NASA/TM—2010-216107

A Comparison of Flow-Through Versus Non-FlowThrough Proton Exchange Membrane Fuel Cell

Systems for NASA’s Exploration Missions

Mark A. Hoberecht

Glenn Research Center, Cleveland, Ohio

May 2010
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NASA/TM—2010-216107

A Comparison of Flow-Through Versus Non-FlowThrough Proton Exchange Membrane Fuel Cell

Systems for NASA’s Exploration Missions

Mark A. Hoberecht

Glenn Research Center, Cleveland, Ohio

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

May 2010
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Available electronically at http://gltrs.grc.nasa.gov
A Comparison of Flow-Through Versus Non-Flow-Through

Proton Exchange Membrane Fuel Cell Systems for

NASA’s Exploration Missions

Mark A. Hoberecht

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Background

As part of the Exploration Technology Development Program (ETDP) under the auspices of the

Exploration Systems Mission Directorate (ESMD), NASA is developing both primary fuel cell power

systems and regenerative fuel cell (RFC) energy storage systems within the fuel cell portion of the Energy

Storage Project. This effort is being led by the NASA Glenn Research Center (GRC) in partnership with

the NASA Johnson Space Center (JSC), Jet Propulsion Laboratory (JPL), NASA Kennedy Space Center

(KSC), and industrial partners. The development goals are to improve fuel cell and electrolysis stack

electrical performance, reduce system mass, volume, and parasitic power requirements, and increase

system life and reliability.

A major focus of this effort has been the parallel development of both flow-through and non-flowthrough proton exchange membrane (PEM) primary fuel cell power systems. The plan has been, at the

appropriate time, to select a single primary fuel cell technology for eventual flight hardware development.

Ideally, that appropriate time would occur after both technologies have achieved a technology readiness

level (TRL) of six, which represents an engineering model fidelity PEM fuel cell system being

successfully tested in a relevant environment. Budget constraints in fiscal year 2009 and beyond have

prevented NASA from continuing to pursue the parallel development of both primary fuel cell options.

Because very limited data exists for either system, a top-level, qualitative assessment based on

engineering judgement was performed expeditiously to provide guidance for a selection. At that time, the

non-flow-through technology was selected for continued development because of potentially major

advantages in terms of weight, volume, parasitic power, reliability, and life. This author believes that the

advantages are significant enough, and the potential benefits great enough, to offset the higher state of

technology readiness of flow-through technology. This paper summarizes the technical considerations

which helped form the engineering judgement that led to the final decision.

Introduction

Space fuel cell systems were first used to generate power during the Gemini program, and are

presently in use on the Shuttle. For future Exploration Missions, NASA’s development approach has been

to leverage the extensive commercial development of PEM fuel cell systems, while also addressing the

unique NASA requirements for space applications. These more stringent space requirements include

operation with pure oxygen (instead of air), and water management in reduced and zero-gravity

environments. All fuel cell systems, whether for space or commercial applications, consist of one or more

fuel cell stacks in combination with appropriate balance-of-plant hardware. The fuel cell stack performs

the electrochemical function of breaking down hydrogen and oxygen to form water and electrical power,

and the ancillary components comprising the balance-of-plant perform the necessary fluid and thermal

management functions of the system.

NASA/TM—2010-216107
System-Level Description

Both the stack and balance-of-plant are considerably different between the flow-through and

non-flow-through PEM fuel cell systems. In order to highlight the major differences between these two

options, a summary description at the system level is presented below.

Flow-Through PEM Fuel Cell Technology

Flow-through PEM fuel cell technology is characterized by a recirculating oxygen reactant stream

that removes product water generated at the cathode surface within each individual cell of the fuel cell

stack. Figure 1 depicts this recirculating oxygen stream. Depending on the operating characteristics of the

stack for a particular fuel cell vendor, a duplicate hydrogen recirculating stream may also be required for

removal of small amounts of water which inadvertently accumulate within the anode chamber of each

individual cell. Periodic venting of each reactant stream may also be required to remove inert gases,

depending on the purity of the reactants.

O2

 / H2O

MEA

f 1 active water

separator

H2O nmm^'- H2O

U

U C

m c —°o

O2

U U

H2O

gas

recirculation

pump

H2O2

Figure 1.—Flow-Through PEM Fuel Cell Schematic (active components).

NASA/TM—2010-216107 2
passive water

separator

;*% H2O

O2

H2O

H2O

Recirculating reactant streams dictate the need for some type of device to initiate and sustain the

recirculating flow, and another device to separate the product water from the two-phase stream exiting the

stack. In the case of existing state-of-the-art (SOA) flow-through PEM fuel cell systems, these devices are

typically active mechanical components, such as the pump and water separator in Figure 1 (Refs. 1 and

2). Any fuel cell system using these active components bears their weight, volume, parasitic power,

reliability, life, and cost penalties. Replacing the active components with passive components can

minimize some of the resulting penalties. Figure 2 depicts the same flow-through technology schematic as

in Figure 1, but with passive components replacing active mechanical components.

For reactant recirculation, various combinations of injectors, ejectors, solenoid valves, and pressure

regulators have been investigated as passive replacements for pumps. In the area of product water

separation, membrane separators, vortex separators, and meniscus separators have all been investigated as

passive replacements for active motorized water separators.

Even though the flow-through PEM fuel cell technology was not selected for continued development,

it is still considered a viable back-up option for Exploration missions. Therefore, ongoing work to

complete the assessment of passive components will continue through 2008. The results of these efforts

will be documented at that time.

O2

 / H2O

MEA A

injector/ejector

H2O2

Figure 2.—Flow-Through PEM Fuel Cell Schematic (passive components).

NASA/TM—2010-216107
Non-Flow-Through PEM Fuel Cell Technology

Non-flow-through PEM fuel cell technology is characterized by product water generated at the

cathode surface wicking through a support structure across an adjacent gas cavity, through a hydrophilic

membrane, into a water cavity within each cell of the stack, as shown in Figure 3. As with flow-through

technology, periodic venting of each reactant cavity may be required to remove inert gases, depending on

the purity of the reactants.

There are no recirculating reactants, and hence no requirement for providing either recirculation or

product water separation from two-phase reactant streams. Therefore, there is no need for components

that provide these functions, whether they are active or passive. The only ancillary component in a nonflow-through system that is not needed in a flow-through system is a back-pressure regulator needed to

maintain the proper differential pressure between the oxygen and water/coolant cavities. This backpressure regulator is critical for water removal within the non-flow-through system.

H2O

MEA

H2O

H2O

H2

O2

Figure 3.—Non-Flow-Through PEMFC Schematic.

NASA/TM—2010-216107 4
Stack-Level Comparison

Even though there are many similarities between a flow-through PEM fuel cell stack and a non-flowthrough stack, there are minor differences that potentially justify the selection of one technology over the

other. The important design parameters that impact this selection at the stack level are efficiency, weight,

volume, life, cost, and TRL. Table 1 highlights these parameters, with a check mark indicating the

technology option having the advantage based on a qualitative assessment by the author.

TABLE 1.—STACK-LEVEL COMPARISON OF FLOW-THROUGH

VERSUS NON-FLOW-THROUGH

Design parameter Flow-through Non-flow-through

Efficiency

Weight 3

Volume 3

Life

Cost

TRL 3

Assuming identical cell performance for both flow-through and non-flow-through PEM fuel cell

stacks, the required active electrode area is inherently the same for both technologies. One difference

between these options is the separate water cavity required in a non-flow-through stack, however it is

possible that this water cavity could also serve as the coolant cavity. Another difference between these

options is the thicker cathode (and possibly anode) reactant chamber needed to accommodate discreet

flow channels in a flow-through stack. It is these flow channels that provide the required high-velocity

recirculating reactant stream within each cell of the stack, thereby sweeping away the product water

droplets generated across the entire active area of each cathode surface. In comparison, the water droplets

generated in a non-flow-through stack are not swept away by a high-velocity recirculating reactant

stream. These droplets travel passively across a support structure in the cathode cavity, then through a

hydrophilic membrane into a water cavity within each cell of the stack. A slight pressure differential

across the hydrophilic membrane drives the water transfer. Simple diffusion is sufficient to provide

reactants to the electrode surfaces in a non-flow-through stack, so this allows the reactant chambers to be

much thinner than those required in a flow-through stack. Diffusional flow is more likely to be acceptable

at lower operating current densities where possible gas transport limitations in non-flow-through stacks

are less likely to occur.

In spite of the possibly separate water chamber in a non-flow-through stack, the difference between

relatively thick flow channels (flow-through) and thin support structures (non-flow-through) accounts for

the minor weight and volume advantage of the non-flow-through stack in Table 1. Preliminary estimates

by this author indicate a possible 5 to 20 percent weight advantage, and a possible 5 to 10 percent volume

advantage at the stack level. Preliminary values have yet to be determined for the other design parameters

in Table 1, however from an electrochemical standpoint, no major differences are anticipated between the

two technology options. Therefore, efficiency and life are not expected to be discriminators. There is also

little or no difference projected between the costs of the systems. Future development and testing will

validate the accuracy of these assessments.

The single major advantage of the flow-through PEM fuel cell stack over the non-flow-through stack

is its higher TRL. Even though an older vintage of non-flow-through fuel cell technology provided power

for the Gemini missions, the technology has seen little development over the past several decades, even

for commercial applications, when compared to flow-through fuel cell technology (Ref. 3). This is

because non-flow-through technology is not applicable to fuel cells with air as the oxidant due to the

continuous purge required to remove inert gases from the oxygen cavities in the cells. Inert gases

NASA/TM—2010-216107
comprise approximately 80 percent of the air stream, and the continuous purge required to remove these

inert gases essentially dictates flow-through operation—hence the minimal development of non-flowthrough technology over the past several decades. Flow-through PEM fuel cell technology for space

applications using pure oxygen as the oxidant has been under development by NASA since the late 1990s,

while non-flow-through development was not initiated until 2005. Non-flow-through PEM fuel cell

technology (presently at the laboratory/breadboard level) therefore lags flow-through technology

(presently at the breadboard/engineering model level) by at least several years in terms of readiness for

flight hardware development.

Balance-of-Plant-Level Comparison

There are very few similarities between a flow-through PEM fuel cell balance-of-plant and a nonflow-through balance-of-plant. The differences are significant, and clearly favor non-flow-through

systems. A summary comparison of these two options at the balance-of-plant level, based on a qualitative

assessment by the author, is presented in Table 2.

TABLE 2.—BALANCE-OF-PLANT-LEVEL COMPARISON OF

FLOW-THROUGH VERSUS NON-FLOW-THROUGH

Design parameter Flow-through Non-flow-through

Weight 3

Volume 3

Parasitic power 3

Reliability 3

Life 3

Cost 3

TRL 3

The differences between the flow-through PEM fuel cell system and non-flow-through system are

more significant at the balance-of-plant level than at the stack level. The major contributors are product

water separation internal to the stack and the lack of reactant recirculation with non-flow-through PEM

fuel cell technology (Fig. 3), which lead to the elimination of a number of ancillary components in the

balance-of-plant when compared to flow-through technology (Figs. 1 and 2).

This reduction should result in an advantage for the non-flow-through balance-of-plant in terms of

weight, volume, parasitic power, reliability, life, and cost. Preliminary estimates by this author indicate a

50 to 70 percent weight and volume advantage at the balance-of-plant level could be achieved.

Preliminary values have yet to be determined for the other design parameters, but can be discussed at a

summary engineering level. Parasitic power would likely be slightly less for a non-flow-through system

compared to a passive flow-through system because of fewer ancillary components, and significantly less

when compared to an active flow-through system because of no motorized active components. Reduced

numbers of ancillary components would likely translate directly into increased life and reliability of the

entire balance-of-plant because there are fewer components capable of failing, whether they be active or

passive. Similarly, reduced numbers of components may also translate into lower costs for a non-flowthrough balance-of-plant. Future development and testing will validate the accuracy of these assessments.

Just as with the stack-level comparison, the single major advantage of the flow-through PEM fuel cell

balance-of-plant over the non-flow-through balance-of-plant is its higher TRL. Even though an older

vintage of non-flow-through technology has flown previously on Gemini missions, flow-through

technology for space applications has been under development by NASA for many more years than the

newer vintage of non-flow-through technology. It therefore has the schedule advantage in terms of

readiness for flight hardware development.

NASA/TM—2010-216107 6
Summary

Over the past several years, NASA has been pursuing parallel development of flow-through and

non-flow-through PEM primary fuel cell power systems under the ETDP Energy Storage Project. Both

systems consist of a stack which generates electrical power and produces water, and a balance-of-plant

which performs the appropriate fluid and thermal management functions of the system. At the stack level,

non-flow-through PEM fuel cell systems have a potentially minor weight and volume advantage over

flow-through systems because of thinner individual cells within the stack. The potential weight and

volume advantage is more significant at the balance-of-plant level because of fewer ancillary components.

The reduced number of ancillary components also likely leads to a reduction in parasitic power and an

increase in reliability and life for non-flow-through PEM fuel cell systems. Costs are also anticipated to

be less because of fewer ancillary components.

The lone major advantage of flow-through PEM fuel cell systems over non-flow-through systems is

the higher level of technology readiness. Flow-through systems for space applications have been under

development by NASA since the late 1990s and have reached the breadboard/engineering model level.

Non-flow-through system development was not initiated until 2005 and this technology is presently at the

laboratory/breadboard level. At least several more years of development will be required to bring nonflow-through PEM fuel cell hardware to the same level of technology readiness as existing flow-through

hardware.

The original plan was to select a single primary fuel cell technology for eventual flight hardware

development once both systems had reached an engineering model hardware fidelity. Budget constraints

in fiscal year 2009 and beyond have led NASA to make an early selection of a single primary PEM fuel

cell technology. A top-level, qualitative assessment was performed expeditiously to provide guidance for

the selection, and based on this assessment; non-flow-through technology was selected for further

development. The justification is based on a number of qualitative technical advantages with respect to

weight, volume, parasitic power, reliability, life, and cost of non-flow-through technology when

compared to flow-through technology. Some of these individual advantages may be minor, but in

aggregate have the potential to be significant. It was this significant potential that drove the selection.

Future development and testing will validate the accuracy of the assessments and the wisdom of the

selection.

References

1. Loyselle, P.L., Prokopius, K.P., NASA Glenn Research Center, “Teledyne Energy Systems, Inc.

Proton Exchange Membrane (PEM) Fuel Cell Engineering Model Powerplant Test Report: Initial

Benchmark Tests in the Original Orientation,” NASA/TM—2010-216225, (to be published).

2. Loyselle, P.L., Prokopius, K.P., NASA Glenn Research Center, “Teledyne Energy Systems, Inc.

Proton Exchange Membrane (PEM) Fuel Cell Engineering Model Powerplant Test Report:

Remaining Benchmark Tests in All Three Spatial Orientations,” NASA/TM—2010-216224,

(to be published).

3. NASA Johnson Space Center, “Fuel Cell Technology Program Final Report,” Contract NAS9-11033,

September 1971.

NASA/TM—2010-216107 7
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13. SUPPLEMENTARY NOTES

14. ABSTRACT

As part of the Exploration Technology Development Program (ETDP) under the auspices of the Exploration Systems Mission Directorate

(ESMD), NASA is developing both primary fuel cell power systems and regenerative fuel cell (RFC) energy storage systems within the fuel

cell portion of the Energy Storage Project. This effort is being led by the NASA Glenn Research Center (GRC) in partnership with the

NASA Johnson Space Center (JSC), Jet Propulsion Laboratory (JPL), NASA Kennedy Space Center (KSC), and industrial partners. The

development goals are to improve fuel cell and electrolysis stack electrical performance, reduce system mass, volume, and parasitic power

requirements, and increase system life and reliability. A major focus of this effort has been the parallel development of both flow-through

and non-flow-through proton exchange membrane (PEM) primary fuel cell power systems. The plan has been, at the appropriate time, to

select a single primary fuel cell technology for eventual flight hardware development. Ideally, that appropriate time would occur after both

technologies have achieved a technology readiness level (TRL) of six, which represents an engineering model fidelity PEM fuel cell system

being successfully tested in a relevant environment. Budget constraints in fiscal year 2009 and beyond have prevented NASA from

continuing to pursue the parallel development of both primary fuel cell options. Because very limited data exists for either system, a toplevel, qualitative assessment based on engineering judgement was performed expeditiously to provide guidance for a selection. At that time,

the non-flow-through technology was selected for continued development because of potentially major advantages in terms of weight,

volume, parasitic power, reliability, and life. This author believes that the advantages are significant enough, and the potential benefits great

enough, to offset the higher state of technology readiness of flow-through technology. This paper summarizes the technical considerations

which helped form the engineering judgement that led to the final decision.

15. SUBJECT TERMS

Fuel cells; Fuel cell power plants; Hydrogen oxygen fuel cells; Energy conversion; Energy storage

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