1

COMPARISON OF ELECTRICAL ENERGY EFFICIENCY OF ATMOSPHERIC AND

HIGH-PRESSURE ELECTROLYSERS

Amitava Roy*, Dr Simon Watson, Prof David Infield

Centre for Renewable Energy Systems Technology (CREST), Loughborough University,

Leicestershire LE11 3TU, UK

ABSTRACT

Efforts are being made to produce highly pressurised electrolysers to increase the overall energy efficiency by

eliminating mechanical compression. However, in-depth modelling of electrolysers suggests that electrolysis at

atmospheric pressure is electrically more energy efficient if parasitic energy consumption and gas losses are

incorporated in both cases. The reversible cell voltage increases with increasing pressures. The electrode activation

and Ohmic losses, leakage current and inevitable heat losses increase the electrolysis voltage beyond the

thermoneutral voltage and consequently heat removal from the stack becomes essential. The expected gas loss at

various operating pressures is incorporated to reveal the energy consumption that would occur in practice.

Comparison of total energy consumption at various operating pressure up to 700atm is performed and atmospheric

electrolysers are found more efficient at all levels. Practical considerations such as corrosion, hydrogen

embrittlement, operational complexity, dynamic response and cost are less favourable for pressurised electrolysers.

Keywords: Atmospheric electrolysers, pressurised electrolysers, thermodynamic analysis, gas loss, energy

efficiency, practical considerations

1. INTRODUCTION

The efficiency of electrolysers can be described in different ways, such as the stack efficiency, voltage

efficiency, overall efficiency, energy efficiency and water to hydrogen conversion efficiency. The gas losses

and some of the major parasitic losses are often ignored when calculating the overall energy efficiency. The

milestone report published by the NREL, [1], suggests that electricity costs comprise 80% of the total selling price

of hydrogen from large-scale electrolysers. This emphasises the requirement for an improvement in the electrical

* Corresponding author: E-mail address: A.Roy@Lboro.ac.uk, Fax: 0044-(0) 1509-610031,Tel: 0044 -(0)-1509-228147

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energy efficiency of electrolysers. The present industry trend is to develop highly pressurised electrolysers to

eliminate conventional mechanical compressors for efficiency improvement. However, the reversible cell voltage

increases significantly with increasing pressure in the first place. The use of a feed water pump becomes essential at

higher operating pressures. Electrolyte circulation pumps are also needed for better removal of the heavier bubbles

formed at extremely high pressure. On the other hand, the enthalpic voltage, thermoneutral voltage and higher

heating voltage of water electrolysis slightly decrease with increasing pressure, which favoures pressurised

electrolysis assuming the electrolysis takes place under perfectly insulated conditions. However, perfectly insulated

conditions are practically difficult to achieve. In reality, the electrode activation and Ohmic losses and leakage

current increase the electrolysis voltage much beyond the thermoneutral voltage and consequently heat removal

from the stack becomes essential. Therefore, optimising the thermoneutral, enthalpic or higher heating voltage at

high-pressure for liquid water electrolysis, to improve the overall energy efficiency is not in practice an effective

approach. The electrical-energy-efficiency is more important than the heat-energy-efficiency in terms of the costs of

hydrogen production, therefore focus should be given on reducing reversible voltage, activation loss and ohmic loss,

to bring the total electrolysis voltage closer to the thermoneutral, enthalpic or higher heating voltages.

The total thermodynamic energy or enthalpic energy required per mole of hydrogen produced is supplied in terms of

electrical energy and heat energy. The change in enthalpic energy with increasing pressure is negligible. However,

the proportion of electrical energy and heat energy varies significantly with temperature and pressure. In this paper,

we have examined the effect of pressure on electrical and heat energy demand for liquid water electrolysis. The

reversible voltage, which represents a major part of the electrical energy consumption, increases with pressure and

the entropy, multiplied by temperature (K), which represents the heat energy demand, decreases with pressure thus

keeping the enthalpic voltage almost unchanged with increasing pressure at any operating temperature below 1000C.

Due to excess heat already generated, we should not reduce the heat demand further by operating the electrolyser at

higher pressure, as that will further increase the cooling demand and reversible voltage. The enthalpic voltage and

reversible voltage reduce with increasing temperature, irrespective of pressure. We have taken 750C as a common

operating temperature from a practical point of view for both low and high-pressure electrolysis. The thermoneutral

voltage, and higher heating voltage increase with increasing temperature, again irrespective of pressure. A key

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Nomenclature

H change in enthalpy (J) G change in Gibbs free energy (J) S change in entropy (J) T temperature (K)

t temperature (0C)

p pressure (atm)

ptH , change in enthalpy at t0C and p atm (J)

0

25H change in enthalpy at 250C and standard pressure

(J) 0

tH change in enthalpy at t0C and standard pressure

(J)

ptV , enthalpic voltage at t0C and p atm (V)

0

tV enthalpic voltage at t0C and standard pressure (V)

n number of electrons transferred (n =2)

F Faraday constant (F = 9648530 C/mol)

VHHV, t higher heating voltage at t0C (V)

Vtn thermoneutral voltage (V)

pw vapour pressures of KOH solution (atm) *

wp vapour pressures of water vapour (atm)

0

)]([ 2 tlOHfG Gibbs free energy of formation of liquid

water at 250C (= -56690.2 cal)

R Universal gas constant =8.314 J K-1mol-1

anode charge transfer coefficient for anode= 0.5

cathode charge transfer coefficient for cathode =0.5 F Faraday constant = 96485.30 C/mol

i operating current density in A/cm2

i0,anode exchange current density for anode = 0.016A/cm2

at 800C

i0,cathode = exchange current density for cathode 0.02A/cm2

at 800C

Cp = specific heat capacity of hydrogen,

14.304kJ/kg/K

m.

= mass flow rate of hydrogen, kg/s

T = temperature difference, Kelvin.

objective of this paper is to estimate the electrical energy consumption for hydrogen production (kWh/Nm3) at

various operating pressures and at 750C. The practical unit of kWh/Nm

3 hydrogen is useful to compare with other

hydrogen production methods. It is also then straightforward to find the electrical energy efficiency using the higher

heating value of hydrogen (i.e. 142MJ/kg, 39kWh/kg, 3.5kWh/Nm3) for liquid water electrolysis.

4

This paper mainly focuses on alkaline electrolysers, however, the thermodynamic analysis, discussion about some of

the gas losses and parasitic losses are also applicable to polymer electrolyte membrane (PEM) electrolysers. The

additional electrical energy needed to power such as the extra safety mechanisms and the sophisticated control

systems required in a pressurised electrolyser should also be considered in the overall electrical energy efficiency

calculation. The inevitable increased rate of hydrogen loss in the de-oxidation unit and controlled venting of gas to

balance the electrolyte levels is also a significant design factor for pressurised electrolysers. The water replenishing

system also leads to gas losses if no pump is employed to fill the water. All these gas losses are directly proportional

to the operating pressure. The higher operating pressure reduces the reaction rate and thus increases the activation

loss. The resistivity of electrodes also increases due to metal hydride formation, the rate of which increases with

higher pressure. It can be seen that the overall practical electrolyser efficiency is a result of complex trade-offs.

Detailed models developed in MATLAB-SIMULINK have been used to quantify the operation and energy

consumption of atmospheric and pressurized electrolysers at various delivery pressures. The results from these

models and the operational data from the electrolyser of the HARI project, [2], are the basis of this paper.

2. THERMODYNAMICS OF WATER ELECTROLYSIS

The electrical energy demand is directly proportional to the reversible voltage. Eq 1 describes the balance between

electrical energy and thermal energy:

STGH Eq 1

LeRoy et. al., [3] describe the thermodynamics of water electrolysis. The enthalpy of water splitting is expressed as

follows:

0,0250025, tpttpt HHHHHH Eq 2

Where

)(

0

25

00

25

00

25

00

25

0

2225.0

lOHtOtHttHHHHHHHH Eq 3

and

5

)(

0

,

0

,

0

,

0

,222

5.0lOHtptOtptHtpttpt

HHHHHHHH Eq 4

The effect of pressure on enthalpic voltage (Vt, p,), for both non-ideal hydrogen and oxygen gases is calculated, [3]

for the values for 1-100atm shown in

Table 1 using:

nFHHVV tpttpt /)(0

,

0

, Eq 5

Table 1is expanded by linear extrapolation up to 700atm. Values up to 700atm pressure are given as at least one

company is engaged in a feasibility study of a 700atm electrolyser under a DOE, USA contract, [4]. It is clear from

Table 1, that the pressure effect on enthalpic voltage is negligible. The enthalpic voltage reduces from 1.473V at

1atm to 1.462V at 700atm at 750C i.e. only a 0.747% reduction. The temperature effect of enthalpic voltage is

expressed as in Eq 6 [3].

2840 1084.910490.14850.1 txtxVt Eq 6

Eq 7 defines the higher-heating-value voltage, (VHHV, t).

nFHHVV lOHtttHHV /)( )(0

25

000

, 2 Eq 7

The values of {(H0

t-H0

25) H2O (l)}/nF are obtained from the Steam tables, [5] giving the following expression for

higher-heating-voltage [3].

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2840

, 1052.110252.24756.1 txtxV tHHV Eq 8

LeRoy et. al. presented the pressure dependence of higher heating value for 1-100atm, which is further extrapolated

up to 700atm. The values for 750C are also interpolated in Table 2. The higher heating voltage for any temperature

and pressure up to 700atm can be calculated using Eq 8 and Table 2. For example, without the effect of pressure,

VHHV, 75C = 1.49257V and the pressure effect for 1atm at 750C = -0.0103mV giving the VHHV, 75C, 1atm = 1.49254V.

The pressure effect is -6.06mV for 700atm from Table 2 giving the VHHV, 75C, 700atm = 1.48648V, which is only 0.4%

reduction compared to the atmospheric-higher-heating-voltage. The percentage increase in reversible voltage for the

same conditions is 12.85%, which is explained later. The higher heating voltage of hydrogen is widely used to

calculate the energy efficiency of electrolysis.

The thermo-neutral voltage (Vtn) is denoted by Eq 9 as follows, [3].

nF

YVV HHVtn

Eq 9

Where,

)/(06682.0762.4042960 2 molJttY Eq 10

)/(5.1 ww ppp Eq 11

*21

ln024.11933.01380.001621.0ln ww pmmp Eq 12

TTpw ln416.3/627604.37ln* Eq 13

The vapour pressures of KOH solution (pw) at various temperatures and concentrations were obtained from the

International Critical Tables [6], and fitted to Eq 11, [3]. The value of m=7.64 for 30 (wt)% KOH solution is taken.

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The vapour pressure of pure water (pw*) is calculated from Eq 13, with less than 1% error over the range 25-2500C,

[3].

Eq 14 gives the reversible voltage, )( 0 ,, ptrevE .

wwwlOHfptrev ppppRTnF

GnF

Et

/)(ln11 *5.10

)]([

0

,, 2 Eq 14

The E0

rev, T (K) at any temperature (Kelvin) without the effect of pressure is given by Eq 15.

28530

)(, 1084.9ln10523.9105421.15184.1 TxTTxTxE KTrev Eq 15

The value of .2291.1025, 0

VECrev The pressure effect on reversible voltage can be calculated with the values of

vapour pressure of pure water (pw*) and vapour pressure of KOH (pw) at various temperatures and concentration.

The values of vapour pressure of KOH and water calculated using Eq 12 and Eq 13 are given in Table 3.

The reversible voltage is tabulated in Table 4 incorporating both pressure and temperature effects. It is clear from

the Table 4 that the reversible voltage has increased by 12.85% at 700atm pressure compared to 1atm pressure at

same 750C temperature, thus increasing the energy consumption by the same percentage. For 100

0C (liquid water)

operating temperature, this value increases by 15.6%, which could be utilised for efficiency improvement by

operating at 1atm pressure rather than at 700atm.

3. ELECTRODE KINETICS

The electrode kinetics has been modelled using MATLAB-SIMULINK, incorporating the exchange current density,

cathodic and anodic transfer rate of reaction. The Butler-Volmer equation is used to estimate the activation

overpotential. Its value also depends on the operating current density, temperature and the OH- ion concentration,

which could be effected to some extent by void fraction of gas bubbles. Figure 1 shows the activation overpotential

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as a function of current density (mA/cm2). Le Chatelier propounded his principle in 1884, which states that the yield

of a chemical reaction will increase with pressure if the volume of the products is less than the volume of the

reactants. In water electrolysis, the molar volume of the gaseous products is more than the condensed molar volume

of liquid water thus the yield of the reaction is decreased with increasing pressure.

The concentration of OH- ions at the electrode surface is reduced to some extent due to gas blanketing or increased

void fraction, especially in the modern zero gap configuration of electrolyser cells, such as the Stuarts (IMET)

Electrolyser. Modelling of bubble nucleation growth rate and the departure velocity of bubbles require detailed

investigation of mass transfer mechanism. The mass transfer depends mainly on concentration of reaction products,

which can be ascertained only by experiments and such experiments are difficult to perform, which were beyond the

scope of this paper. In most cases, these experiments were performed using a tracer, by measuring the transfer of

tracer to the electrodes (Riegel et. al.[7]). Riegel used a reference electrode, to measure the voltage between these

electrodes and determine the concentration of OH- ions. This method is fairly similar to the approach followed by

Mullar et. al.,[8]. Mullar et. al. determined the concentration of reactants as a function of cathode distance. Nagai et.

al. [9], investigated the bubble movements and void fraction incorporating various parameters such as, current

density, with and without separators, system temperature, space between electrodes, height, inclination angles,

surface roughness and wettability of electrodes. The experimental evidence suggests that that the void fraction

increases with the reduction of space and for each specific current density there is an optimum space. Nagai et. al.

also developed a physical model to represent the qualitative tendency of the experimental results. Kikuchi K et. al.

[10] investigated hydrogen concentration in water from an alkaliionwater electrolyser having a platinum-

electroplated titanium electrode. Kikuchi K et. al has also investigated the supersaturated concentration of hydrogen

in electrolysed water, obtained from a flow-type electrolytic cell under various electrolysis conditions, which,

indicates that the hydrogen concentration is related to both the diffusion of dissolved hydrogen from the electrode

surface to the bulk solution and hydrogen bubble growth. All these studies provide very useful knowledge about this

process; however they are based on certain assumption, which do not particularly include the modern zero gap

configuration of the stack of the electrolyser.

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The zero gap configurations between perforated electrodes, membranes, current collectors (generally made of dense

nickel wire mesh) and bipolar plates could increase obstructions for free bubble movement, which ultimately could

hinder easy coalescence of bubbles to some extent. Other factors, such as, the surface roughness, surface structure of

electrodes, viscosity of KOH with varying temperature and the bubbles affinity to stick to the electrodes surface

(hydrophilic nature of surface coating) could also influence the coalescence of bubbles. The nucleation growth rate

of bubbles is affected to some extent due to increase in pressure; for example, the bubble volume will be 700 times

smaller at 700atm pressure. The bubbles will be heavier and would stay longer on the electrode surface. The

buoyancy force does not dominate the displacement of bubbles between the gas-separator and perforated electrodes

(Figure 2) as there is no space for free upward bubble movement. Once the bubbles come out to the outer surface of

electrodes, opposite to the electro-active surface, where the electrolyte is circulated (i.e. the space for current

collectors), then the buoyancy dominates the vertical velocity of bubbles. The greater gas voidage at high current

densities would be more noticeable in a zero gap configuration. It is worth mentioning that this would be a

complicated method to accurately measure and observe the bubble growth and removal from the very compact

layers of electrodes, membranes, current collectors and bipolar plates. Although future work is needed on the bubble

removal mechanism in zero gap configurations and precise measurement technique to accurately model its effects

on cell voltage for modern electrolysers, the present modelling also provides fairly good prediction. The activation

overpotential can be calculated using the Butler Volmer equation, in Eq 16 and its value is shown in Eq 17.

0, 0,

ln( ) ln( )actanode anode cathode cathode

RT i RT i

F i F i

Eq 16

Where, the values of each parameter are given in the nomenclature. The operating temperature is taken as 750C.

8.314 348.15 0.451 8.314 348.15 0.451ln( ) ln( ) 0.3873

0.5 96485.309 0.016 0.5 96485.309 0.02act

x xV

x x Eq 17

The Butler Volmer equation gives fairly accurate prediction of activation overpotential for medium to high net-

current density depending on the rate of bubble removal from the electrode surface.

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4. OHMIC OVERPOTENTIAL

Ohmic resistance in electrolytic cell increases the voltage required for electrolysis beyond the reversible cell voltage,

which is called Ohmic overpotential. The electrodes, current collectors, bipolar plates, gas electrolyte separators and

electrolyte layer between the electrodes and gas separators constitute the Ohmic resistance of the cell, which vary

with temperature and also due to variation in void fraction of gas bubbles. The resistivity of the electrode will

increase slightly due to metal hydride formation as a result of the reaction between molecular hydrogen and nickel

and other metals that are prone to form hydrides contained in the electrodes, [11]. The rate of metal hydride

formation will increase with higher pressure, thus slightly higher Ohmic overpotential. Table 5 describes the initial

Ohmic resistance of various components of the cell of the actual electrolyser, used in the HARI project. The initial

resistance is measured at 273.15K, which is gradually reduced with increasing temperature, given by the empirical

equation as shown in Eq 18.

6 6 2[4 10 ln( 273.15) 4 10 ln ]final initialR R x T x T Eq 18

The Eq 18 is valid when the temperature (T) is in the range of 273.15K < T < 350K. The empirical equation has

been developed by curve fitting of the measured current voltage data-sets in tune with the predicted Ohmic

overpotential as generated by the Matlab-Simulink model after several iterations.

The gas production increases at high operating current density, which increases the void fraction of gases in the

electrolyte due to presence of more gas bubbles. The increased void fraction results in increased resistance of the

electrolyte layer, which finally increases the cell voltage and is called here as the bubble voltage loss. This

phenomenon of bubble voltage loss has been modelled using an empirical equation. The empirical equation has been

developed by curve fitting of the measured current voltage data-sets in tune with the predicted Ohmic overpotential

and overall current-voltage profile as generated by Matlab-Simulink model after several iterations. Nagai et. al. [9]

has developed a relationship between the void fraction and current density, which has been referred while

developing the empirical equation of bubble voltage loss. Bubble voltage loss (V bubble) is measured in Volt, which is

a function of the operating current as shown in Eq 19.

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9 2 9 30.00001* 4*10 * 10 *bubbleV i i i Eq 19

. c. v *final bubbleOverall Ohmi O erpotential R i V Eq 20

The final Ohmic resistance is multiplied by the operating current to find the voltage increase due to final Ohmic

resistance, which is then added to the bubble voltage loss as shown in Eq 20 to calculate the overall Ohmic potential,

which is represented in Figure 3. It shows that the overall Ohmic potential is almost equal to the voltage drop

caused by Ohmic resistance up to 150mA/cm2 current density, which rapidly increases with further increase in

current density. This phenomenon can be explained by the fact that the void fraction of gases increases rapidly in the

zero gap cell structure due to the absence of forced circulation for bubble removal. The resistance of the separator

contributes for about 79% of the total initial resistance (0.00009Ohm out of 0.000114Ohm as shown in Table 5

highlighting the scope of efficiency improvement using better separators. Ion exchange polymer membranes could

be used.

The Ohmic overpotential can change instantly without any time delay with dynamic variation in current unlike the

activation overpotential. Activation loss makes the electrolyser less dynamic, whereas the Ohmic loss allows the

electrolyser to respond immediately. However, activation loss increases logarithmically with increasing current but

the Ohmic overpotential varies linearly due to final Ohmic resistance, therefore the Ohmic resistance will cause

much higher voltage drop at higher current densities than the activation drop. Nevertheless, the bubble voltage loss

increases exponentially with increasing current density thus an optimisation is essential between the activation loss,

Ohmic loss and bubble voltage loss in order to design an electrolyser with adequate dynamic response, better

efficiency and higher gas production rate.

5. PRACTICAL ELECTROLYSIS VOLTAGE

The SIMULINK model predicts the variation of electrolysis voltage with current density for any temperature,

pressure, total number of cycles and total stand by time. The predicted and measured cell voltage incorporating

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temperature and pressure-effects on the reversible voltage, activation overpotential and Ohmic overpotential, for a

particular operating pressure, temperature, number of cycles and stand by time are shown in Figure 4. The relevant

values have been measured on the 34kW alkaline type Vandenborre-Stuart Energy's IMET1000 electrolyser,

operating on the HARI project test site near to Loughborough University, UK [2]. Appropriate devices have been

fitted within the electrolyser to record the average values of voltage, current, pressure, temperature, gas purity and

number of cycles at a minimum interval of every 5s. The measured and predicted stack power for dynamic operation

has been shown in Figure 5.

6. GAS PURITY

The purity of gases depends on many factors. The mixing of electrolyte carrying dissolved gases in the gas-

electrolyte-separators is one of the major causes of impurity. The solubility of gases in the electrolyte increases with

higher pressure. Having a separate anolyte and catholyte loop can increase the gas purity. The molecular diffusion

through the micro-pores of the gas-separator is another major cause of impurity in the gases. This is difficult to stop,

however, encouraging faster nucleation growth of bubbles can reduce this, which is easier at ambient pressure. The

presence of nano- or micro-bubbles at very high pressure in a narrow space for a sufficiently long time has the

undesirable effect of diffusion through the gas separators even at 5mbar (0.005atm) differential pressure.

7. DIFFERENTIAL PRESSURE

This has a direct relationship to gas impurity; but it also causes structural damage to the gas-separators. The

differential pressure-control across the gas-separators is one of the most important operations of any pressurised

electrolyser. The balancing of electrolyte level and associated solenoid valve synchronisation to release the gas from

the system is a complex process. In a bipolar zero gap cell geometry, the pressure between the membrane and either

anode or cathode can vary instantly with fluctuating current input. A high operating pressure for the electrolyser

tends to result in a high differential pressure due to inefficient bubble removal and slightly higher response time

required by the control system for valve opening and closing to release gases. This differential pressure then causes

the gas to diffuse to one gas compartment or the other. The mixing of hydrogen in oxygen has been measured up to

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2.5% at 120atm pressure by a research organisation in Jlich, Germany [12]; therefore, a significantly higher gas-

impurity can be expected at 700atm pressure if the same gas separator is employed.

8. GAS LOSSES AND ENERGY CONSUMPTION

Gas loss occurs during de-oxidation and drying in common to both atmospheric and pressurised systems but at

different proportions. The gas loss in pressurised electrolysers occurs in the water filling system. The water filling

mechanism depends on the electrolyser manufacturer. The KOH is scrubbed from the gas while replenishing the

reactant water. The water filling mechanism in the Hydrogenics (Stuart Energy -Vandenborre) electrolyser installed

in the HARI project [2], is controlled by several solenoid valves (26 nos.) operating synchronously which has about

2.5% hydrogen loss when operating at 25atm pressure. This loss will increase at higher pressure proportionately if

the same method is employed. To avoid that, a feed-water pump is used to fill water at higher pressure (30atm) but

at some cost in terms of energy consumption and investment, [[13] to [20]]. Although, low flow liquid pumps are

available, however, it is sometimes difficult to get a pump operating at a very high output pressure from ambient

pressure for a very small discharge rate. On the other hand, no hydrogen loss occurs in the atmospheric electrolyser

while filling water under gravity force. A KOH pump is often used with a pressurised electrolyser to increase the

bubble removal rate at higher pressure but with an increased energy cost. The impurity of oxygen in the hydrogen

gas is removed by the de-oxidation unit but this consumes hydrogen and produces water vapour in the hydrogen gas.

The drier is used to absorb the moisture from the gas stream. The moisture content of the gas is then adsorbed in the

molecular sieve. When the drier is fully saturated then the stand-by-drier starts working and the moist drier is

depressurised by venting the wet gas to the atmosphere. This depressurisation causes immediate loss of hydrogen.

Electrical coil heating starts at this point to regenerate the drier bed and this continues for a prescribed duration.

Once the drier bed is heated enough, about 10% of the rated hydrogen production is flushed through it to remove the

traces of air and water for approximately 12 hours at about 6atm pressure. The hydrogen loss and electricity

consumption in this stage of regenerating the drier bed depends on the capacity of the drier. The impurity in the

gases depends mainly on the current density and gas-separator's quality. The hydrogen impurity present in the

oxygen gas cannot be reclaimed and is considered as a direct loss of hydrogen as the oxygen is generally vented to

the atmosphere. Pressurised electrolysers need nitrogen purging if the pressure is reduced to near ambient pressure

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causing further hydrogen loss, which is not included when calculating the total gas loss given in Table 6. The

inevitable losses through the couplings and through-leakage of valves are high at elevated operating pressure though

this is also not considered in Table 6. The HARI project has experienced several nitrogen purging cycles and

significant gas losses (about 1500Nm3 from November 2003 to December 2005) mainly due to internal-leakage of

valves.

The moisture content in hydrogen varies inversely with the operating pressure. Therefore, the atmospheric

electrolyser has the drawback of higher moisture content in the gas. However, this helps slightly in reduced cooling

demand due to higher latent heat of evaporation of water. Ultimately, atmospheric electrolysers require a

compression stage and as a result the moisture content from the gas is reduced when compressed due to

condensation. Compressors are available in the market with moisture removal features operating with atmospheric

electrolysers. To take an example, the drier gas loss in atmospheric electrolysis combined with compression up to

700 atm is equal to the drier gas loss of 700atm electrolysis.

Electrolysers having pressures more than 30atm generally employ feed-water pumps. Table 7 shows the feed-water

pump energy for an electrolyser having 8Nm3/h hydrogen production rate, for which the water supply rate would be

2.2 x 10-6

m3/sec with a pump efficiency of 75%. Using Eq 21, the power of the feed water pump can be calculated:

3 /. . * .Watt atm m sPump Power Water pressure flowrate Eq 21

Onda et al., [20] have suggested that 700atm-pressure electrolysers with feed-water pumps would require 5% less

power in total, than atmospheric electrolysers with four-stage compressors, assuming 50% efficiencies for both

compressor and pump. They have also noticed that the total power would be almost equal, if 75% efficiencies of

compressor and pump were assumed. Metal diaphragm compressors can achieve 75% or more compression

efficiency as demonstrated by Andra's Hofer Compressors for large flow rates, [21]. Metal hydride compressors

have a number of features which make them attractive for use with atmospheric electrolysers, e.g. low power

consumption, scalability, cost, etc. The HERA Hydrogen-(Ergenics), [22], metal hydride compressor can absorb

15

hydrogen at ambient pressure and increase the pressure up to 200atm or higher in a multi-stage process. It consumes

recyclable heat energy at 25kJ/mol hydrogen, which is available as a by-product of electrolysers, fuel cells or

internal combustion engines. The electrical energy consumption will be significantly less than that required for

mechanical compression, when powered by waste heat, which reduces the cost of hydrogen production, [22].

However, Onda et. al. did not include activation overpotential and Ohmic losses in their calculation of electrolysis

voltage. They also did not incorporate other parasitic losses such as balance of plant, drier regenerating energy

consumption, gas losses, gas venting, pressure drop, cooling demands, etc. The parasitic energy consumption is

much higher in smaller pressurised electrolysers due to many solenoid valves, added safety and monitoring devices,

control electronics circuits, KOH pump, pressure and level control mechanisms, etc. Also, the difference in parasitic

energy consumption in pressurised and atmospheric electrolysers is much higher for smaller electrolysers compared

to larger ones. Very large (MW size) pressurised electrolysers will have reduced parasitic loss compared with their

smaller counterparts but need added safety devices due to the increased hazards associated with KOH liquid and

hydrogen under high-pressure (700atm).

The reduced rate of evaporation of water in the pressurised electrolyser increases cooling demand. As an estimate,

about 41kJ heat/Nm3 hydrogen is absorbed from the system for evaporation of water in the atmospheric electrolyser,

whereas 0.05kJ/Nm3 is absorbed in 700atm electrolysers. The actual energy consumption (kWh/Nm

3) in pressurised

electrolysers incorporating the loss of hydrogen is described in Table 8. The actual overall energy consumption in

the Hydrogenics (Stuart Energy- Vandenborre-IMET1000), 25atm electrolyser is about 5.56kWh/Nm3 in its first

cycle if the inevitable hydrogen loss is incorporated in the calculation. We also have experienced about 11% cell

voltage rise due to corrosion in the electrolyser, within 250 on-off cycles in the first 6 months of its installation,

which leads to total energy consumption of 6.023kWh/Nm3. However the cell voltage rise due to corrosion slowed

down gradually and it is fairly stabilised after about 1500 on-off cycles. The corrosion rate increases for several

reasons and higher operating pressure is one of the major causes. However, assuming equal corrosion in the 700atm

pressure electrolyser after 250 on-off cycles, the overall energy consumption would be 6.3 kWh/Nm3, incorporating

the gas losses.

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The power of the compressor can be calculated for various pressures depending upon the number of stages of

compression from atmospheric pressure. Eq 22 gives the power of the compressor. For example, up to 50atm

pressure, a two-stage compressor can be suitable. Three stage compressors can be used for pressures between 75atm

and 500atm and four-stage compressors can be used for pressures between 600atm and more than 1000atm. Table 9

describes the compressor energy assuming the gas flow rate as 8Nm3/h and the efficiency of the metal diaphragm

gas compressor as 75%.

, * *compressor kW pW C T m

Eq 22

The total energy consumption in an atmospheric electrolyser system including compressor energy is shown in Table

10, assuming the rated current density of 440mA/cm2 at 75

0C, number of on-off cycles as one and 8Nm

3/h hydrogen

production rate. Figure 6 shows the overall energy consumption for pressurised electrolysers at various pressures

and an atmospheric electrolyser with a compressor at the corresponding pressures incorporating gas losses. The

700atm-pressurised electrolyser consumes 16.6% more energy compared to atmospheric electrolysers with external

compression to 700atm pressure. We have taken a conservative estimate in Table 6 of gas purity, which is reflected

in Figure 6. However, if we assume 2.5% hydrogen in oxygen at 700atm pressure which has been measured at a

significantly lower pressure of 120atm, [12] then the total gas loss will be 5.82% at 700atm, which will increase the

energy consumption to 5.89kWh/Nm3, that is 19.9% higher energy consumption than the atmospheric system. It is

worth mentioning that if the practical hydrogen leaks (e. g. internal-leakage of valves etc) at higher operating

pressures were incorporated in Figure 6, then the difference in energy consumption between the two systems would

be even higher. Table 11 compares the total energy consumptions of atmospheric and pressurised systems and the

percentage increase of energy consumption is also shown. The electrical energy efficiency can now be calculated

based on the higher heating value of hydrogen as 3.5kWh/Nm3. The efficiency of a 700atm-pressurised electrolyser

will be 60.9% and 59.4% respectively for 5.74kWh/Nm3 and 5.89kWh/Nm

3 hydrogen production energy. Similarly,

the efficiency of the atmospheric electrolyser with a compressor will be 71.1% for a total hydrogen production

energy of 4.92kWh/Nm3, including compression up to 700atm pressure. The Hydrogenics (Stuart Energy-

Vandenborres alkaline electrolyser -IMET1000) operating in the HARI project, [2], has an energy efficiency of

17

62.9%, for its energy consumption of 5.56kWh/Nm3 at 25atm pressure, including gas losses. With reference to PEM

electrolysers (HOGEN 380) of Proton Energy Systems, the electrical energy efficiency will be 55.5%, for their

energy consumption of 6.3kWh/Nm3 at 13atm pressure, [1], without incorporating gas losses. It is interesting to note

that the energy efficiency of pressurised electrolysers is better in the range of 50-100atm compared to either 25atm

or 200-700atm due to optimisation between gas loss, parasitic energy and pump energy (Table 11). However

atmospheric electrolysers are more energy efficient at all pressures.

9. PRACTICAL CONSIDERATIONS

The practical aspects of operating electrolysers at higher pressure should be analysed. The membranes or gas

separators and electrodes are the sensitive components of the electrolyser stack. The membranes are subject to

rupture at higher differential pressure. The corrosion and degradation of metallic parts of the stack including the

electrodes, bipolar plates, and current collectors are increased at higher pressure. All these contribute towards

reduced stack life. The solar hydrogen project at Neunburg Vorm Wald, Germany, experienced significant

operational problems and degradation in their high-pressure electrolyser compared to their low-pressure electrolyser

operating under the same conditions. The alkaline type low-pressure electrolyser operated for more than 13 years

and remained the main hydrogen production unit, [23].

The general trend for high-pressure electrolysers often necessitates the encapsulation of the stack and other

components in one pressure vessel made by a thick layer of stainless steel [24], [25]. The pressure vessel is then

filled with water at the same pressure as the stack using a pump. This helps reduce the differential pressure between

the inside and outside of the stack and reduces the chances of leakage from the stack and increases the durability of

seals, O-rings, gaskets, etc. However, this increases the heat capacity of the whole system significantly thus making

it less responsive dynamically. The temperature plays a major role in the efficiency of the electrolysis. It will take

much longer to reach the optimum operating temperature from a cold start. The increase in heat capacity reduces the

thermodynamic response, which makes this type of pressurised electrolyser system less useful when powered by a

fluctuating energy source. As the future sustainable production of large quantities of hydrogen will rely on

fluctuating renewable sources of energy, this is a significant drawback for a pressurized electrolyser. During

18

intermittent power supply from renewable energy sources, it is difficult to maintain the optimum temperature of the

electrolyser stack, which leads to higher cell voltage. The higher heat capacity of the electrolyser further aggravates

this problem. The parasitic loads are generally much higher in pressurised electrolyser due to extra instrumentation

and control systems. These electrolysers when powered by the stand alone renewable energy sources, incur

significant stand-by energy loss over a long period, as they generally spend most of the time in the stand by mode.

In the HARI project, the electrolyser has spend about 90% time in stand by mode in last 2years and consumed

standing power (from 125W to 200W), thus increases the overall energy consumption (kWh/Nm3) significantly. The

high stand by loss in this case is mainly due to over-sizing the electrolyser. It has been found that the average energy

consumption (kWh/Nm3) of electrolyser increases by 14.3% if batteries are used to supply the stand-by loads of the

electrolyser. The energy consumption increases by about 48% in case of fuel cells powering the standing loss. On

the other hand atmospheric electrolysers can be simply switched off, thus no stand by loss. This analysis shows that

the accurate sizing of electrolysers is very crucial for efficient stand-alone energy systems.

The potential risk of splashing or leakage of hot jets of KOH from high-pressure electrolysers must be considered in

a risk assessment and the risk further increases due to higher rate of hydrogen embrittlement. Pressurised

electrolysers have significantly greater numbers of joints and couplings than atmospheric ones, which also increases

the risk of leakage. On the other hand, any potential failure or leakage of KOH from atmospheric electrolysers will

take place slowly and liquid will tend to fall vertically downwards thus greatly reducing the risk of caustic burns.

Recent anecdotal reports of KOH leakage at various electrolyser installations should be considered in the risk

assessment of pressurised electrolysers.

The maintenance problems of pressurised electrolysers are high with the current state of technology, requiring

highly skilled technicians. The cell stack of the 25atm pressurised electrolyser used in the HARI project started

leaking after about 15months of operation, and the stack subsequently has failed after 22 months with 10% run time

due to high impurity of gases and severe KOH leaks from the stack. A new stack has recently been installed using

new separators/membranes and some other modification. The research and operational experience from this project

is being applied to design a low cost novel electrolyser suitable for renewable energy powered operation. On the

19

other hand the maintenance requirements of atmospheric electrolysers are less, although the maintenance of the

compressor can become an issue depending on its type. However, the periodic maintenance of seals and diaphragms

of the compressor is a relatively easy job. In addition, it should be stressed that stand-alone compressors are widely

used in industry in contrast to pressurised electrolysers. A piston compressor has also been in automatic operation

in the HARI project to boost pressure from 25 atm to 137atm without any major problem so far.

10. FUTURE SCOPE OF ATMOSPHERIC AND PRESSURISED ELECTROLYSERS

The analysis, which has been described above would suggest that atmospheric electrolysers are more energy

efficient than pressurised electrolysers. They have fewer gas losses, higher stack life and require less maintenance.

Presently, the combined cost of an atmospheric electrolyser with an external compressor is about 30% cheaper than

a pressurised electrolyser at the equivalent operating pressure, [26]. Atmospheric electrolysers have a proven record

of operation for many years around the world, [27], [28]. Norsk Hydro says their conventional atmospheric

electrolysers are more energy efficient than their pressurised systems for higher production rates, [28]. Large

electrolysers would be needed in future in forecourts for fuelling vehicles or in various manufacturing industries,

where the hydrogen selling price would be less using atmospheric electrolysers. Large-scale atmospheric

electrolysers could be more suitable for the production of hydrogen fuel to be used for electricity grid balancing and

peak-shaving with their improved efficiency and dynamic response. Most hydrogen consumption devices such as

fuel cells, internal combustion engines and catalytic burners operate within the range of 0.5-5atm, which can be

delivered using external compressors relatively easily. Where space is readily available, e.g. a salt cavern or

underground tankers, then large quantities of hydrogen can be stored at low pressure. If both hydrogen production

and consumption are happening in a synchronous way for example in food processing industries, then the

requirements of hydrogen storage will be less important, which is an ideal situation for atmospheric electrolysers. If

the hydrogen needs to be stored for long-term and in large quantities, then the use of high-pressure external

compressors will be an efficient and economical approach.

The Department of Energy, USA has established a goal under the Matsanuga Hydrogen Implementation Act for

efficiency of hydrogen storage of a volumetric density of 70 kg/m3 and 7 wt % gravimetric densities for

20

transportation applications. The Act states: The high-pressure gaseous storage is a relatively simple technology on

the surface, offering dormancy, easy filling and requiring only appropriate valving and metering to supply product

but it has the disadvantages of low volumetric densities and the perception of being excessively hazardous. For this

reason, it is often seen as a transitional stage of hydrogen storage development, [29]. The prospective development

of efficient metal hydride hydrogen storage could become more attractive than the compressed gas storage for

transportation applications due to higher energy density and increased safety. With the development of metal

hydride compressors, atmospheric electrolysers could become economically feasible even for smaller production

volumes and be suitable for the niche market of fuelling vehicles with higher efficiency and lower cost of hydrogen.

Although there are few hydrogen compressors available for inlet ambient pressure and low flow rates, [30], the

problem of small production volumes (100atm) electrolysers for small

production volumes because the pressurised electrolyser needs a feed-water pump to avoid significant amounts of

gas losses. For a hydrogen production rate of 8Nm3/h, the required flow rate of water will be 0.0022l/s, which is

difficult for a small pump to deliver at high pressure. Therefore a bigger pump will be necessary and will operate

periodically but that will increase the capital cost and size.

In the absence of commercial metal hydride compressors coupled with atmospheric electrolysers, grid-connected

steady-state medium-pressure (up to 25-100atm) electrolysers can be suitable for smaller production volumes (5-

25Nm3 /h) assuming small discharge and high-pressure feed water pump should be available. They also need to be

more reliable and durable compared with current technology. In the case of high-pressure (>300atm) electrolysers,

the issues of low energy efficiency, complication, restricted dynamic response in conjunction with renewable energy

sources and higher cost need to be addressed for successful commercial reality in the home fuelling market.

21

Electrolysers which operate at pressures of around 700atm involve unnecessary complication, entail increased risk

and have a high cost. In many cases, their role could be better filled using atmospheric electrolysers.

11. CONCLUSION

This paper has analysed the thermodynamics and gas losses associated with practical water electrolysis. It has been

shown that the reversible voltage increases by more than 12.85% in 700atm-pressurised electrolysers compared to

an atmospheric system. The optimising of thermo-neutral voltage with higher operating pressure for efficiency

improvement is not appropriate as, in practical electrolysers, excess heat is generated due to activation and Ohmic

losses. The reversible voltage should be kept low to increase electrical efficiency, which is possible with an

atmospheric system. The energy consumption in using feed-water pumps and gas compressors is incorporated in the

total energy consumption at various pressures. The energy consumption in 700atm electrolysers is about 16% higher

than the combined energy consumption of atmospheric electrolysers and compressors. Comparison of total energy

consumption at various operating pressures up to 700atm is performed and atmospheric electrolysers are found more

efficient at all levels. The durability, on-off cycle, differential pressure, pressure drop, gas purity and dynamic

response and cost issues do not favour the pressurised electrolysers.

The efficiency of a 700atm-pressurised electrolyser and an atmospheric electrolyser including compression up to

700atm pressure will be 60.9% and 71.1% respectively. The Hydrogenics alkaline electrolyser (IMET1000)

operating in the HARI project, [2], has an average energy efficiency of 62.9%, for its energy consumption of

5.56kWh/Nm3 at 25atm pressure, including gas losses.

Atmospheric electrolysers will be appropriate for large-scale hydrogen production, such as, for vehicle fuelling in

forecourts, balancing of electrical grids and peak shaving. They are also suitable for long-term, mass storage of

hydrogen using external compressors. In addition, their better dynamic response makes them more suitable than

pressurised electrolysers for hydrogen generation powered by fluctuating renewable energy sources. However for

long-term but small quantity storage, pressurised electrolysers might be preferable where energy efficiency is less

important, provided the reliability of operation and durability is improved from the current status. Moreover, with

22

higher reliability and commercialisation of small-scale metal hydride hydrogen compressors, atmospheric

electrolysers could become more attractive than pressurised electrolysers even for small production capacity such as

home fuelling of cars.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Prof Tony Marmont for his financial support in the HARI project. The

authors thank the team members of the HARI project, Rupert Gammon, Murray Thomson, Matt Little and, for their

mutual support. The Government of India fellowship to conduct research in the UK is gratefully acknowledged. The

first author also acknowledges the National Institute of Technical Teachers' Training and Research (NITTTR),

Bhopal, India for sponsoring his study leave.

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[9] Nagaia N., Takeuchia M., Kimurab T., Okaa T., Existence of optimum space between electrodes on hydrogen

production by water electrolysis, Int. J. Hydrogen Energy 28 (2003) 35 41.

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Forschungszentrum Jlich GmbH Institut fr Werkstoffe und Verfahren der Energietechnik (IWV 3).

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No.5, pp. 453-458.

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Nagasaka, Yokosuka city, 240-0194, Japan.

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Hydrogen Energy 1998; 23, No.12, pp. 1113-1120.

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1996, Contract No. JOU2-CT93-0413.

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Grimma, Germany, Wissenschaftliche Schriftenreihe, Nummer 2: Regenerative Energien.

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pressure electrolysis; Journal of power sources 132 (2004) 64-70.

[21] Andra's Hofer Compressors, Website last accessed on 02-02-2005.

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24

Advanced thermal hydrogen compressors, DaCosta D. H., Golben M. Website last accessed on 02-02-2005.

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[23] Szyszka A., Toward a (solar) hydrogen energy scheme, Thirteen successful years of SWB's Solar Hydrogen

Demonstration Project at Neunburg vorm Wald, Germany, Field of Solar Hydrogen Solar Hydrogen Plant

No. 23, Manuscript dated July 30, 1999. http://www.neuhaus.com/swb/web_index_e.htm Website last

accessed on 02-02-2005.

[24] GHW Presents Advanced Pressurized Alkaline Electrolyzer at HYFORUM,

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last accessed on 02-02-2005.

[25] Mitsubishi Corporation Succeeds in Generating 35MPa High-Pressure Hydrogen Without a Compressor

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[26] Mr Jrgen RINN, ELT Elektrolyse Technik GmbH, Private communication by email with, dated Sun, 02 Nov

2003 08:43:48 +0100

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[28] Norsk Hydro Electrolysers

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Development Areas (U), SANDIA REPORT, SAND94-8229 *UC-406, Unlimited Release, Printed April

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[30] Low inlet pressure and low flow hydrogen compressor, http://www.rixindustries.com/products_hydr.html

25

Table 1 Pressure dependence of the enthalpic voltage (mV), Vt, p

t (0C) Pressure

1 atm 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm

25 -0.0152 -0.578 -1.143 -1.686 -2.208 -4.43 -6.64 -8.85 -11.07 -13.28 -15.5

75 -0.0103 -0.429 -0.844 -1.241 -1.616 -3.24 -4.86 -6.48 -8.1 -9.73 -11.35

100 -0.0078 -0.354 -0.694 -1.018 -1.320 -2.65 -3.97 -5.29 -6.62 -7.95 -9.27

26

Table 2 Pressure dependence of higher heating voltage (mV), (VHHV, t, p - V0HHV, t) (mV)

t (0C) Pressure

1 atm 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm

25 -0.0152 -0.370 -0.720 -1.048 -1.354 -2.71 -4.06 -5.41 -6.76 -8.15 -9.47

75 -0.0103 -0.247 -0.473 -0.681 -0.867 -1.73 -2.6 -3.463 -4.328 -5.19 -6.06

100 -0.0078 -0.185 -0.350 -0.497 -0.624 -1.25 -1.87 -2.49 -3.11 -3.74 -4.36

27

Table 3 Vapour pressure (atm)

Vapour

pressure

298.15K

(250C)

323.15K

(500C)

348.15K

(750C)

373.15K

(1000C)

pw*(water) 0.03103 0.1201 0.3755 0.99157

pw (KOH) 0.01725 0.0689 0.2216 0.599

28

Table 4 Reversible voltage at various temperature and pressure

Tempera

ture

Pressure

1 atm 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm

250C 1.229 1.299 1.312 1.32 1.325 1.339 1.347 1.352 1.356 1.36 1.354

750C 1.19 1.268 1.283 1.292 1.299 1.315 1.324 1.33 1.335 1.339 1.343

1000C 1.153 1.252 1.269 1.279 1.286 1.303 1.313 1.32 1.325 1.329 1.333

29

Table 5 Ohmic resistance of individual components of a cell at 00C

Components Electrode

area, m2

Resistivity, - m

2/m

Thickness,

m

Number

of unit

Resistance,

KOH layer 0.1 0.012 0.0001 2 0.000024

Nickel electrode 0.1 6.8x10-8

0.001 2 0.00000000136

Nickel current collector

(wire mesh)

0.1 5x10-8

0.003 2 0.000000003

Nickel bipolar plate 0.1 6.8x10-8

0.002 1 0.00000000136

Gas separator (VITO) 0.1 0.000009 - m2 - 1 0.00009

Total initial resistance 0.000114

30

Table 6 Hydrogen gas loss (%) at various operating pressure

Gas loss Pressure 1 atm 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm

Water* 0 2.5 - - - - - - - - - Drier** - 4.93 2.8 2.08 1.73 1.2 1.02 0.93 0.88 0.84 0.82

Due to O2 in

H2

0.4x2 0.407x2 0.414x2 0.42x2 0.43x2 0.46x2 0.48x2 0.51x2 0.54x2 0.57x2 0.6x2

H2 in O2 1 1 1.01 1.02 1.03 1.06 1.08 1.12 1.14 1.16 1.2

Total 1.8 9.244 4.64 3.94 3.62 3.18 3.06 3.07 3.1 3.14 3.22 * Assuming a pump is employed from 50atm to 700atm to fill the water into the electrolyser stack to prevent gas losses. The

absence of feed water pump at 25atm results into gas loss.

** Assuming the drier bed has adsorbing capacity of 225Nm3 moist hydrogen at 25atm at 250C temperature according to

Vandenborre-Stuart electrolyser. The same drier would adsorb 6300Nm3 moist gas at 700atm. The gas impurity values beyond 100atm pressure are conservative estimates in the absence of actual data. It could

significantly increase with higher pressure [9].

The drier gas loss corresponding to the compressor's output pressure must be added to this value.

31

Table 7 Power of the feed-water pump

Pressure

1 atm 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm Feed water

Pump (kW) - - 0.01467 0.022 0.02935 0.05869 0.088 0.1174 0.147 0.176 0.2054

Energy

(kWh/Nm3) - - 0.0018 0.0028 0.0037 0.00734 0.011 0.015 0.018 0.022 0.026

32

Table 8 Energy consumption (kWh/Nm3) in pressurised electrolysers

Pressure

1 atm 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm Gas loss (%) 9.244 4.64 3.94 3.62 3.18 3.06 3.07 3.1 3.14 3.22 Stack energy

kWh/Nm3* 4.15 4.345 4.384 4.407 4.422 4.459 4.482 4.498 4.51 4.52 4.53

Parasitic energy

kWh/Nm3* 0.3 0.6 0.63 0.64 0.66 0.72 0.77 0.84 0.89 0.95 1

Pump energy

kWh/Nm3 - - 0.0018 0.0028 0.0037 0.00734 0.011 0.015 0.018 0.022 0.026

Total energy*

without gas loss

kWh/Nm3

- 5.045 5.015 5.05 5.09 5.17 5.26 5.35 5.418 5.492 5.556

Total energy*

with gas loss

kWh/Nm3

- 5.56 5.259 5.257 5.28 5.34 5.42 5.52 5.6 5.67 5.74

* Assuming the rated current density of 440mA/cm2 at 750C and number of on-off cycle as one and 8Nm3/h hydrogen

production rate. The parasitic power loss will be less for larger systems.

33

Table 9 Power of the hydrogen compressor

Pressure

Compressor stages 2-stage 3-stage 4-stage Pressure (atm) 25 atm 50 atm 75 atm 100 atm 200 atm 300 atm 400 atm 500 atm 600 atm 700 atm

Gas Compressor

power (kW) 1.339 1.719 1.75 1.894 2.26 2.485 2.65 2.782 2.658 2.738

Energy

(kWh/Nm3)* 0.1674 0.2149 0.2187 0.2368 0.2824 0.3106 0.3313 0.3477 0.3322 0.3423

* The Energy (kWh/Nm3) is required to compress hydrogen at the corresponding pressures from 1atm.

34

Table 10 Energy consumption (kWh/Nm3) in an atmospheric electrolyser with a compressor.

Pressure

1atm 25atm 50atm 75atm 100atm 200atm 300atm 400atm 500atm 600atm 700atm Gas loss (%) 6.73 4.6 3.88 3.53 3 2.82 2.73 2.68 2.64 2.62 Stack energy

(kWh/Nm3) 4.15 - - - - - - - - - -

Parasitic energy

(kWh/Nm3) 0.3 - - - - - - - - - -

Compressor energy

(kWh/Nm3) 0.1674 0.2149 0.2187 0.2368 0.2824 0.3106 0.3313 0.3477 0.3322 0.3423

Total energy without

gas loss

(kWh/Nm3)

4.45 4.6174 4.6649 4.6687 4.6868 4.7324 4.7606 4.7813 4.7977 4.7822 4.7923

Total energy with

gas loss

(kWh/Nm3)

- 4.9506 4.889 4.857 4.858 4.878 4.897 4.915 4.929 4.912 4.92

35

Table 11 Comparison of total energy consumptions of atmospheric and pressurised systems

Pressure (atm) 25 50 75 100 200 300 400 500 600 700

Total energy atmospheric

system (kWh/Nm3) 4.9506 4.889 4.857 4.858 4.878 4.897 4.915 4.929 4.912 4.92

Total energy pressurised

system (kWh/Nm3) 5.56 5.259 5.257 5.28 5.34 5.42 5.52 5.6

5.67 5.74

Percentage increase (%) 12.31% 7.57% 8.24% 8.69% 9.47% 10.68% 12.31% 13.61% 15.43% 16.66%

36

Activation Overpotential temperature=750C, i0, anode=

0.016A/cm2, i0, cathode=0.02A/cm

2, charge transfer

coefficient for anode and cathode=0.5

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0

0.0

5

0.1

0.1

5

0.2

0.2

5

0.3

0.3

5

0.4

0.4

5

Current density (A/cm2)

Activation o

verp

ote

ntial

(V)

Figure 1 Activation overpotential

37

Figure 2 Schematic diagram of cell structure

38

Ohmic overpotential including bubble voltage

Temperature=348.15K

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 50 100 150 200 250 300 350 400 450

Current density (mA/cm2)

Ohm

ic o

verp

ote

ntial (V

)

overpotential based on Ohmic

resistance only

overpotential including bubble voltage

loss and Ohmic resistance

Figure 3 Overall Ohmic overpotential including bubble voltage loss

39

Electrolysis cell voltage

Temperature=348.15K, Pressure=18atm, On-of f cycle=712, Stand by time=4933.17

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

0

25

50

75

100

125

150

175

200

225

250

275

300

32

5

35

0

37

5

40

0

42

5

45

0

47

5

Current Density (mA/cm2)

Cell

vo

ltag

e (

V)

Predicted voltage, V

Measured voltage, V

Figure 4 Practical electrolysis voltage

40

0 200 400 600 800 1000 1200 1400 1600 1800 2000 22000.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

4

Time (s)

Measure

d a

nd p

redic

ted s

tack p

ow

er

(W)

Measured and predicted stack power

Predicted Power (resolution 0.01s)

Measured Power (resolution 5s)

Figure 5 Measured and predicted stack power

41

Figure 6 Comparison of energy consumption

0

1

2

3

4

5

6

Energ

y c

onsum

ption

(kW

h/N

m3)

25 a

tm

50 a

tm

75 a

tm

100 a

tm

200 a

tm

300 a

tm

400 a

tm

500 a

tm

600 a

tm

700 a

tm

Pressure

atmospheric electrolyserpressurised electrolyser

42

List of Figures .................................................................................................................... Page No.

Figure 1 Activation overpotential ........................................................................................... 36

Figure 2 Schematic diagram of cell structure .......................................................................... 37

Figure 3 Overall Ohmic overpotential including bubble voltage loss ..................................... 38

Figure 4 Practical electrolysis voltage ..................................................................................... 39

Figure 5 Measured and predicted stack power ........................................................................ 40

Figure 6 Comparison of energy consumption ......................................................................... 41

NOTICE: this is the authors version of a work that was accepted for publication in International Journal of Hydrogen Energy. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Roy A, Watson S J and Infield D G (2006). Comparison of electrical energy efficiency of atmospheric and high-pressure electrolysers. International Journal of Hydrogen Energy 31 pp 1964-1979. DOI: http://dx.doi.org/10.1016/j.ijhydene.2006.01.018 .