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Hydrogen production using alkaline electrolyzer andphotovoltaic (PV) module

Ankica Ðuki�c*, Mihajlo Firak

University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, I. Lu�ci�ca 5, 10000 Zagreb, Croatia

a r t i c l e i n f o

Article history:

Received 18 March 2010

Received in revised form

3 December 2010

Accepted 31 January 2011

Available online 4 March 2011

Keywords:

Alkaline electrolyzer

Ni metal foam

Photovoltaic module

* Corresponding author. Tel.: þ385 1 6168 21E-mail address: ankica.djukic@fsb.hr (A.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.01.180

a b s t r a c t

In this paper it is presented hydrogen production using alkaline water electrolysis where

a 30 W photovoltaic (PV) module was involved as a source of electric energy. Therefore, the

process is without emitting CO2. There is constructed and tested an alkaline electrolyzer

with 50� 50� 2 mm Ni metal foam electrodes, 50� 50� 0.4 mm Zirphon� membrane and

25% alkaline (KOH) solution electrolyte. Electrolyzer UI characteristics for natural and

forced flow of electrolyte with PV module UI characteristics are presented. The results are

in favor of forced flow circulation, and these are better if the flow velocities are higher.

Calculated Energy efficiency (based on hydrogen high heating value) for both types of

circulation is above 55%. There are much evidence for further improvement of the system

components and consequently electrolyzer and system efficiency.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction There was no electronic equipment between PV module and

In this paper there is presented hydrogen and oxygen

production using photovoltaic (PV) module coupled with an

alkaline water electrolyzer. This is the only existing commer-

cial technology of hydrogen production without CO2 emission

[1e4]. The industrial standard of purity of hydrogen produced

using alkaline electrolyzer is 99.3e99.8%. Electrolyzer effi-

ciency expressed as a quotient of amount of hydrogen high

heating value and electric energy needed to produce it can be

about 0.6 [5]. The aim of this work is to research the potenti-

alities for further improving and simplifying of this technology

by using newest materials and design concepts. In this

research 3D porous Ni metal foam [6,7] for electrode material

was usedwith porosity of 95e97%andnominal pore diameters

of 600 mm. The liquid 25% potassiumhydroxide (KOH) solution

electrolyte was forced to flow through the electrode’s pores by

two centrifugal pumps. PV module was designed to cover

electricity for the electrolyzer and for pumps simultaneously.

8; fax: þ385 1 6156 940.Ðuki�c).2011, Hydrogen Energy P

the rest of the system aimed for PV module Maximum Power

Point (MPP) tracking or pump’s speed control. In this work,

hydrodynamic microscopic level of hydrogen and oxygen

bubbles growth, detachment, coalescence and flowwith liquid

electrolyte phase through the electrodes was not studied.

There are presented the macroscopic level of the electrolyzer

functioning through the electrolyzer UI characteristics,

hydrogen production and efficiency. Also, the majority of the

experiments were carried out indoor using laboratory DC

power supply, but there is also shown an example of the

outdoor functioning under real solar irradiance. As the whole

system was built mainly of transparent Plexiglas, it was

possible to study visually the gas bubbles separation from

liquid electrolyte in the hydrogen and oxygen separators and

concentration of the gas micro bubbles in electrolyte circula-

tion loops. Further research of the mentioned phenomenon

will consequently lead to the enhancement of the system and

system components.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Nomenclature

A electrodes area, 0.0025 m2

a parameter related to ohmic resistance of

electrolyte, Um2

b coefficient for overvoltage on electrodes, V

c coefficient for overvoltage on electrodes, A�1m2

c1 parameter for temperature dependence,

3e�3 m2 V�1

c2 parameter for temperature dependence,

0.1e�6 m2 (V�1 K�1)

CT thermal capacitance of PV module, J K�1m�2

e elementary charge, 1.60219e�19 C

E global solar irradiance, Wm�2

HH hydrogen high heating value, 12.745e6 Jm�3

I electrolyzer operating current, A

Iph photo current, A

IPV PV module operating current, V

Is saturation dark current, A

Isc short circuit current, A

k Boltzmann’s constant, 1.3806e�23 J K�1

m non ideality factor

np number of parallel strings

ns number of series-connected PV cells

PV photovoltaic

q fluid flow, kg s�1

qL overall heat loss coefficient, Jm�2

RPM revolutions per minute, min�1

Rp parallel resistance, U

Rs series resistance, U

t time, s

Ta atmospheric temperature, K

Tc solar cell temperature, K

U electrolyzer operating voltage, V

Uoc measured PV module open circuit voltage, V

UPV PV module operating voltage, V

Urev reversible voltage, 1.23 V

UT thermal voltage, V

UI voltageeelectric current

VH2 hydrogen produced volume, m3

DP pump head pressure, Nm�2

hc PV cell efficiency

hE electrolyzer energy efficiency, %

sa transmittanceeabsorptance product of PV cells

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 67800

2. Effects of the different electrodesdistances on the electrolyzer UI characteristics

The first part of this work covers experimental researches of

electrodes distance effects during the alkaline electrolysis

process [8]. There are performed the experiments with elec-

trode distances from 5, 10, 15, 20, 30, 40, 50 mm and Zirphon�

[9] membrane thickness of 0.4 mm. In these experiments

electric current, voltage and temperature of electrolyte are

Fig. 1 e Measurement scheme for U

measured. Fig. 1 shows experimental setup which includes

pool electrolyzerwithelectrolyteof25%KOHaqueoussolution.

In the first step, 3D electrodes of Ni metal foam are

included in the experiments with mentioned different elec-

trode distances and results for 0.01, 0.03 and 0.05 mmdistance

are given in Fig. 2a.

From these experimental results it is obvious a large effect

of electrode distance during the electrolysis, firstly in the

electric current valuewhich gives high or lowUI characteristic

slope. In this context, the thickness of the Zirphon� material

I characteristics determination.

0 1 2 3 40

5

10

15

20

25

Voltage, U [V]

]A[I,tnerruc

cirtcelE

d = 0.01 md = 0.03 md = 0.05 m

Urev= 1.23

0 1 2 3 40

5

10

15

20

25

Voltage, U [V]

]A[I,tnerruc

cirtcelE

3D Ni metal foam& Zirphon®Ni coated carbon steel plate& Zirphon®

Urev= 1.23

a

b

Fig. 2 e (a) Electrolyzer UI characteristics with different

electrodes distances. (b) Electrolyzer UI characteristics

comparison with different electrodes material.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 6 7801

membrane is proved to be an electrode distance with highest

slope. According to this conclusion, there is performed an

experimentwith an electrode of Ni plated carbon steel plate as

a referent electrode and Zirphon� material membrane. These

experimental results are given in Fig. 2bwhere it is shown that

3D electrodes of Ni metal foam are manifested as better in

comparisonwithNi plated carbon steel plate for the process of

electrolysis with membrane of Zirphon� material with thick-

ness of 0.4 mm.

The electrolyte reservoir was made of transparent glass.

This allowed the gas bubbles convective flow to be photo-

graphed using high speed video camera. There were observed

generally two types of bubbles: the first and the bigger onewas

able to pass through the border of liquid electrolyte in the

reservoir to the open atmosphere, and the second and the very

small one (micro bubbles, hardly visible by human eye) which

cannot penetrate through the liquid level and slow circulate in

the electrolyte making it as nontransparent white fluid. These

facts, together with inherent porosity of the Ni metal foam

and noticed electrode distance influence the electrolyzer UI

characteristics and good mechanical characteristics of the

membrane material, suggested that forced flow convection of

the electrolyte through the pores might show better results in

terms of UI characteristic slope. To test this idea, there was

constructed and tested one electrode pair electrolyzer with

forced electrolyte circulation driven by two independent

centrifugal pumps in two independent loops. The whole

system includes two gas separators and two gas storages.

3. System of electrolyzer and photovoltaic(PV) module

3.1. The system and the components description

There are performed several experiments with laboratory DC

power supply and PV module as sources of electric energy

with 3D Ni metal foam electrodes. Electrolyzer is designed as

device of 60W maximum power with single pair of

50� 50� 2 mm electrodes and membrane of thickness of

0.4 mm and 490 mm. Housing of electrolyzer is designed as

complex of four parts: two 120� 120� 5 mm internal parts

and two 120� 120� 8 mm external parts. PV module is

designed as 30 Wdevice consisted of 125� 65 mm18 PV-m-Si-

cells connected in two parallel strings of 9 solar cells. The aim

of these experiments is to research phenomenon of the

process of electrolysis with and without circulated electrolyte

and to design an electrolyzer and other components with

better understanding in further researches. The measure-

ments scheme is shown in Fig. 3 (switch can be set to DC

power supply or to PVmodule). From the given characteristics

of the system components it is obvious that electrolyzer

maximum power of 60 W is double in comparison with PV

module of 30 W. This is because the goal of this work was not

operation of the system at maximum power of electrolyzer,

but testing of coupled system in natural and forced flow of

electrolyte at the level of thermal hydraulic problems.

3.1.1. ElectrolyzerFor the purpose of research in this paper, there is designed

a small alkaline electrolyzer with alkaline (KOH) 25% solution

as an electrolyte, and maximum power of 60 W. The electro-

lyzer housing is made of Plexiglas with one electrode pair.

Two types of electrodes important for this work can be used

alternatively: one pair of electrodes of Ni plated carbon steel

plate (thickness of 1 mm) and one pair of electrodes of Ni

metal foam as 3D electrodes (thickness of 2 mm). The

dimensions of electrodes in both cases are 50� 50 mm and

Zirphon� material membrane, 490 mm, and thickness of

0.4 mm. The electrolyte circulate through the porous Ni metal

foam by natural convection because of hydrogen and oxygen

bubbles generation or by forced convection driven by two

small centrifugal pumps. The electrolyzer design ensures

electrolyte/bubbles convection from inlet to outlet through

the porous electrode uniform cross-section of 50� 2 mm

(Fig. 4).

Fig. 3 e Measurements scheme of experiments.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 67802

3.1.2. The separatorThe electrolyzer is attached to the electrolyte/bubbles separa-

tors (Fig. 5). The volume of the separator amounts 0.23 dm3.

Within thiswork it is notmodeled separator individually, but it

is tested in real experiments as the component of the coupled

O2 and electrolyteoutlet

Electrolyteinlet

Electrolyteinlet

H2 and electrolyteoutlet

2

Membrane

Porouselectrode

Fig. 4 e The electro

systemwhat resultedwith courses in further improving of the

system which steps are explained in Section 3.3.4.

It can be seen that separators at the same time provide gas

separation and electrolyte storage. There is no electrolyte

supply during the electrolyzer operation.

50

50

lyzer concept.

Gas inlet

Gas

Water

(p, T)

400

φ94

120

φ46

Fig. 6 e The storage tank concept.

Gas andelectrolyteinlet

Gas outlet

Liquidelectrolyteoutlet

φ44

155

Fig. 5 e The separator concept.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 6 7803

3.1.3. Gas storageSeparated hydrogen is conducted to the hydrogen storage and

oxygen to the oxygen storage (Fig. 6). Volume of the gas

storage amounts 0.7 dm3.

Gas storage tanks are designed in the way that accumu-

lated gas raises the water level in the upper part giving the

pressure of the gas in the storage in the form of water column

height. At the same time, the water level in the lower part of

the storage shows the volume of the accumulated gas. The

temperature of the gas is also measured. All of these

mentioned parameters (including the nailed air captured

before start of the electrolyzer operation) allow calculating the

mass of the produced H2 gas. Separators and both storage

vessels are made of Plexiglas. This allows visual studying of

the bubbles/liquid separation process and consequently

better future separator and storage tank design.

3.1.4. Centrifugal pumpDC electricity can be supplied by the laboratory DC power

supply (during indoormeasurements) or by PVmodule (during

the outdoor measurements). This electricity is led to the

electrolyzer and to the pump electric engines simultaneously.

Because of the inequality of the hydraulic resistance of the

tubing and porous electrodes and pump characteristics, two

potentiometers were put in the pump engines circuits aimed

to control the Revolution per minute (RPM) of the pump’s

engines. Thismeasure ensures proper operation of the system

during the time needed to complete filling of the hydrogen

storage tank with hydrogen without controller (up to 15 min).

3.1.5. PV moduleAfter the indoor measurements, the outdoor measurements

were performed. Instead of laboratory DC source, the real PV

modulewas connected to the rest of the system (Fig. 3, switch to

PV module). It is seen from Fig. 3 that three new measurement

sensorswereadded.Theseare theglobal solar irradiance sensor

in theplanewithPVmodule, representativePVcell temperature

sensor (under the covering glass) and the sensor of the atmo-

spheric temperature close to the PVmodule. In this work, there

is an intention to tune roughly electrolyzerUI characteristic and

PV module MPP trajectory. The reason for this is to build the

systemwithoutMPP tracker. The 30WPVmodule of 18 PVm-Si

cells of 125� 65 mm (nominal efficiency of 14%), connected in

two parallel strings of 9 solar cells was designed (Fig. 3).

3.1.6. Data acquisitionAll thenecessary electric and temperature quantitieswere led to

the computer controlled data acquisition system. Because of the

lack of thewater level sensors, thewater levels in the gas storage

tank were recorded by video camera through the transparent

walls and afterward synchronized and stored on the computer.

3.2. Measurements

3.2.1. Electrolyzer UI characteristicsFig. 7 shows the UI characteristics for the described elec-

trolyzer for two modes of the electrolyte circulation

0 1 2 3 4 50

5

10

15

20

25

Elec

tric

curre

nt,I

/A

Natural convectionForced convection

Voltage, U / VUrev =1.23

Fig. 7 e UI characteristics with natural and forced

convections.x PV module experimental data

PV module simulation

Experimental MPP trajectorySimulation MPP trajectory

0 1 2 3 4 50

1

2

3

4

5

6

7

Voltage, U / V

Elec

tric

curre

nt,I

/A

E=1000 Wm-2 Tc=47.8 °C

E=900, Tc=48.6

E=800, Tc=44.5

E=700, Tc=41.0

E=600, Tc=38.5

E=500, Tc=35.8

E=380, Tc=33.1

E=260, Tc=30.9

E=160, Tc=29.7

Fig. 8 e PV module UI characteristics [7].

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 67804

through the porous electrodes. Both were obtained during

the indoor experiments by random change of the voltage as

the output variable from the laboratory DC power supply. It

is seen that the current limit was reached at 18 A. A forced

convection of the electrolyte (circle indication) gives more

stable and steeper characteristic. UI points obtained during

natural convection of the electrolyte (no pumps operation)

were right-dispersed because sensitivity of the electrolyte/

bubbles flow regime depends on the direction of the supply

voltage slope, temperature, electrolyzer and housing

design.

3.2.2. PV module UI characteristicsPV module was examined in detail by authors of this paper in

[10]. PVmodule UI characteristics for different solar irradiance

and cell temperatures were measured and the mathematical

model of the one-diode type was derived and compared with

the measurements. Also the real and the simulated MPP

trajectory were plotted. Fig. 8 shows results.

3.2.3. Comparison between PV module and electrolyzer UIcharacteristicsFrom Fig. 3 it is seen that PV module supplies DC electricity to

theelectrolyzer andpumpssimultaneously. If pumpswouldbe

switched off, the trajectory of the working point of PV module

andelectrolyzer couldbe found in the intersectionof the twoUI

characteristics. For the purpose of assessment these two

characteristics are both presented in Fig. 9 e including PV

module MPP trajectory. It is clear that PV module should be

designed to cover pumps energy consumption using these UI

characteristics. Even without the pump characteristics it is

clear that when PV module would have more parallel con-

nected cells, the positive effect of the electrolyte forced flow to

the hydrogen production would be much stronger.

3.2.4. Measuring of hydrogen production and energyefficiency calculationUI characteristicwas obtained by using up and down changing

supply voltage. For the purpose of the measuring gas produc-

tion, voltage has been hold nearly constant during indoor and

outdoor tests. During the outdoor tests, solar irradiance was

almost constant and amounts about 1000Wm�2. The

temporal lengthof the recordedfiles equals 600 s. For example,

in Fig. 10a and b there are presented changes of the solar irra-

diance, electrical currents, voltages and accumulated

hydrogen.Using thisquantitiesand temperatures, it ispossible

to calculate energyefficiencyof the electrolyzer or of thewhole

system according to equation (1):

hE ¼ VH2HHZt2

t1

VðtÞIðtÞ dt(1)

The values of the electrolyzer energy efficiency calculated

according to different test conditions (natural or forced flow,

indoor or outdoor tests) were up to 55%. During the tests,

several design ideas were identified as a space for system

improving as there are gasket leakages between hydrogen and

oxygen circulation loop, poor gas separation in the separators

and cable heating. By the system improving with design

changes, it could be obtained higher efficiency.

3.3. Mathematical models

The author’s intention is on further improving of described

system. This would be supported using appropriate mathemat-

ical models and computer simulations. While mathematical

0 200 400 600 8000

200

400

600

800

1000

Time, t / s

Glo

bals

olar

irrad

ianc

e,E

/Wm

-2

0

0.5

1

1.5

2

2.5

3

3.5x 10-4

Producedhydrogen,m

/kg

Global solar irradiance

Produced hydrogen

0 200 400 600 8000

1

2

3

4

5

6

7

Time, t / s

Elec

tric

curre

nt,I

/A

0.5

1

1.5

2

2.5

3

Voltage,U/VPV module voltage

Pump electric current at the O2 loopPump electric current at the H2 loopPump voltage at the O2 loopPump voltage at the H2 loop

PV module electric currentElectrolyzer electric current

a

b

Fig. 10 e (a) Global solar irradiance and produced hydrogen

in function of time. (b) PV module, electrolyzer and pumps

electric current and voltage in function of time.

Fig. 9 e The coupled system with and without circulating

electrolyte (diagrams from Figs. 7 and 8 combined).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 6 7805

model and simulation of the PV module are already done [10],

mathematical models of the electrolyzer and the rest of the

systemareyet tobedeveloped. Itwouldbedoneas it isdescribed

in following sections.

3.3.1. Mathematical model of electrolyzerMathematical model of electrolyzer is given in equation (2)

according to [11].

U ¼ Urev þ aAIþ b log

� cAIþ 1

�(2)

Coefficientsa,band c canbecalculated respecting temperature

changes, but in this case there should be also taken into

account certain influence of the electrolyte/bubbles mix

velocity through theporous electrodesasnext stepof research.

3.3.2. Mathematical model of PV moduleAccording to [10,12e14] mathematical model of PV module is

studied. Within this work it is given model with no further

description of effective electric parameters. The authors of

this work gave deep analysis of this model in Ref. [10]. Simu-

lation results are given in Fig. 8. Mathematical modeling is

performed as follows:

IPV ¼ Iph � Is

�exp

�UPV þ IPVRs

UT

�� 1

�� UPV þ IPVRs

Rp(3)

Iph ¼ ðc1 þ c2TcÞE (4)

Isc ¼ Iph (5)

Is ¼ IscexpðUoc=UTÞ � 1

(6)

UT ¼ mnskTc

e(7)

Equations (2)e(7) should be accompanied by PV module heat

balance. This is because for proper determination of PV

performances, definition of its operating temperature is

needed. This temperature depends on atmospheric condition

and PV module operation as well. Thermal model is based on

energybalanceand its general form is given inequation (8) [14].

CTdTc

dt¼ saE� hcE� qLðTc � TaÞ (8)

The expression CT¼ dTc/dt is thermal energy stored in the PV

module and three expressions on the right side of equation are

as follows: heat gain because of solar irradiance absorption,

produced PV module electric energy and heat losses to the

atmosphere.

3.3.3. Mathematical model of the pumpsPumps will be modeled through their DPeq characteristics

taking into account pumps motor variable RPM and piping

hydraulics resistance. This will be a semi-empirical type of

model because datasheet obtained from pumps producer is

appropriate primarily forwater as aworkingmedia andnot for

KOH electrolyte. In further research, if coupled PV module/

electrolyzer systemwill keep thepumps, theirmodelingwill be

taken into account within modeling of the whole system, too.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 7 7 9 9e7 8 0 67806

3.3.4. Mathematical model of the gas separatorsThis model depends a lot on certain assumption and negli-

gence of the two-phase flow in it. Separator is at the same time

electrolyte storage and it will be based on themass balance for

the volume occupied by the gas and liquid. Mathematical

model of the gas separators within this work is not simulated,

but further research will include this. New design of gas

separators is in the process. Comparison of these two types of

separators is expected to give a course in solving problems of

bubbles: new design of separators is expected to allow bubble

unilateral transfer.

3.3.5. Mathematical model of the gas storagesGas storages mathematical models will be based on the gas

and liquidmass balances and gas equation of state. These will

be presented in near future of research.

3.3.6. Mathematical models of the heat flow lossesAbout 45% of the expensive PV module DC current is lost in

this system in the form of heat and low percentage of it on

pump operation. From this fact it is clear that the PV module

and electrolyzer heat losses should be carefully managed. At

the working temperature range of both, PV module and elec-

trolyzer, higher temperature means lower PV module effi-

ciency and higher electrolyte temperature means higher

electrolyzer efficiency. Obviously, some kind of managing the

heat would give some benefits to the system. Mathematical

modeling will be an important research tool.

4. Conclusion and further work

The original system consisting of an alkaline electrolyzer

(60 W), photovoltaic module (30 W), two centrifugal pumps,

two gas/electrolyte separators, two gas storage vessels and

silicon tubing of inner diameter equal to 8 mmis designed. The

systemwas testedat indoor andoutdoorconditions. For indoor

tests, insteadof thePVmodule, the laboratoryDCpowersupply

(20 A, 30 V) was applied. Original features of this system are Ni

metal foam electrodes with porosity of 95e97%, nominal pore

diameters of 600 mm throughwhich two phasesmixture of the

electrolyte and gas bubbles can flow by natural or forced flow

convection. Moreover, both pumps which ensure the forced

flow are powered by the energy from the same PV module as

the electrolyzer. It means that at a higher solar irradiance and

applyingabiggeramountof electricity to theelectrolyzer,more

bubbles will be produced, but at the same time the pumps will

circulate the mixture flow faster and more bubbles will be

carried away. This fact has a positive effect on the electrolyzer

UI characteristic in comparison with UI characteristic with

natural convection of the mixture in the porous electrodes

(Fig. 7). It is also experimentally shown that this positive effect

rises with the electric current through the electrolyzer. Two

main deficiencies of the systemwere observed: The first one is

the higher pressure on the hydrogen sidewhich stimulates the

electrolyte/hydrogen mixture to flow to the oxygen side. The

second one is the separator design and construction. Very

small bubbles (having diameters below 0.5 mm) can hardly be

separated from the liquid electrolyte and forced again to flow

through the pump and electrolyzer electrodes what causes

reduced pump and electrolyzer efficiency. Because of these,

the next step will be redesigned of the separators and gasket

system in the electrolyzer. The fact that the pumps have

a positive effect on the UI characteristic of electrolyzer above

the UI characteristic of the PV module used (Fig. 9), suggests

also redesign of the PV module.

r e f e r e n c e s

[1] Clarke RE, et al. Direct coupling of an electrolyser to a solarPV system for generating hydrogen. International Journal ofHydrogen Energy 2009;34:2531e42.

[2] Bilgen E. Solar hydrogen from photovoltaiceelectrolyzersystems. Energy Conversion and Management 2001;42(9):1004e57.

[3] Biddyut P, Andrews J. Optimal coupling of PV arrays to PEMelectrolysers in solar-hydrogen systems for remote areapower supply. International Journal of Hydrogen Energy2008;33:490e8.

[4] Gibson TL, Kelly NA. Optimization of solar poweredhydrogen production using photovoltaic electrolysis devices.International Journal of Hydrogen Energy 2008;33:5931e40.

[5] H2 Logic. http://www.h2logic.com.[6] MitsubishiMaterials. http://www.mmc.com.[7] Bidault F, et al. A new application for nickel foam in alkaline

fuel cells. International Journal of Hydrogen Energy 2009;34:6799e808.

[8] Nagai N, et al. Existence of optimum space betweenelectrodes on hydrogen production by water electrolysis.International Journal of Hydrogen Energy 2003;28:35e41.

[9] VITO. http://wwwa.vito.be.[10] Firak M, Ðuki�c A. An investigation into the effect of

photovoltaic module electric properties on maximum powerpoint trajectory with the aim of its alignment withelectrolyzer U-I characteristic. Thermal Science 2010;14:729e38.

[11] Ulleberg Q. Modeling of advanced electrolyzers: a systemsimulation approach. International Journal of HydrogenEnergy 2003;28:21e33.

[12] Quaschning V. Understanding renewable energy systems.London, UK: Bath Press; 2005.

[13] Chenni R, et al. A detailed modeling method for photovoltaiccells. Energy 2007;32:1724e30.

[14] Ulleberg Q. Stand-alone power systems for the future:optimal design, operation & control of solar-hydrogen energysystems. PhD thesis, Norwegian University of Science andTechnology, Trondheim; 1998.