Conference Paper

NREL/CP-550-47302

January 2010

Hydrogen Production: 

Fundamentals and Cas Study

Summaries 

e

Preprint

K.W. Harrison, R. Remick, and G.D. Martin

National Renewable Energy Laboratory

A. Hoskin

Natural Resources Canada

To be presented at the 18th World Hydrogen Energy Conference

Essen, Germany

May 16–21, 2010
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iii

List of Acronyms and Abbreviations

AC alternating current

AC/DC AC-to-DC

atm atmosphere

BEPC Basin Electric Power Cooperative

Btu British thermal units

DC direct current

DC/DC DC-to-DC

DOE U.S. Department of Energy

F Faraday constant

G Gibbs free energy

H enthalpy

HARI Hydrogen and Renewables Integration

HHV higher heating value

IC internal combustion

JANAF Joint Army-Navy-Air Force

kJ kilojoule

kWe kilowatt electrical

kWh kilowatt hours

LHV lower heating value

mol mole

MPPT maximum power point tracking

NREL National Renewable Energy Laboratory

Nm3 normal cubic meter

NWTC National Wind Technology Center

PEM polymer electrolyte membrane

PV photovoltaic

RE renewable energy

S entropy

V volt

Wind2H2 Wind-to-Hydrogen project
iv

Table of Contents

List of Acronyms and Abbreviations.............................................................................................iii

Heating Value, Heat of Reaction, and Free Energy........................................................................ 1

Heat of Combustion and Heat of Reaction ................................................................................. 1

Standard Conditions.................................................................................................................... 1

Free Energy................................................................................................................................. 2

Heat of Formation and Free Energy of Formation.......................................................................... 2

Calculating Fuel Cell System Efficiency........................................................................................ 3

Voltage Efficiency of Fuel Cells and Stacks .............................................................................. 4

Other Efficiency Calculations..................................................................................................... 5

Water Electrolysis........................................................................................................................... 5

Voltage Efficiency of Electrolysis Cells and Stacks................................................................... 6

Steam Electrolysis and High-Temperature Cells........................................................................ 7

Recommendations....................................................................................................................... 8

Case Studies of Wind/Hydrogen Projects....................................................................................... 9

Canada......................................................................................................................................... 9

Ramea Island........................................................................................................................... 9

Prince Edward Island .............................................................................................................. 9

Greece ......................................................................................................................................... 9

RES2H2 Project...................................................................................................................... 9

Spain ......................................................................................................................................... 10

RES2H2 Project.................................................................................................................... 10

ITHER Project ...................................................................................................................... 10

Tahivilla Project.................................................................................................................... 11

United Kingdom........................................................................................................................ 11

HARI Project ........................................................................................................................ 11

PURE Project........................................................................................................................ 12

United States............................................................................................................................. 12

Basin Electric, Wind-to-Hydrogen Energy Pilot Project...................................................... 12

National Renewable Energy Laboratory and Xcel Energy, Wind-to-Hydrogen Project...... 13

Summary....................................................................................................................................... 15

References..................................................................................................................................... 16
1

Heating Value, Heat of Reaction, and Free Energy

One of the issues that arises when discussing the calculation of the electrical efficiency of a fuel 

cell or an electrolysis cell is confusion about the terms heat of combustion (often called the 

heating value), the heat of reaction, the heat of formation, the free energy of reaction, and the 

free energy of formation. Hydrogen and hydrocarbon fuels also pose a source of confusion about

whether the water vapor produced during combustion is condensed back into liquid water or is 

lost as a vapor diluted in the combustion products. Condensing the water vapor produces 

additional heat. Finally, there is confusion about standard conditions.

Total energy is composed of both electrical and thermal energy known as enthalpy (H). The 

amount of electrical energy is known as the Gibbs free energy (G) and corresponds to the maximum amount of useable electrical energy available when hydrogen recombines with oxygen. 

Irreversible energy or entropy (S) is the “cost of doing business” and is dependent on the temperature at which the reaction takes place. The loss due to entropy is similar to how a bouncing ball 

loses energy when hitting the floor, as friction from the action of bouncing causes a transfer of 

thermal energy to atoms in the floor. The energy transferred to those floor atoms dissipates and is 

not recoverable. Consequently, the change (Δ) in these quantities from a standard set of 

conditions follows the form shown below.

∆H = ∆G + T∆S

Heat of Combustion and Heat of Reaction

By definition, the heat of combustion is the amount of heat released when 1 gram molecular 

weight of a substance is burned in oxygen. (Heat of reaction is a more generic term and refers to 

the heat released in any chemical reaction, combustion or otherwise.) Heat of combustion 

measurements usually are made in a calorimeter under controlled conditions in which the water 

vapor, if produced, is condensed and the additional heat of condensation is included. The heat of 

combustion of methane, for example, is 890.3 kilojoule (kJ) per mole when measured at 25°C in 

a calorimeter.

CH4 + 2 O2 → CO2 + 2 H2O(liquid) + 890.3 kJ/mole heat

If the water vapor produced is not condensed, then the heat of combustion of methane is 

802.3 kJ/mole, with the difference being the latent heat of condensation of the water vapor. The 

greater value—based on forming liquid water—is known in the industry as the higher heating 

value (HHV) of methane. The lesser value—based on forming water vapor—is the lower heating 

value (LHV) of methane. Most standard natural gas appliances (e.g., water heaters, gas furnaces, 

gas ranges) operate with an excess of air, and the water vapor does not condense but remains 

dissolved in the exhaust stream. This also is true of internal combustion engines and gas turbines.

In the United States, however, the efficiencies of appliances and heat engines usually are rated 

based on the HHV, whereas in European communities LHV is used. The use of LHV in 

calculating heat engine efficiencies yields greater efficiency numbers than those using HHV.

Standard Conditions

The heat of combustion changes with temperature. For example, the heat of combustion of 

methane (LHV) at 1,000 K (727°C or 1,340°F) is 800.9 kJ/mole, which is slightly less than the 

LHV value of 25°C. It is helpful, therefore, to use a standard set of conditions for calculating 
2

efficiency. The usual practice in chemical thermodynamics is to choose 1 atmosphere (atm) 

pressure and 25°C (298 K), although other conditions are used as standards by different 

industries and in other parts of the world.

Free Energy

There are two definitions of free energy and both are related to work done by the system. The 

Helmholtz free energy is the maximum amount of work that can be obtained from a system 

under perfectly reversible conditions, and is used with thermodynamic calculations of heat 

engines. Gibbs free energy is the net work that can be done by a system. In an electrochemical 

cell working reversibly at constant temperature and pressure, the net work is equal to the 

electrical work. Because we want electrical work, the Gibbs free energy is important to fuel cell 

calculations.

Heat is released in the combustion of hydrogen in oxygen. In the electrochemical reaction 

between hydrogen and oxygen in a fuel cell, electricity and heat are produced. Although the 

chemical equation is written the same way for both reactions, the energy values are not equal.

The heat produced by combustion does not equal the electricity produced in the fuel cell.

H2 + 1⁄2 O2 → H2O(liquid)

The HHV for the combustion of hydrogen is 285.8 kJ/mole, but the Gibbs free energy for the 

reaction—and therefore the maximum electricity produced by a fuel cell—is only 237.2 kJ/mole.

The difference, 48.6 kJ/mole, appears as heat produced in the fuel cell. We can show this by the 

following equation for the fuel cell reaction.

H2 + 1⁄2 O2 → H2O(liquid) + 237.2 kJ/mole electricity + 48.6 kJ/mole heat

In sum, all fuel cells operating on hydrogen and oxygen produce heat in addition to the electricity. The distribution of energy produced between electricity and heat shown above, however, 

is for a perfect fuel cell operating in a thermodynamically reversible manner. Actual, practical 

fuel-cell devices incur losses due to inefficiencies of the electrochemical reactions and due to 

electrical and ionic resistance as the current flows through the fuel cell. These generically are 

classed as internal resistance losses, and manifest as additional heat produced by the fuel cell at 

the cost of the electrical generation. Nevertheless, the sum of the electricity and the heat 

produced by the fuel cell must equal the HHV (or LHV, if the water vapor produced is not 

condensed).

Heat of Formation and Free Energy of Formation

The definition of the heat of formation is the heat released or required when chemicals are

formed from their elements in their standard state. The heat of formation of the elements in their 

standard state is, by definition, zero.

The equation for the formation of methane from its elements is given below.

2 H2 + C → CH4

The heat of formation for this reaction at 25°C is listed in the Joint Army-Navy-Air Force 

(JANAF) Thermodynamic Tables as -74.9 kJ/mole. The minus sign indicates that the methane 
3

contains less energy than the elements do, and that heat would be released by this reaction were 

it possible to cause the reaction under standard conditions. Likewise, the free energy of 

formation of a compound is the work that could be recovered from a reaction in which the 

compound is formed from its elements under standard conditions.

In the unique case of hydrogen reacting with oxygen to form water, the heat of formation of 

water has the same values as the heat of combustion of the hydrogen. This is true only because,

in the combustion reaction, water is formed from its elements. This is a unique situation that is 

shared by only a few other chemical reactions. Although the numerical values are the same, the 

concepts of the heat of formation and the heat of combustion are distinctly different. In the 

discussion that follows, we will use the heats of combustion, HHV and LHV, without further 

reference to heat of formation or the heat or reaction.

Calculating Fuel Cell System Efficiency

The standard method for calculating the efficiency of a fuel cell power plant or other electrical 

generation device is to divide the electricity produced by the HHV of the fuel used. This is a 

reasonable method for calculating power plant efficiency, because the power-plant operator 

purchases fuel (natural gas is sold by heating value) and sells electricity.

HHV of fuel used

Electricity produced Electrical Efficiency =

Therefore, there is a maximum theoretical limit to the electrical efficiency attainable by a fuel 

cell system represented by the Gibbs free energy divided by the heat of combustion of the fuel.

In the case of the hydrogen fuel cell this value is the Gibbs free energy/HHV (237.2 kJ/mole ∕

285.8 kJ/mole = 83%). The use of HHV here is in keeping with the method used in the United 

States to calculate efficiency for internal combustion (IC) engine/generators and gas 

turbine/generator systems.

Some U.S. developers of high-temperature fuel cells, however, prefer the European convention 

and instead use the LHV of hydrogen for efficiency calculation. The JANAF Tables list the 

Gibbs free energy for the formation of water vapor from hydrogen and oxygen as 228.6 kJ/mole,

so the maximum theoretical efficiency of a complete fuel cell system based on LHV of hydrogen 

is 228.6 kJ/mole ∕ 241.8 kJ/mole or 94.5%. Using the LHV convention for calculating the 

efficiency of an electrical generator always yields numbers greater than those yielded by 

calculations using the HHV for the same system. When quoting electrical efficiency of an 

electric generator it is important to indicate whether it is based on the HHV or the LHV 

calculation method.

Practical fuel cells cannot achieve these maximum electrical efficiency numbers because of 

internal resistance losses. For example, a practical fuel cell operating near its maximum power 

output might be able to produce only 154 kJ of electricity per mole of hydrogen consumed, with 

the rest of the heating value appearing as heat produced by the fuel cell. The calculation for such 

a fuel cell is: 154 kJ/mole ∕ 285.8 kJ/mole = 54% efficient (HHV). The remaining 46% of the 

energy produced can be recovered from the fuel cell system as co-generated heat.
4

Voltage Efficiency of Fuel Cells and Stacks

The electrical system efficiency calculations discussed above are applied globally to complete 

fuel cell systems that include many individual components, such as fuel processors, humidifiers, 

fuel cell stacks, power conditioners, and controls. Many experimenters and developers, however,

also wish to assess the efficiency of the fuel cell stack separate from the efficiency of the system.

For this reason it is convenient to use the concept of voltage efficiency, which is defined as the 

actual cell (or cell stack) operating voltage divided by the thermodynamic cell voltage.

( )

( )

Thermodynamic voltage E

Operating voltage V Voltage efficiency =

The thermodynamic cell voltage can be calculated using the Gibbs free energy and the Nernst 

equation.

1/ 2

2 2

2

[ ][ ]

[ ] ln

H O

H O

nF

RT E = E° −

Where:

• E = the thermodynamic voltage under prevailing conditions

• E° = the thermodynamic voltage under standard conditions

• R = the gas constant (8.314 J deg K-1 mole

-1

)

• T = temperature in degrees Kelvin (25°C = 298 K)

• n = the number of electrons transferred (2 in this case)

• F = Faraday constant (96,485 Coulomb mole

-1

)

• [ ] = the thermodynamic activity of the reactants and products, which, for gases, can be 

approximated by their partial pressure in atmospheres.

Gibbs free energy can be converted into a thermodynamic voltage using the formula in which

free energy is in joules per mole, as shown below.

Gibbs free energy (standard conditions) = nFE°

Under standard conditions, the Gibbs free energy for the hydrogen-oxygen reaction is 

237.2 kJ/mole for the production of liquid water at 25°C. Therefore, the thermodynamic voltage 

for a hydrogen-oxygen fuel cell operating at standard temperature and pressure is 1.229 volts.

V J E 1.229

2(96,485)

237,200 ° = =

The Nernst equation is useful for calculating the thermodynamic voltage at varying pressures and 

reactant concentrations, but it should be noted that the Gibbs free energy of the hydrogen/oxygen 

reaction changes with temperature. Therefore, to calculate the thermodynamic voltage of a cell at 

a temperature other than standard temperature requires looking up the Gibbs free energy for the 

reaction at that temperature in a thermodynamic table. For example, the thermodynamic voltage 

for a high-temperature fuel cell operating on hydrogen/oxygen at atmospheric pressure at 1,000 

K is 0.998 volts. Experimenters and developers of higher-temperature fuel cells typically use the 
5

free energy for the formation of water vapor rather than liquid water in their calculations. Both 

are listed in most thermodynamic tables.

A good fuel cell with well-sealed components and a properly functioning electrolyte should 

exhibit a voltage close to the thermodynamic voltage when it is not producing power (no load).

This is also known as the open-circuit voltage. Thus, a fuel cell operating at 25°C with 1 atm of 

hydrogen at the anode and 1 atm of pure oxygen at the cathode should exhibit a voltage of 

1.23 volts with no load. Comparison of the actual open-circuit voltage with the thermodynamic 

voltage can be used to determine the integrity of the cell. Pinholes in the electrolyte that allow 

fuel and oxidant to mix, for example, reduce the open-circuit voltage and indicate a problem.

The approximate efficiency for a fuel cell stack that is producing electrical power can be 

calculated by dividing the operating voltage by the thermodynamic voltage. Thus, a polymer 

electrolyte membrane (PEM) fuel cell operating at 0.800 volts under standard conditions has a 

voltage efficiency of 0.800 V ∕ 1.229 V = 65%.

Other Efficiency Calculations

Another calculation that often is used to describe fuel cell stack performance is fuel cell stack 

efficiency, which is calculated as the direct current (DC) electrical output of the fuel cell stack 

divided by the LHV of the fuel consumed in the stack. This calculation is similar to the global 

system efficiency calculation, except that parasitic electrical losses due to auxiliary systems are 

not included in the calculation and the LHV of the fuel used by the stack does not include 

parasitic fuel use upstream or downstream of the fuel cell. It also is a more difficult calculation to 

perform because accurate measurements of the amount of fuel consumed by the fuel cell stack 

are not easy to obtain.

Water Electrolysis

Water electrolysis is the reverse of the fuel cell reaction. In fact, many fuel cells based on PEM 

and solid-oxide technology can work both as a fuel cell or a water electrolysis cell, depending on 

the direction of the electrical current. The equation for the water electrolysis reaction simply is 

the reverse of the fuel cell equation.

H2O(liquid) + 237.2 kJ/mole electricity + 48.6 kJ/mole heat → H2 + 1⁄2 O2

The efficiency calculations, therefore, can be inverted as well. The efficiency of an electrolysis 

system, for example, can be calculated as the heating value of the hydrogen produced divided by 

the electrical energy input.

Electricity used

HHV of H produced Electrical efficiency HHV

2

( ) =

or

Electricity used

LHV of H produced Electrical efficiency(LHV)

2 =
6

Other efficiency measures often are listed on the product brochures of electrolyzer 

manufacturers, the most frequently used being kilowatt hours (kWh) per normal cubic meter 

(Nm3

) of dry hydrogen produced (kWh/Nm3). The kilowatt-hours per kilogram term also appears 

frequently in the literature. These measures have more meaning to the customers of electrolysis 

units who, after all, want hydrogen gas and not heat.

Voltage Efficiency of Electrolysis Cells and Stacks

A problem exists, however. Using this approach to calculate the maximum thermodynamic efficiency of an electrolysis cell operating reversibly produces nonsense numbers that exceed 100%.

(HHV) 285.8 kJ/mole ∕ 237.2 kJ/mole = 120.5%

(LHV) 241.8 kJ/mole ∕ 228.6 kJ/mole = 105.8%

The problem is that it takes both electricity and heat to split water electrochemically and the heat 

is not being included in the above calculation of the energy input.

Although the thermodynamic voltage for splitting water under standard conditions is the same 

1.229 volts as the fuel-cell reaction, practical electrolysis cells—like fuel cells—do not operate 

near this voltage. Whereas the practical fuel cell operates well below 1.23 volts (in the range of 

0.750 to 0.900 volts), the practical electrolysis cell operates above this voltage in the range of 

1.60 to 2.00 volts. System efficiencies of practical systems calculated using the above approach, 

although inflated, always are less than 100%; therefore, the problem is not obvious. It is when 

we attempt to develop a method for calculating individual cell and multiple cell stack efficiency 

that we see the problem.

Splitting a mole of liquid water to produce a mole of hydrogen at 25°C requires 285.8 kJ of 

energy—237.2 kJ as electricity and 48.6 kJ as heat; there is no way around this fact. In PEM and 

alkaline electrolysis cells the heat requirement is supplied from the extra heat generated, due to 

internal resistance as the electric and ionic currents flow through the cell. This heat requirement 

is directly traceable back to the electricity supplied. In other words, 285.8 kJ—not 237.2 kJ—of 

electricity is the minimum required to split water in these cells. This translates into a cell voltage 

of 1.481 volts, not the 1.229 volts used in calculating the theoretical maximum electrical 

efficiency of a fuel cell.

The electrochemical potential (standard potential) corresponding to the HHV is 1.481 V/cell as 

shown below. This represents the thermoneutral voltage at which hydrogen and oxygen are 

produced with 100% thermal efficiency (i.e., no waste heat produced from the reaction). This is 

determined using Faraday’s Law, and dividing the HHV (285,840 J/mole) by the Faraday 

constant (F = 96,485 coulombs mole-1

) and the number of electrons needed to create a molecule 

of hydrogen (z = 2).

cell

Volts 1.481

mol

C 2 96,485

mol

J 285,840

zF

Δ H

E

o

f O =

∗

= =
7

This voltage, 1.481 volts, is required for splitting liquid water. It is the voltage at which an 

electrolysis cell operating at 25°C can operate without producing excess heat. (Practical cells 

operate above this voltage and produce excess heat.) It also is the voltage that corresponds to the 

HHV of hydrogen and therefore represents a more reasonable value to use when calculating cell 

and stack voltage efficiency. The formula for calculating the voltage efficiency of a cell or cell 

stack thus becomes the following.

Cell operating voltage (V)

Thermal neutral voltage (E) Voltage efficiency =

A similar calculation can be performed for water vapor using the LHV. The thermoneutral

voltage for splitting water vapor at 25°C is 1.253 volts.

Steam Electrolysis and High-Temperature Cells

The above discussion applies primarily to electrolysis cells operating at temperatures that are less 

than the boiling point of water; these include the PEM and alkaline electrolysis cells. There is a 

class of high-temperature steam electrolysis cells under development, however, that operates in 

the 800° to 1,000°C temperature range, where the thermodynamics are significantly different. As 

the temperature climbs, the LHV of hydrogen increases and the Gibbs free energy decreases. At 

1,000°C, for example, the LHV of hydrogen is 249.2 J/mole and the Gibbs free energy for the 

reaction is 179.9 kJ/mole. The water-splitting reaction at 1,000°C can thus be written as follows.

H2O(steam) + 179.9 kJ/mole electricity + 69.3 kJ/mole heat → H2 + 1⁄2 O2

The thermodynamic voltage for this reaction—which corresponds to both the open-circuit 

voltage for the solid oxide fuel cell and the solid oxide electrolyzer cell—is 0.932 volts. The 

thermoneutral voltage for the electrolysis reaction is 1.291 volts. These high-temperature cells 

are considerably more efficient in that they have lesser internal resistance losses and improved 

reaction kinetics as compared to their low-temperature PEM counterparts.

It is well within the realm of possibility that a practical high-temperature electrolysis cell could 

operate below the thermoneutral voltage. In this case, the heat requirement must be made up by 

an external heat source. A high-temperature electrolyzer operating at 1,000°C and 1.200 volts, 

for example, would not generate sufficient heat via internal resistance to keep the electrochemical reaction going. As the cell operated, the electrochemical reaction would withdraw heat from 

the cell components and cool the cell to the point that it ceases operating. Therefore, to maintain 

temperature, sensible heat must be supplied to the cell components from an outside source.

In the high-temperature case, calculating the voltage efficiency of the cell can be straightforward 

and the thermodynamic voltage can be used. In the case of global system efficiency, however, 

both the electrical input and the heat input from the external source must be included, otherwise 

the calculation produces a nonsensical answer and an efficiency that is greater than 100%.

Operating voltage (V)

Thermo dynamic voltage (E) Voltage efficiency(cell) =
8

Electricity used Heat supplied

HHV of H produced Electrical efficiency(system) + = 2

Recommendations

The following quote is drawn from a 2002 publication on hydrogen by the Bellona Foundation, 

of Oslo, Norway. “When calculating the efficiency in a fuel cell, the lower heating value is used.

In the electrolysis process, the high heating value is used.” The full report is available at 

http://bellona.org/filearchive/fil_Hydrogen_6-2002.pdf, and it basically sums up the European 

point of view.

In the United States, however, the situation is more equivocal. Developers of natural gas fuel 

cells do use LHV. Gas turbine manufacturers and manufacturers of IC engine generator sets,

however, use HHV to calculate electrical efficiency. These groups complain that comparisons of 

fuel cells with heat engines are unfair because of the different basis used for the efficiency calculations. In the United States, natural gas is sold by the therm (1 therm = 100,000 Btu), but it is 

measured by the cubic foot. The conversion from cubic feet to therms used by a gas supplier in

billing customers uses either the measured gas composition and the sum of the HHV of each 

fractional component, or the actual calorimeter measurements of the HHV of the gas supplied to 

the customer. In either case, the customer pays based on the HHV of the gas used. It therefore 

makes sense that calculations of system efficiency for all electric generators should use the HHV 

of the fuel.

Electrolyzer manufacturers appear to have standardized on kWh/Nm3 or kWh/kg as a measure of 

system efficiency, which sidesteps the LHV versus HHV controversy. As noted above, 

Europeans prefer HHV for calculating electrolyzer efficiency on the basis of heating value.

As stated, splitting a mole of liquid water to produce a mole of hydrogen at 25°C requires 285.8 

kJ of energy—237.2 kJ as electricity and 48.6 kJ as heat. It then follows that the ratio of 

reversible free energy potential (1.229 V) over the thermoneutral voltage (1.481 V) is 83%. This 

represents the highest efficiency attainable when using the LHV to determine stack voltage 

efficiency. Likewise, the same can be said for electrolyzer system efficiency calculations. 

Therefore, it is worth stating that the highest attainable efficiency is 83% when referencing

electrolyzer system and stack efficiencies to the LHV.

The HHV easily is converted into more common forms of the higher heating value.

kg

MJ 141.8

kg

J 141,799,781

1kg

1,000g

2.0158 g

1mol H

mol

J 285,840 2 ∗ ∗ = =

kg

kWh 39.4

3,600sec

1h

1,000w

1kW

J

1watt sec

kg

MJ 141.8 ∗ ∗ = − ∗

The U.S. Department of Energy (DOE) Fuel Cell Technologies Program Multi-Year Plan [1]

includes targets for distributed water electrolysis and for central wind water electrolysis using 

three measures of efficiency: HHV, LHV, and kWh/kg. It also sets goals [2]. In all other 

hydrogen-production schemes, however—for example in natural gas reforming and biomass 

gasification production scenarios—“energy efficiency is defined as the energy in the hydrogen 
9

produced (on a LHV basis) divided by the sum of the feedstock energy (in LHV) plus all other 

energy used in the process.” [3]

Case Studies of Wind/Hydrogen Projects

Canada

Ramea Island

Ramea Island is a small island located in the Atlantic Ocean off the Southern coast of 

Newfoundland and Labrador in eastern Canada. It was the site of Canada’s first wind-diesel 

demonstration project, in 2004. In an attempt to increase the proportion of the island’s electrical 

generation coming from wind, a project to integrate hydrogen storage to the existing wind-diesel 

system was initiated. The system consists of three new 100-kW turbines in addition to the six 

existing 65-kW wind turbines and a 90 m3

/h alkaline electrolyzer with 2000-m3 hydrogen storage 

(10 bar). The stored hydrogen will be converted back to electricity, as need be, via four 62.5-kW 

hydrogen internal combustion engine generators. All system components are now delivered to 

this remote location with integration efforts underway.

Prince Edward Island

Prince Edward Island is home to Canada’s Wind Energy Institute and boasts one of the strongest 

wind regimes in Canada with 44 MW installed and another 30 MW planned. The Wind Energy 

Institute is located on North Cape area of the island and is well positioned to host a wind-tohydrogen demonstration project. 

The system’s components consist of:

• Wind Turbine: Vergnet 60-kW variable speed, or the local grid

• Electrolyzer: Unipolar Alkaline (66 Nm3/h)

• Storage: 4000 Nm3/h (at 17 bar) with plans to add 90 kg at 450 bar

• 120-kW retrofitted diesel genset

This project will provide valuable data, resulting from the direct connection of the variable speed 

turbine to the electrolyzer. It is also the only ongoing project that currently employs a unipolar 

alkaline electrolyzer and should provide information about the durability of this type of 

electrolyzer with variable input.

Aside from the scientifically interesting data that will be generated, the project has a number of 

applications for the hydrogen that will be produced. Hydrogen is used with a fuel cell as part of a 

back-up/auxiliary power unit and is also being used to fuel two 12-seat hydrogen internal 

combustion engine buses. These buses are an integral part of the public transit system in 

Charlottetown (the provincial capital).

Greece

RES2H2 Project

For the Greek test site of RES2H2, a Casale Chemicals 25-kW electrolysis unit operating at a 

pressure of up to 20 bar is connected to a 500-kW gearless, synchronous, multi-pole Enercon 

E40 wind turbine. The electrolysis unit was developed with special cells to be able to withstand 
10

rapid changes of input power (15% to 100% capacity in 1 sec). The electrolyzer operates in 

various modes (percentage of wind turbine production, “peak-shaving,” etc.), with excess energy 

from the wind turbine being fed to the grid. The electrolytic hydrogen is purified prior to 

entering a buffer tank. Part of the produced hydrogen is stored in novel metal hydride tanks 

having capacities of approximately 40 Nm3 H2. The rest of the produced hydrogen is compressed 

to 220 bar and fed to cylinders at a filling station.

Several conclusions were reached from this undertaking:

• Besides meeting technical and cost targets and addressing safety issues, the design of a 

hydrogen energy system must be done in relation to what is market ready—there is no 

point to optimizing a system that specifies units having capacity that is not available or

that are still at an early development phase.

• The transportation and installation of hardware is something to be considered for such 

installations that in many cases are remote and have poor access. In terms of size and 

weight in combination with the poor access road quality, the capacities of the systems 

involved at the present site were the limit for conventional trucks and lifting equipment.

Spain

RES2H2 Project

A second component of the RES2H2 project is located at the Instituto Tecnológico de Canarias, 

on Gran Canaria Island, Spain. The projects major components are:

• Wind turbine: 500 kW (ENERCON)

• Electrolyzer: 5 Nm3

/h H2 per 20 bar

• Hydrogen compressor: 7.5 kW per 220 bar.

The goal for this site is to optimize the energy produced by a wind turbine by providing 

electricity to the grid, and producing drinking water through a reverse-osmosis plant and 

hydrogen through an electrolyzer (this will be stored in a tank and used in a fuel cell for reelectrification purposes).

ITHER Project

The aim of the ITHER Project is the start-up of an installation that enables testing of hydrogen 

generation by electrolysis, using electricity obtained from renewable sources, with the most 

diverse available technologies. The project tries to cover all of the hydrogen chain (production, 

management, and efficient use), obtaining the primary energy from renewable sources by means 

of processes currently available (photovoltaic and wind).

The project consists of three turbines, each with a different type of technology and on an average 

range of powers (80 kW, 225 kW, and 330 kW). With this infrastructure, it is expected to be 

pioneering not only in Spain, but also at an international level due to the range of powers 

handled.

For hydrogen storage, the project includes three metal hydride tanks with a capacity of 7 Nm3

each used to store 100 kWh of energy. In addition to the metal hydride tanks two other options

exist, one is to store hydrogen gas at 45 bar in a tank and the second is using a trailer of cylinders 
11

at 200 bar, with a total capacity of 400 Nm3. The hydrogen is used in two Ballard 1.2-kW fuel 

cells and a Plug Power 5 kW, which are integrated as a back-up system in the building.

Tahivilla Project

This project involves a wind-hydrogen pilot plant located near Cadiz which is a part of a research 

project led by ENDESA Generation with Green Power Technologies, AICIA, and INERCO as 

partners. After a preliminary study of electricity production by the wind farm where the pilot 

plant is located (by comparing the production and prediction curves of the last 3 years), 

simulations were made to optimize wind energy generation by means of an integrated system of 

hydrogen and electric energy generation. This system, whose main components are an 

electrolyzer, a fuel cell, and a hydrogen tank, allows the generation of hydrogen by using part of 

the energy produced by a variable-speed wind turbine.

The system is located on-site at an 80-MW park in the south of Spain (1,900 equivalent production hours per year, “MADE 800” wind turbines). It is composed of an electrolyzer with a maximum electricity consumption of 41 kWe. After the hydrolysis is complete the resulting oxygen is 

vented and the hydrogen is stored at medium pressure (15 bar) in a storage tank. The system also 

has a compressor, for storage at 200 bar, and a fuel cell capable of generating 12 kWe.

United Kingdom

HARI Project

The Hydrogen and Renewables Integration (HARI) project was established in 2001, on the site 

of an existing renewable energy system at West Beacon Farm, in Leicestershire, England. The 

two main objectives of this project were to demonstrate and gain experience in the integration of 

hydrogen energy storage systems with renewable energy systems, and to develop software 

models which could be used for the design of future systems of this type.

Prior to the installation of the hydrogen energy system, the existing renewable energy systems at 

the site included two 25-kW wind turbines, 13-kW photovoltaics, and two micro-hydroelectric 

turbines with combined output of 3 kW. The addition of a hydrogen energy storage system to the 

existing renewable energy (RE) supply network was seen as a means of balancing the varying

supply with the fluctuating demand, and enabling the evaluation of the feasibility of a standalone 

RE system. Three key components added to the existing network, a 36-kW alkaline electrolyzer 

(with 25-bar output pressure), 2,856 Nm3 of pressurized (137 bar) hydrogen storage, and 2 fuel 

cells (2 kW, 5 kW).

During the operation of this site between 2001 and 2006, several lessons were learned, and

suggested a number of ways that the overall efficiency could be optimized when designing 

similar systems. Importantly, matching the output and input requirements of all components 

ensures the most efficient energy conversion and hydrogen production. Additionally, the power 

conversion electronics were found to be the most significant parasitic losses in the system. Over 

time, the electrolyzer module’s efficiency also declined. The variable input from the wind 

turbine caused the electrolyzer to cycle, which lead to degradation of the stacks. When it was 

first installed the electrolyzer was rated at 36 kW, but over 2 years this had risen to 39 kW for 

the same hydrogen output.
12

PURE Project

The standalone small-size wind hydrogen energy system PURE Project was a joint project of 

Unst (community of the Shetland Islands), siGEN (system integrator), and AccaGen SA for the 

PURE Community of Shetland-Islands, and is supported by the European Union. The project 

aims to demonstrate how wind power and hydrogen technology can be combined to meet the 

energy needs of a remote rural industrial estate. PURE was conceived to test and demonstrate 

safe and effective long-term use and storage of hydrogen produced by renewable energy using 

wind-powered electrolysis of water, and to regenerate the stored energy into electric energy with 

a fuel cell. The key components of the system are:

• Wind turbines: Two 15 kW (Proven Ltd)

• Electrolyzer: 15 kW alkaline operating at 55 bar (AccaGen SA)

• Hydrogen storage: 44 Nm3 in H2 cylinders

• PEM fuel cell: 5 kW (Plug Power).

The electrolyzer section consists of an AccaGen electrolyzer unit assembled with advanced cells 

specifically designed and manufactured by AccaGen SA for wind application, capable of operating up to 55 bar. Apart from high energy efficiency and good dynamic performance in variable

operation, a particularly important requirement for a wind-operated water electrolyzer is the 

possibility of operating the electrolyzer over a wide range with high current yields and sufficient 

gas purities.

United States

Basin Electric, Wind-to-Hydrogen Energy Pilot Project

The goal of this project was to research the application of hydrogen production from wind 

energy, allowing for continued wind energy development in remote wind-rich areas and 

mitigating the necessity for electrical transmission expansion. [4]

Four modes of operation were considered in the feasibility report to evaluate technical and 

economic merits. It should be noted that all the modes studied represent hydrogen production 

efficiencies that are less than those achievable if the system were operated at full production on 

“grid” electricity. The modes of operation studied were:

• Mode 1—Scaled wind

• Mode 2—Scaled wind with off-peak

• Mode 3—Full wind

• Mode 4—Full wind with off-peak.

In summary, the feasibility report, completed on August 11, 2005, found that the proposed 

hydrogen production system would produce between 8,000 kg and 20,000 kg of hydrogen 

annually, depending on the mode of operation. This estimate was based on actual wind energy 

production of one of the North Dakota wind farms of which the Basin Electric Power 

Cooperative (BEPC) is the electrical off-taker. The cost of the hydrogen produced ranged from 

$20 to $10 per kilogram (again depending on the mode of operation).

The hydrogen-production system utilizes a bipolar alkaline electrolyzer nominally capable of 

producing 30 Nm3

/h (2.7 kg/h). The hydrogen is compressed to 6,000 psi and delivered to an onsite three-bank cascading storage assembly with 80 kg of storage capacity. Vehicle fueling is 
13

made possible through a Hydrogenics-provided gas control panel and dispenser, able to fuel 

vehicles to 5,000 psi.

A key component of this project was the development of a dynamic scheduling system to control 

the wind energy’s variable output to the electrolyzer cell stacks. The dynamic scheduling system 

received an output signal from the wind farm, processed this signal based on the operational 

mode, and dispatched the appropriate signal to the electrolyzer cell stacks.

Unfortunately, chronic shutdown issues prevented consistent operation, and therefore did not 

allow accurate economic analysis as originally intended. Much valuable experience was gained 

in the form of “lessons learned,” however, and the project served as an extremely valuable 

platform for educating the public.

National Renewable Energy Laboratory and Xcel Energy,

Wind-to-Hydrogen Project

Xcel Energy and the U.S. Department of Energy’s National Renewable Energy Laboratory 

(NREL) have collaborated to design, install, and operate the Wind-to-Hydrogen (Wind2H2) 

project. As the largest provider of wind-generated electricity in the United States, Xcel Energy is 

working with NREL to establish and understand state-of-the-art renewable electrolysis equipment and the operation of a renewable hydrogen production facility. Hosted at NREL’s National 

Wind Technology Center (NWTC), the Wind2H2 system was approved for initial operation in 

March 2007 and is enjoying success as a demonstration project, producing hydrogen directly 

from renewable energy sources. This unique research-oriented project uses solar and wind 

energy to produce and store hydrogen. The stored hydrogen can be used both as a transportation 

fuel and as an energy-storage medium, effectively allowing renewable energy to be stored and 

then converted back to electricity at a later time.

The Wind2H2 project is helping researchers understand the hurdles and potential areas for 

improvement in emerging renewable electrolysis technologies. By allowing engineers to operate 

and configure an integrated electrolysis facility, this project has enabled the investigation and 

analysis of hydrogen production, compression, storage, and electricity generation. This project is 

generating valuable data that are being used to improve the designs of future renewable 

electrolysis systems. The Wind2H2 project provides important guidance to industry and key 

stakeholders for development of future renewable electrolysis systems. The Wind2H2 project is 

the only renewable hydrogen production facility in the world that can operate multiple 

electrolyzers in any of the following configurations: 

• Grid connected

• Directly connected from the output of a photovoltaic array to the electrolyzer stack

• Real-time electrolyzer stack current control based on a power signal from a wind turbine

• Closely coupled photovoltaic (PV) and wind energy sources to the electrolyzer stack with 

custom designed and built power electronics.

NREL and Xcel Energy have undertaken the Wind2H2 project with several key objectives in 

mind. First and foremost the Wind2H2 project is being used to demonstrate the operation of a 

renewable electrolysis system, allowing researchers to evaluate actual system performance and 

costs and to identify areas for cost and efficiency improvements. Additionally, the project 
14

provides operational experience with a renewable electrolysis hydrogen-production facility, 

enabling project engineers to investigate operational challenges and to explore system-level 

integration issues and opportunities for performance and cost improvements resulting from 

system-level optimization. The project investigates how to maximize the use of renewable 

energy resources in renewable hydrogen-production systems by optimizing energy transfer from 

photovoltaic arrays and wind turbines to the stacks of commercial electrolyzers. Finally, the 

project is designed to explore operational challenges and opportunities related to energy storage 

systems and their potential for addressing electric system integration issues inherent with high 

penetrations of variable renewable energy resources.

To help enable greater penetration of renewable energy sources, hydrogen production from 

renewable electrolysis must be cost-competitive. DOE has a target of reducing the cost of central 

production of hydrogen from wind-based water electrolysis to $3.10/kg by 2012; by 2017 DOE 

seeks to reduce this cost to less than $2/kg. [1] Electrolyzer manufacturers are improving 

performance and reducing the capital cost of electrolyzer systems. At the same time, the 

complete renewable electrolysis system—including the renewable power source, electrolyzer, 

and interfacing power electronics—must be integrated and optimized to improve system 

performance and to reduce costs. The Wind2H2 project presents an excellent research platform 

to investigate these integration and optimization opportunities.

To achieve the objectives of the Wind2H2 project, NREL engineers have been working to 

complete a number of project tasks. These tasks include:

• Designing, building, and testing dedicated wind- and PV-to-electrolyzer stack power 

electronics to more closely integrate the renewable energy resources and electrolyzer 

stacks

• Modeling and simulating renewable electrolysis system performance to enable improved 

hydrogen-production system designs

• Characterizing renewable energy system impacts on commercial electrolyzer technology 

and the ability to accommodate the varying energy input from wind and PV sources

• Sequencing multiple electrolyzer systems to improve overall system efficiency, 

responsiveness, and performance with varying renewable energy sources.

The challenge of renewable electrolysis is designing and implementing systems that can produce 

hydrogen from renewable sources in a cost-effective way using streamlined, robust, and efficient 

processes. When the wind turbine or solar array is co-located with the electrolysis system, more 

direct connection between the source and the electrolyzer stack is possible. This close-coupling 

eliminates the need for long-distance transportation of electricity and reduces the number of 

electrical conversions, resulting in a more efficient, cost-effective system.

Valuable operational experience is shared through testing, daily operations, and troubleshooting 

the Wind2H2 system. Equipment errors are logged to help evaluate the reliability of the system. 

The valuable lessons from this system operational experience will lead to improved design, 

implementation, and operational plans of renewable electrolysis systems. For example, 

integrated renewable electrolysis systems require that system components from different 

manufacturers be configured to function together smoothly. Consequently, programmable logic 
15

control systems must be able to communicate with all major elements of the system 

(e.g., electrolyzers, compressors, power converters, load-transfer switches).

As another primary goal of the project, NREL engineers investigated methods to maximize 

renewable energy use and to optimize energy transfer within the system. Such systemoptimization efforts can significantly reduce the cost of renewable hydrogen. In an analysis of 

the potential improvements to a wind-electrolysis system, project engineers estimated that 

optimized power electronics would result in a cost improvement of 7%—reducing the cost of 

hydrogen produced from wind to $5.83/kg from a baseline of $6.25/kg. For reference, DOE has 

set a target for reducing the cost of central production of hydrogen from wind-based electrolysis 

to $3.10/kg by 2012. To investigate such optimization opportunities, NREL developed multiple 

power-electronics configurations that convert varying electricity from a solar PV array and wind 

turbines into the electricity used by the electrolyzer stacks directly. These power converters have 

the added benefit of executing maximum power point tracking (MPPT) from the wind turbine or 

PV array to enable greater energy transfer to the electrolyzer stack.

The PV array connected to the electrolyzer stack both with and without an intermediary power 

converter was tested and analyzed. The PV array was configured to supply different input 

voltages to a step-down, DC-to-DC (DC/DC) power converter over many days of testing. We 

found that using the power converter increases the energy delivered to the electrolyzer stack by 

10% to 20%, depending on the PV array input voltage to the power converter. The efficiency of 

the power converter decreased as the input voltage from the PV array increased. The greatest 

input voltage from the PV array to the power converter still provided the maximum energy 

capture to the electrolyzer stack. In other words, although this configuration has the greatest 

difference between the PV voltage and electrolyzer stack voltage (ΔV), it provided the most

energy to the stack over a given day.

A 10-kW wind turbine was connected to one of the PEM electrolyzer stacks through an MPPT

alternating current to direct current (AC/DC) power converter. This configuration represents a 

non-grid-tied (i.e., stand-alone) configuration closely coupling a wind turbine to the electrolyzer 

stack. Testing and analysis shows that the AC/DC converter has the ability to both maintain 

optimal operation of the turbine and deliver power to the electrolyzer stack without a battery 

link. It eliminates several power electronics conversions inherent in grid-tied electrolyzer configurations as well as a battery and its associated maintenance. This AC/DC power converter is 

undergoing upgrades expected to further increase the energy capture from the wind turbine.

One of the major tasks going forward is to determine how the findings of the Wind2H2 project 

can improve performance and reduce the cost of renewable electrolysis production systems. The 

results of system-optimization efforts, performance measurements, and evaluation data will be 

used in economic models to better understand how these system improvements can reduce the 

cost of hydrogen produced via renewable electrolysis.

Summary

It is important that parties interested in hydrogen technologies standardize methods of evaluating 

the performance and efficiency of these technologies. A detailed description of the chemical and 

electrical processes for electrolysis and fuel cells is presented. Important terminology, units of 
16

measure, constants, and chemical reactions are discussed. Recommendations for calculating 

electrolyzer and fuel cell performance are suggested. 

As interest grows about using hydrogen for grid energy storage and as a transportation fuel, pilot 

projects and research efforts are underway to experiment with hydrogen production and 

utilization technologies. The focus of many of these projects is coupling wind energy with 

hydrogen production (via water electrolysis) in an effort to use all available wind energy and to 

store that energy to be used during times of high electricity demand. The body of knowledge and 

lessons learned from designing and operating renewable electrolysis plants is growing. As 

interested parties learn more about the best way to utilize hydrogen as an energy carrier, the cost 

of hydrogen produced from renewable sources is decreasing.

References

1. U.S. Department of Energy. “Hydrogen, Fuel Cells & Infrastructure Technologies Program: 

Multi-Year Research, Development and Demonstration Plan.” Office of Energy Efficiency 

and Renewable Energy. www.eere.energy.gov/hydrogenandfuelcells/mypp/. Accessed 

December 27, 2009.

2. U.S. Department of Energy. “Hydrogen, Fuel Cells & Infrastructure Technologies Program: 

Multi-Year Research, Development and Demonstration Plan.” Office of Energy Efficiency 

and Renewable Energy.

www.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/production.pdf, Table 3.1.4, footnote 

f; and Table 3.1.5, footnote e. Accessed December 29, 2009.

3. U.S. Department of Energy. “Hydrogen, Fuel Cells & Infrastructure Technologies Program: 

Multi-Year Research, Development and Demonstration Plan.” Office of Energy Efficiency 

and Renewable Energy. 

www.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/production.pdf, Table 3.1.2, footnote 

f; Table 3.1.8, footnote g. Accessed December 29, 2009.

4. Rebenitsch et al. “Wind-to-Hydrogen Energy Pilot Project: Basin Electric Power 

Cooperative.” www.osti.gov/bridge/servlets/purl/951588-ejIotC/951588.pdf. Accessed 

December 29, 2009.
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This paper presents a detailed description of the chemical and electrical processes for electrolysis and fuel cells, 

discusses important terminology, units of measure, constants, and chemical reactions, and suggests 

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hydrogen; hydrogen production; electrolysis; chemical electrolysis; fuel cells; electrolyzer; fuel cell performance; 18th 

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