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Alkaline electrolysers: Model and real data analysis

Paola Artuso a,*, Rupert Gammon b, Fabio Orecchini c, Simon J. Watson d

aSapienza University of Rome, CIRPS, Interuniversity Research Centre for Sustainable Development, Piazza S. Pietro in Vincoli,

10, 00184 Rome, ItalybBryte Energy Ltd., Loughborough Innovation Centre, Epinal way, Loughborough, Leicestershire LE11 3EH, United KingdomcUniversita degli Studi “Guglielmo Marconi”, Via Plinio, 44, 00193 Roma, 00184 Rome, Italyd Loughborough University, Centre for Renewable Energy Systems Technology, Department of Electronic and Electrical Engineering,

Loughborough, Leicestershire LE11 3TU, United Kingdom

a r t i c l e i n f o

Article history:

Received 24 May 2010

Received in revised form

17 December 2010

Accepted 18 January 2011

Available online 8 April 2011

Keywords:

Alkaline electrolyser

Simulation programme

Experimental data

Electrolyser model

* Corresponding author. Tel.: þ39 06 4458540E-mail address: paola.artuso@uniroma1.i

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.01.094

a b s t r a c t

This paper presents an analysis of the data collected during a test of the 36 kW alkaline

electrolyser at West Bacon Farm (WBF), Loughborough, UK. This data is then used to verify

a software model of an electrolyser. The test consisted of collecting data under different

operating conditions, in particular controlling the power supplied to the electrolyser.

The experiment was divided in four phases. In the first two phases, the electrolyser was

operated at full power, in the third phase it was operated at 60% of maximum power and in

the forth it was operated at 20% of maximum power, which is the minimum permitted

level. Each phase lasted approximately an hour.

During phase 1, the initial warm up period, the voltage remained almost constant, while

the current increased to a maximum of 428 A. After about an hour, the current suddenly

dropped to 310 A, the voltage decreased from 92 V to 88 V and hydrogen production

decreased from 7.3 Nm3/h to 6.6 Nm3/h, even though no change had been made to the

control parameters, which were still set to maximum power input. The temperature

continued upwards with only a slight reduction in its rate of increase and the pressure

stopped rising, remaining at 22 bar(g). This point marks the transition from phase 1 to

phase 2 and the reasons for the sudden discontinuity are investigated in this study.

Once the optimum operating temperature was reached, during phase 2, it was main-

tained within a limited range by the cooling system. The initial stack temperature, at the

beginning of phase 1, had been 17 �C. The net power draw increased until the stack

temperature reached its maximum at 76 �C after about 1 h 20 min.

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

reserved.

1. Introduction [4,5]. Electrodes are therefore located within an ion-conduct-

Renewable technologies, such as wind turbines and photo-

voltaic arrays, can generate electricity, whichmight be used to

produce hydrogen via electrolysis with no resulting green-

house gas emissions [1e3]. In an electrolyser, water is split

into hydrogen and oxygen by the input of electrical energy

0; fax: þ39 06 44585779.t (P. Artuso).2011, Hydrogen Energy P

ing electrolyte (usually an aqueous alkaline solution of 30%

potassium hydroxide KOH) and gaseous hydrogen is produced

at the negative electrode (cathode) while oxygen is produced

at the positive electrode (anode). The necessary exchange of

charge occurs through the flow of OHe ions in the electrolyte

and current (electrons) in the electric circuit. In order to

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

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 9 5 6e7 9 6 2 7957

prevent a mixing of the product gases, the two reaction areas

are separated by a gas-tight, ion-conducting membrane. This

type of system can produce high purity hydrogen that is

suitable for use in fuel cells (FCs) in a range of stationary,

portable or vehicular applications.

2. Validation of the electrolyser model. HARIproject

This work focused on the analysis of the data acquired from

the electrolyser located in the West Bacon Farm (WBF), which

is part of the demonstration project carried out in Lough-

borough, in United Kingdom. The project is called HARI. The

researchers working on this project developed the system

integration of several environmental friendly innovative

technologies to supply the energy required by theWBF [6,7]. In

particular, the activity described in this paper dealt with the

electrolyser and with the analysis of the possibility to apply

a model already available in literature [8,9]. During the test,

real data was collected from the operating 36 kW alkaline

electrolyser. The electrolyser worked in different operating

conditions and the power was varied in three steps.

The data acquired were:

� Current in Amperes

� Voltage in Volts

� Pressure in bar(g)

� Temperature in degrees Celsius

� Hydrogen produced in normal cubic metres.

Fig. 1 e The data plots div

The experiment was divided in three phases, in corre-

spondence of three different levels of the supplied power: in

the first phase the power was set to themaximum level, in the

second phase a 60% maximum power was considered, during

the last phase 20%maximum power was supplied, taking into

account it is the minimum switching on power of the

electrolyser.

The first phase lasted 2 h, while the other two phases kept

about 1 h each. Thewhole collected data is plotted in the Fig. 1,

where the several quantities featuring the electrolyser are

reported as a function of the time from 11.00 o’clock in the

morning to 3.00 pm.

During the start up, while the voltage was almost constant,

the current increased until a maximum value of 428 A,

reached after 1 h 20 min. After that, even if the control system

was not modified and it was set to the maximum value, the

current suddenly dropped down to 310 A. In this instant, the

voltage decreased from 92 V to 88 V, hydrogen production

decreased from 7.3 Nm3/h to 6.6 Nm3/h, the temperature

curve had a modification of the slope, the pressure reached

the maximum equals to 22 bar(g). After 1 h 20 min a disconti-

nuity was noticed and in correspondence of that point, at the

same time, the electricity, the voltage and the hydrogen yield

dropped down, the temperature slope curve varied, the pres-

sure draw reached amaximum and after that it was stabilized

at 22 bar(g). Because of this discontinuity, the analysis of the

100% power phase has been split into two parts.

To process the data analysis, data from the electrolyser’s

internal data acquisition (DAQ) system had to be synchro-

nized with that of the site’s own DAQ system, which was

ided by four phases.

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 9 5 6e7 9 6 27958

operating at a different sample rate. Once synchronised, the

acquired data allowed analysis of the operating conditions of

the electrolyser, both during the initial warm up period and

during steady-state conditions at power inputs of 100%, 60%

and 20% of the maximum power. Analyzing the plots,

summarized in the Fig. 1, four phases are finally individual-

ized, each of which had a duration of about 1 h:

- Phase 1: thewarmup, providing 100% of the power usable by

the electrolyser;

- Phase 2: the provided power was set to maximum, but the

electrolyser was working in steady-state conditions;

- Phase 3: the input powerwas decreased to a value 60% of the

maximum electrolyser working power;

- Phase 4: the power was supplied at 20% of the maximum

electrolyser working power, which is the minimum oper-

ating level.

All this phases will be further discussed in detail in the

following paragraphs. For each phase, with the intent to

eliminate some noise, the data had been averaged in the range

of 1 min.

Excluding the warm up, in the phases 2, 3, 4 the working

conditions were quite stable and the averaged quantities of

current, pressure, temperature, voltage, hydrogen production

could be considered as characteristic of each phase, as

reported in Table 1.

2.1. Phase 1

During the warm up, all the quantities increased constantly.

The hydrogen production was characterized by a disconti-

nuity after about 10 min, due to the fact the compressor

started to work when the pressure reached 19 bar(g). During

the warm up, the pump switched on after 7 min.

2.2. Phase 2

After 1 h 20 min, without modifying any control system

quantity, the electrolyser changed suddenly the working

conditions. Albeit the data reported in Fig. 1 were averaged in

the range of 1 min, the current draw was still variable and it

had a shape pretty different from the other phases plots.

The reasonof this unstable input currenthasbeenanalyzed,

in accordance with the other quantities and considering the

several components of the electrolyser. The attention focused

on the sudden variation in the electrolyser working conditions.

The rising questions was why the electrolyser suddenly

changed the working conditions and all the plots had

a discontinuity in the same instant.

Considering that all the quantities varied in one instant,

the conclusion was that there should be a control system

Table 1 e The operating current, pressure, temperature, voltag

Phase Current (A) Pressure (bar) Temper

2 335.7 22.0

3 258.9 19.6

4 108.8 16.8

inside the electrolyser which switches on at a certain time.

The problemwas to individualize what kind of control system

switched on and why, because it was designed directly by the

electrolyser manufacturers. Analyzing the plots in the Fig. 1,

the control system seemed to be activated by the pressure

level. In fact, in the phase 1 the pressure increased to reach

a certain point. After that, during the phase 2, the pressure

was constant, therefore the control system works in order to

stabilize the pressure, once it joined the maximum plate

value. The pressure control insured safe working conditions.

In addition, the constant pressure is suitable for conventional

electrolyser applications in industrial sector. In this kind of

applications an important target is to maintain a constant

hydrogen flow, and it can be obtained by means of a uniform

pressure difference between the electrolyser and the ambient

where the hydrogen is used.

2.3. Phase 3

During this phase, the current and power draws were very

smooth and the pressure reached the new working value,

equals to 18 bar(g), after about half an hour.

The hydrogen production decreased during the transition

phase, reaching the value equals to 19.6 bar(g) when the

pressurewas stabilized. Decreasing the energy provided to the

electrolyser, a contemporary temperature reduction had been

recorded, because of the correspondent abatement of the heat

production inside the electrolyser. With regard to the

temperature, it had a wave shape between the values 67.7 �Cand 73.7 �C, excluding the transition phase from the 100%e

60% feeding power.

2.4. Phase 4

Decreasing the feeding power, the attention focused on the

minimumworking power required by the electrolyser. Looking

to the global graph reported in Fig. 1, there was a 10 min break

during which the electrolyser was switched off. It was not

provoked by purpose, but it gave the opportunity tomake some

considerations, with particular attention to the temperature

which decreased from 75.6 �C to 67.6 �C, that means the heat

was quickly dispelled. By the analysis of this fact, the conclu-

sion was that if the electrolyser has to work in variable condi-

tions, characterized by frequent switch on/off cycles, the stack

should be insulated to reduce the heat exchange between the

stackand theambient inorder tomaintainhigh temperatureas

long as possible. In variable working conditions, the system

insulation could be a crucial aspect to be further analyzed in

particular if Cool-Heat-Power (CHP) generating systems will be

designed in order to optimize the efficiency.

During the phase 3, the current was variable, but in

a limited range between 102 A and 117 A. The wave shape of

e and hydrogen production in each phase.

ature (�C) Voltage (V) H2 production (Nm3/h)

71 89 6.56

70 86 5.37

64 80 2.35

Table 2e Comparison between the Julich Electrolyser andthe WBF one.

Size Electrolyser Julich Electrolyser BEF

Plate Power 26 kW 36 kW

Plate Pressure 6 bar 25 bar

Cell Number 21 46

Cell Area 0.25 m2 0.1 m2

Temperature 80 �C 70 �C

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 9 5 6e7 9 6 2 7959

the pressure plot was due to the compressor which switches

on when the pressure in the buffer was higher than 19 bar(g).

When the compressor was working, the hydrogen pumped

into the tank was more than the quantity produced by the

electrolyser, so that the pressure decreased until the

compressor switched off when the pressure level was lower

than 16 bar(g). With reference to the Fig. 2, how the variation

of the hydrogen productionwas reported. The inversion of the

pressure draw trend is every 603000, which is the distance

between the points A, B, C, D, E, F, G. Starting from the point A

to the point B, the compressor was working, thus the

hydrogen flowed and the pressure decreased until the level

was so low that the compressor stopped working. From B to C

the hydrogen flow was low, because the compressor was not

operating, but the pressure was becoming higher and higher

until it reached 18 bar, which is 1 bar less than the plate value.

At the point C the compressor switched on again and the cycle

was repeated other two times.

3. Comparison between the real data and themodel

Considering the available experimental data of the electro-

lyser in WBF, it was not possible to calculate all the parame-

ters following the procedure described in the paper [3]

reported as in the Eq. (1):

U ¼ Urev þ r1 þ r2$TA

$Iþ s$log

0B@t1 þ t2

Tþ t3T2

A$Iþ 1

1CA (1)

In fact, for each power, the electrolyser was characterized

by a certain temperature, current, pressure and it was not

possible to vary only one of those quantities, but they all

changed contemporary because the control system could

operate only on the voltage. Taken this, it was considered the

possibility to apply the model as it is proposed in the paper.

The differences between the two alkaline electroysers are

summarized in the Table 2.

The input of the model was the current and the tempera-

ture, by means of which the voltage and the hydrogen

production were calculated. In the Fig. 3 the real voltage data

(in grey) and the calculated voltage data (in black) are reported

in order to visualize the difference. The plots shapewere quite

similar, although theywere translated each other (ref. plot a of

the Fig. 3). For this reason, it was investigated what would had

happen if only one parameter was changed. In other words,

the adopted procedure was to calculate the mean error

between the real voltage and calculated values. After that the

parameter value was computed in order to minimize this

error. All the procedure was processed for each parameter (r1,

r2, s, t1, t2, t3) and then the computed value was substituted in

Fig. 2 e The cycles of the phase 4.

themodel formula. The best result was foundmodifying the t1parameter from 1.002 A-1 m2 �C to �0.0596 A-1 m2 �C.

Once t1 was modified, the calculated voltage draws the

black plot in the Fig. 3 b. The voltage computed and the real

voltage values are very similar with a maximum relative error

equals to 2%, excluding the period when the electrolyser was

switched off. To deeper analyze the correspondence between

the model data and the real one, the currentevoltage plot has

been drawn (ref. plot Fig. 4 a). The indicators are respectively:

rhombus for the phase 1, square for the phase 2, triangles for

the phase 3 and circles for the phase 4. The black continues

line refers to the values of the paper [3], considering the

modified value of the t1 parameter, as it is reported above, and

considering a constant temperature of 70 �C. The real data is

very close to the calculated one if the warm up phase is

excluded. About this last, it is also interesting to observe that

the voltage is almost constant during this phase. The reason is

that the power control system which can control either the

voltage or the current, in this case it is clearly controlling the

voltage at the maximum level, because the working condi-

tions were set to the 100% power. During this phase the

current constantly increased until the steady-stable

Fig. 3 e Comparison U real and U model.

Fig. 4 e Comparison graphs between real and model data.

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 9 5 6e7 9 6 27960

conditions were reached. During the second phase, albeit the

current is quickly variable, theworking electrolyser points lied

on the characteristic curve. In the course of the third phase,

the values of both the current and of the voltage were slightly

variable and all values were very close to the characteristic

curve, as it happened during the last phase.

To verify the model effectiveness, the second step was to

analyze the hydrogen production using Eq. (2) and Eq.(3):

hF ¼ðI=AÞ2

f1 þ ðI=AÞ2 f2 (2)

_nH2¼ hF

ncIzF

(3)

In the second plot of the Fig. 4 b, the real data plot is

reported together with other two, drawn considering two

different conditions:

1.‘H2 model’: the f1 and f2 parameters were considered

constant and equals to the ones in the paper [3];

2.‘H2 model f2’: f1 and f2 were considered dependent on the

current.

Analyzing the graph, the first solution seems not onlymore

simple, considering constant parameters, but also more

similar to the real data. The differences between the real data

and the first model solution were analyzed, phase by phase.

Starting from the left side of the Fig. 4 b, the real data is

characterized by a discontinuity when the compressor

switched on. In fact, only when the hydrogen production

reached 19 bar(g), the compressor started to work. The model

does not simulate the compressor and this caused noteworthy

errors during the start up and during low power supply level,

in the phase 4 (ref. Fig. 4 c), when the compressor switches on

and off during regular intervals.

During the phase 2, the computed hydrogen production is

more variable than the real quantities, because the model

does not consider the elasticity of the gas (compression-

decompression). In fact the model considers the gas produc-

tion as a function of the supplied electricity. Also other

quantities influenced the hydrogen yield, like the temperature

and the voltage, but they had lower effects. In conclusion, the

actual range of the hydrogen flow was actually smaller than

the range foreseen by the simulation, but this last included the

first one.

During the phase 3, at the beginning there was a transition

period when the real data was higher than the simulated one.

During this stage, which has persevered for about half an

hour, the real data was characterized by a descending trend,

while the simulated data had a constant trend. The real

behaviour was influenced by the working conditions of the

previous phase. When the stationary conditions were

reached, the calculated data was correspondent to the real

one.

During the last phase, the real hydrogen yield was depen-

dent on the compressor working conditions. This provoked

a step trend of the real hydrogen flow. The model did not

include what was downstream the electrolyser and for this

reason the hydrogen production was almost constant, but the

averaged values of the hydrogen flow (marked by circles in the

Fig. 4 c) are very similar to the real ones (marked with rhom-

buses). Although the moment by moment hydrogen produc-

tion was not always well foreseen by the model, the integral

quantity of the produced hydrogen has been calculated in

order to verify if the model values matched the real ones. The

integral of the hydrogen yield during the whole experimental

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 9 5 6e7 9 6 2 7961

period has been plotted in the Fig. 4 d. By means of this graph,

the good correspondence between the simulation and the real

data was evident, even though the real hydrogen quantity was

always a bit lower than the calculated one.

If the whole period is taken into account, the relative error

equalled 0.89%, while the maximum absolute error was

0.51 Nm3.

In conclusion, the model results matched moment by

moment real data when the electrolyser working conditions

were steady-stable. During the transition periods, the real

data was affected by factors whichwere not considered by the

model, as the compressor influence and the dynamic

hydrogen flow phenomena, but the model can be considered

efficient to evaluate the produced hydrogen quantity in

a certain time, that is the integral of the hydrogen flow.

4. Conclusions

By the analysis of the real data of the electrolyser located in

the WBF, in Loughborough, UK, some insights outcropped.

The electrolyser was realized to have an inside pressure

control system designed by the manufacturer, which made

the electrolyser to work at constant pressure after the

preliminary warm up. The electrolyser was designed to be fed

by RES, which are insecure and variable, but still the start up

required 1 h 20 min and the temperature drastically dropped

down when the electrolyser was switched off. Therefore, the

design of the electrolyser should be further improved in terms

of thermal management and the stack should be properly

insulated, in particular if the electrolyser is intended to be

used in a waste heat recovery system which contemporary

produces hydrogen and heat.

By a comparison between the model and the real data,

a good correspondence was pointed out, but slightly modi-

fying one of the originalmodel parameters. In fact, using the t1value in paper [3], the real voltage would have been always

lower than the real one, while using the t1 value computed in

this paper, the simulated voltage matches the real quantities.

Despite the moment by moment hydrogen flow was not

suitably computed by the model, the simulation could care-

fully evaluate the total hydrogen produced during the test

with maximum absolute error of 0.51 Nm3 and the relative

error equalled 0.89%.

The experiments had to be terminated because the elec-

trolyser washers collapsed on the action of the inside stack

pressure, thus the reliability needs to be further improved in

forthcoming products.

Acknowledgements

The research has been supported by Regione Lazio Assessor-

ato all’Ambiente e alla Cooperazione tra i popoli, in the

framework of the Programme “Polo IdrogenoLazio”

(2006e2009). In addition, the authors wish to tank Tony Mar-

mont and his wife, Angela, for funding the HARI project.

Nomenclature

A area of electrode, m2

I power, A

ncells number of electrolyzer cells

r parameter related to ohmic resistance of electrolyte,

Wm2

s coefficient for overvoltage on electrodes, V

t coefficient for overvoltage on electrodes, A-1 m

T temperature

Ta ambient temperature, �CTr Array reference temperature, 28 �C for commercial

policristalline PV modules.

Tmodule PV module temperature, �CG radiance, W/m2

U voltage, V

Urev reversible voltage. For water, it equals to 1,228 V at

6 bar and 80 �Cr1 8.05E-5 W m2

r2 �2.5E-7 W m2 �C�1

s 0.185 V

t1 1.002 A�1 m2 �Ct2 8.424 A�1 m2 �Ct3 247.3 A�1 m2 �C2

hF Faraday efficiency, which is the ratio between the

ideal current necessary to obtain 1 mol of

a substance (e.g. hydrogen) and the true required

current to produce the same quantity.

f1 250 mA cm�4

f2 0.96_nH2 molar flow rate

F 96485 Cmol�1 Faraday constant

z number of electrons transferred per reaction (2 in the

analysed case)

nc number of electrolyser cells

r e f e r e n c e s

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