Concept Analysis of an Electrical Fuel CellIntegration in Modern Aircraft

A Lucken1, T. Kut2, H. Langkowski1, S. Dickmann2, Senior Member IEEE and D. Schulz1, Member IEEE1Electrical Power Systems, Helmut Schmidt University, 22043 Hamburg, Germany

2Fundamentals of Electrical Engineering, Helmut Schmidt University, 22043 Hamburg, Germany

arno.luecken@hsu-hh.de

Abstract—The reduction of pollutant emissions and hence theincreasing of the eco efficiency of future aircraft is one ofthe major challenges for the aviation industry. Executing theMore Electric Aircraft concept can be one approach to achievethese high targets. One idea in this context is to substitutethe traditional Auxiliary Power Unit by a Multifunctional FuelCell System. It is necessary to transform the load-dependentfuel cell stack output voltage to the intended electrical onboardgrid voltage level. To increase the efficiency and reduce thesystem weight an optimized electrical integration is presented.Measurement results on a test bench confirm the theoreticalconcepts.

Index Terms—Fuel Cell Integration, More Electric Aircraft

I. INTRODUCTION

In the recent years, the international air transport has

increased more and more. The global aircaft fleet is expected

to double in the next years. Therefore, the trend is towards

higher efficiency for modern commercial aircraft. A significant

reason are the limited resources and therefore the increase of

the kerosene price, which leads to enforce the development of

weight optimized electrical architectures. An approach could

be the implementation of the More Eletric Aircraft (MEA)

concept. Within this concept, hydraulic and pneumatic systems

should be substituted by electrical ones, which lead to an

increase of the electrical power, see Figure 1 [1].

1

2

3

4

5

B767 B777 A380 B787 C Series

Next Gen.

(long range)

Next Gen.

(short range)

Conventionalaircraft

More ElectricAircraft

Elec

trica

l pow

er (k

W/s

eat)

Fig. 1: Increase of the electrical power demand [2].

The rising demand of the electrical energy leads to an

increase of the voltage level for power transmission on a

modern aircraft. Therefore, a High Voltage DC (HVDC) grid

(540 VDC or ±270 VDC) could be used [3]. One reason for

the decision of 270 VDC is the transformation of the traditional

three phases 115 VAC aircraft grids. A thyristor bridge rectifier

(B6) with a trigger delay angle of α = 0◦ could be used and

is described as follows [4]:

VDC =3 ·√3 ·√2

π· cos(α) ·VAC (1)

The HVDC has several positive effects compared to

conventional AC grid. The decrease of the cable diameter

due to the reduced current flow, while transmitting the same

amount of electrical power leads to weight savings. The

reduction of the dissipation power due to the higher voltage

level increase the system efficiency. Moreover, it is possible

to design a lightweight DC/DC converter due to the small

number of electronic components which leads to an average

high power density [5]. Figure 2 shows the increase power of

different conventional Auxiliary Power Unit (APU) systems

of the last 50 years. It is presented that the share of the

shaft power compared to the total power decreases while

the electrical power increases. Normally, the conventional

APU has to provide both, electrical energy and compressed

air. Modern aircraft have an electrical engine start and

an electrical environmental control system and substitute

the pneumatic ones, which reduce the share of the shaft power.

0

325

700

1050

1400

1963

1983

1988

1993

2007

2008

Pow

er (k

W)

Overall Power

Shaft PowerElectrical Power

year aircraft APU-type196319831988199320072008

B727A310A320B747A380B787

GTCP 85GTCP 331GTCP 131GTCP 600PW 980AAPS 5000

Fig. 2: Increasing APU power within the last 50 years [6], [7], [8],[9].

The trend to create an electrical engine start leads to the

approach to use a Multi Functional Fuel Cell System (MFFCS)

to increase the efficiency of a modern APU concept. The

MFFCS could replace the noisy and inefficient APU in the

tail of the aircraft. In addition, to the high efficiency it must

543978-1-4673-4430-2/13/$31.00 ©2013 IEEE

be ensured that the system weight of the MFFCS does not

exceeds the overall weight of the substituted components. The

required DC/DC converter, as well as the fuel cell stacks are

the heaviest single components. Therefore, a weight optimized

electrical integration is required. As a part of the project

cabin technology and multifunctional fuel cell systems, the

Institutes of Electrical Power Systems and the Fundamentals

of Electrical Engineering are dealing with a high efficiency

weight optimized power supply based on a multi functional

fuel cell system in cooperation with Airbus Operation GmbH.

A. The advantages of MFFCS

The reduction of emissions such as carbon dioxide (CO2)

gas, the water production and the noise level reduction are

important benefits. The Oxygen Depleted Air (ODA) can also

be used for fuel tank inerting and for fire suppression inside

the aircraft. Moreover, the ram air turbine (RAT) could be sub-

stituted due to the independency of the MFFCS from kerosene

[10],[11]. The main possible applications are illustrated in

Figure 3.

Fig. 3: Possible application scenario for the MFFCS [12].

The emission saving effect of the MFFCS is 100 %, because

no pollutants are produced during the reaction of hydro-

gen and air. With the assumption that the combustion of

1 kg kerosene produced 3.15 kg CO2 and a consumption of

kerosene of 800 l/(day·aircraft) is required, leads to approxi-

mately 2000 kg/(day·aircraft) CO2, which can be avoided.

B. Aim of Investigation

The aim of this paper is to present the methodology to confirm

the theoretical concepts for a weight optimized buck converter

based on an optimal fuel cell output voltage. The theoretical

concept will be presented. Simulation as well as measurement

results supporting these theoretical concepts.

II. ELECTRICAL INTEGRATION

A significant impact for increasing the electrical power in

modern aircraft is the application of a full electrical envi-

ronmental control system. The MFFCS has to be designed

for maximum power in order to ensure that all connected

consumers are supplied with electrical energy, even under hot

ambient conditions (ISA hot day (Tamb = 311.15K)). The fuel

cell output voltage strongly depends on the connected load.

DC/DC converters could be used to transform the fuel cell

(FC) voltage into the HVDC grid voltage. Figure 4 shows a

typical polarization curve of a fuel cell system.

0 10.4

1

Fuel cell current (normalized)

Fuel

cel

l vol

tage

(nor

mal

ized

)

Converter PowerMaximum Power

HVDC range

Fig. 4: Fuel cell polarization curve with the allowed HVDC range.The upper limit is set to ±280 VDC and the lower limit to ±250 VDC

according to the aircraft electrical power characteristics (MIL-STD.704F).

To achieve a lightweight electrical FC system one approach is

to use a buck converter structure with additional bypass, based

on an optimal fuel cell output voltage. When the fuel cell volt-

age level is equal to the HVDC grid voltage, it is possible to

connect the FC with a bypass converter directly to the HVDC

grid [13],[14]. Due to the allowed voltage range of the HVDC

grid, the converter can be designed for approximately 30% of

the maximum installed fuel cell power. An overall FC power

between 200 kW and 300 kW is expected, which leads to a

possible weight saving of 35 kg to 52.5 kg, with an assumed

power density of 4 kW/kg based on the characteristic out of

Figure 4. Figure 5 shows a possible electrical architecture

for connecting the MFFCS to the Primary Electrical Power

Distribution Center (PEPDC) of the aircraft. The electrical

aircraft grid could be divided into two sides to isolate the

opposite grid if a failure occurs.

DC

DC

DC

DC

FC FC

HVDC Bus 1

+270V -270V

540V

DC

DC

DC

DC

FC FC

HVDC Bus 2

+270V -270V

540V

PEPDC

Fig. 5: Possible electrical integration of two independent HVDCbuses, both with two FC multistacks.

544

Within this paper the electrical grid sides are named as

HVDC bus 1 and HVDC bus 2. The single fuel cells (FC)

are connected in series. The tap of each sides between the

single FC systems could be based on ground potential to

separate the positive and negative voltage level. The main

advantages of the presented integration can be summarized to

the following:

• connection of different loads (540 VDC, ±270VDC)

• higher efficiency due to the reduce current flow

• two independent single grids increase the availability

• modular construction of the single components

The DC/DC converter could be subdivided into several smaller

DC/DC converter to reduce weight and increase efficiency.

Moreover, the subdivided DC/DC convertes could be designed

as an interleaved converter. Figure 6 presents a 4 level in-

terleaved converter with integrated bypass. Within this paper

the semiconductor switches S1 to S6 are designed as a Metal

Oxide Semiconductor Field Effect Transistor (MOSFET). The

angle delay α has to be addapted for each switch. Therefore,

a period offset of T/4 must be considered [15]. For a buck

converter the single α can be described as follows:

α1 =T4

; α2 =T2

; α3 =3 ·T

4; α4 = T (2)

A detailed describtion of the buck converter design can be

taken from [13],[14].

Vin VoutI1

I2

I3

I4

Iin Iout

Ibypass

D

D2

D3

D1 L1

L2

L3

L4

S1

S2

S3

S

4S

S6

C

4

5

Fig. 6: 4 level interleaved buck converter with integrated bypass path.

A. Advanced bypass mode

Depending on the electrical power demand the switches S5 and

S6 are closed (bypass mode) or open (converter mode). When

the bypass mode is active, all switches S1 to S6 are closed to

decrease the power dissipation Ploss according to Equation 3,

with the number n of parallel switches and the resistance R.

Ploss =∞

∑n=1

(In

)2

·R (3)

For converter mode the output current Iout can be described as

follows:

Iout1 = I1 + I2 + I3 + I4 (4)

respectively for bypass mode

Iout2 = I1 + I2 + I3 + I4 + Ibypass (5)

The efficiency advantage Δη of the additional closed switches

S1 to S4 during the bypass mode can be calculated with

Equation 6. Ploss(1) describes the power dissipation, if only

the bypass carries the output current Iout and Ploss(2) describes

the power dissipation of the divided current flow:

Δη = 1− ΔPloss(2)

ΔPloss(1)(6)

With the help of the drain source resistance RDS(on) and

the inductance resistance Rind, the power dissipation Ploss(2),

respectively Ploss(1) can be calculated as follows:

ΔPloss(1) = 2 · ( Iout2

)2 ·RDS(on) (7)

respectively

ΔPloss(2) =

4 ·[(

Iout−Ibypass

4

)2 ·RDS(on) +(

Iout−Ibypass

4

)2 ·Rind

]

+2 ·(

Ibypass

2

)2 ·RDS(on)

(8)

According to Equation 6, an increase of the efficiency up

to 35% could be achieved based on the divided current

flow. When the bypass is active, an operation efficiency of

approximately 100% could be assumed, due to the minimal

power dissipation.

B. Current commutation

The operating mode changes from the bypass mode into the

converter mode has to be considered more specifically. The

current commutates from the lower inductance path into the

higher inductance path. Depending on the current rise time

on the inductance L an overvoltage at the output could occur.

To prevent such an overvoltage a soft switching strategy was

created for opening the bypass switches S5 and S6 [16].

Therefore, the maximum power dissipation of the single switch

has to be calculated and must be compared to the term

comprising the junction Tj and ambient temperature Ta, the

heat sink Rth3, the resistance of the thermal pad Rth2 and the

transient thermal resistance ZthJC.

∫Ids ·Vds ≤

Tj −Ta

ZthJC +Rth2 +Rth3(9)

545

C. Fuel cell multistack

As well as the DC/DC converters also each of the FC systems

could be divided into several single stacks. Normally, the

number of single cells n for a stack is limited to approximately

n = 200 due to the mechanical stability and optimize gas

distribution. To create the required fuel cell output voltage

of approximately 600 VDC (3 ·n) a multistack approach could

be integrated. An antiparallel diode for each stack prevents the

total loss of a FC-System, if a single stack is out of operation.

Figure 7 shows the multistack architecture with the integrated

antiparallel diodes.

Stack 1 Stack 2 Stack 3

Fig. 7: Fuel cell multistack with antiparallel diodes.

D. Simulation results

Figure 8 shows the simulation results, which confirm the

theoretical concept. At time t = 40 ms the operating mode

change from the converter into the bypass mode. The bypass

current does not increase immediately to the nominal value

beacause the inductances L1 to L4 have to be discharged first.

0.02 0.04 0.06 0.08 0.10

50100

Output current

0.02 0.04 0.06 0.08 0.10

50

100

Cur

rent

(A)

0.02 0.04 0.06 0.08 0.10

50

Time (s)

Bypass current

One of the parallel currents

Fig. 8: Simulation results of the operating mode change from theconverter mode into the bypass mode.

If the bypass mode is active, it is illustrated that the current

Iout is divided to 2/3 on the bypass (S5,S6) and to 1/3 on the

closed DC/DC path (S1,S2,S3,S4).

III. FUEL CELL INTEGRATION TEST BENCH

To verify the theoretical concepts, a test bench with up to

10 kW was created. Therefore, the essential components can

be described as follows:

• programmable voltage source

• converter with integrated bypass

• programmable electrical load

• hardware in the loop system

• measuring equipment

Figure 9 shows the test bench. Due to the integration of

a programmable voltage source, controlled by the hardware

in the loop system, it is possible to create each type of

polarization curves, see Figure 4. Moreover, also the different

transient impacts like the double layer effect or the diffusion

effect can be simulated.

Controlsystem

MATLAB/Simulink

- Rated Values- Request of Actual Values- Visualization of Measurements

Current

Vol

tage

DC - Source

- Fuel Cell Model- Current and Voltage Characteristics

RS-

232

Current

Vol

tage

Electronic Load

DC/DC - Converter

Am

plitu

de

Period

RS-232

Control

- Simulation of Loadsteps- Request of Actual Values- Constant Output Voltage

Fig. 9: Schematic illustration of the test bench.

For implementation of the hardware in the loop system a

control system from the Bachmann company was selected.

The communication is done by Matlab/Simulink. Moreover, all

the measurement values can be processed by Matlab/Simulink

models. The application of the test bench is not restricted

only for the considered approaches. New concepts to fulfill the

required EMC standards are also investigated on the presented

test bench [17].

A. Measurement results

Figure 10 show the measurement results of the 4 level inter-

leaved converter.

0 1 2 3 4 5 6 7 8 9 1094

100

1 parallel 2 parallel4 parallel

Effiz

ienc

y (%

)

Power (kW)

Fig. 10: Efficiency measurements of 1, 2 and 4 level interleavedconverter.

The highest system efficiency can be reached at part load

operation, when all 4 level are active. However, it must

be considered that the efficiency of the 4 level interleaved

converter in the lower part load operation is below the 2 or

546

1 level interleaved converter. Therefore, a converter control

strategy could be implemented which reduce the number of

parallel DC/DC converter to achieve a high efficiency over

the entire operating mode. Moreover, it is possible to activate

the bypass mode already at lower requested power. That

leads to efficiency improvements up to 3% based on the

bypass mode. According to the simulation results of Figure 8

the measurement results, presented in Figure 11 confirm the

theoretical concept of the current distribution.

0 5 10 15 20 25 30 35 402

4

6

0 5 10 15 20 25 30 35 40-100

1020

Time (ms)

Vol

tage

(V)

Cur

rent

(A)

Fig. 11: Gate source voltage for activating the bypass mode (uppercurve). Current flow of one of the parallel interleaved converters(bottom curve).

At time t = 20 ms the operating mode changes from the

converter mode into the bypass mode. The required gate source

voltage rise immediately while the converter current (1 parallel

path) need some time to reach the new operating value of about

2.5 A due to the discharge effect of the inductance.

IV. CONCLUSION

The aim of the investigation was to present a weight optimized

electrical integration concept of a MFFCS-APU for modern

aircraft. Due to the high efficiency and the multifunctional

application regarding the use of the inert gas, water production

as well as the possibility to substitute the ram air turbine,

a MFFCS could achieve a lot of advantages. The fuel cell

stack as well as the required DC/DC converter are the heaviest

single components from the electrical part of the MFFCS.

Therefore, a weight optimized electrical integration of the

MFFCS is one of the main challanges. When the fuel cell

output voltage is equal to the HVDC grid, it is possible to

connect the FC directly (without DC/DC converter) to the

HVDC grid. However, the fuel cell output voltage is load

dependent, therefore, especially for the part load operation a

DC/DC converter is nessecary. To increase the efficiency a 4

level interleaved buck converter with additional bypass was

presented. Based on the investigated method, it is possible

to reduce the converter weight and increase the efficiency.

Therefore, a fuel cell output voltage above the HVDC voltage

is required to use the buck converter structure. A multistack

approach with a series connection of single stacks is presented.

The overall electrical MFFCS integration to the PEPDC could

be divided into two equal sides to isolate each other if a failure

occurs. For the bypass operation, the advantage bypass mode

to increase the efficiency, as well as the current commutation

from the low inductance path to the high inductance path

is presented. To verify the theoretical concepts a test bench

was built for a prototyp application up to 10 kW. The mea-

surement results confirm the theoretical results. In summary,

the presented method could be a good alternative and is not

restricted only for aircraft application. However, the amount of

the benefits depends on the allowed HVDC range and on the

internal fuel cell resistance (slope of the polarization curve).

ACKNOWLEDGMENT

This work as part of the project cabin technology and mul-

tifunctional fuel cell systems has been supported by Airbus

Operations GmbH and the German Federal Ministry of Edu-

cation and Research. (support code: 03CL03A)

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