Life Cycle Assessment of Hydrogen Production via Electrolysis – A Review Ramchandra Bhandari, Clemens A. Trudewind, Petra Zapp 1) 1) Forschungszentrum Jülich, Institute of Energy and Climate Research ‐ Systems Analysis and Technology Evaluation (IEK‐STE), D‐52425 Jülich, Germany Executive Summary There are several hydrogen production methods. They range from the most widely used fos‐
sil fuel based systems such as natural gas steam methane reforming to the least used re‐
newable energy based systems such as wind electrolysis. Currently almost all the industrial hydrogen need worldwide is produced using fossil fuels. Electrolytic hydrogen production based on renewable resources generated electricity and hydrogen’s energetic use could con‐
tribute to the global needs for a sustainable energy supply. However, also these methods are not free from environmental burdens. A life cycle assessment (LCA) helps to identify such impacts considering the entire life cycle of the process chains. This paper reviews twenty‐one studies that address the LCA of hydrogen production meth‐
ods, majority of them addressing also the electrolytic methods. It has been observed that the global warming potential (GWP) is the impact category analyzed by almost all those au‐
thors. The acidification potential (AP) ranks the second. Other categories such as toxicity potential are not often analyzed. The environmental concern of electrolytic hydrogen pro‐
duction process is mainly associated with the electricity supply. GWP contribution of the electrolyzer unit is relatively small (e.g. only about 4% for wind based electrolysis for hydro‐
gen production and storage system). From LCA perspective, it can be concluded that elec‐
trolysis using wind or hydropower generated electricity is one of the best hydrogen produc‐
tion methods over that using conventional grid electricity mix or conventional fossil fuel feedstock methods. Keywords LCA, electrolyzer, global warming potential, environmental impact, green electrolysis Contribution to Journal of Cleaner Production 3
Abbreviations ADP AP ATP CCS CED CExD CMM COD EP FCV GHG GWP HHV HTE HTP ICE ISO LCA LCI LHV LNG MEA mPt Nm3 NREL ODP OECD PEM POCP PSA PV R SMR SOE SPE UCTE Abiotic depletion potential Acidification potential Aquatic toxicity potential Carbon dioxide capture and storage Cumulative energy demand Cumulative exergy demand Coal mine methane mitigation Chemical oxygen demand Eutrophication potential Fuel cell vehicle Greenhouse gas Global warming potential Higher heating value High temperature electrolysis Human toxicity potential Internal combustion engine International organization for standardization Life cycle assessment Life cycle inventory Lower heating value Liquefied natural gas Membrane electrode assembly Milli‐points Normal cubic meter US national renewable energy laboratory Ozone depletion potential Organization for economic co‐operation and development Polymer electrolyte membrane Photochemical ozone creation potential Pressure swing adsorption Photovoltaics Radiation Steam methane reforming Solid state electrolysis Solid polymer electrolyte Union for the co‐ordination of transmission of electricity 4
I
Introduction Global energy consumption is expected to increase continuously in the next decades, driven by rising standards of living and growing population worldwide. The increased need for more energy will require growth in energy generation capacity and more secure and diversified energy sources [USDOE, 2009]. The need for a sustainable energy supply is obvious due to declining fossil energy resources, environmental pollution, climate change, and increasing dependency on fossil fuels exporting countries. Fossil fuel combustion or reformation causes adverse environmental impacts, which motivate researchers to look for environmentally sustainable alternative fuels. Those alternative fuels are required to fulfill criteria such as no or less release of carbon dioxide (CO2), suitability for both mobile and stationary sector and affordable price range [Romagnoli et al., 2011]. Hydrogen is one of such candidates [Cetinkaya et al., 2012]. It has several advantages associated with its use: it can be produced using renewable energy resources, high yields in fuel cells, clean combustion without emis‐
sions of CO2 and oxides of nitrogen and sulfur (NOx, SOx), and feasible indirect storage of the energy from intermittent renewable energy resources [Balat, 2008, Muradov & Veziroglu, 2008]. Like electricity, hydrogen is an energy carrier and not a primary energy source. However, hydrogen is so far mainly used for non‐energetic purpose in different industrial applications (>95% of global hydrogen production), with the largest consumer sector being ammonia production (about 62.4%) and its energetic use is very small [Spath & Mann, 2001]. Some reports state the global hydrogen production amount in an order of about 500 billion Nm3/yr [Saur, 2008]. Although there might be mass production of hydrogen using renewable energy in the long term, the fossil fuels are today the major sources for its production [Dufour et al., 2011]. In 2006, the global sources for hydrogen production were about 48% from natural gas, 30% from oil, 18% from coal and only 4% by electricity via water electrolysis [PE International, 2010]. This share may not drastically change in a near future; though coal, being the most abundant primary energy source in many countries worldwide, might play a bigger role than natural gas. Electrolytic hydrogen production will also be on focus because hydrogen can be produced using the electricity generated from renewable resources in this method. Although hydrogen is generally considered to be a clean fuel during its use phase (direct combustion or use in fuel cells), its production has negative impacts to the environment. Examining the resource consumption, energy requirements, and emissions from a life cycle point of view gives a complete picture of the environmental burdens associated with hydro‐
gen production [Spath & Mann, 2004]. The production of hydrogen can be categorized into three phases: plant (hardware) manufacturing and its installation, plant operation to pro‐
duce hydrogen (energy used to operate the plant as well as feedstock for hydrogen), and the storage and/or delivery of the produced hydrogen (use phase could also be included as the fourth phase). The environmental impacts associated with hydrogen production in almost all methods, i.e. from steam methane reforming to electrolysis, are mainly in the plant opera‐
5
tion phase. In steam methane reforming, it is due to the consumption of natural gas as feed‐
stock, and in electrolysis it is due to the use of electricity to operate the electrolyzer. These impacts can be minimized in electrolysis process when the hydrogen is produced by using electricity from renewable energy sources (a schematic process flow diagram for electrolysis based on renewable energy electricity is shown in Fig. 4). During the operation of such sys‐
tem, there are almost none or less direct emissions. However, manufacturing and installa‐
tion of renewable energy power plants are mainly responsible for environmental impacts. Life cycle assessment (LCA) can be used to understand these impacts in different phases of the process chain. LCA is an established and internationally accepted method that is defined in ISO standards: ISO 14040 [ISO, 2006a] and ISO 14044 [ISO, 2006b]. The life cycle refers to the major activi‐
ties during the product’s lifetime from its manufacturing, use, and maintenance to its final disposal, including the raw material acquisition required manufacturing the product [Curran, 2006]. There are numbers of impact assessment methods in use [Frischknecht & Jungbluth, 2007]. The CML 2001 [Guinee, 2001] and eco‐indicator 95 [Goedkoop, 1997] methods are mostly used for the life cycle impact assessment of the hydrogen production methods re‐
viewed under this paper. European Union has developed and recommended a guideline (FC‐
Hy Guide) on how to carry out the LCA of hydrogen production methods [FC HyGuide, 2011]. This method also complies with the ISO series: 14040 and 14044. Since this guideline is re‐
cently developed, its use in LCA analysis has not been noticed in the published literatures. The aim of this paper is to analyze the environmental LCA of different hydrogen production routes with the help of published literatures in this field. For this purpose, an intensive liter‐
ature search was carried out. LCA related papers and reports for hydrogen production methods were in focus of the review. Altogether twenty one studies (Fehler! Verweisquelle konnte nicht gefunden werden.), consisting of 17 peer‐reviewed papers and 4 reports or pre‐prints, were selected for a thorough review. Regionally, articles for every regions of the world were considered. In terms of publication years, articles and reports published be‐
tween 2000 and 2012 were considered. An extensive review provides the state of the art knowledge on LCA of current hydrogen production routes and the comparison among differ‐
ent methods. Although the paper’s focus is to analyze the LCA of electrolytic hydrogen pro‐
duction, other different hydrogen production methods (e.g. conventional production routes using fossil fuels) are also included in the review for comparison of the data/results. The following section II introduces the different types of electrolytic hydrogen production methods. In section III, the studies under review are classified under two categories: tech‐
nical parameters of the production methods considered by these studies and LCA methods employed in these studies. Detailed review results on LCA of hydrogen production methods have been presented in section IV under environmental performance of electrolysis. Section V concludes the main body of this paper. 6
II
Hydrogen Production via Electrolysis Hydrogen can be produced from a variety of feed‐stocks. These include fossil resources such as natural gas and coal, and renewable resources such as biomass; which can be directly converted to hydrogen via gasification and reformation. Water is used as feedstock in elec‐
trolytic methods, where the electricity needed for electrolysis may be from renewable (e.g. solar, wind, hydropower, etc.) or non‐renewable (fossil fuel or nuclear based) resources. Such diversity of these sources makes hydrogen a promising energy carrier of future [USDOE, 2012]. A variety of processes/technologies can be used for its production, e.g. chemical, biological, electrolytic, photolytic, thermochemical, etc. Each technology is in a different stage of development and commercialization, and each has its benefits and draw‐
backs. Local availability of feedstock, maturity of the technology, market applications and demand, policy issues, and costs influence the choice of the suitable options for hydrogen production [IEA, 2006]. Several technologies are already available in the market for the industrial production of hy‐
drogen. The first commercial technology to produce pure hydrogen, dating back to the late 1920s, was the electrolysis of water. In the 1960s, the industrial production of hydrogen shifted towards fossil based feedstock, which is the main source for hydrogen production until today [IEA, 2006]. Currently, only about 4‐5% of global hydrogen production is based on electrolysis [Padro & Putsche, 1999, Häussinger et al., 2007]. Other studies already give de‐
tailed technical overview on different hydrogen production techniques, e.g. [Holladay et al., 2009]. The various hydrogen production methods by outlining the economics, environmental impact, applications, and hydrogen energy status around the world are examined in [Momirlan & Veziroglu, 2002]. State of the art of different electrolytic hydrogen production methods is reviewed thoroughly in [Ursua et al., 2012], and energy saving possibilities in electrolytic methods are discussed in [Stojic et al., 2003]. Nevertheless the electrolytic hy‐
drogen production method is briefly introduced in the following paragraphs, as it is the focus technology for LCA in this paper. Water electrolysis is a process whereby water is split into hydrogen and oxygen through the application of electrical energy (eq. 1). The electrolysis cell is the basic element of the elec‐
trolytic hydrogen production system (Fig. 1). The cells are connected in parallel or in series to form the electrolysis module [Ursua et al., 2012]. directcurrent
→
(eq. 1) 7
Fig. 1: Typical alkaline electrolysis cell Source: [Koroneos et al., 2004] IEK‐STE 2013 The hydrogen and oxygen generated from the electrolysis are cooled, purified, compressed and stored. In many installations, the oxygen is not stored but vented to the atmosphere instead. There are electrolyzers that produce hydrogen also at high pressure, thus avoiding the compression stage and associated economic and energy costs. Water coming into the unit is previously treated to fulfill purity requirements to avoid mineral deposition in the cells and non‐desired electrochemical reactions. In general, electrolyzers do not require con‐
tinuous maintenance since they hardly include mobile elements. They are silent and have a high degree of modularity, thus they are suitable for decentralized applications in residen‐
tial, commercial and industrial areas [Ursua, 2010]. Although electrolyzers have been used for decades, there is still a need for improvements in many aspects ‐ reduction of manufac‐
turing, distribution, and installation costs; efficiency improvement: electrolysis module, power supply, control system, etc.; operation under variable electricity supply profiles; in‐
crease of the operating temperature and pressure; etc. [Ursua, 2010]. Currently there are three broad types of electrolyzers available: alkaline, polymer electrolyte membrane and high temperature solid oxide electrolyzers. They are briefly described in following sub‐
sections. II.1
Alkaline electrolyzers The alkaline water electrolysis cell consists of two electrodes separated by a gas‐tight dia‐
phragm (Fig. 1). If a direct current is connected to the electrodes, hydrogen is produced at the cathode and oxygen at the anode. This assembly is immersed in a liquid electrolyte that is usually a highly concentrated aqueous solution of KOH (25–30 wt.%) to maximize its ionic conductivity. Other possible electrolytes solutions of NaOH or NaCl are less commonly used. The main drawback of the alkaline electrolyte is its corrosive character. Hydrogen gas evolves from cathode, where water is reduced yielding hydroxide anions that circulate across the diaphragm to anode within the electric field established by external power 8
source. The hydroxide anions recombine on anode surface to produce oxygen. The following reactions take place in an alkaline electrolysis cell: →
:4
:4
:2
→
4
2
4
→2
2
(eq. 2) (eq. 3) (eq. 4) The diaphragm separates the reaction zones from each other and prevents mixing of the product gases. Hence, the diaphragm is permeable to water ions but impermeable to gas [Nitsch, 2003]. In conventional electrolyzers asbestos (with thickness in the range of 3 mm [Neumann, 2007]) is used as diaphragm material [Häussinger et al., 2007]. Due to the mate‐
rial properties of asbestos the operating temperature of conventional alkaline electrolyzers is generally limited to 80°C [Angloher & Dreier, 1999]. The health hazards of asbestos are forcing manufacturers to replace it with other non‐hazardous material. Adjacent to both sides of the diaphragm electrodes are placed. The anode is usually made of nickel or nickel coated steel, while the cathode is made of steel activated by a coating with different cata‐
lysts. The distance between the anode and cathode is about 5 mm in conventional electro‐
lyzers [Neumann, 2007]. Hydrogen and oxygen can be generated at atmospheric as well as at elevated pressures. At‐
mospheric electrolyzers operate at the pressure of up to 6 bar, while high pressure electro‐
lyzers generate hydrogen at pressures from 6 to 30 bar. However, the higher pressures of up to 200 bar are also demonstrated in laboratory [Emonts, 2002]. An important advantage of high pressure is the reduction in energy expenditures for compressing hydrogen to storage pressure levels. Its disadvantage is a reduced purity of the product gases caused by the fact that high pressures and high temperatures increase the permeability for gases through the membrane. The energy demand of alkaline electrolyzers is dependent on the characteristics of the electrodes and operational conditions. Atmospheric electrolyzers have specific energy demand of about 4.1 to 4.5 kWh/Nm³H2 and pressurized electrolyzers have this demand of about 4.5 to 5 kWh/Nm³H2. However, the atmospheric electrolyzers need energy for com‐
pression of the produced hydrogen making the total body of plant energy demand of about 4.5 to 7 kWh/Nm³H2 [Smolinka et al., 2011]. The atmospheric electrolyzers have efficiencies of up to 85% (HHV) and high pressure ones have up to 78% (HHV) [Smolinka et al., 2011]. Life time of alkaline electrolyzers is reported to be up to 30 years, though every 7 to 15 years a general overhaul is necessary to replace/reactivate the electrodes and to replace the dia‐
phragms [Pehnt, 2001, Wenske, 2008, Smolinka et al., 2011]. Alkaline water electrolysis is a mature technology. Those electrolyzers are reliable and safe [Smolinka et al., 2011]. As a result, they constitute the most extended electrolysis technolo‐
gy at a commercial level worldwide [Ivy, 2004]. The investment costs are in the range of 1000‐5000 $/kW depending on the production capacity [Rajeshwar et al., 2008]. There are many manufacturers worldwide offering these systems. In the last years, the efficiency of the electrolyzers has been improved with the aim of reducing operating costs associated to the consumption of electricity. And, the operating current densities have been increased in 9
order to reduce the investment costs. The investment costs are almost proportional to the electrolysis cells surface area. Among others, these improvements include development of new advanced materials to be used as diaphragms replacing the previous ones made of as‐
bestos. In this regard, the use of ion exchange inorganic membrane has become an alterna‐
tive [Vandenborre et al., 1980, Vermeiren et al., 1998]. Also, the advanced alkaline water electrolyzers with working temperatures up to 150oC are developed. Advanced alkaline elec‐
trolyzers are suitable for large scale hydrogen production. The purity levels of hydrogen and oxygen can reach 99.9 and 99.7 vol.%, respectively, without auxiliary purification equipment [Ivy, 2004]. The purity could be improved by catalytic conversion and adsorptive drying units. II.2
Polymer electrolyte membrane electrolyzers This technology is also referred to as proton exchange membrane (PEM) or solid polymer electrolyte (SPE) electrolyzers. In contrast to alkaline electrolyzers PEM electrolyzers do not require a liquid electrolyte. The electrolyte is a gas tight thin polymer membrane. The most commonly used membrane is Nafion with a thickness of less than 0.2 mm [Ursúa et al., 2012]. As this is the critical component within PEM electrolyzers, lifetime of PEM is limited in comparison to alkaline electrolyzers [Smolinka et al., 2011]. Anode, cathode, and membrane set a membrane electrode assembly constitute. The electrodes typically consist of noble metals, e.g. platinum or iridium. The following reactions take place in a PEM cell: →
:
:2
2
2
→
2
(eq. 5) (eq. 6) At the anode, water is oxidized to produce oxygen, electrons, and protons that circulate across the membrane to the cathode where they are reduced closing the circuit and produc‐
ing hydrogen (eqs. 5‐6). PEM electrolyzers are commercially available for low scale produc‐
tion applications. The hydrogen purity is typically above 99.99 vol.% (in some cases up to 99.999 vol.%) without the need of auxiliary purification equipment [Ursua et al., 2012]. Moreover, low gaseous permeability of the polymeric membranes lowers the risk of flam‐
mable mixtures formation. PEM electrolyzers operate at temperature of around 80°C and pressure of up to 15 bar. Typical PEM electrolyzers have production capacities of 0.06 to 30 Nm³H2/hr. Specific energy demand is typically in the range of 6 to 8 kWh/Nm³H2 but it could also be less than 6 kWh/Nm³H2 in large‐scale systems having production rate higher than 10 Nm³H2/hr. PEM electrolyzers’ efficiencies are in the range of 67‐82% [Smolinka et al., 2011, Ursua et al., 2012]. PEM electrolyzers have ability to work under variable power supply. This is due to the fact that the proton transport across the polymeric membrane responds quickly to power fluctu‐
ations. This is in contrast with alkaline electrolyzers, where the ionic transport in liquid elec‐
trolytes shows a greater inertia [Rajeshwar et al., 2008]. Although commercially available, PEM electrolyzers have some drawbacks. The main problem is their high investment costs, associated to the membranes and the noble metal based electrodes. The production capaci‐
ty needs to be increased for their wider commercialization [Grigoriev et al., 2006]. 10
II.3
Solid oxide electrolyzers Solid oxide electrolyzers (SOEs) constitute an advanced concept enabling water, or rather, steam electrolysis at high temperatures up to 1000oC, which results in higher efficiencies compared to alkaline or PEM electrolyzers [Brisse et al., 2008]. Both steam and recycled hy‐
drogen are fed to the cathode, where water is reduced to produce hydrogen. The oxide ani‐
ons generated in cathode pass through the solid electrolyte to anode, where they recombine forming oxygen and closing the circuit with the released electrons. The cathode is a cermet made of nickel and yttrium stabilized zirconia (YSZ) [Ursua et al., 2012], solid electrolyte is made of YSZ and the anode is made of perovskite [Bello & Junker, 2006, Ursua et al., 2012]. Steam electrolysis emerged with the aim of reducing the energy demand and thus the oper‐
ating costs of conventional water electrolysis [Salzano et al., 1985]. Theoretically up to 40% of the energy required to produce hydrogen from steam electrolysis can be supplied as heat at 1000oC [Brisse et al., 2008] and electricity demand could be reduced by up to 25% [Pham et al., 2000, Brisse et al., 2008]. Therefore, it is expected that those electrolyzers can reach higher efficiencies than alkaline and PEM electrolyzers [Bello & Junker, 2006]. This feature makes SOEs attractive for hydrogen production when a high temperature heat source is available; e.g. nuclear reactors, geothermal energy, solar thermal energy, etc. Currently, SOEs are at the research and development stage and they are operated only in laboratory with hydrogen production rates of up to 5.7 Nm³/hr and power rating of up to 18 kW [Bello & Junker, 2006, Smolinka et al., 2011]. The disadvantages associated with high temperature electrolyzers are due to material problems. Load changes result heat losses and changes in cell temperature that cause micro cracks to the membrane. These cracks reduce electrolyz‐
er’s lifetime significantly. Hence, high temperature electrolyzers are not suitable to be cou‐
pled with sole intermittent renewable energy based electricity [Smolinka et al., 2011]. The typical specifications of alkaline, polymer and solid oxide electrolyzers have been sum‐
marized in Table 1 below. 11
Table 1: Typical specification of alkaline, PEM and high temperature (solid‐oxide) electro‐
lyzers Specification Technology maturity Unit Alkaline a PEM b SOEs c State of the Demonstra‐
art tion R & D Cell temperature o
60‐80 50‐80 900‐1000 Cell pressure bar < 30 < 30 < 30 Current density A/cm2 0.2‐0.4 0.6‐2.0 0.3‐1.0 Cell voltage V 1.8‐2.4 1.8‐2.2 0.95‐1.3 Power density W/cm2 Up to 1.0 Up to 4.4 ‐ Voltage efficiency % 62‐82 67‐82 81‐86 Specific energy consumption, system kWh/Nm3 4.5‐7.0 4.5‐7.5 2.5‐3.5 Partial load range % 20‐40 0‐10 ‐ Cell area m2 < 4 < 300 ‐ Hydrogen production, system Nm3/hr < 760 < 30 ‐ Life time, stack hr < 90000 < 20000 < 40000 System life time yr 20‐30 10‐20 ‐ Purity of hydrogen produced % > 99.8 99.999 ‐ Cold start up time min. 15 < 15 > 60 C Source: most of the data compiled and modified from a,b [Smolinka et al., 2011] and c [NEEDS, 2008] IEK‐STE 2013 III Classification of the Studies The following sub‐sections present the classification of reviewed literatures on LCA of water electrolysis for hydrogen production. Although the study’s focus lies on electrolytic methods, few studies with other types of hydrogen production processes, such as fossil fuel reforming and thermochemical water splitting, are also analyzed for comparison purpose. Because of differences in system boundary assumptions, system sizes, methods used for environmental impact assessment, functional units and other several such parameters, it is difficult to make a direct comparison of results from one LCA study to the other one. Similar problems were also mentioned in a LCA review report by Geerken et al. [Geerken et al., 2004]. However, an aggregated comparison has been made in this paper. Details on the studies that were as‐
sessed as relevant to fulfill this study’s aim on LCA review of electrolytic hydrogen produc‐
tion methods are listed in Fehler! Verweisquelle konnte nicht gefunden werden.. 12
Altogether twenty one studies were reviewed thoroughly. They include four re‐
ports/preprints and 17 peer reviewed articles. Most of the studies were published in recent years (Fig. 2). The trend shows that there are recently more publications in this field, mean‐
ing that interest and importance of LCA in hydrogen production process has been growing. Fig. 2: Number of studies and publication years Source: data from [Bhandari et al., 2012] IEK‐STE 2013 Regionally, most of the studies are published either in Europe or in North America as shown in Fig. 3. Many publications, mainly dealing with the hydrogen production methods using nuclear heated thermochemical cycles, are authored or coauthored in the same institute in Canada (University of Ontario Institute of Technology; six studies [Cetinkaya et al., 2012, Granovskii et al., 2006, Hacatoglu et al., 2012, Kalinci et al., 2012, Ozbilen et al., 2011, Ozbilen et al., 2012]. Two papers were generic (not country or region specific) and they were using mostly secondary data from different regions of the globe. Fig. 3: Regional distribution of the studies Source: data from [Bhandari et al., 2012] IEK‐STE 2013 13
Indeed it is difficult to distribute the studies spatially, as the whole process chain for hydro‐
gen production is complex; and in cradle to grave analysis, many processes undertake in dif‐
ferent countries. However, it is important to know the grid electricity mix in grid based elec‐
trolytic process and those cases have been paid special attention in this paper. Different studies have different goals and thereby the focus is sometimes on different hy‐
drogen production technologies. Most authors analyze environmental LCA of one or two hydrogen production technologies in detail and compare the results with the others using secondary literatures data. All the studies focus their analysis in greenhouse gas emissions and most of them extend their emissions analysis up to GWP calculations. Few authors ana‐
lyze the cumulative fossil energy demand for hydrogen production. The majority of the stud‐
ies have their focus on the hydrogen production and storage phases without considering its use phase. Only three papers look after the use of hydrogen to drive vehicles and compare its GWP results with the use of gasoline. Also, only a few studies (e.g.[NEEDS, 2008, Spath & Mann, 2004]) present the resources used in electrolytic hydrogen production process and thereby resulted emissions in detail. Others do not detail the LCI data. Even if many studies somehow discuss and compare the LCA of electrolytic methods, the majority of these stud‐
ies’ focus is not the analysis of electrolytic methods; rather it is of other fossil fuel based methods or of nuclear based thermochemical cycles. None of the study focuses in comparing one electrolyzer with the other from LCA viewpoint. However, one electricity source used for electrolysis has been compared to the other as well as one hydrogen production method to the other. In the following sub‐sections, the reviewed studies are analyzed from two perspectives: technical aspects and LCA methodological choices. III.1 Technical aspects Alkaline electrolyzer has been used in most of the studies that discuss electrolysis. Among the electrolytic methods analyzed, only one study [Utgikar & Thiesen, 2006] specifically mention the use of high temperature SOE, whereas none of them analyze the LCA of PEM electrolyzers. Different hydrogen production capacities have been presented for different methods. Electrolytic methods have generally smaller capacities (e.g. about 30 Nm3/hr for wind based; 160 kg/day for PV based electrolyzers presented in the reviewed studies). The theoretical capacities of nuclear based high temperature electrolyzers have been presented quite high, i.e. about 7.7 ton/hr in [Utgikar & Thiesen, 2006], though such plants are not op‐
erational and commercially available yet. Fossil fuel based hydrogen production methods have fairly bigger capacities, e.g. 1.5 Million Nm3/day for a SMR unit used in [Spath & Mann, 2001]. Also coal gasification and theoretical nuclear thermochemical cycle based plants have large production capacities, about 284 ton/day and 125 ton/day, respectively. Smaller capac‐
ities of commercially available alkaline electrolyzers have been one of the limitations for widespread use of electrolysis for hydrogen production, along with economic factors. Alt‐
hough not analyzed in the reviewed papers, there are alkaline electrolyzers commercially available in Europe with relatively higher capacities (e.g. 760 Nm3/hr from IHT in Switzer‐
land). As the electrolyzer units can be built modular, the hydrogen production capacity could 14
be increased in line with increased power supply capacity from the renewable energy power plant. A high energy efficiency value of up to 85% [Spath & Mann, 2004] has been used in the liter‐
atures for alkaline electrolyzers, though these values differ from study to study (e.g. 80% in [Dufour et al., 2012], 77% in [Koroneos et al., 2004]). For comparison, the energy efficiency of SMR process is higher, e.g. 89% [Spath & Mann, 2001], whereas that of nuclear based thermochemical water splitting process is lower, e.g. 52% [Solli et al., 2006]. Heat loss in high temperature processes, such nuclear based ones, is relatively higher. Efficiency ranges of different electrolyzer types have been already discussed in section 2. Different types of electricity sources have been used for electrolysis. Most common among them is wind energy, followed by solar PV and grid supply. Solar thermal, biomass‐
gasification‐electrolysis, nuclear based high temperature electrolysis has also been men‐
tioned by each one author. Almost all the authors discussing electrolytic methods compare their results with that of other non‐electrolytic methods – using either primary data or sec‐
ondary data from the other publications. Steam methane reforming of natural gas is the most widely studied method. An overview on the hydrogen production technology ranges considered in this paper is given in Fehler! Verweisquelle konnte nicht gefunden werden.. III.2 LCA methodological choices As mentioned in section I, LCA has been widely used for the environmental impact analysis of hydrogen production technologies. A generic methodology on how to carry out an LCA has been detailed in ISO standards [ISO, 2006a, ISO, 2006b]. Those standards are the most widely used standards so far for performing the LCA of hydrogen production methods. Un‐
der the framework of an EU project, a specific methodology guideline on how to carry out LCA of fuel cell and hydrogen production and use has been developed [FC HyGuide, 2011]. Since this is a new document, none of the reviewed studies has adapted this guideline while carrying out their LCA. Since the different authors have different focus technologies and different goals of their studies, the corresponding system boundary also varies highly. As a general trend, it can be said that the most often used system boundary is cradle to gate among the reviewed papers. In the electrolytic methods, energy and material resources used in manufacturing, installa‐
tion, and operation of renewable energy power plants (e.g. wind, solar PV, solar thermal, etc.) has been considered within the system boundary. Unlike in conventional fossil fuel based methods, majority of the environmental impacts occur during this phase in electrolytic methods. Hydrogen production plant manufacturing and installation have been included in all the studies. The difference is only on the fact that some studies explicitly detail the mate‐
rial and resources consumption during this phase and others use only aggregated data or the secondary data. Plant operation (i.e. production of hydrogen) has been included within the system boundary in every study, whereas end of life of the energy production plant and electrolyzer unit has not been specified in most cases. The studies that consider the use of grid electricity for electrolysis take the aggregated environmental impact values for the grid 15
electricity mix, without giving detail upstream data regarding the power plants and beyond. A typical system boundary for the electrolytic hydrogen production system using power sup‐
ply from renewable energy has been shown in Fig. 4. Outer dashed line represents the sys‐
tem boundary for such plant, a typical boundary used by most of the reviewed papers. Use phase of hydrogen, outside of the boundary in Fig. 4, has been included within the boundary only in few cases. Fig. 4: LCA relevant stages for renewable energy based alkaline electrolyzer Source: [Bhandari et al., 2012] IEK‐STE 2013 Generally, studies that use fossil fuels as feedstock to produce hydrogen with conventional methods have included the extraction of these fossil fuels (natural gas, coal, gasoline, etc.) within their system boundary. Few studies [Dufour et al., 2009, Solli et al., 2006] even con‐
sider the environmental impacts mitigation methods while extracting these resources, e.g. carbon dioxide capture and storage (CCS) during natural gas extraction. Similar CCS ap‐
proaches and coal mining methane mitigation (CMM) measures have also been employed in one study [Solli et al., 2006]. However, the consideration of CCS or CMM measures is not a general trend observed so far. In plant operation phase of the conventional fossil fuel based methods, focus has been made in feedstock that indeed contributes the most greenhouse gas emissions rather than other energy and water resources needed for hydrogen produc‐
tion. In nuclear based methods, mining of uranium and nuclear fuel production has been included within the system boundary. Studies that use biomass either as feedstock or as power supply source for hydrogen pro‐
duction have considered the biomass production and collection phase within their system boundary. Also here, the specifically detailed information such as land preparation, fertilizers use, water used for irrigation, other machinery and processes used for biomass harvesting, etc. could not be found for each biomass resources. 16
Majority of the studies do not consider the use phase of hydrogen, thus their system bound‐
ary is limited to the hydrogen production at the production plant. The physical properties of produced hydrogen differ within the studies; e.g. gas pressure, storage in gas or liquid phase, etc. Only three studies [Granovskii et al., 2006, Hacatoglu et al., 2012, Lee et al., 2010] con‐
sider the use of hydrogen in fuel cell vehicles and compare the hydrogen fuelled fuel cell vehicles with gasoline fuelled internal combustion engine vehicles. While doing so, the stud‐
ies do not include the vehicle manufacturing phase within their system boundary, but the fuel cell life cycle has been included. Studies that deal with electrolytic methods use the state of the art technologies and they do not analyze the potential advanced technology scenarios for future. Same is true for the conventional fossil fuel based methods as well. However, some authors using grid electricity for electrolysis have sometimes considered different future scenarios of electricity mix (share of fossil fuel and renewables as electricity generation resources). Studies that deal with nuclear based thermochemical cycles discuss the technology that is not yet commer‐
cially available in the market, rather only the theoretical and design data for such hydrogen production plants have been used. This could be thus considered as future technology as well. Regarding the environmental emissions, most of the studies convert their emissions results into the impact categories, whereas one study [NEEDS, 2008] present the results as only in‐
dividual air emissions without calculating the impact categories. Detailed LCI data are availa‐
ble mainly in the reports [NEEDS, 2008, NETL, 2006, Spath & Mann, 2001, Spath & Mann, 2004], whereas the papers use mostly aggregated LCI data and summarized impact category results. Both CML and eco indicator 95 methods are the mostly used methods for impact assessment. GWP is the most widely analyzed impact category. It is followed by AP. Few au‐
thors have analyzed also the other impact categories e.g. eutrophication potential (EP), cu‐
mulative energy demand (CED), radiation (R), ozone depletion potential (ODP) and solid par‐
ticulate matters (SPM) (for details, refer to Fehler! Verweisquelle konnte nicht gefunden werden.). Five studies also present the single score points derived via eco indicator meth‐
ods. Use of the primary data has been very limited, especially for renewable energy based elec‐
trolytic methods. A detailed report on wind based electrolysis prepared at NREL [Spath & Mann, 2004] is the major data source for every papers discussing LCA of this method and it is cited by all of them. None of the studies present detailed LCI data on the electrolyzer unit, thus it has not been the focus of any studies reviewed here. This is most likely due to the fact that the impacts associated with the electricity supply dominate the LCA results and not the electrolyzer unit itself. Another report, also prepared at NREL [Spath & Mann, 2001], on LCA of SMR method is the major data source for papers discussing the LCA of this method. In general, secondary data source dominates the LCI database and thereby the results of the LCA. Different functional units have been used in these studies. One kg of hydrogen produced at the plant has been the mostly used functional unit. Depending upon the hydrogen produc‐
17
tion methods considered, its pressure and purity level differ among the studies. Some au‐
thors extend the functional unit to one kg of stored hydrogen at high pressure or in liquid form. It should be noted that a significant amount of electricity is needed to compress the hydrogen at plant outlet (generally at about 30 bar in alkaline electrolytic methods) pressure to tank storage pressure (generally at 200 bar). Also, the steel use in manufacturing of stor‐
age tanks plays a significant role. This issue of whether the same functional unit has been used should be made clear when the LCA of two methods are compared with each other. One study [Wulf & Kaltschmitt, 2012] uses the functional unit as one kg of hydrogen at refu‐
eling station at 700 bar. After one kg of hydrogen, the second mostly used unit in the litera‐
tures is one normal cubic meter (Nm3) of hydrogen at plat gate. Others use one MJ or TJ of hydrogen produced. One study considers the use of one MJ of hydrogen in PEMFC vehicles. Another study uses two functional units – one MJ of hydrogen produced and one MJ of me‐
chanical work done (use of hydrogen to drive a fuel cell vehicle, indirectly MJ/km driving). The other uses gCO2eq./km of driving (use in fuel cell vehicle) and one MJ hydrogen pro‐
duced. Fehler! Verweisquelle konnte nicht gefunden werden. shows the summarized in‐
formation on the studies reviewed under this paper. Nuclear B SMR natural gas Coal gasifica‐
tion Biomass gasi‐
fication GWP AP EP x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x m x x x x x x x x x x x x x
x x x x x x a
c
f
x
x f
h
j
x
x
p
q
Single score (points)
Remarks Grid based x x x x x x x x x x Others Solar PV Non‐electrolytic Hydro Global (aggregated)
North America Europe (OECD) Europe Mexico North America North America North America Europe (Greece) Europe (Greece) Asia (Korea) Global (generic) Europe North America North America North America Europe (Norway) North America North America Impact categories Wind Region A Electrolytic 2011
2012
2009
2012
2012
2006
2012
2012
2004
2008
2010
2002
2008
2006
2011
2012
2006
2001
2004 Authors (alphabetical) Boyano et al. [Boyano et al., 2011] Cetinkaya et al. [Cetinkaya et al., 2012] Dufour et al. [Dufour et al., 2009] Dufour et al. [Dufour et al., 2012] Giraldi et al. [Giraldi et al., 2012] Granovskii et al. [Granovskii et al., 2006] Hacatoglu et al. [Hacatoglu et al., 2012] Kalinci et al. [Kalinci et al., 2012] Koroneos et al. [Koroneos et al., 2004] Koroneos et al. [Koroneos et al., 2008] Lee et al. [Lee et al., 2010] Marquevich et al. [Marquevich et al., 2002] NEEDS [NEEDS, 2008] NETL [NETL, 2006] Ozbilen et al. [Ozbilen et al., 2011] Ozbilen et al. [Ozbilen et al., 2012] Solli et al. [Solli et al., 2006] Spath & Mann [Spath & Mann, 2001] Spath & Mann [Spath & Mann, 2004] Hydrogen production methods Year Table 2: Reviewed literatures on LCA of hydrogen production (alphabetical list after author’s name) x x x x x b d e g g i k l n o 19
Utgikar & Thiesen [Utgikar & Thiesen, 2006] 2006 North America Wulf & Kaltschmitt [Wulf & Kaltschmitt, 2012] 2012 Europe (Germany) Source: compiled from [Bhandari et al., 2012] A B a b c d e x x x r
r
r
x x x x x x x x x IEK‐STE 2013 q country is relevant mainly in grid based electrolysis both high temperature electrolysis and thermochemical water splitting also ozone depletion potential and winter/summer smog considered hydrogen production via thermal and auto‐catalytic decomposition of methane (from natural gas) also analyzed both cumulative energy and exergy demand auto‐maintained methane steam methane reforming (SMR), solar based thermochemical cycles and water photo‐splitting methods analyzed GWP values for aggregated energy sources: fossil, biomass and renewables are also presented (secondary data); different electricity supply scenarios for S‐I cycle method cumulative energy demand utilization of gasoline fuel in internal combustion engine) ICE and hydrogen fuel in proton exchange membrane fuel cell (PEMFC) vehicles con‐
sidered solid particulate matters biomass gasification – electricity – electrolysis route abiotic resource depletion; cumulative energy demand, radiation life cycle costing also carried out hydrogen production by steam reforming of hydrocarbon feedstock (methane (from natural gas) and naphtha) and vegetable oils (rapeseed oil, soybean and palm oil) GWP not calculated but the individual gas emission listed for LNG steam methane reforming, the application of CCS considered; and for coal, CMM option considered biomass gasification – electricity – electrolysis route eutrophication potential, abiotic resource depletion potential, ozone depletion potential, photochemical oxygen creation potential, and radia‐
tion human toxicity potential, eutrophication potential, and radiation r PV, wind, hydro and biomass renewable electricity mix considered as green electricity f g h i j k l m n o p IV Environmental Performance of Electrolysis Under this section, the environmental impact results of different studies will be discussed. Being the electrolytic method in focus of this paper, the inventory data for this system will be first presented using the exemplary values for wind based electrolysis prepared at NREL [Spath & Mann, 2004]. In that report, the resource consumption and energy use, material and energy balances are performed in a cradle to grave manner. The system incorporates three 50 kW wind turbines with an electrolyzer with hydrogen production capacity of 30 Nm3/hr. This electrolyzer system converts the electricity to hydrogen at an efficiency of about 85% (HHV). Thus produced hydrogen is compressed to a pressure of 20 MPa, stored, and dispensed at the fueling station. For this wind/electrolysis system, the materials re‐
quired to construct the wind turbines, electrolyzer, and hydrogen storage tanks were taken within the system boundary. Fossil fuels, metals, and minerals are used as major inputs in this process. The iron, which is mostly used in manufacturing the wind turbines and hydrogen storage vessels, accounts for 37.4% of the resources used. The large amount of limestone, 35.5% of the major resources, is used for the turbines’ concrete foundations. Coal, which is consumed primarily to produce the steel, iron, and concrete, accounts for 20.8% of the remaining resources. This is followed by oil at 4.7%, and natural gas at 1.6%, which are primarily used in manufacturing the wind turbines. Water is consumed not only in the electrolysis operation, but also in upstream pro‐
cesses and its total consumption rate is about 26.7 liters/kgH2. Nearly 45% of this water is used by the electrolyzer, while 38% and 17% is used in wind turbines and hydrogen storage vessels manufacturing, respectively. The average resources consumed in the process are summarized in Table 3. Table 3: Average consumption of resources in wind electricity electrolysis system Resource Total (g/kgH2) For wind tur‐
For electrolysis (%)* For storage (%)*
bines (%)* Coal 214.7 68 5 27 Iron (Fe, ore) 212.2 64 6 30 Iron scrap 174.2 53 8 39 Limestone 366.6 96 1 (0.3 real value) 3 Natural gas 16.2 72 15 13 Oil 48.3 76 13 11 Source: [Spath & Mann, 2004] Note: * percentage numbers are rounded IEK‐STE 2013 21
For this system, the GWP is presented to be about 0.97 kgCO2 eq./kgH2. About 78% of it was from wind turbine production and operation; about 4.4% was from electrolyzer production and operation; and about 17.6% was from hydrogen compression and storage. Average en‐
ergy consumption was 9.1 MJ/kgH2. The majority of the energy consumption, i.e. 72.6%, was from manufacturing of the wind turbines (LHV). Contribution form electrolysis was 4.8% and that from storage was 31.6%. In emissions CO2 is emitted at the highest rate, about 95% by weight. In general, majority of air emissions come from the process steps in manufacturing and installation of wind turbines. Table 4 shows air emission values for the described system. Table 4: Average air emissions from wind based electrolysis (% numbers rounded) Air emission Total (g/kgH2) From wind tur‐ From electrol‐ From stor‐
bines (%) ysis (%) age (%) Carbon dioxide 950 78 4 18 Carbon monoxide 0.9 80 4 16 Methane 0.3 92 3 5 Nitrogen oxides 4.7 46 47 7 Nitrous oxides 0.05 67 6 27 4.4 63 7 30 Particulates 28.7 94 1 5 Sulfur dioxide 6.1 62 26 12 Non‐methane drocarbons hy‐
Source: [Spath & Mann, 2004] IEK‐STE 2013 Life cycle approach for hydrogen production have been documented in detail also in NEEDS project report [NEEDS, 2008]. It contains relatively detailed LCI data on electrolytic hydrogen production plant; though for wind based system often the cross‐referencing has been made to the data in [Spath & Mann, 2004]. The LCA analysis here considers four phases: construc‐
tion (of electrolyzer components and accessories), operation (component replacement, maintenance, etc. excluding electricity supply), fuel supply (i.e. electricity needed for elec‐
trolysis, including electricity generation system), and disposal. Fig. 5 shows the contribution analysis for electrolytic hydrogen production phases by using then UCTE grid mix (17% hydro and renewables, 29% nuclear and the rest fossil fuel based electricity). 22
Fig. 5: Contribution analysis for electrolytic hydrogen production Source: [NEEDS, 2008] IEK‐STE 2013 A huge contribution in all emission categories comes from electricity supply, namely from the fuel used to generate the grid electricity, thus the source of electricity is very important while carrying out environmental analysis. This can viewed in Fig. 6 where the same study has compared different air emissions values while operating the same electrolyzer system using two power supply sources: aforementioned UCTE grid electricity mix and then Iceland‐
ic electricity grid mix (82% hydro and 18% geothermal). Fig. 6: Comparison of contributions: UCTE (UCTE mix) vs. Icelandic grid (IS mix) Source: [NEEDS, 2008] IEK‐STE 2013 23
The contribution values shown in Fig. 6 are given in percentage. It can be seen that the emis‐
sions could be reduced by more than 90% (for CO about 88%) if the renewable sources sup‐
ply the electricity needs instead of fossil fuel dominated conventional grids. As discussed in section 3, there are many differences in individual studies. An attempt has been made for a summarized comparison of different environmental impact categories of different hydrogen production methods in the following sub‐sections. IV.1 Global warming potential Global warming potential has been analyzed/presented by almost all the authors with an exception. Summarized GWP values for different electrolytic methods are shown in Fig. 7. These values vary from one study to the other and those varying ranges are shown by ex‐
tended thin lines on tips of the bars. Even if many studies analyzed the GWP for wind based electrolysis, they all present the same GWP value of 0.97 kg/CO2 eq./KgH2, initially reported by [Spath & Mann, 2004]. Also, no such variation in the GWP values for electrolytic methods using hydro, biomass and high temperature nuclear based electrolytic methods can be seen in Fig. 7. This is due to the fact that either one study has analyzed each technology or the values presented are similar. For solar PV based electrolysis this values vary more than the factor of two. A small variation can be seen also for grid based electrolysis. In some cases, the GWP values are derived from the graphs published in the reviewed papers, since no nu‐
meric data was detailed. This may have caused slight deviation on these presented values in Fig. 7 from the exact original value. Wind based electrolysis ranks as the best method followed by hydro. GWP values for solar PV, solar thermal and biomass based electrolytic methods are also only slightly higher than the hydro or wind electrolysis. This shows that the electrolytic methods based on renewable energy generated electricity are one of the best methods to produce hydrogen. Fig. 7: GWP of electrolytic hydrogen production methods Source: [Bhandari et al., 2012] IEK‐STE 2013 24
As shown previously also in Fig. 5, the environmental concern of electrolytic process for hy‐
drogen production is mainly in its operation phase – i.e. electricity supply (based on fossil fuel or renewable resources generated). This is why, the GWP values for different electrolyt‐
ic methods highly vary (e. g. electrolysis with German (UCTE) grid based electrolysis is over 30 times than that based on wind based electrolysis). Highest value for GWP can be seen for grid based electrolysis. This is due to high share of fossil fuel based resources (about 54% excluding nuclear in 2010) in the grid electricity mix. LCA of electrolytic hydrogen production using wind energy shows that adverse environmen‐
tal contribution from electrolyzer is relatively small. In contrast, the wind turbine itself is the major contributor to GWP and other impact categories (Fig. 8; values from [Spath & Mann, 2004]). Fig. 8: A Share of GWP (970gCO2 eq./kgH2) in wind electrolysis methods Source: [Bhandari et al., 2012] IEK‐STE 2013 A comparison of GWP values for different electrolytic and non‐electrolytic methods is shown in Fig. 9. Similar to in Fig. 7 the GWP ranges of different studies is shown by extended lines on the tips of bars (e.g. value for steam methane reforming of natural gas varies from 8.9 to 12.9 kgCO2 eq./kgH2). Nuclear based thermochemical cycles seem to be the best methods of hydrogen production from GWP perspective. Coal gasification performs the worst among conventional methods. However, when the CCS and CMM measures are applied during coal mining and coal gasifi‐
cation, this method is competitive with renewable energy based electrolytic methods from GWP perspectives. Steam methane reforming follows the coal gasification method. Because of efficiency losses in multi‐step energy conversion processes in fossil fuel based electrolytic methods, the conventional (non‐electrolytic) fossil fuel based hydrogen production methods perform better in terms of GWP. Surprisingly, the biomass gasification performed worse than biomass‐electrolysis route. One of the main reasons for this was the assumption that the biomass‐gasification‐electricity power plant produces all the electricity required for elec‐
trolysis and hydrogen liquefaction steps without need of additional power source for these provisions. On the other hand the gasification steam reforming plant requires additional electricity due to compression requirements that involve the steam reforming and PSA pro‐
25
cesses. This electricity is assumed to come from the grid, which is mainly fuelled with non‐
renewable energy sources. Fig. 9: GWP values of different hydrogen production methods Source: data from [Bhandari et al., 2012] IEK‐STE 2013 Other impact categories beyond the GWP should also be analyzed before one method is claimed to be superior to the other when the overall environmental impacts of the hydrogen production methods are concerned. Unfortunately, many studies limit their scope to GWP. IV.2 Acidification potential As already summarized in Fehler! Verweisquelle konnte nicht gefunden werden., the sec‐
ond mostly analyzed impact category is acidification potential (AP). The average AP values discussed in the reviewed literatures are given in Fig. 10. For the remaining hydrogen pro‐
duction technologies shown in Fig. 9, no analysis of this impact category has been made by the respective authors. Nevertheless, wind based electrolytic method ranks the second also in AP, followed by hy‐
dro. Solar PV has relatively higher values of GWP and AP, mainly due to the material and energy intensive process chains involved during the manufacturing of solar PV modules. Bi‐
omass based electrolysis performs the worst under this category, and the values are almost double to that of natural gas based SMR. Emissions during the biomass production process (plantation, fertilizers use, etc.) are the major causes behind it. 26
Fig. 10: Average AP values of different hydrogen production methods Source: data from [Bhandari et al., 2012] IEK‐STE 2013 Also under this category, the nuclear based thermochemical cycle performs the best. The environmental impacts (both global warming and acidification) of the nuclear system are primarily due to the activities associated with mining, fabrication and construction of the plants. Authors of the nuclear based methods, e.g. [Utgikar & Thiesen, 2006], suggest that these activities will occur over a short time period compared to the operational life time of the plant, which itself has relatively minor adverse impact on the environment and it may be easier to manage these short‐term emissions through carbon dioxide sequestration and acid gas neutralization technologies to mitigate their environmental impact. However, the socie‐
tal concerns of use of nuclear fuel management and nuclear waste disposal may need atten‐
tion before making the conclusions that nuclear based option is better than the other meth‐
ods. IV.3 Other categories The rest categories such as EP, HTP, etc. are discussed only occasionally in the reviewed lit‐
eratures. Few authors consider the primary energy demand and resources use including the water consumption in electrolytic process. Others mentioned the solid particulate matters. Two studies [Ozbilen et al., 2012, Solli et al., 2006] mentioned human toxicity and radiation impacts of using nuclear based water splitting methods. Also EP and winter smog (solid par‐
ticulate matters, SPM) along with GWP and AP are analyzed in [Koroneos et al., 2004] for high pressure alkaline electrolysis using renewable energy electricity (wind, hydropower, solar PV, solar thermal and biomass) and steam reforming of natural gas. For all these four categories, electrolysis via wind electricity shows the best results followed by hydro and so‐
lar thermal electricity. GWP, AP, EP, ADP, ODP, POCP and R for nuclear‐based hydrogen pro‐
duction via thermochemical water splitting using the Cu‐Cl cycle (three‐, four‐ and five‐step 27
Cu‐Cl cycles) are analyzed in [Ozbilen et al., 2012]. Solli et al. [Solli et al., 2006] presented a comparative hybrid LCA (combining process LCA and input‐output model) for two hydrogen production methods: nuclear based water splitting and natural gas steam reforming with CCS. The natural gas based system performs better for HTP and R whereas the nuclear alter‐
native has a better score for GWP, AP and EP; the impacts often associated with emissions from combustion. Except for radiation, the numbers are in the same order of magnitude, ratio from natural gas to nuclear varying from factor of 1.4 to 4.5. For these categories no detailed quantitative data could be found and thus their comparison has not been graphical‐
ly presented in this paper, unlike for GWP and AP. IV.4 Single score comparison Some authors present their results as single score point and claim a technology group win‐
ner over the other. Sometimes, due to different assumptions in weighting procedure, differ‐
ent winners could also be possible for the same emissions. In the compilation of LCA results in [Boyano et al., 2011], the eco‐indicator points for elec‐
trolytic hydrogen production vary from 0.05 mPt/Nm3H2 for wind electrolysis to 10.30 mPt/Nm3H2 for biomass‐gasification‐electricity‐electrolysis route. The values for hydropower and solar PV based electrolysis are given as 0.08 and 0.52 mPt/Nm3H2, respectively. As a comparison, their value of 0.4 mPt/Nm3H2 for natural gas SMR makes this process better than solar PV based electrolysis. Also the single score results in [Koroneos et al., 2004] show the solar PV as the worst option with a score of 0.05 mPt/MJH2. This is followed by biomass, natural gas, solar thermal, hydropower, and wind electrolysis, which has the score nearly 0.005 mPt/MJH2. It is important to note in this study that CO2 equivalent emission for natu‐
ral gas option is double to that for PV system (0.08 vs. 0.04 kg/MJH2), whereas the overall single score point for the former one is only about 0.75% of the later one. This shows the clear risk of considering only one impact category in LCA analysis and making the decision on superiority of one method to the other from whole environmental perspectives. Dufour et al. [Dufour et al., 2009] studied the thermal and autocatalytic decomposition of natural gas (methane) and compared the results with steam reforming with and without CCS. Their eco‐
indicator single score results for full autocatalytic decomposition is about 0.24 mPt/Nm3H2. Steam reforming with CCS is far better than without CCS in terms of CO2 emissions (0.31 vs 0.95 kgCO2 eq./Nm3H2); however, surprisingly, the latter is slightly better for single score point (0.45 vs. 0.47 mPt/Nm3H2). This is due to the fact that the CCS process requires elec‐
tricity leading to more NOx emissions than conventional SMR and thus leading to higher acidification and winter smog impacts (reference has been made for World Energy Outlook electricity generation mix for year 2004). Only the use renewable electricity would make SMR with CCS attractive in this case. There are also authors (e.g. [Ozbilen et al., 2011]) who present the single score for natural gas SMR the worst (more than the factor of 3) among solar, wind and biomass based electrolysis systems. Nuclear based thermochemical cycle has been given the best score, though the radiation and nuclear waste management issues have not been considered. 28
While looking at these single score results, no straight conclusion on the best technology can be drawn regarding the environmental impact of different hydrogen production methods. Such comparisons are plausible only when exactly similar assumptions on weighting factors and single score calculation methods are applied for the methods under investigation. Oth‐
erwise, comparing either individual impact category (e.g. defined by CML) or individual envi‐
ronmental emissions would be more accurate than comparing a single score value of one method to the other. V Conclusions Several studies are published on LCA of hydrogen production methods. Major hydrogen pro‐
duction methods are steam methane reforming of natural gas followed by coal gasification. Share of electrolysis in global hydrogen production is still small, i.e. about 4%. Three types of electrolysis are discussed in the literature: alkaline, polymer membrane electrolyte, and solid oxide electrolysis. Most of the LCA studies compare environmental impacts of electrolytic hydrogen production methods with conventional fossil fuel based methods. Studies dealing with electrolysis com‐
pare the environmental impacts of hydrogen production via conventional grid electricity to via renewable energy (mainly wind and solar) electricity. Hydro and wind energy based elec‐
trolytic hydrogen production methods have been proven to be the best methods for hydro‐
gen production from ecological point of view. Although the nuclear based high temperature methods would perform better in GWP and AP categories, analysis of other impact catego‐
ries is necessary before declaring it as the best method. Solar PV based electrolysis is also better than the grid based electrolysis and fossil fuel feedstock based methods. Although biomass based methods perform very well in GWP category, they are problematic in other categories, mainly AP and EP. Biomass plantation processes and fertilizer use during biomass growth are the main reasons behind it. Electrolyzer has been analyzed as a single component. Therefore details on the contribution from individual components of electrolyzer such as electrodes, membrane, etc. could not be understood from this review. An electrolyzer unit focused LCA study needs to be carried out to understand individual component’s contribution. In future, it is recommended to broaden the scope of the LCA studies on hydrogen production methods by including other impact categories beyond the GWP. It would only be possible to compare the different electrolytic methods (alkaline, PEM and SOE) from LCA perspective if the electrolyzer is not analysed as a black‐box of hydrogen production process chain, rather the contribution from individual components of the electrolyzer are also detailed. Such electrolyzer focused LCA analysis would also be important, for example when the toxicity related impacts caused by asbestos membranes in alkaline electrolyzers are to be analyzed. This membrane material is of specif‐
ic interest as some countries have already banned the use of asbestos in membranes citing health related issues. The electrolyzer focused LCA study – taking an example of wind elec‐
tricity based alkaline electrolyzer – will be our next research in this context. 29
Acknowledgements Authors acknowledge the financial support from the European Union under the 7th Frame‐
work Program for the Elygrid project. 30
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Systems Analysis and Technology Evaluation at the Research Centre Jülich Many of the issues at the centre of public attention can only be dealt with by an interdisci‐
plinary energy systems analysis. Technical, economic and ecological subsystems which inter‐
act with each other often have to be investigated simultaneously. The group Systems Analy‐
sis and Technology Evaluation (STE) takes up this challenge focusing on the long‐term sup‐
ply‐ and demand‐side characteristics of energy systems. It follows, in particular, the idea of a holistic, interdisciplinary approach taking an inter‐linkage of technical systems with econom‐
ics, environment and society into account and thus looking at the security of supply, eco‐
nomic efficiency and environmental protection. This triple strategy is oriented here to socie‐
tal/political guiding principles such as sustainable development. In these fields, STE analyses the consequences of technical developments and provides scientific aids to decision making for politics and industry. This work is based on the further methodological development of systems analysis tools and their application as well as cooperation between scientists from different institutions. Leitung/Head: Prof. Jürgen‐Friedrich Hake Forschungszentrum Jülich Institute of Energy and Climate Research IEK‐STE: Systems Analysis and Technology Evaluation 52428 Jülich Germany Tel.: +49‐2461‐61‐6363 Fax: +49‐2461‐61‐2540, Email: preprint‐ste@fz‐juelich.de Web: www.fz‐juelich.de/ste