transient response of the system. When theload power is suddenly increased, Fig. 7(a) (t = 7 s, 25 s), the FCpower slowly adjusts to the new load level as shown in Fig. 7(b)and responds as a low-bandpass filter with a time response of a fewseconds (LF). During this time, it can be observed from the UCcurrent, the BAT current (Fig. 8(a), and the power (Fig. 8(a)) thatHESS reacts immediately to supply the transient energy demandwhich is not supplied by the FC (MF, HF); the UC behaves asa high-bandpass filter with a time response of less than a fewmilliseconds (HF) when the BAT ensures its complement (MF).Otherwise, in spite of these load power fluctuations, the DC busvoltage VBUS is closely regulated to its reference value VBUSref

of 410 V (Fig. 8(b), and the required load power is supplied bythree complementary sources. Its output power sequence followsthe optimal control law. For the FC, when its power is not requiredfor traction, it is fully turned off.

Table I. Electrical parameters of the hybrid system

Fuel cell Value

Open circuit voltage 140 VRated voltage 90 VRated current (iFCmax) 120 AUltracapacitor Value

Capacitance 47 FRated voltage 150 VRated current (iUCmax) 180 A

Battery ValueOpen circuit voltage 180 VRated current (iBATmax) 150 AOptimal voltage (VBATref) 120 V

Inductors and capacitors ValueInductors LFC /LBAT /LUC 150/300/300 μHRated currents LFC /LBAT /LUC 150/200/200 ACapacitity CBUS 14 mFOptimal DC bus voltage (VBUSref) 410 V

-1

-0.5

0

0.5

1

1.5

0 20 40 60 100

-1.5

-1

-0.5

0

0.5

1

1.5x 10

t [s]

PLoad [10*kW]

PFC [10*kW]

PUC [10*kW]

PBAT [10*kW]

Power demand cycle

Power response of the sources

(b)

(a)

Fig. 7. System response. (a) Power demand cycle. (b) Powerresponse of the sources.

Note that for each power load reduction (Fig. 7(a)) (t = 50 s,75 s), deceleration and braking mode, the HESS current Fig. 8(a)(iBATref, iUCref) sign changes in order to absorb excess energy fromthe DC bus during the recovery mode and, as long as the FCproduces electrical energy, inducing the increase of the UC andBAT state of charge (Fig. 8(d) and (c)).

The frequency splitting control strategy effectively prevents theFC from responding to a large current slope, which ensures asmooth increase of 2.5 A.s−1 maximal slope (Fig. 8(a)) and forceseach source to respect its own characteristics (currents loops), andensures a very precise bus voltage regulation to its reference value(Fig. 8(b) using fast transient UC and BAT. Obviously, this desiredbehavior induces a change in the UC and BAT SOC (Fig. 8(d)and (c)). However, in this case, the HESS requires energy thatcan be provided without reaching either the low voltage limit orthe high voltage limit. The FC power rises and falls smoothly andtends to reload the UC source to its nominal value, which showsthat the compensation action is very effective according to thedynamic variation of the power demand. In steady-state conditions,the global system rebalances. This fact confirms the performanceof the fast internal current loop and the outer loop of the EMS.

Furthermore, for the currents in closed loops, as shown in Fig. 9,the currents follow their reference trajectories well without staticerror, which makes the inner control relevant. It demonstrates thepower enhancement capability of a controlled hybrid system andthe complementarity of the sources that are suitably used togetherto meet the transient and steady-state loads.

4.2. Control strategy flexibility The previous test val-idated that the frequency splitting control strategy was fully effec-tive and successfully managed the energy in the hybrid system.In this section, a flexibility test is conducted to verify whetherthe proposed EMS is also capable of being easily adjusted (split-ting frequencies) to obtain different power splittings (degree ofhybridization) according to each source dynamics. The correspond-ing results are shown in Fig. 10. Two case studies have beensimulated to guarantee more or less raised FC dynamics. Theregulation of this dynamics takes place regarding the choice ofthe frequency splitting (fFS1, fFS2), which are the degrees of free-dom of this technique fixed (at fFS1 = 50 mHz, fFS2 = 200 mHz)according to the FC and BAT requirements. The control parame-ters that have been used are fFS1 = 25 mHz, fFS2 = 250 mHz for

552 IEEJ Trans 9: 548–554 (2014)

ENERGY MANAGEMENT STRATEGY FOR FUEL CELL POWER SYSTEM

-150

-100

-50

0

50

100

150

410

408

412

0 20 40 60 100

100

110

120

130

100

120

140

160

t [s]

iFC [A]

iUC [A]

iBAT [A]

VBUS [V]

VBUSref [V]

VBATref [V]

VBAT [V]

VUC [V]

VUCref [V]

Sources currents

DC bus voltage

BAT voltage

UC voltage

(d)

(c)

(b)

(a)

Fig. 8. System performance. (a) Source current. (b) DC busvoltage. (c) BAT voltage. (d) UC voltage.

the first test (Fig. 10(a) and fFS1 = 40 mHz, fFS2 = 150 mHz forthe second test (Fig. 10(b). Therefore, in the first case, most partof the power demand is extracted from BAT. As a result, FC andUC dynamic responses are considerably limited. Conversely, inthe second test, the UC absorbs/supplies the power transient (fastpower variation) instead of the BAT, which avoids chattering andprovides smoother response of FC, thereby improving its life time.As can be seen from Fig. 10(a) and (b), the proposed EMS allowsdetermining suitable operating points for FC, BAT, and UC.

According to these results, the EMS proposed is endowed withthe functionality to deal with operating constraints using the degreeof freedom given by splitting frequencies, which allows the adap-tation of the global system behavior according to desired con-straints/objectives (constituents limits, system design, applications,etc.), which makes the control approach more effective.

5. Conclusion

In this paper, a new EMS was developed for a hybridFC/BAT/UC power source to manage the instantaneous powersplitting between the sources. Given the constraint of the FCdynamics and the complexity of the energy management, a com-plete control structure has been elaborated and successfully tested.In this way, local controllers were designed to drive the currentof each source and to guarantee tracking to their references andsafe operation. A simple and effective algorithm was used to

0 20 40 60 100

-100

0

100

-50

0

50

0

20

40

60

80

100

iFC [A]

t [s]

iFCref [A]

iBAT [A]iBATref [A]

iUC [A]iUCref [A]

(c)

(b)

(a)

Fig. 9. Current response. (a) FC current. (b) BAT current. (c) UCcurrent.

-2

-1

0

1

2

0 20 40 60 100-2

-1

0

1

2

t [s]

PFC [10*kW]PBAT [10*kW]

PUC [10*kW]

PFC [10*kW]PBAT [10*kW]

PUC [10*kW]

(b)

(a)

Fig. 10. Power response of the sources; (a) fFS1 = 25 mHz, fFS2 =250 mHz. (b) fFS1 = 40 mHz, fFS2 = 150 mHz.

produce current references using a power splitting technique,allowing a frequency decomposition of the power demands. Ituses two low-bandwidth filters with two splitting frequencies toassign the load power demands to the appropriate source. Simu-lation results clearly showed that the system performances wereimproved, especially in the following aspects: the hybrid sourcewas able to support the dynamics of unpredictable load demandsfor different operating modes and to satisfy the sources’ inherentcharacteristics, FC, BAT, and UC voltage and current limits; aswell as slow FC dynamics (FC system can be operated in its mostefficient region). In addition, another control parameter was inves-tigated to further validate the control strategy and to show theflexibility and generality of the hybrid source design. Thus, thestudied hybrid system can be easily adapted to different FC, BAT,

553 IEEJ Trans 9: 548–554 (2014)

T. AZIB ET AL.

and UC specification requirements and can be scaled to differentapplications.

References

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Toufik Azib (Non-member) received the Ph.D. degree in ElectricalEngineering from the University of Paris SouthXI, France, in 2010. Since 2011, he has been anAssociate Professor with the Control and Sys-tems laboratory, ESTACA Engineering School,France. His current research interests includepower electronics and new electrical devices

(fuel cell, batteries, and ultracapacitors).

Cherif Larouci (Non-member) received the Ph.D. degree inelectrical engineering from the Institut NationalPolytechnique de Grenoble, France, in 2002.He is currently a Research Professor with theControl and Systems laboratory, ESTACA Engi-neering School, France. His research interestsinclude the design of power electronic applica-

tions for automotive, aeronautics, and railway industries.

Ahmed Chaibet (Non-member) received the Ph.D. degree inautomatics from the University of Evry Val-d’Essonne, France, in June 2006. He joined theControl and Systems laboratory of ESTACAEngineering School in May 2007. His researchinterests include robust control of electricalmachines.

Moussa Boukhnifer (Non-member) received the Ph.D. degreein Control Engineering from the Universityof Orleans, France, in 2005. He is currentlya Research Professor with the Control andSystems laboratory of ESTACA EngineeringSchool, France. His research interests includedesign, modeling, control, and optimization with

applications to power electronics and electrical drives.

554 IEEJ Trans 9: 548–554 (2014)