ww.sciencedirect.com

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 3

Available online at w

journal homepage: www.elsevier .com/locate/he

Life cycle assessment of alternatives for hydrogen productionfrom renewable and fossil sources

Javier Dufour a,b,*, David P. Serrano a,b, Jose L. Galvez c, Antonio Gonzalez c, Enrique Soria d,Jose L.G. Fierro e

aDepartment of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, c/Tulipan s/n, 28933 Mostoles, Madrid, Spainb Instituto IMDEA Energıa, c/Tulipan s/n, 28933 Mostoles, Madrid, SpaincRenewable Energy Area, National Institute of Aerospace Technology (INTA), Crtra. Ajalvir Km 4, 28850 Torrejon de Ardoz, Madrid, SpaindRenewable Energy Division, CIEMAT, Avenida Complutense 22, 28040 Madrid, Spaine Instituto de Catalisis y Petroleoquımica, CSIC, Cantoblanco, 28049 Madrid, Spain

a r t i c l e i n f o

Article history:

Received 30 May 2011

Received in revised form

21 September 2011

Accepted 22 September 2011

Available online 22 October 2011

Keywords:

Life Cycle Assessment

Hydrogen production

Methane decomposition

Photosplitting

Thermochemical cycles

Conventional production methods

* Corresponding author. IMDEA Energıa, Ed914888138; fax: þ34 914887068.

E-mail address: javier.dufour@urjc.es (J. D0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.09.135

a b s t r a c t

New processes under development for producing hydrogen have been assessed using a life

cycle methodology and compared to conventional ones. The aim of this paper is to

determine the main obstacles to be beaten or the critical aspects to be addressed to ensure

the feasibility of these processes. Water photosplitting, solar two-step thermochemical

cycles and automaintained methane decomposition with different lay-outs were studied.

They have been compared to methane steam reforming with CCS and electrolysis with

different electricity sources.

The results show the good behaviour of the automaintained methane decomposition.

This process is one of the best options when the greenhouse effect emissions are evalu-

ated. Nevertheless, the consumption of a great amount of a non-renewable resource, i.e.,

natural gas, as reagent can be negative. The two-step thermochemical cycles based on

NiFe2O4 is also an interesting option, but its behaviour depends largely on the infrastruc-

ture materials employed on the installations. The most promising option is photosplitting

with CdS as catalysts. This process shows the best performance.

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

reserved.

1. Introduction high heating value. Its storage can solve the problems of the

The global warming, the need for increasing the energy

independence of regions and the search for new sustainable

energy generation scenarios have made researchers to focus

on alternative ways to the conventional fuels and current

energy systems. During the last years, a very important effort

has been made in the development of hydrogen and its

applications as an energy carrier. Hydrogen is a clean fuel with

ificio CAT, Universidad R

ufour).2011, Hydrogen Energy P

intermittent character of renewable sources and the use of

fuel cells with hydrogen produces higher yields than the

combustion of gasoline or diesel in conventional thermal

engines. However, the development of some of these tech-

nologies is still in an early stage as well as the market pene-

tration would be slow due to the lack of infrastructures.

During the recent years, research on hydrogen has been

focused on all the life cycle stages (production, storage,

ey Juan Carlos, c/Tulipan s/n, Mostoles 28933, Spain. Tel.: þ34

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

Nomenclature

AD automaintained decomposition

CCS carbon capture and storage

CED cumulative energy demand

CExD cumulative exergy demand

E electrolysis

IPCC Intergovernmental Panel for Climate Change

ISO International Organization for Standardization

LCA life cycle assessment

PD photodecomposition

PSA pressure swing adsorption

SMR steam methane reforming

TSTC two-step thermochemical cycles

Table 1 e Assessed processes for hydrogen production.

Process Acronym

Automaintained decomposition of AD-I

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 31174

transport and use) and always looking for the most sustain-

able option, i.e. the one that minimizes resources, emissions

and costs [1]. Thus, Life Cycle Assessment (LCA) is a key tool

for these research purposes. An LCA is the systematic study of

all the inputs and outputs of a product system for the evalu-

ation of the environmental impact which is associated to all

the life cycle stages, from raw materials extraction to final

wastes management [2]. This kind of study has been essential

for the development of the ecodesign of products, but its

application can also be crucial to the development of new

sustainable energy sources [3,4].

There is an important number of hydrogen LCAs studied in

the literature: some of them are focused on production [5,6],

storage [7] and use [8], and other studies are aimed at the

evaluation of integrated systems [9,10]. All of these studies are

centred on the environmental benefits of hydrogen from the

global warming potential point of view, evaluating also the life

cycle efficiency and the global impact in some of them. In this

work, a comprehensive assessment of alternatives for

hydrogen production is shown. We have evaluated three

technologies at their early developing stages: methane

decompositionwithcarboncatalysts,waterphotosplittingand

water decomposition with two-step thermochemical cycles.

The focus has beenmade on greenhouse gases emissions, the

consumption of fossil resources, and the energy and exergy

balances. The assessment has been also made over conven-

tional processes as steam reforming of natural gas and water

electrolysis, in order to compare the results of the studied

alternatives. A sensitivity analysis has also been performed in

order to determine the influence of the reaction conversion or

the thermal efficiency of the assessed processes.

methane, heated with natural gas

Automaintained decomposition

of methane, heated with solar

concentrator

AD-II

Water photosplitting, CdZnS as

photocatalyst

PD-I

Water photosplitting, CdSeZnSeZnO

as photocatalyst

PD-II

Two-step thermochemical cycles

with nickel ferrite, NiFe2O4

TSTC-I

Two-step thermochemical cycles

with zinc oxide, ZnO

TSTC-II

Natural gas reforming with CO2

capture and storage system

SMR-CCS

Water electrolysis, electricity

from grid

Grid-E

Water electrolysis, electricity

from wind turbine

Wind-E

Water electrolysis, electricity

from photovoltaic panels

Solar-E

2. Systems definition

The methodology used in this work corresponds to the LCA

standards (ISO 14040:2006 and ISO 14044:2006). For all the

assessed processes, the system boundary has been located at

the end of the production schemes, including the raw mate-

rials acquisition stage, because the aim of the study is to

compare manufacturing technologies, although it is remark-

able that the stages after the production process usually have

a low influence in the impact of the hydrogen life cycle [9,10].

The main purpose is not to establish the accurate impact, but

the potential capacity of each process from the environmental

point of view, with a special regard on the processes under

development and their comparison to the conventional

production schemes. Table 1 shows all the evaluated cases in

this work, which are described in the next sections.

2.1. Automaintained decomposition of methane

The reaction for methane decomposition is CH4$Cþ 2H2. It

is endothermic and needs high temperatures to be carried out.

This reaction is used in the carbon black production at the

process known as Thermal Black, developed in 1970 [11]. The

temperature in the reactor of this process is 1400 �C and the

produced hydrogen is consumed as fuel for heating purposes.

Recent trends show that the operating temperature can be

reduced to 500e600 �C if metallic catalysts are used [12,13] or

to 800e900 �C if carbonaceous catalysts are employed [14e16].

The use of carbon-based materials as catalysts has made

researchers to suggest the use of the carbon being produced

during the reaction as the reaction catalyst after an activation

process. Then, the reaction can be called automaintained

[17e19]. Anyway, it is important to remark that no CO2

emissions are produced during the reaction since C is kept as

solid carbon.

The flowchart of the assessed process, Fig. 1, is based on

the scheme proposed by Muradov et al. [19] and analyzed by

Dufour et al. [17].

As shown in Fig. 1, the natural gas is preheated at the

reaction temperature, 850 �C, and fed to a fluidized bed reactor

where the methane is decomposed in carbon and hydrogen.

Fig. 1 e Flowchart of the automaintained decomposition of

methane.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 3 1175

The produced carbon is assumed to be lifted and separated by

a cyclone series. The hydrogen-rich stream is cooled,

compressed and fed to a PSA (pressure swing adsorption) unit,

where a 99.99% hydrogen stream is obtained. The non-reacted

methane is mixed with the raw natural gas to be fed to the

reactor. A part of the produced carbon is separated to avoid its

accumulation in the system and the other part is regenerated

by means of a surface gasification [19], in which non-reacted

methane and hydrogen from the PSA unit is burnt for heat-

ing purposes.

For the AD-I configuration, it has been assumed that the

heat for the reaction is obtained from the combustion of an

excess of natural gas, while in the AD-II system it is assumed

that the needed heat is obtained from a solar concentration

system. In this case, the design of the fluidized bed is assumed

to include a quartz window for the admission of concentrated

radiation [20,21]. Both processes reach an overall conversion

of 99% because of the recycling of internal streams. A fraction

of 60% of the residual heat of the streams is assumed to be

used in the high-pressure steam production. The main

parameters of the material and energy balances for the AD-I

and AD-II processes are shown in Table 2.

Table 2 e Energy and material balances for the ADprocesses (per Nm3 of hydrogen).

Parameter AD-I AD-II

Conversion in reactor 50% 50%

Reaction temperature 850 �C 850 �CNatural gas consumed

as reactive, kg

0.390 0.390

Natural gas, consumed for

heating purposes, MJ

3.04 0

Direct emissions of

CO2, kg of eq. CO2

0.042 0.042

Heat from solar tower

concentrator, MJ

0 3.04

Part of chemical plant

per Nm3 of produced H2

2.7 � 10�10 2.8 � 10�10

Catalyst consumption, kg 0.001 0.001

Electricity for compression, MJ 0.67 0.67

Produced steam, kg 0.53 0.53

2.2. Water photosplitting

Water photosplitting is the photocatalytic decomposition of

water, in which the needed energy comes from the photons of

visible spectrum which are accumulated in a photosensitive

material that produces electrons and holes capable to reduce

H2O molecules into H2 and oxidize H2O molecules into O2,

respectively (H2Oþ hn/H2þ 1/2O2). In the literature, many

photosensitive materials can be found, from organic dyes to

metallic oxides [22e24]. Titanium oxide is one of the preferred

photosensitive materials, but its yield is quite low in the

visible spectrum, needing UV radiation. Bak et al. [25] estab-

lished that applicable photosensitive materials for hydrogen

production would have high efficiency, durability, low

manufacturing and maintenance costs. Researchers have

found CdS to be one of the best options from the technical and

economical point of view, but it is quite sensitive to photo-

corrosion. CdS is active in the UVevisible spectra, up to about

500 nm wavelenght. This limit varies and depends on the

composition of the photocatalyst. Average yields with visible

light have been taken into account for the calculationmade in

this work. Two alternative CdS formulations have been used

in this assessment:

- PD-I: CdSeCdOeZnO [26]

- PD-II: Cd1�xZnxS (x ¼ 0.2e0.4) [27]

The proposed process for water photosplitting is quite

simple, as shown in Fig. 2.

Sacrificial reagents, Na2S and Na2SO3, are needed to avoid

the catalyst photocorrosion by the formation of O2 and the

degradation of the photocatalytic material. They also avoid

the recombination of the hydrogen produced. These reagents

react with the electrons and avoid the electronehole recom-

bination. The produced hydrogen is separated from the elec-

trolyte and is compressed for its storage. The main

parameters of the material and energy balances of the

proposed processes are shown in Table 3.

The energy efficiency of both processes is assumed to be

the same in order to compare the behaviour of the photo-

catalytic materials in the hydrogen life cycle. The lifespan of

the installation has been assumed to be 5 years (approxi-

mately 10,000working hours). This durability is optimistic, but

it can be assumed as the lifetime objective for this kind of

processes in order to be competitive with other production

techniques.

Fig. 2 e Flowchart of the water photosplitting.

Table 3 e Energy and material balances for the PDprocesses (per Nm3 of H2)

Parameter PD-I PD-II

Energy efficiency 0.5% 0.5%

Photocatalyst Cd1�xZnxS

(x ¼ 0.3)

CdSeCdOeZnO

Photocatalyst consumption, kg 0.0016 0.0023

Na2S consumption, kg 0.015 0.015

Na2SO3 consumption, kg 0.0097 0.0097

Land use, m2a 5930 5930

Deionised water, kg 0.812 0.812

Heat as natural gas,

associated to photocatalyst

manufacturing, MJ

0.742 0.77

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 31176

2.3. Two-step thermochemical cycles

The thermochemical cycles are reaction networks designed as

indirect routes for water splitting. There are several thermo-

chemical cycles, but only a few are capable of producing

hydrogen in a two-step process [28e31]. The assessed process

in this work is shown in Fig. 3, where a nickel ferrite, NiFe2O4

is reduced and reoxidized in a cyclic process.

The reduced phase is NixFe1�xO, which represents the

inclusion of nickel in the FeO structure. For the development

and calculation of the material and energy balances, the

subscript x is assumed to be 1/3. Then, the global reactions

would be:

NiFe2O4/3Ni1=3Fe2=3Oþ 0:5O2 ðreductionÞ (1)

3Ni1=3Fe2=3OþH2O/NiFe2O4 þH2 ðhydrolysisÞ (2)

The global reaction would correspond to the water decom-

position: H2O/H2þ 0.5O2. For the calculations of the mate-

rial and energy balances, the conversion of reactions (1) and

(2) has been defined as the ratio of reduced material to total

ferrite (w/w) for the reduction step and the ratio of the

oxidized material to total ferrite for the oxidation stage.

According to Kodama [28], the global conversion can be

Fig. 3 e Flowchart of the two-step thermoch

assumed as 72% (80% for the reduction reaction and 90% for

the hydrolysis reaction). This reaction is carried out in a solar

reactor, with a quartz window, an alumina coating and a solid

steel body. The lifetime of the ferrite has been assumed to be

10,000working hours. The reaction temperature is assumed to

be 1773 K.

The need for an inert gas has been discussed in the liter-

ature [30,32]. Argon is used as the carrier of the produced

oxygen in the reduction process because it avoids the

recombination of oxygen with the reduced phase and

displaces the equilibrium reaction to the products formation.

Nitrogen cannot be used because of the high rate of NOx

production at high temperatures, so argon becomes the best

option. For the economy of the process, argon should be

recovered in a separation process, an atmospheric-vacuum

PSA process [33].

The hydrolysis reaction is carried out at 1273 K and it is an

exotermic process. The produced hydrogen is quite pure,

although it must be separated from the residual steam. The

hydrolyzed and the unconverted ferrite is recycled to the

reduction stage to close the cycle. The main input of thermal

energy is assumed to be fed by solar concentration to the

reactor. As in the AD case, a fraction of 60% of the residual

heat is used to produce high-pressure steam and for pre-

heating the input streams.

In this study, an alternative thermochemical cycle based

on ZnO has also been proposed. The use of zinc oxide as redox

material (reduced as metallic Zn) has been deeply studied

[34e36]:

ZnO/ Znþ 1/2O2 (reduction) (3)

ZnþH2O/ ZnOþH2 (oxidation by steam) (4)

The global conversion of Zn reaches values of 80% and the

reduction temperature is higher, 2300 K. One of the key

characteristics of this process is that metallic Zn is in the

gaseous phase at the operating temperature, so a fast

emical cycle for water decomposition.

Table 5 e Energy and material balances for the SMR-CCSprocess (per Nm3 of H2).

Parameter SMR-CCS

Overall conversion 85%

Reaction temperature 850 �CNatural gas consumption as reagent, kg 0.26

Natural gas consumption as heat, MJ 1.15

Direct, pre-capture CO2 emissions, kg 0.8

Steam consumption, kg 1.24

Steam production, kg 0.86

Catalysts consumption, kg 4.4 � 10�5

Electricity consumption, MJ 1.24

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 3 1177

quenching process should be included to avoid the quick

oxygenezinc recombination. Argon is also used as the gas-

carrier for this process, as it increases the conversion and

avoids recombination. The main parameters from the energy

and material balances for the ferrite and the zinc oxide cycles

are shown in Table 4.

2.3. Steam methane reforming

The steam reforming of the methane contained in natural gas

is a reference process for the production of hydrogen. In this

work, the data from some previous studies have been used

[37,38]. The process temperature is assumed to be 850 �C and

the global conversion 85%. Residual heat is also used to

produce high-pressure steam. The water gas shift reaction is

carried out in two stages (high and low temperature stages)

and hydrogen is separated using a PSA. This process produces

CO2 from natural gas, so a carbon capture and storage (CCS)

system is assumed. The CCS system consists of the absorption

of carbon dioxide with amines, high-pressure compression

and injection in an exhausted oil field [39,40]. The heat needed

for the process comes from the combustion of an excess of

natural gas. High-pressure steam is also produced from the

residual heat. The results from the energy and material

balances of this process are shown in Table 5.

2.4. Water electrolysis

As in the case of steammethane reforming, water electrolysis

is other conventional process to produce hydrogen. Water is

decomposed when a direct current is fed to an electrolyser

with enough voltage to reduce protons and oxidize hydroxyl

groups. Fig. 4 shows the flowchart for the conventional alka-

line water electrolysis.

Water should be preconditioned by an ion exchange

process. Then, a 25% potassium hydroxide electrolyte is

prepared and fed to the electrolyser at 70 �C. The gases

produced are dried by condensation and compressed to 37 bar,

in the case of hydrogen, and released to the atmosphere in the

case of oxygen. Design parameters have been obtained from

the literature [22,41].

Three electrolysis processes will be assessed, each one

with different electricity origin for the water splitting: Grid-E,

where the electricity is assumed to be obtained from the

average European electricity generation mix (33% coal, 9%

Table 4 e Energy and material balances for the TSTCprocesses (per Nm3 of H2).

Parameter TSTC-I TSTC-II

Redox material NiFe2O4 ZnO

Redox material consumption, kg 6.25 � 10�4 6.5 � 10�4

Conversion (global) 72% 40%

Reduction temperature, K 1773 2300

Oxidation temperature, K 1273 1273

Heat (solar tower concentration), MJ 32.9 33.6

Part of solar reactor 4.9 � 10�6 1.1 � 10�5

Deionised water, kg 0.81 0.81

Argon, kg 0.54 0.54

hydro, 19% natural gas and oil, 34% nuclear, 5% renewable),

Solar-E, where the electricity would be obtained from

a photovoltaic panels installation, and Wind-E, where the

electricity would be obtained from a wind turbine. In Table 6,

the main parameters from the material and energy balances

are summarized, whereas Table 7 shows the main input

parameters of the life cycle of the photovoltaic panels and

wind turbines.

3. Methodology for the impact assessment

The functional unit is defined as 1 Nm3 of hydrogen with

99.99% of purity to be used in a fuel cell system. The inventory

analysis of the indirect stages of the life cycle has been carried

out through the Ecoinvent database and the impact assess-

ment has been estimated with the SimaPro 7.1 software.

Three impact assessment methods were employed: Global

warming potential with the Intergovernmental Panel for

Climate Change method, reviewed in 2007 and with a time

scope of 100 years; Cumulative Energy Demand (CED) and

Cumulative Exergy Demand (CExD).

3.1. IPCC 2007 global warming potential, 100 years

Thismethodmeasures the amount of equivalent CO2 from the

gaseous emissions of a life cycle. This category is one of the

most employed methods for evaluating the climate change

potential of a life cycle. It was modified in 2007 by the IPCC.

The characterization factors are proposed for a list of

substances contributing to climate change where the impact

is measured as the equivalent amount of carbon dioxide that

would produce the same impact as 1 kg of the evaluated

Fig. 4 e Flowchart of the water electrolysis process.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 31178

substance. The stability of the emissions is a key factor to

calculate the global warming potential, so three sets of factors

are proposed for three different time scopes: 25 years, 100

years and 500 years. The medium-term, 100 years, has been

chosen for this assessment.

3.2. Cumulative energy demand (CED)

This method is focused on the use of energy in each stage of

a life cycle. It was developed in the mid-1970s as a conse-

quence of the energy crisis and it was aimed to detect the

stages of a life cycle where main savings could be achieved

[42]. Although it has a strong link with the environmental

impact, it has to be employed as a complement of other

assessment methods [43]. The method is divided in two main

categories, renewable and non-renewable, where the subcat-

egories are divided by the type of resource (fossil, nuclear,

biomass,.). In this work, twomain characteristics of each life

cycle have been determined:

- Fossil resources consumption, which is the direct result of

the subcategory ‘non-renewable, fossil’ from CED. In the

evaluation of the fossil resources consumption, a distinction

has been made between energy purposes and reagent

purposes, as the methane decomposition or the steam

reforming consume CH4 as reagent for H2 production.

- Renewability, defined by Neelis et al. [7], and calculatedwith

Eq. (5), has been used to determine the renewable character

of the produced hydrogen with the evaluated alternatives.

H2 renewability; % ¼ renewable energy inputrenewable energyþ non� renewable energy inputs

(5)

3.3. Cumulative exergy demand (CExD)

Cumulative exergy demand is quite similar to CED, but it

evaluates the flow of available work, i.e. exergy, in a life cycle

[44]. This method is used to calculate the exergy efficiency in

the whole life cycle of hydrogen. The main property of this

method is that the exergy consumed in the generation of

renewable energy is null, i.e., the renewable energy is ‘free’

from the exergetic point of view [45,46]. In this work, the

exergy efficiency has been calculated from the definition given

by Dincer et al. [45]:

Exergy efficiency ¼ Hydrogen exergy ð10:6 MJ=Nm�3ÞLife cycle exergy consumption ðfossil;nuclear and minerals extracionÞ (6)

This parameter was defined by Granovskii et al. [46] for

determining the production of available work, i.e., exergy,

from 1 MJ of non-renewable exergy. Then, the exergy effi-

ciency can be higher than 1, if the life cycle is producing net

exergy, and lower than 1 if there is net consumption of non-

renewable exergy.

3.4. Sensitivity analysis

The influence of the process efficiency on the results of the

LCA has been evaluated for the proposed processes: auto-

maintained decomposition ofmethane, where conversion has

been varied in the AD-II alternative, water photosplitting,

where the energy efficiency has been varied in the PD-I option

and the water decomposition with two-step thermochemical

cycles, where the conversion of the reduction stage has been

evaluated.

4. Results and discussion

4.1. Greenhouse gases emissions

The calculated emissions of each alternative are shown in

Fig. 5. It should be noticed the high amount of CO2 emissions

associated to the production of hydrogen through electrolysis

with grid electricity. There is a large contribution of the

associated emissions from the carbon combustion and other

fossil fuels coming from the electricity generation. One of the

main features of Fig. 5 is how the solar processes (Solar-E and

the two options for thermochemical cycles) have higher

impact on this category.

An important focus must be put on the life cycle stages no

directly related to the production process of hydrogen because

it has a negligible contribution to this category. For the ther-

mochemical cycles, the great amount of steel and construc-

tion materials needed for the installation is the key factor

influencing the environmental behaviour. There is a low

difference between both alternatives, although the NiFe2O4

process gets lower emissions due to less energy requirements

of the thermochemical cycle.

In the case of the photovoltaic-electrolytic hydrogen, the

manufacturing of panels is the responsible of the high thermal

energy consumption. The high emissions associated to

photovoltaic hydrogen can also be observed in the literature:

0.5 kg CO2 per Nm3 of hydrogen calculated by Utgikar et al. [47]

and Koroneos et al. [6], which is close to the value obtained in

this work, 0.59 kg CO2. The crucial parameter of the photovol-

taic panels life cycle is the durability: Granovskii et al. [10]

calculated 0.22 kg CO2 per Nm3 of hydrogen when the lifetime

was 30 years. However, in this work, 30,000 solar hours of

working time have been assumed as average to consider the

timeof a solar cellworking at themaximumefficiency (varying

Fig. 5 e Emissions of greenhouse gases, measured as

equivalent CO2, of the assessed production alternatives.

Table 6 e Energy and material balances for theelectrolysis process (per Nm3 of H2).

Parameter Electrolysis

Reference flow, 60

Yield 80%

Electrical energy consumption, MJ 17.56

Heat, as natural gas, MJ 0.754

Electrodes (stainless steel þ nickel), kg 1.1 � 10�6

Electrolyte consumption, kg 3.9 � 10�7

Diaphragm and other materials, kg 4.4 � 10�4

Deionised water, kg 1.0

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 3 1179

from 1000 to 2500 h per year in Europe). This value absorbs the

variability of isolation, efficiency loss due to aging, etc. There-

fore, the calculated emissions are higher.

The fossil fuel-based processes show a remarkable

performance. Both steam methane reforming and the auto-

maintained decomposition alternatives have emissions quite

similar to those produced by the wind turbine-electrolysis

option. In the SMR process, the achieved reduction of emis-

sions with the CCS system is 70%, from 0.9 kg/Nm3 to 0.3 kg/

Nm3. However, it has a high energy consumption associated to

the CO2 storage system. To avoid this impact, the auto-

maintained decomposition of methane has shown better

performances [17], showing that decarbonisation is an

environmental-friendly technique to avoid CO2 emissions,

keeping C as a solid. When the automaintained decomposi-

tion is heated with solar-concentrated radiation, the emis-

sions are not significantly reduced, due to the balance

between the avoided combustion of natural gas and the

inclusion of solar concentration process, with high embodied

emissions in the construction materials.

The best option is thewater photoelectrolysis (PD-I and PD-

II), with emissions under 0.1 kg CO2 per hydrogen cubicmeter.

The simplicity and the assumed durability of the photo-

catalysts make these processes to have the best results in the

greenhouse gases emissions impact category.

4.2. Fossil resources consumption

The consumption of fossil resources in the whole life cycle has

been evaluated through the CED methodology. This parameter

is related to the renewability and the efficiency of the assessed

hydrogen life cycle. Fig. 6 shows the consumption of fossil

resources foreachassessed lifecycle. It is important tonotehow

theconsumptionof fossil fuels isvery important ingeneral even

for classical renewableoptionsassolarorwind-basedprocesses

Table 7e Life cycle parameters of photovoltaic panels andwind turbine.

Parameter PV panel(3 kW)

Wind turbine(>1 MW)

Lifetime, hours 30,000 160,000

Yield 15% 25%

Type Si, polycrystalline Terrestrial

Embodied Energy 4610 MJ/panel 1.63 � 107 MJ/turbine

Embodied

CO2 emissions

213 kg/panel 8.65 � 105 kg/turbine

As expected, the worst behaviour was obtained for the

electrolysis with grid electricity due to the average European

mix, mainly based on fossil fuels. The three following

processes are those which use natural gas as raw material. In

fact, the main contribution is associated to the consumption

of CH4 as reagent to produce H2. For example, in the case of

AD-II, the use of natural gas as reagent ismore than 80% of the

total fossil resource consumption. This also implies the higher

consumption in AD processes as the reaction involved needs

more methane to produce hydrogen (CH4$Cþ 2H2) than

steam reforming coupled to WGS reaction

(CH4þ 2H2O$ 4H2þCO2).

If the energy use is evaluated, it can be observed that the

energy required from fossil fuels in the AD-I, AD-II and SMR-

CCS options (the grey bar of Fig. 6) are lower than the

assessed solar options. This aspect reveals the high efficiency

of the energy use in AD and SMR processes. In order to reduce

the impact associated to the resources depletion, the use of

biogenic hydrocarbons would be the ideal option, as they

would avoid the consumption of fossil resources as reagent

(the black bars of Fig. 6) as well as the main contribution to

climate change would be reduced due to the biogenic origin of

the CO2 produced in the life cycle of such process.

Fig. 6 e Fossil fuels consumed in each assessed option for

hydrogen production.

Fig. 8 e Life cycle exergy efficiency for each assessed

alternative.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 31180

The solar processes show a relatively high consumption of

fossil sources due to the high embodied fossil energy use in

the installation for hydrogen production. In the case of the

photodecomposition of water and the wind-electrolysis, the

fossil consumptions are the lowest, although they are still

significant and cannot be neglected.

4.3. Renewability

The renewability of the produced hydrogen from each

assessed alternative is shown in Fig. 7.

The concept of renewability was first introduced by Neelis

et al. [7], when the authors compared the renewable character

of hydrogen produced by wind electricity-electrolysis and

natural gas reforming. Their results showed that, in spite of

the low efficiency, the wind electricity-electrolysis option

produces the higher renewability of hydrogen: 98%. This value

is also obtained in this work, as shown in Fig. 7, and it is

improved in the case of the photosplitting options. When

a whole life cycle is evaluated, there is always an input from

fossil resources that make the renewable options, like solar

photovoltaic or wind turbine alternatives, to have not 100% of

renewable character. In this sense, it must be noticed the good

results for the water photodecomposition options, which are

very close to 100%. This is explained by the direct conversion

of a renewable source, solar radiation, into hydrogen, without

intermediate carriers as electricity. If the development of

photocatalysts reaches the characteristics assumed in this

work, i.e. high durability and resistance, it will be the most

efficient and renewable option to produce hydrogen.

On the other hand, the hydrogen produced from non-

renewable sources, as natural gas, has a low renewable

character as expected. It can be improved by the use of

renewable rawmaterials as biogas. The renewability observed

for the grid electrolysis option comes from the assumed mix

for electricity production, although its net energy consump-

tion is very high. The methane decomposition with solar heat

has a low renewability value due to the high consumption of

natural gas as reagent.

4.4. Life cycle exergy efficiency

Fig. 8 shows the results from the analysis of the exergy flows

in the assessed life cycles with the CExD assessment method.

Fig. 7 e Hydrogen renewability for each assessed process.

As expected, the life cycle of hydrogen based on fossil fuels

(SMR-CCS, AD and grid electrolysis) has low exergy efficien-

cies because the input of raw gas, used as reagent, is also

considered as a loss of exergy.

One of the most important aspects of Fig. 8 is the position

held by the solar processes. All of the considered solar-

renewable processes for hydrogen production, ecept those

based on photosplitting (Solar-E and TSTC) have exergy effi-

ciencies lower than 1. The efficiency achieved by the photo-

voltaic electrolysis is the same value as observed in the

literature: 0.69 [45,46]. The low value of the exergy efficiency is

due to the high input of exergy in the life cycle of materials

used for the photovoltaic panels or for the construction of

solar reactors in the two-step thermochemical cycles.

The production of hydrogen through a wind turbine-

electrolysis and water photosplitting have exergy efficiencies

higher than 1. Therefore, there is a net production of available

work from the produced hydrogen, although part of the fossil

exergy input is being consumed in the life cycle. The observed

maximum is 9.6 MJ of exergy from each MJ of fossil exergy

input. There are two main factors affecting this result: the

simplicity of the PD life cycle, with very low fossil resources

consumption, and the durability of the installation involved in

the life cycle.

4.5. Sensitivity analysis

The main objective of this work is to assess the potential

environmental performance of new processes for hydrogen

production. In this way, we have considered some sensitivity

analyses to study the possibilities of improving the proposed

processes. Therefore, the conventional processes (SMR and

electrolysis) have not been included in this section.

For the automaintained methane decomposition, the

influence of the conversion ofmethane in the reactor has been

studied. The AD-II process has been assessed, with solar

concentration as the heat source. The values of conversion

have been varied from 30% to 90%. The overall amount of

methane reacted in the process is assumed to be constant, so

the conversion would mainly affect the internal recycling of

Fig. 10 e Fossil fuel consumption vs. methane conversion

in AD-II process, vs. energy efficiency in PD-I process and

vs. conversion of ferrite in the reduction stage of process

TSTC-I.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 3 1181

streams. Accordingly, the electricity consumption and heat

flows are strongly affected.

For the water photosplitting, the energy efficiency, defined

as the energy contained in the produced hydrogen divided by

the energy received by the solar radiation to produce it, has

been varied. Some authors [22] propose the objective of 10% as

the limit for achieving a cost efficient photosplitting solution.

The values of the energy efficiency are usually low, and they

have been varied from 0.5% to 7.5% in this assessment.

The main objective for the water decomposition by two-

step thermochemical cycles was to evaluate and compare

the redox processes, so the reaction conversion has been

varied from 60% to 95%.

The environmental aspects selected in this sensitivity

analysis are the CO2 emissions, evaluated according to the

IPCC 2007 method, and the consumption of fossil resources,

assessed with the CED method. Both aspects are shown in

Figs. 9 and 10, respectively. In order to simplify the graphs, the

ratio between the results and the obtained data in previous

sections has been drawn.

A general trend is observed in Fig. 9, the increase in the

conversion or the efficiency reduces the CO2 emissions. This is

specially remarked for water photosplitting with values close

to zero-emissions for an efficiency higher than 5%. Regarding

AD process, the emissions are decreased down to 20% when

conversion is 90%. Finally, similar reductions (20%) are ach-

ieved for TSTC-I process. However, wemust bear in mind that

the emissions are higher for this alternative and they are

mostly due to the energy (and the subsequent emissions) used

in the manufacturing of the materials for the necessary

facilities.

According to the results shown in Fig. 10, an important

fossil fuels consumption reduction is reached for the water

photodecomposition, where an 80% of reduction from the

reference case is accomplished when the energy efficiency is

increased from0.5% till 5%. Again the influence of the facilities

materials are observed for the TSTC-I option, when the

Fig. 9 e Greenhouse gases emissions vs. methane

conversion in AD-II process (a), vs. energy efficiency in PD-I

process (b) and vs. conversion of ferrite in the reduction

stage of process TSTC-I.

conversion is increased till 95%, only a 20% of reduction in

fossil fuels is achieved. The automaintained decomposition of

methane is almost not affected because the main use of the

fossil fuels (mainly gas natural) is as raw material for

producing H2 (more than 80%, as shown in Fig. 6). Therefore,

this amount cannot be decreased. Only the volume of the

internal recycling streams and the energy needed for

compression are influenced.

5. Conclusions

Water photosplitting shows a high potential for hydrogen

production from the environmental point of view, although its

efficiency is quite low. It produces almost 100% renewable

hydrogen and shows the highest exergy efficiency and the

lowest greenhouse gases emissions.

On the other hand, methane decomposition is a promising

technology for hydrogen production, as the associated CO2

emissions are low, but it needs high amounts of fossil fuels.

The production of hydrogen through two-step thermochem-

ical cycles shows a good process performance, but several

concerns arise when the life cycle of the materials needed for

the production facilities is considered.

The renewable character of the produced hydrogen has

been determined, being correlated to the energy input to its

life cycle. The solar options usually produce less renewable

hydrogen than the wind turbine-electrolysis of hydrogen,

except for the photosplitting, as above mentioned, which

produces almost 100% renewable hydrogen. In terms of exergy

efficiency, only the wind-electrolysis and photodecomposi-

tion alternatives are able to producemore available work than

the fossil exergy consumed.

Finally, the main aspects to be addressed to improve the

environmental performance of the processes studied are:

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 31182

- Water photoelectrolysis shows good results even for the low

efficiencies studied, but an increment on it would yield an

improvement in its environmental impacts. As well, more

durable and resistant photocatalysts are important aspects.

- The automaintained decomposition of methane is also

a clean process, but the fossil origin of methane makes this

option to be far from sustainability. The change of the origin

of methane and other hydrocarbons to biogenic sources

would improve its environmental feasibility.

- Thermochemical cycles with solar-concentrated heat

shows a good performance for the production stage that

could be improved if the reaction conversion is increased.

The main drawback comes from the materials used in the

production facilities whose manufacturing demands large

amounts of energy, yielding important environmental

impacts. Therefore, the sustainability of the materials

manufacturing is one of the main aspects to be solved.

Acknowledgements

The authors wish to acknowledge the funding received from

‘Comunidad de Madrid’ through PHISICO2 (S-0505/ENE/

000404) and RESTOENE (S2009/ENE-1743) programmes. J.L.G.

acknowledges the funding received for his contract from

European Social Fund and ‘Comunidad de Madrid’.

r e f e r e n c e s

[1] Sorensen B. Hydrogen and fuel cells. London: ElsevierAcademic Press; 2005.

[2] International Organization for Standardization. ISO 14040:2006 and ISO 14044:2006: environmental management e lifecycle assessment e principles and framework.

[3] Udo de Haes HA, Heijungs R. Life-cycle assessment forenergy analysis and management. Appl Energy 2007;84:817e27.

[4] Wagner U, Geiger B, Schaefer H. Energy life cycle analysis ofhydrogen systems. Int J Hydrogen Energy 1998;23(1):1e6.

[5] Solli C, Stromman AH, Hergtwich EG. Fission or fossil: lifecycle assessment of hydrogen production. Proc IEEE 2006;94(10):1785e94.

[6] Koroneos C, Dompros A, Roumbas G, Moussiopoulos N. Lifecycle assessment of hydrogen fuel production processes. IntJ Hydrogen Energy 2004;29:1443e50.

[7] Neelis M-L, van der Kooi HJ, Geerlings JJC. Exergetic life cycleanalysis of hydrogen production and storage systems forautomotive applications. Int J Hydrogen Energy 2004;29:537e45.

[8] Koroneos C, Dompros A, Roumbas G, Moussiopoulos N.Advantages of the use of hydrogen fuel as compared tokerosene. Resour Conserv Recycl 2005;44:99e113.

[9] Ally J, Pryor T. Life-cycle assessment of diesel, natural gasand hydrogen fuel cell bus transportation systems. J PowerSources 2007;170:401e11.

[10] Granovskii M, Dincer I, Rosen MA. Life cycle assessment ofhydrogen fuel cell and gasoline vehicles. Int J HydrogenEnergy 2006;21:337e52.

[11] Horn JH, Morehead WR. Thermal carbon black process.United States Patent US3523010; 1970.

[12] Bonura G, Di Blasi O, Spadaro L, Arena F, Frusteri F. A basicassessment of the reactivity of Ni catalysts in thedecomposition of methane for the production of ‘‘COx-free’’hydrogen for fuel cells application. Catal Today 2006;116:298e303.

[13] Otsuka K, Takenaka S, Ohtsuki H. Production of purehydrogen by cyclic decomposition of methane and oxidativeelimination of carbon nanofibers on supported-Ni-basedcatalysts. Appl Catal A Gen 2004;273:113e24.

[14] Serrano DP, Botas JA, Guil-Lopez R. H2 production frommethane pyrolysis over commercial carbon catalysts: kineticand deactivation study. Int J Hydrogen Energy 2009;34(10):4488e94.

[15] Jung JU, Nam W, Yoon KJ, Han GY. Hydrogen production bycatalytic decomposition of methane over carbon catalysts ina fluidized bed. Korean J Chem Eng 2007;24(4):674e8.

[16] Suelves I, Lazaro MJ, Moliner R, Pinilla JL, Cubero H.Hydrogen production by methane decarbonization:carbonaceous catalysts. Int J Hydrogen Energy 2007;32:3320e6.

[17] Dufour J, Serrano DP, Galvez JL, Moreno J, Garcıa C. Life cycleassessment of processes for hydrogen production:environmental feasibility and reduction of greenhouse gasesemissions. Int J Hydrogen Energy 2009;34:1370e6.

[18] Pinilla JL, Suelves I, Utrilla R, Galvez ME, Lazaro MJ, Moliner R.Hydrogen production by termo-catalytic decomposition ofmethane: regeneration of active carbons using CO2. J PowerSources 2007;169:103e9.

[19] Muradov N, Veziroglu TN. From hydrocarbon to hydrogen-carbon to hydrogen economy. Int J Hydrogen Energy 2005;30:225e37.

[20] Abanades S, Flamant G. Solar hydrogen production from thethermal splitting of methane in a high temperature solarchemical reactor. Sol Energy 2006;80:1321e32.

[21] Kogan M, Kogan A. Production of hydrogen and carbon bysolar thermal methane splitting. I. The unseeded reactor. IntJ Hydrogen Energy 2003;28:1187e98.

[22] Rajeshwar K, McConnell R, Licht S. Chapters 3 and 7. In: Solarhydrogen generation. Solar hydrogen generation. Towarda renewable energy future. New York: Springer Science; 2008.

[23] Aroutiounian VM, Arakelyan VM, Shahnazaryan GE. Metaloxide photoelectrodes for hydrogen using solar radiation-driven water splitting. Sol Energy 2005;78:581e92.

[24] Amouyal E. Photochemical production of hydrogen andoxygen from water: a review and state of the art. Sol EnergyMater Sol Cells 1995;38:249e76.

[25] Bak T, Nowotny J, Rekas M, Sorrell CC. Photo-electrochemicalhydrogengeneration fromwaterusing solar energy.Materials-related aspects. Int J Hydrogen Energy 2002;27:991e1022.

[26] Navarro RM, del Valle F, Fierro JLG. Photocatalytic hydrogenevolution from CdS-ZnOeCdO systems under visible lightirradiation: effect of thermal treatment and presence of Ptand Ru cocatalysts. Int J Hydrogen Energy 2008;33:4265e73.

[27] Del Valle F, Ishikawa A, Domen K, Villoria de la Mano JA,Sanchez-Sanchez MC, Gonzalez ID, et al. Influence of Znconcentration in the activity of Cd1�xZnxS solid solutions forwater splitting under visible light. Catal Today 2009;143:51e6.

[28] Kodama T, Gokon N, Yamamoto R. Thermochemical two-step water splitting by ZrO2-supported NixFe3�xO4 for solarhydrogen production. Sol Energy 2008;82:73e9.

[29] Diver RB, Miller JE, Allendorf MD, Siegel N, Hogan RE. Solarthermochemical water-splitting ferrite-cycle heat engine. In:Morehouse JH, Krarti M, editors. Proceedings of the ASMEInternational Solar Energy Conference, Solar Engineering2006. Nueva York: ASME; 2006.

[30] Kodama T, Gokon N. Thermochemical Cycles for High-Temperature Solar Hydrogen Production. Chem Rev 2007;107:4048e77.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 7 3e1 1 8 3 1183

[31] Abanades S, Charvin P, Flamant G, Neveu P. Screening ofwater-splitting thermochemical cycles potentially attractivefor hydrogen production by concentrated solar energy.Energy 2006;31:2805e22.

[32] Perkins C, Weimer AW. Likely near-term solar-thermal watersplitting technologies. Int J Hydrogen Energy 2004;29:1587e99.

[33] Rege SU, Yang RT. Kinetic separation of oxygen and argonusing molecular sieve carbon. Adsorpt J Int Adsorpt Soc 2000;6:15e22.

[34] Charvin P, Abanades S, Lemort F, Flamant G. Analysis ofsolar chemical processes for hydrogen production fromwater splitting thermochemical cycles. Energy ConversManage 2008;49:1547e56.

[35] Steinfeld A. Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redoxreactions. Int J Hydrogen Energy 2002;27:611e9.

[36] Steinfeld A. Solar thermochemical production of hydrogen e

a review. Sol Energy 2005;78:603e15.[37] Dufour J, Serrano DP, Galvez JL, Moreno J, Gonzalez A.

Hydrogen production from fossil fuels: life cycle assessmentof technologies with low greenhouse gases emissions.Energy Fuels 2011;25:2194e202.

[38] Spath, M.L., Mann, M.K. Life cycle assessment of hydrogenproduction via natural gas steam reforming. NREL technicalreport NREL/TP-570-27637; 2001. Available from: www.osti.gov/bridge.

[39] Odeh NA, Cockerill TT. Life cycle GHG assessment of fossilfuel power plants with carbon capture and storage. EnergyPol 2008;36:367e80.

[40] Damen K, Van Troost M, Faaij A, Turkenburg W. Acomparison of electricity and hydrogen production systemswith CO2 capture and storage. Part A: review and selection ofpromising conversion and capture technologies. Prog EnergyCombust Sci 2006;32:215e46.

[41] Ivy J. Summary of electrolytic hydrogen production. NRELreport MP-560-36734; 2004. Available from: www.nrel.gov.

[42] Bousted I, Hancock GF. Handbook of industrial energyanalysis. Chapter 1. Chichester: Ellis Horwood Ltd.; 1979.

[43] Frischknecht R, Jungbluth N. (Eds). Implementation of lifecycle impact assessment methods. Ecoinvent report (no. 3);2007. Available from: www.ecoinvent.org.

[44] Boesch ME, Hellweg S, Huijbregts MAJ, Frischknet R.Applying cumulative exergy demand (CExD) indicators to theecoinvent database. Int J Life Cycle Assess 2007;12(3):181e90.

[45] Dincer I, Rosen MA. Exergy. Chapter 19. Oxford: Elsevier;2008.

[46] Granovskii M, Dincer I, Rosen MA. Exergetic life cycleassessment of hydrogen production from renewables. JPower Sources 2007;167:461e71.

[47] Utgikar V, Thiesen T. Life cycle assessment of hightemperature electrolysis for hydrogen production via nuclearenergy. Int J Hydrogen Energy 2006;31:939e44.