Solar Energy 79 (2005) 544–550

www.elsevier.com/locate/solener

Solar-powered regenerative PEM electrolyzer/fuel cell system

Daniel Shapiro a, John Duffy a,*, Michael Kimble b,1, Michael Pien b

a University of Massachusetts Lowell, Solar Engineering Program, 1 University Avenue, Lowell, MA 01854, USAb ElectroChem, Inc., 400 W. Cummings Park, Woburn, MA 01801, USA

Received 23 June 2003; received in revised form 2 August 2004; accepted 8 October 2004

Available online 10 February 2005

Communicated by: Associate Editor A.T. Raissi

Abstract

An electrolyzer/fuel cell energy storage system is a promising alternative to batteries for storing energy from solar

electric power systems. Such a system was designed, including a proton-exchange membrane (PEM) electrolyzer,

high-pressure hydrogen and oxygen storage, and a PEM fuel cell. The system operates in a closed water loop. A pro-

totype system was constructed, including an experimental PEM electrolyzer and combined gas/water storage tanks.

Testing goals included general system feasibility, characterization of the electrolyzer performance (target was sustain-

able 1.0 A/cm2 at 2.0 V per cell), performance of the electrolyzer as a compressor, and evaluation of the system for

direct-coupled use with a PV array. When integrated with a photovoltaic array, this type of system is expected to pro-

vide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array)

would provide high-quality backup power to critical systems such as telecommunications and medical facilities.

� 2005 Published by Elsevier Ltd.

1. Introduction

1.1. Context

It is often cited that at least 2 billion people live with-

out access to reliable electricity (Flavin and O�Meara,

1997). Even modest amounts of electric power can tre-

mendously improve the quality of life of people in

underserved regions; two of the authors have seen this

0038-092X/$ - see front matter � 2005 Published by Elsevier Ltd.

doi:10.1016/j.solener.2004.10.013

* Corresponding author. Tel.: +1 978 934 2968; fax: +1 978

934 3048.

E-mail addresses: shapiro_solar@yahoo.com (D. Shapiro),

John_duffy@uml.edu (J. Duffy), fuelcell@fuelcell.com (M.

Pien).1 Now with MicroCell Technologies, 410 Great Road, Suite

C-2, Littleton, MA 01460, USA.

firsthand through rural electrification work in Peru.

Among the basic applications are: lights, vaccine refrig-

erators, radio transceivers, and nebulizers.

There is a growing consensus (although by no means

unanimous) that the global petroleum-based industry is

unsustainable from an ecological viewpoint (UN IPCC,

2001), and that alternative sources of energy must be

developed. Renewable energy technologies, including

solar photovoltaic, solar thermal, geothermal, tidal, wind

and others, offer the hope of sustainable development.

They are inherently scalable, and lend themselves extre-

mely well to distributed power generation. This develop-

ment model (many small generation facilities rather than

a few large ones) holds the potential for eliminating the

need for an all-encompassing grid to transmit power to

users—especially helpful in areas where such a grid does

not already exist.

Fig. 1. Regenerative PV electrolyzer/fuel cell system.

Fuel Cell

O2H2

B

C

A

E

D

H2 O

H2OH2O

F

Fig. 2. Target system components: (A) accumulator, (B)

differential-pressure relief, (C) electrolyzer, (D) external power

lead, (E) water circulating pump, (F) fuel cell power out.

D. Shapiro et al. / Solar Energy 79 (2005) 544–550 545

1.2. Regenerative fuel cell system: A definition

Regenerative fuel cells, including those using water

electrolysis, are not a new concept (McElroy, 1993).

Such a system functions like a secondary (or recharge-

able) battery. In a regenerative scheme, the energy con-

version device (fuel cell) is combined with other

components to become a true energy storage system.

An external power source supplies initial energy, which

is converted within the system for storage. On demand,

the stored energy is reconverted into electricity. When

the stored energy is exhausted, the system may be

replenished by the outside power source.

1.3. Scope of project

Recent developments in PEM fuel cells are beginning

to make possible a promising alternative to batteries for

storage of energy from solar electric power systems

(Cisar et al., 1999). With this in mind, UML and Electro-

Chem combined efforts to design an improved energy

production/storage system based upon the regenerative

fuel cell concept. When integrated with a photovoltaic

array, this type of system is expected to provide reliable,

environmentally benign power to remote installations. If

grid-coupled, this system (without PV array) would pro-

vide high-quality backup power to critical systems such

as telecommunications and medical facilities.

Major project goals include:

• Design a system (the ‘‘target system’’) capable of

delivering 4 kW for 4 h, and of recharging itself

within 40 h with gases stored at up to 2000 psig

(13.8 MPa).

• Design, build, and test a 200 psig (1379 kPa)-capable

prototype system.

• Collect operating data from PEM electrolyzer: target

was 1.0 A/cm2 at 2.0 V per cell.

• Evaluate the system�s suitability for direct-coupled

use with a PV array.

The aim of this study is to contribute to existing

knowledge in several ways:

• Evaluate performance of experimental PEM device.

• Make system design innovations.

• Evaluate performance of PEM electrolyzer as

compressor.

2. Target system

2.1. Overview

The system concept is shown in Fig. 1, with a more

detailed view in Fig. 2. A photovoltaic array drives a

PEM (Proton-Exchange Membrane) electrolyzer, which

breaks water into hydrogen and oxygen and compresses

them into high-pressure storage tanks. The gases are

used to run a PEM fuel cell, producing on-demand elec-

tricity to power a load. The fuel cell�s only byproducts

are heat and pure water, which is recycled for use by

the electrolyzer. The only required input is energy to

drive the electrolyzer—the water and gases cycle in a

closed loop. The round-trip efficiency of the storage sys-

tem would be roughly 25%, but the marginal cost of

added energy storage (i.e., larger gas tanks) would be

relatively low.

2.2. Elements of target system design

2.2.1. Simplification

From the outset, a reductionist approach was

adopted: simplification was the watchword. Fewer com-

ponents would mean lower system cost and fewer possi-

ble points of failure. To that end, one of the first design

Fig. 3. Membrane electrode assembly (MEA).

546 D. Shapiro et al. / Solar Energy 79 (2005) 544–550

decisions was to integrate gas and water storage in the

same pressure vessels. High-pressure, corrosion-resistant

containers are expensive, so any reduction in the re-

quired number was significant. This decision also meant

that the system could dispense with components other-

wise needed to produce dry gas: phase separators and

dryers to remove moisture from the electrolyzer�s two

product gas streams.

2.2.2. Pressure balancing

The electrolyzer is a filter-press style stack of individ-

ual electrolytic cells, capped with stiff endplates and held

together with compression bolts around the perimeter.

This design was intended for use in free atmosphere at

moderate internal pressures, up to about 50 psig

(345 kPa). Above this pressure, leakage from the seals

between cells is expected to be significant. Similarly, if

the ambient pressure were much greater than the inter-

nal pressure, the stack would leak. To operate success-

fully at high pressure, therefore, the electrolyzer�s seals

must not experience a large differential pressure between

the internal flow fields and the stack�s environment. It

was decided to enclose the electrolyzer in a pressurized

environment. Electrolyzers with reinforced plates to

operate at high pressure are available commercially,

but these would be more costly than those under discus-

sion here.

If the electrolyzer were simply enclosed in a pre-pres-

surized vessel, the system�s operating pressure range

would still be limited by this pressure—i.e., it could

not operate more than about 50 psi (345 kPa) above or

below the electrolyzer vessel pressure, whatever that

pressure might be. To access the full desired range of

system operating pressures, then, the challenge was to

match continuously the electrolyzer�s ambient pressure

to that of the fluid within the electrolyzer, entirely elim-

inating pressure differences across the seals.

This requirement led to a radically simplified system

design: the electrolyzer is contained within the hydrogen/

water pressure vessel. This concept automatically pro-

vides continuous pressure-matching; as the system pres-

sure changes (and thus the pressure of the electrolyzer�sworking fluid), the electrolyzer�s ambient pressure is of

necessity equal to its internal pressure. In addition, this

system design reduces the number of fittings and compo-

nents exposed to the difference between system and

room pressure: tubing and connections for hydrogen

gas and hydrogen-side water transport are reduced or

eliminated.

To address the possibility of an oxygen leak within

the hydrogen vessel, catalyst-treated material would be

placed around the electrolyzer. Any escaping oxygen

would immediately recombine on the catalyzed surface

with the ambient hydrogen, forming water. This sponta-

neous reaction is the same as takes place in a PEM fuel

cell.

2.2.3. Pressure damping

Another key issue in the system design was to mini-

mize any pressure differences within the electrolyzer

itself. The core of a PEM device (fuel cell or electrolyzer)

is the thin, flexible proton-exchange membrane itself,

usually laminated between catalyzed carbon electrodes.

It separates the hydrogen side of the system from the

oxygen side. This membrane electrode assembly (MEA,

see Fig. 3) is relatively fragile; undue stress could

compromise the intimate bond between membrane and

electrodes or even result in partial or complete rupture,

with consequences ranging from impaired system per-

formance to uncontrolled mixing of H2 and O2. It was

essential to prevent transient pressure surges, such as

might be caused by automated valve actuation, from

being transmitted to the MEA.

A hydraulic accumulator was chosen to provide a

transfer-barrier, pressure–damping interface between

the hydrogen and oxygen volumes. The accumulator

(at (A) in Fig. 2) is a steel shell containing a flexible blad-

der. The bladder volume would be connected to the sys-

tem�s hydrogen side, while the shell would be connected

to the oxygen side. The accumulator acts as a gas dam-

per, minimizing the magnitude of any pressure surges in

the system.

2.2.4. Volume imbalance

The accumulator has the additional virtue of com-

pensating for imbalances in water distribution. Under

normal operation, water is consumed only on the oxy-

gen side of the system, creating a volume imbalance.

Compounding the problem, each proton migrating

through the membrane osmotically ‘‘pulls’’ between 1

and 2.5 water molecules along with it to the hydrogen

side of the system (US DOE, 2000). Thus for each H2

molecule produced by the electrolyzer, the oxygen side

loses 3–6 molecules of H2O, while the hydrogen side

gains 2–5 H2O molecules. This cumulative imbalance

limits the electrolyzer�s maximum run time in two ways:

first, at some point the oxygen side will run out of water,

and second, the mounting pressure imbalance will

endanger the electrolyzer. By allowing the relative vol-

ume change between the two gases needed to equalize

their pressures, the accumulator will extend system run

time between cycles, and will permit operation even

under conditions where a system problem such as a slow

oxygen leak creates improper proportions of oxygen and

hydrogen.

D. Shapiro et al. / Solar Energy 79 (2005) 544–550 547

2.2.5. Differential-pressure (D-p) relief

Large or sustained pressure imbalances between the

oxygen and hydrogen sides are avoided with a differen-

tial-pressure relief valve (at (B) in Fig. 2). Excessive pres-

sure from either side causes a piston to move to one side,

allowing the higher-pressure gas an avenue to escape.

After sufficient gas has escaped, the piston moves back

into a central position, again sealing the system.

3. Prototype system

With the target system design in hand, a prototype

was designed much like the target system, but with the

electrolyzer and fuel cell scaled down. However, as it be-

came evident that the project budget would not support

the cost of high-pressure hardware, it was decided that

the critical mechanisms and concepts could be demon-

strated by a prototype designed to operate at up to

200 psig (1379 kPa). The prototype (Fig. 4) produces en-

ough gas at high enough pressures to validate the system

design and to serve as a testbed for the experimental

electrolyzer. As the analysis of safety issues concerning

placing the electrolyzer in a hydrogen atmosphere was

incomplete at the time, the electrolyzer was placed in

its own pressure vessel and a nitrogen pressurization

subsystem was added.

ElectrolyzerVessel

Electrolyzer

Nitrogen

HydrogenTank

OxygenTank

SightGlasses

Pump

BleedValve

Bleed Valve

ReliefValve

ReliefValve

System Schematic

Fig. 4. Schematic of prototype system.

The hydraulic accumulator was omitted due to con-

cerns that it would create difficulty in isolating pressure

and volume phenomena—one could not know the quan-

tity of either gas in the system when the gas volumes

would be affected by the accumulator�s presence. It

was desirable, therefore, to collect data without the

accumulator. ElectroChem had meanwhile redesigned

their electrolyzer stack, greatly improving the support

of the proton-exchange membranes at the heart of each

cell. This change made the stack much more resistant to

damage by pressure transients and imbalances between

the cathode and anode sides. Testing could therefore

be conducted without the use of an accumulator for

pressure balancing.

The prototype includes the electrolyzer in its contain-

ment vessel, gas/water storage tanks, circulating pump,

manual controls and data-acquisition sensors. Also in

place are a full complement of relief valves, bleed valves,

and a catalyzed hood for the electrolyzer (as described

above, for the target system). The electrolyzer is driven

by a regulated DC power supply rather than a PV array,

for convenience in selecting precise and consistent power

levels. This system was tested with very encouraging

results.

4. Testing

A PC-based data-acquisition system was used to re-

cord most system measurements. Thermocouples were

placed at the inlet, both outlets, and on the body of

the electrolyzer, as well as in each gas tank and in the

electrolyzer pressure vessel. Pressure transducers re-

ported the hydrogen and oxygen pressures. Precision

current shunts gave the circulating pump and electro-

lyzer input currents, while the input voltages were mea-

sured directly.

4.1. Initial function tests

The prototype�s ‘‘shakedown cruise’’ was a series of

gas-generation tests. The system was switched on, H2

and O2 were produced, and the system pressure was al-

lowed to mount. Temperatures were closely monitored,

as it was not known how much heat the electrolyzer

would release.

4.2. Stepwise constant-pressure test

The primary means of investigation was a series of

‘‘stepwise constant-pressure’’ tests. Through the use of

back-pressure regulators, the system was tested under

constant-pressure operation at a variety of pressure lev-

els. Once each desired pressure level was reached, the

pressure was held constant by bleeding additional gas

from the system as it was produced. Care was taken to

548 D. Shapiro et al. / Solar Energy 79 (2005) 544–550

vary the pattern of pressure levels to avoid confusion

due to possible hysteresis effects. Throughout the above

procedure, the intent was to supply the electrolyzer with

power at a constant current level, allowing the stack

voltage to fluctuate as the electrolyzer�s demand for

power changed with changing system pressure.

276277278279280281282283284

Elec

trol

yzer

Pow

er (w

atts

)

405060708090100110120

Syst

em P

ress

ure

(psi

g)

Electrolyzer Power

.

5. Results

5.1. Overall system performance

The basic design has proven quite satisfactory. Both

the strengths and shortcomings of the prototype have

provided crucial lessons for refining the target system�sdesign. For example, integrating the pressure vessels

has greatly simplified the system. On the other hand,

the original design had to be modified by adding a phase

separator at the oxygen sample port, to prevent the es-

cape of water during oxygen gas venting. Pressure bal-

ancing across the electrolyzer seals was successfully

demonstrated; the electrolyzer stack, designed for

50 psig (345 kPa) use, was operated up to 220 psig

(1517 kPa) internal pressure with no evidence of hydro-

gen or oxygen leakage.

5.2. Electrolyzer polarization

The experimental electrolyzer was tested up to its tar-

get current density of 1.0 A/cm2 at a range of pressures,

at an average temperature of 65 �C (see Fig. 5). At this

current level, cell voltage averaged 2.5 V, well above

the ideal but approaching the voltage expected from this

prototype. Note that the 200 psig data series exhibited

an apparent hysteresis effect as current density was

changed.

5.3. Electrolyzer as compressor

It is known that an electrolyzer can generate gas at

high pressure with little more energy than required at

low pressure (Cisar et al., 1999). Current work aims

at proposing a model for the incremental compression

0

200

400

600

800

1000

1200

1.7 1.9 2.1 2.3 2. 5

Cell Potential (volts / cell)

Cur

r. D

ensi

ty (m

A /

cm2 ) 200 psig

150 psig100 psig50 psig

Fig. 5. Polarization curves, 50 cm2 electrolyzer.

process. It appears that the ‘‘missing energy’’ is drawn

from the stack overvoltage, the amount by which the

cells� voltages exceed the thermodynamic ideal minimum

voltage of 1.23 V, and that the electrolyzer just becomes

less inefficient as the pressure rises. The Nernst equation

(Eq. (1)) is often used to describe how the ideal cell

potential varies with the concentrations of the products

and reactants.

E ¼ E0 � RTnF

lnj Cprod jj Creact j

; ð1Þ

where E = ideal cell potential at a given state; E0 = ideal

cell potential at STP (‘‘standard’’ potential); n = number

of moles of electrons being transferred in the half-reac-

tions; F = Faraday�s constant, amount of charge carried

by 1 mol of electrons, 96,485 C/mol e�; R = gas con-

stant, 8.3145 J/K mol; T = temperature in Kelvins;

C = concentrations of products or reactants.

Here, the products are pure H2 and O2 gas, and their

concentration may be taken as their partial pressures

(Zumdahl, 1989). If the product gas pressure increases

by a given factor X, the ideal cell potential increases

by the amount (RT/2F) * ln(X3/2).

In one stepwise constant-pressure test, the electro-

lyzer generated enough gas to raise the system pressure

from 32 to 110 psig (221–758 kPa) (see Fig. 6). The mea-

sured power demand rose minimally, from 278.1 W to

282.2 W, or an increase of only 1.5%. This type of result

has been verified in multiple tests, up to 210 psig

(1448 kPa).

During this test, the electrolyzer showed a voltage

rise significantly greater than the ideal (see Table 1 and

Fig. 7). This is being explored in further tests, but is

274275

0:00

0:04

0:08

0:12

0:16

0:20

0:25

0:29

0:33

0:37

0:41

0:45

Elapsed Time (hours:minutes)

20304P O2 (EL out)

Fig. 6. Power increase of 4-cell, 50 cm2 electrolyzer.

Temp = 65.4 �C, current density = 0.6 A/cm2.

Table 1

Pressure and voltage rise, ideal vs. actual

Press. factor X Ideal DV, mV Actual DV, mV

2.67 20.5 32

Fig. 8. PV module I–V curves and load line.

9.1

9.15

9.2

9.25

9.3

9.35

9.4

9.45

0:00

0:04

0:08

0:12

0:16

0:20

0:25

0:29

0:33

0:37

0:41

0:45

Elapsed Time (hours:minutes)

Volta

ge (v

olts

)

0

20

40

60

80

100

120

140

Pres

sure

(psi

g)

Electrolyzer VoltageOxygen Pressure

Fig. 7. Voltage rise of 4-cell, 50 cm2 electrolyzer (at 65 �C).

D. Shapiro et al. / Solar Energy 79 (2005) 544–550 549

probably related to the electrolyzer�s stack potential at

all pressures being well above the ideal value.

5.4. Direct-coupling PEM electrolyzer with PV

Fig. 8 illustrates conceptually the very favorable

match between the PEM electrolyzer�s load characteris-

tics and the maximum power points of typical PV mod-

ules. The ‘‘sample electrolyzer load line’’ is from actual

test data, and is superimposed upon a set of current–

voltage curves from a typical commercial PV module

(here, an ASE Americas ASE-50). Since the electrolyzer

stack is modular, at approximately 2 V per cell, an array

of PV modules and an electrolyzer can be custom-fit to

each other. This indicates that PEM electrolyzer-based

systems can probably dispense with the complication,

expense, and potential unreliability of interfaces such

as maximum power point trackers. Previous work (such

as Morimoto et al., 1986; and Arkin and Duffy, 2001)

also supports this approach.

6. Discussion

Novel pressure regulation concepts have been incor-

porated into the design of a battery replacement system

with a PEM electrolyzer, gas storage, and fuel cell:

electrolyzer in a storage tank, accumulators, and differ-

ential-pressure relief valve. The experience of building

and operating the prototype system with pressures up

to 200 psig (1379 kPa) has established proof-of-concept

of the placement of the electrolyzer in the hydrogen

storage tank. The prototype has also provided valuable

lessons for refining the design of a larger scale higher-

pressure regenerative electrolyzer/fuel cell system. A

larger 4 kW fuel cell system has been built and is under-

going initial testing. This system could be grid-coupled

to supply ‘‘premium power’’ needs for high-reliability,

high-quality backup power, or integrated with a photo-

voltaic array to serve remote locations.

Several relevant issues were not addressed in this

work, and remain as subjects for further testing and

modeling:

Thermal management of this type of system requires

investigation. Das et al. (2003) did simulate the heat

transfer of this full-scale system with the addition of

cooling fins on the gas tubing and oversized fuel cell

plates of graphite and showed adequate cooling.

Further work is necessary to determine whether the

electrolyzer may be a less efficient compressor at high

pressures (up to 3000 psig, 20.7 MPa). Aurora (2003)

developed a detailed energy flow model of this type of

system and predicted the electrolyzer to be more efficient

than a mechanical compressor.

Especially at high pressures and current densities,

side reactions and material breakdown may affect both

the purity of the product gases and the longevity and

performance of the system. Replaceable ion-exchange

resins could be used to mitigate impurity issues.

Acknowledgment

The authors would like to thank the Massachusetts

Toxics Use Reduction Institute (TURI) and Electro-

Chem, Inc. for partial support of this research.

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