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Design and development of a 7kW polymer electrolytemembrane fuel cell stack for UPS application

G. Squadrito a,*, G. Giacoppo a, O. Barbera a, F. Urbani a, E. Passalacqua a, L. Borello b,A. Musso b, I. Rosso b

a CNR - Istituto di Tecnologie Avanzate per l’Energia ‘‘Nicola Giordano’’ (CNR-ITAE), Salita S. Lucia sopra Contesse 5,

98126 Messina, Italyb Electro Power Systems spa (EPS), Via Grange Palmero 104, 10091 Alpignano, Italy

a r t i c l e i n f o

Article history:

Received 19 June 2009

Received in revised form

30 October 2009

Accepted 3 November 2009

Available online 22 November 2009

Keywords:

Polymer electrolyte fuel cells

Stack design

UPS

* Corresponding author. Tel.: þ39-090624231E-mail addresses: gaetano.squadrito@ita

(O. Barbera), urbani@itae.cnr.it (F. Urbani), emusso@electrops.it (A. Musso), ilaria.rosso@0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.11.019

a b s t r a c t

This work presents the PEMFC stack design methodology developed at CNR-ITAE, in the

frame of a collaboration with an industrial partner, Electro Power Systems (EPS), operating

in the Uninterruptable Power Supply (UPS) market. A detailed description of the design

procedure of a 7 kW PEMFC stack is reported, starting from technical requirements of the

UPS system to experimental tests. Bipolar plate layout, active area surface and shape,

maximum (OCV) and minimum voltage, maximum cooling circuit pressure drop,

maximum cathodic flow-field pressure drop, were the main constraint that influenced the

constructive solutions. The electrochemical performances of Gore Primea 5621 MEA with

SGL Sigracet GDL were chosen as reference to select the appropriate operating point in

terms of current density and single cell voltage. A current density of 800 mA/cm2 was

imposed as operating point of the stack, subsequently main stack parameters were

calculated. Three different cathodic flow fields, that were designed to fulfill UPS system

requirements, were tested in a single cell arrangement, to find the best gas flow path in

terms of compromise between cell performance and pressure drop. Also a specific study

was dedicated to the selection of gasket material to find the best compromise between cell

performance and limited mechanical stress. The assembled 70 cells unit was tested in

a test bench simulating the power system. Preliminary tests of the full unit yielded to

a power of 6.2kW at 36 V.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction systems. Furthermore, recent advances in PEFC technology

Polymer Electrolyte (Membrane) Fuel Cells (PEMFC or PEFC) are

considered as an environmentally friendly and high efficiency

power generator for many applications, and are becoming an

interesting power source in different application fields, from

small electronic portable units to medium size stationary

; fax: þ39-090624247.e.cnr.it (G. Squadrito), gionza.passalacqua@itae.cnelectrops.it (I. Rosso).sor T. Nejat Veziroglu. Pu

have opened the way for commercialisation in several appli-

cation field, especially UPS and backup power market,

showing the possibility to reach a large scale application [1–6].

Although, PEFC technology is far from being commercially

cost effective, single components, (membrane, catalyst, gas

diffusion layers, catalysed membranes) and stacks have been

sue.giacoppo@itae.cnr.it (G. Giacoppo), orazio.barbera@itae.cnr.itr.it (E. Passalacqua), luisa.borello@electrops.it (L. Borello), andrea.

blished by Elsevier Ltd. All rights reserved.

Table 1 – Stack dimensioning parameters.

Input parameters Value Units

Maximum OCV 70 V

Electric Power 7.0 kW

Minimum output voltage 36 V

Reference MEA characteristics (Gore Primea 5621)

Voltage 0.667 V

Current density 800 mA/cm2

Power density 533.3 mW/cm2

OCV 1.0 V

Design parameters

Single cell to Stack efficiency 90 %

Single cell voltage 0.6 V

Current density 800 mA/cm2

Power density 480 W/cm2

Operation Stack Voltage 42.0 V

Cell number 70 –

Cell Active Area w200 cm2

Total current 170.4 A

Power 7.0 kW

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commercially available for many years, allowing the birth of

stack manufactures and system integrators. Nevertheless, the

number of commercially available products is limited,

reducing the possibility for system integrators to find the

stack that matches their technical requirements. Develop-

ment of a PEFC stack meeting specific technical requirements

needs knowledge about the interaction between the flow field

and the MEA (membrane, electrodes and gas diffusion layer),

and a design methodology allowing the fuel cell stack design

to be carried out in a short time. Although many studies about

the influence of different parameters on stack performance

and efficiency have been conducted [7–12], to date a standard

methodology to approach the stack design has not yet been

developed. A design methodology previously developed

[13–15] has permitted the single cell performance to be

directly scaled up to the 70 cells, 7kW unit, avoiding inter-

mediate test phases to be conducted on a sub-stack.

In the present paper, the results of a PEFC stack developed

for UPS purpose are reported. The unit was designed, manu-

factured and tested jointly by CNR-ITAE and the Electro Power

System.

The whole, from the stack dimensioning to the first runs,

was developed over 10 months of work.

Table 2 – Serpentine geometrical prameters range.

Parameter Range

Active area 200 cm2

Active area shape factor 1� 2%

Open area 50–65 %

Channel width 0.5–2.5 mm

Channel height 0.5–1.0 mm

Rib width 0.8–1.0 mm

Targeted pressure drops 50, 100, 150 mbar

2. Experimental

2.1. Fuel cell stack specification

Electrical parameters of the stack such as electrical power,

maximum (OCV) and minimum voltage, were imposed by the

UPS device provider. Fuel cell stack was designed to operate

with pure hydrogen and air at ambient pressure and 70 �C,

using distilled water as coolant. A maximum open circuit

voltage of 70 V and a minimum voltage of 36 V at 7kW were

imposed because of the UPS system DC/DC converter limita-

tion. Gore Primea� 5621 coupled with SGL Sigracet GDL was

selected as MEA for the single cell and stack. Because of a total

active area of 200 cm2 was imposed by MEA manufacturer for

cost reduction concerns, a current density of 800 mA/cm2 was

selected as operating point of the stack. At this current density

a voltage of 0.667 V was achievable as indicated by GORE.

Considering that electrochemical performance provided by

manufacturer were obtained in a small scale single cell

(generally 25 cm2) a voltage loss it could reasonably be

expected with an increase in the MEA active area, a change in

flow path and a connection in series of several cell. Therefore

a voltage loss of 10%, was assumed.

Table 1 resumes the input and design parameters adopted

for stack dimensioning.

2.2. Flow field, bipolar plate and stack design

Design of reactants flow path involved the respect of the plate

layout provided by MEA manufacturer in which manifolds,

active area position and dimensions were assigned. Other

constraints concerned the respect of the cathodic flow field

pressure drop, that had to range from 50 to 150 mbar.

According to recent studies, multiple serpentine flow field

offers a good compromise between pressure drop and

electrochemical performance. Thus a multi serpentine flow

path was adopted for both the cathode and the anode side.

A proprietary software [13,14] was used to find the serpentine

channel arrangement that fits into the imposed active area

and induce the requested pressure drop. The inputs of

the software are the relevant geometrical parameters of the

serpentine and their range of values among which the

software was allowed to explore, see Table 2.

Considering a maximum blower head loss of 200 mbar,

three different cathodic flow fields with a nominal pressure

drop of 50, 100 and 150 mbar, and one anodic flow field of

50 mbar, were designed. Single cell test were performed to

figure out the best gas flow path in terms of cell performance

and pressure drop. Anodic and cathodic plate were made of

4 mm thick composite graphite plates where the flow field and

cooling paths were machined in front and back. Then, the

anodic and cathodic plates are coupled with an expanded

graphite foil frame (conductive gasket) in between, to assure

coolant and gas sealing and electronic conductivity.

Coolant distribution over the active area surface was

assured by a mirrored serpentine path machined in the rear

part of bipolar plates. This solution has permitted to maintain

the pressure drop below 10 mbar and to minimize localized

high temperature phenomena due to coolant stagnation. Cell

sealing was obtained by plane gaskets (non conductive

Fig. 1 – Section view of stack virtual prototipe.

Fig. 2 – Single cell mounted on the test bench. P1 [ cathode

inlet pressure trasducer, P2 [ cathode outlet pressure

trasducer, H1 [ Cathode humidity sensor, T1 [ Air inlet

termocuple, T2 [ Air outlet termocouple, T3 [ H2 inlet

termocouple, T4 [ H2 outlet termocouple, T5 [ Cell

termocouple, T6 [ Cooling fan termocouple, F1 [ Cooling

fan, Ht [ cell heaters.

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gasket), Gore-Tex GR, EPDM, PTFE and NBR were considered.

Reactants and cooling water internal distribution were

arranged in ‘‘Z’’ shape configuration, thus inlets and outlets

ports were placed on opposite clamping plates. The whole

assembly was kept between two copper sheets for current

collection and two aluminum end plates for tightening the

stack with the help of 14 M8 bolts. Two PTFE sheets were

inserted between end plates and current collector for safety

issues. In this way end plates were electrically insulated from

the current collectors. All stack components were designed by

a 3D CAD software and recorded in a database, so that

a prototype stack was virtually assembled, to check compo-

nent interferences and internal distribution circuits correct-

ness (Fig. 1).

Fig. 3 – Layout of 7kW test bench.

Table 3 – Cathodic pressure drops (mbar) measured inthree different single cell arrangements.

Nominal Pressuredrop

Measured Pressuredrop

Assembly 1 50 mbar 20 mbar

Assembly 2 100 mbar 40 mbar

Assembly 3 150 mbar 115 mbar

Fig. 5 – Current density at 0.6 V, cell resistance at OCV and

optimal torque moment values measured with different

material gaskets.

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2.3. Single cell test bench and test procedure

A 1kW test station (Lynntech Ltd) equipped with a gas

handling unit, flash evaporator humidifier unit, electronic

load, control and data logger computer, was used to test the

single cells. Pressure transducers were placed at air inlet and

outlet ports, to monitor the pressure drop at the cathode.

Collected pressure drop data were used to verify the flow field

pressure drop under operation. The temperature of inlet and

outlet gases and fuel cell temperature was also recorded

(Fig. 2).

I-V curves at constant stoichiometric ratio and constant

voltage stability tests (conducted at 0.6 V) were carried out to

characterize the cell. Operative conditions were set in accor-

dance with UPS specifications: T1¼T3¼T5¼ 70 �C, 1.2/1 bar

outlet pressure for H2/air, air inlet dew point of 65 �C, anode

inlet dew point of 45 �C. A multiplexer data acquisition system

allowed the electrical data to be recorded. High frequency

resistance data at a frequency of 1 kHz was also collected with

an Agilent milliohm meter at the beginning and end of the

test.

2.4. Stack test bench and stack test procedure

The stack performance was evaluated in a test bench that was

built with the same components of the commercial UPS

developed and commercialized by Electro Power System.

Therefore the stack was operated in a set-up that reproduces

the real working conditions, a raw scheme of the bench is

depicted in Fig. 3.

Air was supplied to the stack by a blower controlled in rpm.

An air filter was used to prevent MEA contamination from

Fig. 4 – Representative I-V curve of assembly 1–3, obtained

with Gore gasket, compared with the PRIMEA reference

curve.

polluting substances. Oxidant humidification was assured by

a gas to gas heat and water vapour exchanger, heat and

moisture being extracted from the exhaust air to the inlet

stream. Hydrogen coming from bottles was directly fed to the

stack at 1.2barabs. A recirculation system, completed by

a temporized purge valve, connected fuel inlet and outlet.

Intermittent purging helped for removing inert materials

including liquid water and impure fuel and alleviated unequal

gas distribution due to water flooding. The hydrogen recircu-

lation system maximized fuel use by reducing purge

frequency and duration and improving anode humidification.

Electrochemical reaction produces heat that has to be

properly managed to prevent membrane dehydration and

thus maintaining an optimal proton conductivity. Stack

temperature has been controlled by a two stage cooling

circuit. Excess heat drained from the stack using demineral-

ised water was transferred to a second cooling stage by

a liquid to liquid heat exchanger. Successively a radiator was

used to release in the environment the secondary cooling

media heat. Depending on variation of system load, rotation

speed of air fan was adjusted. The test bench allowed the

main stack data to be measured and recorded (stack and

single cell voltages, current, temperature) and also allowed

several operating parameters such as reactants fluxes,

Fig. 6 – Comparative I-V curve with different gasket.

Fig. 7 – a) Assembling procedure, b) Full stack assembled.

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primary and secondary cooling temperatures, and air

humidification to be controlled. A specific digital acquisition

and control unit was developed by the industrial partner (EPS)

for this purpose.

Constant current tests were performed with an electronic

load of 10kW.

Fig. 8 – Seventy cell stack on test bench.

3. Results and discussion

3.1. Flow field selection tests

The flow field selection was performed in a single cell

arrangement, by using MEAs sealed with a GORE-TEX GR

gasket. Comparative tests of three cathodic flow fields has

drove the choice toward the final flow field looking for the best

compromise between pressure drop and electrochemical

performance. A nominal pressure drop calculated for the

channel paths only, ignoring the GDL porosity, was compared

with experimental data obtained in a constant voltage mode

(0.60 V), using fluxes of 1.2/2 for H2/Air at 0.8 mA/cm2. Pressure

drop measurements that were obtained by averaging values

over one hour, pointed out a lower value than the calculated.

This discrepancy is due to the GDL superposition that allows

the gas flow to split in two main flows through both the flow

field channel and the GDL, thus reducing the flow field

pressure drop.

Table 3 shows the measured and calculated value of

pressure drop.

In Fig. 4 the polarisation curves of single cell with different

cathodic flow fields and the GORE Primea reference are

reported. As expected the cathode with a higher pressure drop

(assembly 3) always yielded the best cell performance [11].

At 0.6 V a difference in current density of about 17% between

assembly 3 and assembly 1 and of about 4% between assembly

3 and assembly 2 was revealed; this trend would confirm that

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the increase in performance is not directly proportional to the

flow-field pressure drop increase.

All the assemblies yielded to a lower voltage performance

than the reference provided by the MEA manufacturer. This

behaviour could be justified by the low gas humidification

level adopted for the tests, as indicated in section 2.3. In this

regard, the best choice in terms of parasitic losses and

electrical performance was assembly 2 that achieved an

acceptable power density at 0.6 V which corresponded to the

design operation point.

3.2. Gasket material selection

Another aspect that was dealt with, concerned the gasket

material selection because of its importance not only for the

stack sealing but also to assure an optimal contact resistance,

since they act as spacers between the GDL and bipolar plate.

The aim of this investigation was to find the gasket material

that lead to the best performance with low contact resistance

and clamping torque. Four different materials were consid-

ered, Gore Tex GR, PTFE, NBR and EPDM using the assembly 2

as cell test. Typical electrochemical characterization such as

I-V curves and short stability tests were performed. Fig. 5

shows the maximum current density that was obtained at

0.6 V and the measured cell resistance at the corresponding

torque.

Resistance measurement showed that the PTFE assembly

had the lowest resistance and consequently the best average

current density. Similar behaviour was observed in Fig. 6 that

reports I-V curves, in which PTFE and NBR assembly achieved

the best performance.

In conclusion, PTFE and NBR gave similar performance in

terms of short stability test and I-V curves, although NBR

required a higher torque (12 Nm) in respect to the PTFE (9 Nm).

Therefore the latter was selected for the full stack assembly.

3.3. Stack assembly procedure

The full stack size was assembled by hand, using a specific

fixture to minimize component misalignments. Fig. 7a) shows

Fig. 9 – Preliminary tests. Run1: start-up, Ru

the initial phase of the assembling procedure, where six

alignment rods permitted the operator to correctly stack 428

layers that composed the full unit. Once all components were

stacked, the first 8 tightening roads were placed. At this stage

a pre-assembly torque of 5 Nm was applied to fasten the stack

and remove the 6 alignment rods from the assembly bench.

Finally an intermediate clamping torque of 7 Nm was applied

after the remaining six tie rods were inserted. The entire

assembly process lasted for 3 hours.

The assembled unit was finally clamped at 9 Nm Fig. 7b).

After the fuel cell stack was assembled, the clamping plate

electrical insulation and the internal resistance were checked

and leak tests were performed.

3.4. Preliminary stack performance

The 70 cells size stack was connected to the above described

test bench. After the setting of the reactant flows, humidifi-

cation and cooling conditions, in order to match the design

operative conditions, the achieved temperatures were 10 �C

less than those aimed at. Nevertheless, the stack operated

stably at 60 �C ever showing a continuous increase in

performance.

Fig. 8 is a picture of the stack under test, where on the right

side the electronic load and the DAQ interface are visible,

whereas on the left, the power unit connected to the piping

system for the reactants distribution and humidification

section is observable.

As shown in Fig. 9 a conditioning phase was performed

increasing the current by step of 10A until it reached 160A,

with a single cell potential of 0.5 V (Run1). Run 2 permitted the

power of 6.2kW to be reached with a total voltage of 36 V.

During this run the performance of the stack increased

because higher currents could be reached whit a voltage

almost constant. Fig. 10 shows single cell voltage distribution

for the stack at a current density of 800 mA/cm2. The

average cell voltage was 510 mV with a standard deviation of

15 mV. The stack performance was below than the expected

(0,6 V @ 800 mA/cm2). This was due to a too low level of

humidification.

n2 current increase until 6.2kW @170A.

Fig. 10 – Stack voltage distribution at 170A.

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4. Conclusion

A project aimed at the development of a 7kW PEFC stack for UPS

application was successfully completed in 10 months of work,

from the dimensioning to the first test runs. About one half of

this time was spent in component acquisition or manufacture,

and the final results matched the project target. The flow field

and gasket selection were performed on a unit cell of the entire

stack. These tests allowed the choice of a cathodic flow path

with the best compromise between cell performance and

pressure losses; moreover studies on gasket material allowed

the choice of PTFE as the best compromise between cell

performance and limited mechanical stress. A preliminary test

performed on the full unit led to encouraging results.

Indeed the unit reached 6.2kW in the second run. The

obtained results show the effectiveness of the developed

design procedure. It permitted the scale-up of a single cell to

a full stack avoiding intermediate sub unit tests. A complete

validation of the design procedure requires lifetime and cycle

tests on the stack. This will be the object of future investigation.

Acknowledgements

The reported data were carried out in the framework of

a CNR-ITAE/Electro Power System (EPS) SpA collaborative

research project and was financially supported by Torino

Wireless Foundation.

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