Hydrogen emissions from a hydrogen economy and their potential global warming impact : summary report of the Clean Hydrogen Joint Undertaking expert workshop on the Environmental Impacts of Hydrogen.
Date: 2022-08-19
Institutions: Joint Research Centre
Subjects: carbon neutrality, clean technology, climate change, global warming, greenhouse gas, hydrogen, reduction of gas emissions, research report

Hydrogen emissions from a hydrogen economy 
and their potential global warming impact 

Summary report of the Clean Hydrogen 
Joint Undertaking Expert Workshop on 
the Environmental Impacts of Hydrogen 

Arrigoni, A., Bravo Diaz, L. 

2022 

 EUR 31188 EN 

 
 
This publication is a Technical report by the Joint Research Centre (JRC), the European Commission’s science and knowledge service. It aims to provide 
evidence-based scientific support to the European policymaking process. The scientific output expressed does not imply a policy position of the European 
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Contact information 
Name: Alessandro Arrigoni 
Address: Westerduinweg 3, 1755 LE Petten, Netherlands 
Email: alessandro.arrigoni-marocco@ec.europa.eu 
Tel.: +31 (0)224 56 5951 

EU Science Hub 
https://ec.europa.eu/jrc 

JRC130362 

EUR 31188 EN 

PDF 

ISBN 978-92-76-55848-4 

ISSN 1831-9424 

doi:10.2760/065589 

Luxembourg: Publications Office of the European Union, 2022 

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How to cite: Arrigoni, A. and Bravo Diaz, L., Hydrogen emissions from a hydrogen economy and their potential global warming impact, EUR 31188 EN, 
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Contents 

Acknowledgements .......................................................................................................................................................................................................................................... 1 

Abstract ....................................................................................................................................................................................................................................................................... 2 

1 

Introduction..................................................................................................................................................................................................................................................... 3 

1.1  Workshop objectives and speakers ............................................................................................................................................................................ 3 

1.2  Report structure ............................................................................................................................................................................................................................ 5 

2  Global warming impact of hydrogen emissions to the atmosphere ....................................................................................................... 6 

2.1  Hydrogen atmospheric budget ....................................................................................................................................................................................... 6 

2.1.1 

Current budget ............................................................................................................................................................................................................. 6 

2.1.2 

Historical changes .................................................................................................................................................................................................... 7 

2.1.3 

Hydrogen concentration ...................................................................................................................................................................................... 8 

2.2  Global Warming Potential (GWP) and other metrics ................................................................................................................................. 8 

2.2.1 

GWP100 ................................................................................................................................................................................................................................. 9 

2.2.2 

GWP100 vs. GWP20 ................................................................................................................................................................................................... 11 

2.2.3 

Pulse vs. continuous emissions................................................................................................................................................................. 12 

2.2.4  Which metric to use? ........................................................................................................................................................................................... 13 

3  Global warming impact of a hydrogen economy .................................................................................................................................................... 14 

3.1  H2 demand ...................................................................................................................................................................................................................................... 14 

3.2  H2 losses............................................................................................................................................................................................................................................ 15 

3.2.1 

Sources of hydrogen losses across the value chain ............................................................................................................. 15 

3.2.2  Overall hydrogen losses .................................................................................................................................................................................. 17 

3.3  A life cycle view on H2 economy emissions .................................................................................................................................................... 18 

3.3.1 

Hydrogen delivery: ammonia ...................................................................................................................................................................... 18 

3.3.2 

Hydrogen use: fuel for aviation ................................................................................................................................................................ 18 

3.4  Methane............................................................................................................................................................................................................................................. 19 

3.4.1 

Hydrogen production: implications for methane sources to the atmosphere.............................................. 20 

3.4.2 

Hydrogen delivery: repurposing of natural gas pipelines ................................................................................................ 21 

3.5  Overall climate consequences ..................................................................................................................................................................................... 21 

4  Prevention and monitoring of hydrogen losses ........................................................................................................................................................ 24 

4.1  Leakage prevention: design considerations .................................................................................................................................................... 24 

4.2  Leakage monitoring: detection and quantification methods .......................................................................................................... 25 

4.2.1  Overview of sensor technologies for hydrogen detection .............................................................................................. 26 

4.2.1.1 

Hydrogen-methane blends .............................................................................................................................................................. 28 

4.2.2  Overview of optical techniques for hydrogen detection ................................................................................................... 29 

4.2.2.1 

Hydrogen-methane blends .............................................................................................................................................................. 31 

4.2.3  Other leak quantification methods: industry compatible techniques .................................................................. 32 

4.2.4  Monitoring tools for hydrogen detection in the atmosphere ....................................................................................... 35 

5  Conclusions .................................................................................................................................................................................................................................................. 36 

i 

References ............................................................................................................................................................................................................................................................. 38 

List of abbreviations and definitions ........................................................................................................................................................................................... 42 

List of figures ..................................................................................................................................................................................................................................................... 43 

List of tables ........................................................................................................................................................................................................................................................ 45 

ii 

 
 
 
 
 
 
 
 
 
 
 
 
 
Acknowledgements 

The  workshop  on  the  Environmental  Impacts  of  Hydrogen  has  been  organised  by  the  European  Clean 
Hydrogen Joint Undertaking and the U.S. Department of Energy.  

The authors would like to acknowledge the following people and organizations for their contributions to the 
workshop and in the preparation of this report: 

Firstly, we would like to thank Bart Biebuyck, Executive Director of the Clean Hydrogen Joint Undertaking and 
Sunita  Satyapal,  Director  of  EERE's  Hydrogen  and  Fuel  Cell  Technologies  Office  of  the  U.S.  Department  of 
Energy,  for  conceiving  and  organizing  the  workshop  and  for  giving  JRC  the  opportunity  to  summarize  the 
outcomes of the workshop as an independent and impartial body. 

Then,  we  would  like  to  thank  Hydrogen  Europe,  Hydrogen  Europe  Research,  the  Hydrogen  Council,  and  the 
International  Partnership  for  Hydrogen  and  Fuel  Cells  in  the  Economy,  for  their  support  in  designing  the 
workshop and additional contributions and feedback during the revision of this manuscript.  

We would also like to warmly thank the following speakers and the organisations that they represent for their 
contributions to the workshop and for reviewing the manuscript:  

• 
Stephen Bradley, European Industrial Gas Association (EIGA);  
•  William Buttner, National Renewable Energy Laboratory (NREL);  
•  Cathy Clerbaux, National Centre for Scientific Research (CNRS);  
•  Guy de Reals, Air Liquide;  
•  Austin M. Glover, Sandia National Laboratories;  
•  Volker Grewe, German Aerospace Center (DLR);  
• 
•  Kevin Hartmann, National Renewable Energy Laboratory (NREL);  
•  Didier Hauglustaine, Laboratory for Sciences of Climate and Environment (LSCE);  
• 
•  Didier Jamois, MultHyFuel project and French National Institute for Industrial Environment and Risks 

Steven Hamburg, Environmental Defense Fund (EDF);  

Ethan S. Hecht, Sandia National Laboratories;  

(INERIS);  
Pere Margalef, SNAM;  
Fabien Paulot, National Oceanic and Atmospheric Administration (NOAA);  

• 
• 
•  Christophe Proust, MultHyFuel project and French National Institute for Industrial Environment and 

• 

Risks (INERIS);  
Rod Robinson, National Physical Laboratory (NPL) and European Association of National Metrology 
Institutes (EURAMET);  

•  Maria Sand, Centre for International Climate Research (CICERO);  
•  Davide Scrocchi, SNAM;  
•  David Stevenson, University of Edinburgh.  

Finally,  we  would  like  to  thank  our  colleagues  at  JRC  for  the  thorough  review  of  the  document:  Bertrand 
Bessagnet, Frank Dentener, Pietro Moretto, and Eveline Weidner. 

Disclaimer 

This  report  is  not  an  opinion  paper.  It  is  the  honest  and  balanced  summary  of  the  data,  models,  and 
viewpoints presented at the workshop, some of which may conflict with each other. The views expressed are 
purely  those  of  the  writers  and  contributors  and  may  not  in  any  circumstances  be  regarded  as  stating  an 
official position of the European Commission, the Clean Hydrogen Joint Undertaking, the U.S. Department of 
Energy or the U.S. Government. 

Authors 

Alessandro Arrigoni - Joint Research Centre, Energy Storage Unit, Petten, Netherlands 
Laura Bravo Diaz - Joint Research Centre, Energy Storage Unit, Petten, Netherlands 

1 

 
Abstract 

Hydrogen (H2) is expected to be a key instrument to meet the European Union (EU) Green Deal main objective: 
i.e.,  climate  neutrality  by  2050.  Renewable  hydrogen  deployment  is  expected  to  significantly  reduce  EU 
greenhouse  gas  (GHG)  emissions  by  displacing  carbon-intensive  sources  of  energy.  However,  concerns  have 
been raised recently regarding the potential global warming impact caused by hydrogen emissions. Although 
hydrogen is neither intentionally emitted to the atmosphere when used nor a direct greenhouse gas, hydrogen 
losses  affect  atmospheric  chemistry,  indirectly  contributing  to  global  warming.  To  better  understand  the 
potential environmental impact of a hydrogen economy and to assess the need for action in this respect, the 
Clean Hydrogen Joint  Undertaking and the U.S.  Department of Energy jointly organised, with the  support of 
the  European  Commission,  Hydrogen  Europe,  Hydrogen  Europe  Research,  the  Hydrogen  Council,  and  the 
International  Partnership  for  Hydrogen  and  Fuel  Cells  in  the  Economy,  a  2-day  expert  workshop.  Experts 
agreed  that  a  low-carbon  and,  in  particular,  a  renewable  hydrogen  economy  would  significantly  reduce  the 
global warming impact compared to a fossil fuel economy. However, hydrogen losses to the atmosphere will 
impact  the  lifetime  of  other  greenhouse  gases,  namely  methane,  ozone,  and  water  vapour,  indirectly 
contributing to the increase of the Earth’s temperature in the near-term. To minimise the climate impact of a 
hydrogen  economy,  losses  should  therefore  be  minimised,  prevented,  and  monitored.  Unfortunately,  current 
loss rates along the hydrogen supply chain are not well constrained and are currently estimated to go from 
few  percents  for  compressed  hydrogen  (1-4%),  up  to  10-20%  for  liquefied  hydrogen.  Both  the  global 
warming  impact  of  hydrogen  emissions  and  the  leakage  rates  from  a  developed  hydrogen  economy  are 
subject  to  a  high  level  of  uncertainty.  It  is  therefore  of  paramount  importance  to  invest  in  developing  the 
ability to accurately quantify hydrogen emissions, as well as engage in more research on hydrogen leakage 
prevention  and  monitoring  systems.  More  data  from  the  hydrogen  industry  and  improved  observational 
capacity  are  needed  to  improve  the  accuracy  of  the  global  hydrogen  budget.  Finally,  it  is  recommended  to 
always  report  the  amount  and  location  of  hydrogen  emissions  when  environmental  assessments  are 
performed. There is a range  of emission  metrics and time scales that are  designed to evaluate the climate 
impacts of short-lived GHG emissions compared to CO2 (i.e., CO2 equivalents). The metric choice must depend 
on the specific  policy goal, as they  can provide  very  different perspectives on the relative importance of H2 
emissions on the climate depending on the time horizon of concern. These differences need to be viewed in 
the context of the specific policy objectives.  

2 

1 

Introduction 

Hydrogen is expected to be a key instrument to meet the European Union (EU) Green Deal main objective: i.e., 
climate neutrality by 2050 (European Commission, 2019). To align with that objective, the EU is planning to 
ramp  up  its  renewable  hydrogen  production:  the  initial  target  of  10  Mt  per  year  of  renewable  hydrogen 
production  by  2030  set  out  in  the  hydrogen  strategy  for  a  climate-neutral  Europe  (European  Commission, 
2020) has been recently raised to 20 Mt in the framework of the REPowerEU strategy for more affordable, 
secure  and  sustainable  energy  (European  Commission,  2022).  By  2050,  hydrogen  could  provide  more  than 
20% of the total energy  demand in Europe (Fuel Cells and  Hydrogen Joint Undertaking, 2019) and globally 
(Hydrogen Council and McKinsey & Company, 2021). 

Renewable hydrogen deployment is expected to significantly reduce EU greenhouse gas (GHG) emissions by 
displacing carbon-intensive sources of energy. On average, the  Hydrogen Council (2017) estimated that the 
production  of  each  kilogram  of  renewable  hydrogen  would  allow  the  saving  of  10.9  kg  of  CO2  equivalents 
(CO2e). A hydrogen economy could hence abate some 80 Gt of CO2e by mid-century and some 730 Mt of CO2e 
by 2030 (Hydrogen Council and McKinsey & Company, 2021). However, concerns have been raised regarding 
the  potential  greenhouse  gas  effect  caused  by  hydrogen  emissions  (Derwent  et  al.,  2001;  Warwick  et  al., 
2022).  Although  hydrogen  is  neither  intentionally  emitted  to  the  atmosphere  when  used  nor  a  direct 
greenhouse gas (i.e., the hydrogen molecule does not interact with the incoming solar radiation, nor with the 
infrared  radiation  emitted  back  by  our  planet),  hydrogen  losses  (and  natural  emissions)  affect  the 
atmospheric chemistry, contributing to global warming. 

To better understand the potential climate impact of a hydrogen economy and to assess the need for action 
in  this  respect,  the  Clean  Hydrogen  Joint  Undertaking  (JU)1  and  the  U.S.  Department  of  Energy  organised 
jointly,  with  the  support  of  the  European  Commission,  Hydrogen  Europe,  Hydrogen  Europe  Research,  the 
Hydrogen  Council,  and  the  International  Partnership  for  Hydrogen  and  Fuel  Cells  in  the  Economy,  a  2-days 
expert workshop.  

1.1  Workshop objectives and speakers 

The workshop took place on the 31st of March and the 1st of April 2022, and was divided into two main sub-
topics: the global warming impact of hydrogen, and detection methods and quantification of hydrogen losses. 
The workshop was chaired by Alberto Garcia Hombrados, project officer of the Clean Hydrogen JU. Welcoming 
remarks were given by the executive director of the Clean Hydrogen JU (Bart Biebuyck), the head of unit of 
Directorate-General for Climate Action of the European Commission (Willem Van Ierland), and the director of 
the Hydrogen and Fuel Cell Technologies Office of the U.S. Department of Energy (Sunita Satyapal).  

The three main objectives of the workshop were: 

1.  Provide insights on the current knowledge on the global warming impact of hydrogen; 

2.  Share evidence-based information on human-made hydrogen emissions (sources, quantities, etc.);  

3.  Support reaching a better understanding of the climate impact of hydrogen, and assessing the need 

for action.  

The list of speakers for day 1 and 2 of the workshop is reported in Table 1 and Table 2, respectively. After the 
presentations,  joint  discussions  took  place  on  both  days.  The  discussion  was  moderated  by  the  Clean 
Hydrogen  JU,  and  also  saw  input  from  Hydrogen  Europe  (Grzegorz  Pawelec  and  Jorgo  Chatzimarkakis), 
Hydrogen Europe Research (Laurent Antoni and Louis Mazurkiewicz), Hydrogen Council (Andrei V. Tchouvelev), 
European  Commission  (Pietro  Moretto  from  the  Joint  Research  Centre),  U.S.  Department  of  Energy  (Sunita 
Satyapal  and  Christine  Watson),  and  International  Partnership  for  Hydrogen  and  Fuel  Cells  in  the  Economy 
(Tim Karlsson). 

1 The Clean Hydrogen JU is a unique public private partnership supporting research and innovation activities in 
hydrogen  technologies  in  Europe,  with  the  goal  of  contributing  to  the  European  climate  neutrality.  The 
three members of the JU are the European Commission, Hydrogen Europe and Hydrogen Europe Research. 

3 

                                           
Table 1. Day 1 presentations  

Presenter 

Organisation 

Title 

Maria Sand 

Fabien Paulot 

Centre for International 
Climate Research (CICERO) 

Climate and environmental impacts of hydrogen 
emissions 

National Oceanic and 
Atmospheric Administration 
(NOAA) 

Atmospheric budget of hydrogen: implications for 
assessing the environmental impact of a hydrogen 
transition 

David Stevenson 

University of Edinburgh 

Modelling atmospheric H2 and estimating its  
Global Warming Potential 

Steven Hamburg 

Environmental Defense Fund 
(EDF) 

Future hydrogen economy: Leakage can undermine  
climate benefits of decarbonisation 

Didier Hauglustaine 

Laboratory for Sciences of 
Climate and Environment 
(LSCE) 

Climate benefit of a future hydrogen economy 

Volker Grewe 

German Aerospace Center 
(DLR) 

AHEAD results in terms of CO2 and H2O atmospheric 
impact 

Cathy Clerbaux 

National Centre for Scientific 
Research (CNRS) 

Environmental impacts of hydrogen: Ammonia (NH3) 
- impact on air quality and climate and detection 
from space 

Table 2. Day 2 presentations 

Presenter 

Organisation 

Title 

Stephen Bradley 

European Industrial Gas 
Association (EIGA)  

Hydrogen fugitive emissions 

Guy de Reals  

Air Liquide 

Hydrogen release: a supply chain overview 

Pere Margalef and 
Davide Scrocchi 

SNAM 

Hydrogen leakage climate impact in the gas 
midstream 

William Buttner and 
Kevin Hartmann 

National Renewable Energy 
Laboratory (NREL) 

Hydrogen releases quantification: I) Sensors and 
detection tools; II) Leak rate quantification 

Ethan S. Hecht and 
Austin M. Glover 

Didier Jamois and 
Christophe Proust 

Rod Robinson  

Sandia National Laboratories   Detection and monitoring tools for quantification of 

hydrogen releases, benchmark against methane 

MultHyFuel project; French 
National Institute for 
Industrial Environment and 
Risks (INERIS) 

National Physical Laboratory 
(NPL); European Association 
of National Metrology 
Institutes (EURAMET) 

Hydrogen release: detection, monitoring tools for 
quantification; Industrial compatible techniques for 
detection and flowrate estimates 

EURAMET Project Decarb and metrology aspects of 
hydrogen emission measurement 

4 

 
1.2  Report structure 

This report aims to provide an overview of the presentations and discussions that took place during the 2-day 
workshop,  and  to  summarise  the  main  outcomes.  Instead  of  summarising  each  individual  presentation,  the 
report combines the information provided by the different speakers. First, an overview of the hydrogen budget 
in  the  atmosphere  is  provided  (chapter  2),  highlighting  the  main  sources  and  sinks  of  hydrogen  to  the 
atmosphere  and  the  resulting  role  in  warming  the  planet.  Chapter  3  presents  an  overview  of  the  potential 
implications for climate change of a future hydrogen economy, including the potential replacement of fossil 
fuels and the impact of hydrogen losses. In Chapter 4 the state of the art and current challenges for hydrogen 
leakage  prevention,  monitoring,  and  quantification  are  presented.  Finally,  conclusions  and  recommendations 
based on the presentations and discussions sparked during the workshop are provided in chapter 5. 

In the report the terms “source”, “emission”, “loss”, “release”, and “leak” (or “leakage”) will be used related to 
hydrogen with the following meaning: 

• 

• 

• 

• 

• 

Source:  any  activity  adding  hydrogen  to  the  atmosphere  (anthropogenic  or  natural,  including  H2 
indirect production via chemical reactions in the atmosphere); 

Emission:  any  activity  adding  hydrogen  to  the  atmosphere  directly  (anthropogenic  or  natural, 
excluding H2 production in the atmosphere resulting from chemical reactions); 

Loss: any H2 addition to the atmosphere from anthropogenic activities, including releases and leaks; 

Release: any planned, foreseen, and/or necessary H2 addition to the atmosphere from anthropogenic 
activities (e.g., for safety reasons, depressurisation events); 

Leak or leakage: any unexpected, accidental, and/or unintended H2 addition to the atmosphere from 
anthropogenic activities. 

5 

 
2  Global warming impact of hydrogen emissions to the atmosphere 

In  this  section  the  potential  global  warming  impact  from  hydrogen  emissions  is  presented,  first  introducing 
the  hydrogen  atmospheric  budget  (2.1),  then  presenting  the  current  best  estimates  for  the  different  global 
warming metrics (2.2).  

2.1  Hydrogen atmospheric budget 

2.1.1  Current budget 

Hydrogen is fairly abundant in the atmosphere, with a concentration of approximately 0.5 parts per million by 
volume (ppmv), and an average lifetime of approximately 2 years (1.4-2.5 years). The atmospheric hydrogen 
budget is presented in Figure 1, combining the ranges for the different sources and sinks presented during the 
workshop. Sources of hydrogen are depicted in red, while sinks are in green.  

Figure 1. Global hydrogen budget  

Source: JRC (2022) adapted from the figures presented by Sand, Paulot, and Stevenson 

The dominant source of hydrogen in the atmosphere is not a direct emission, but it is the result of chemical 
oxidation  reactions  of  other  compounds  released  to  the  atmosphere.  In  particular,  the  photo-oxidation  of 
methane and non-methane hydrocarbons (NMHC) generates approximately 40 Mt of hydrogen per year (29-
51 Tg yr-1). Most NMHC form formaldehyde (HCHO) when they are oxidised, which in turn produces hydrogen 
when it photolyses (HCHO + hν → H2 + CO). The main direct sources of hydrogen emissions to the atmosphere 
are fossil fuels (14-20 Tg yr-1), primarily from combustion processes associated with transportation, biomass 
burning (9-20 Tg yr-1), and nitrogen fixation both on land and oceans (6-12  Tg yr-1).  Other sources that are 
currently not included in most models are geological sources (0-31 Tg yr-1), with the largest contribution likely 
arising from the seepage of hydrogen from underground reservoirs (Zgonnik, 2020), and industrial hydrogen 
losses. Assuming a 10% loss rate, the current best estimate of the maximum potential average emission rate, 
the contribution of hydrogen emissions from the current hydrogen industry is 8 Tg yr-1. Adding up all sources, 
total  hydrogen  production  is  believed  to  be  60-140  Tg  yr-1,  with  the  direct  anthropogenic  emissions 
accounting  for  approximately  15%2.  The  main  sink  for  hydrogen  is  soil  uptake.  The  process  is  driven  by 

2 The figure includes only direct anthropogenic hydrogen emissions (i.e., from fossil fuels, biomass burning, and 
hydrogen industry), and does not include hydrogen production caused indirectly by anthropogenic activities 

6 

 
 
                                           
 
bacteria, and it accounts for over 75% of total removals. The magnitude of the soil sink is modulated by soil 
temperature, soil moisture, and the activity of hydrogen consuming organisms (Leung et al., 2020). Although 
there  has  been  recent  progress  in  understanding  the  behaviour  of  these  hydrogen  consuming  bacteria,  our 
knowledge  is  limited  regarding  their  distribution  and  the  effect  of  soil  moisture  on  uptake  rates.  Current 
understanding is that too much soil water impedes hydrogen diffusion into the soil, while too little water (e.g., 
in  deserts)  impedes  microbial  activity.  To  better  constrain  the  hydrogen  uptake,  it  will  be  necessary  to 
investigate  the  temporal  dynamics  of  soil  moisture  and  its  relationship  to  hydrogen  consumption  across 
specific regions (Bertagni, Paulot, and Porporato, 2021). Hydrogen in the atmosphere not removed by bacteria 
is oxidised, mainly via reaction with the naturally-occurring hydroxyl radical (OH) in the troposphere (R1). This 
reaction is the driver for the indirect global warming impact of hydrogen. Total hydrogen sinks amount to 50-
110 Tg yr-1.  

R1: 

𝐻𝐻2   +  𝑂𝑂𝐻𝐻 (+𝑂𝑂2)   →   𝐻𝐻2𝑂𝑂  +  𝐻𝐻𝑂𝑂2
The magnitude of individual sources and sinks are subject to a high level of uncertainty. The main reason is 
the  scarcity  of  extensive  studies  and  observations,  which  constrains  the  ability  to  obtain  robust  bottom-up 
estimates  of  hydrogen  concentration  in  the  atmosphere.  Starting  with  estimated  hydrogen  emissions,  the 
global  hydrogen  budget  is  modelled  by  tuning  the  land  sink  to  match  the  hydrogen  concentration 
observations. This leads to large uncertainties, in the size of both the sources and sinks (approximately 30% 
on individual terms) as well as in the atmospheric lifetime of hydrogen. 

2.1.2  Historical changes 

The  model  developed  by  Paulot  et  al.  allowed  historical  changes  in  hydrogen  sources  to  be  modelled.  The 
results  are  presented  in  Figure  2,  where  the  black  line  represents  total  sources.  The  model  predicts  that 
hydrogen  sources  increased  from  1900  to  the  present  day  by  approximately  65%  due  to  anthropogenic 
activities (not including potential hydrogen losses from the hydrogen industry). The rise in hydrogen sources is 
due  both  to  direct  emissions  (red  line)  and  indirect  production  (blue  line).  The  model  also  predicts  that 
hydrogen  sources  plateaued  around  1980,  due  to  a  reduction  in  hydrogen  emissions  from  transportation 
(while  indirect  sources  kept  increasing).  This  result  of  the  modelling  has  not  been  corroborated  by 
observations. 

Figure 2. Historical changes of hydrogen sources. Chemical production refers to the natural chemical reactions in the 
atmosphere producing hydrogen. 

Source: presentation by F. Paulot. 

(e.g., hydrogen from the oxidation of anthropogenic methane emissions, hydrogen from wildfires caused by 
anthropogenic climate change, etc.)   

7 

 
 
                                                                                                                                    
2.1.3  Hydrogen concentration 

Hydrogen concentrations in the atmosphere are not uniform across the  globe.  As presented by Paulot et al. 
(Figure 3), concentrations over the continents in the northern hemisphere are much lower than the ones in the 
southern hemisphere. This is due to the larger land surface in the northern hemisphere, confirming the role of 
soils as the major hydrogen sink.  

Figure 3. Hydrogen concentration. Diamonds in the figure show the observation points. 

Source: Paulot al. (2021) 

2.2  Global Warming Potential (GWP) and other metrics 

The hydrogen molecule is not a direct greenhouse gas since the lack of a dipole moment prevents absorption 
of  infrared  radiation.  However,  hydrogen  acts  as  an  indirect  greenhouse  gas.  All  of  the  indirect  greenhouse 
gas effects of hydrogen are driven by its oxidation in the atmosphere (R1), which affects the lifetime of other 
greenhouse gases in the atmosphere. In this section, the latest scientific understanding of the potential global 
warming caused by an increase of hydrogen concentration in the atmosphere is presented. The estimations do 
not include potential global warming effects which are not fully understood nor assessed, such as the impact 
of hydrogen emissions on aerosols and nitrous oxide 3. Moreover, the estimates do not include all of the global 
warming consequences that a hydrogen economy might entail. In addition to direct hydrogen emissions, non-
H2  greenhouse  gases  may  be  emitted  throughout  the  renewable  hydrogen  value  chain:  from  the  CO2 
emissions  to  produce  the  concrete  and  steel  used  in  the  infrastructure,  through  the  electricity  to  power  the 
electrolysers, to the emissions associated with transport and use the hydrogen. At the same time, a hydrogen 
economy  may  reduce  methane  emissions  thanks  to  a  lower  demand  for  fossil  fuels.  These  aspects  are  not 
taken  into  account  in  the  models  presented  in  this  section,  focused  only  on  the  global  warming  potentially 
caused by hydrogen emissions, but they are presented in more detail in section 3. 

The  scientific  community  agrees  that  an  increase  in  the  concentration  of  hydrogen  in  the  atmosphere 
indirectly leads to a warming of the planet. Overall, Paulot et al. estimated via the NOAA’s GFDL-AM4 model 
an effective radiative forcing (ERF) for hydrogen of 0.13 mW/m2/ppbv (Paulot et al., 2021). This means that 
an instantaneous increase in hydrogen concentration in the atmosphere by 1 part per billion by volume (ppbv) 
leads  to  a  change 4  in  the  energy  balance  of  the  Earth  system,  increasing  the  energy  flux  directed  towards 

3 Although not assessed yet, the direct impact of hydrogen emissions on aerosols and nitrous oxide is expected 
to be small. Nevertheless, the transition from fossil fuels to hydrogen will affect their concentrations.  
4  The  change  in  radiative  forcing  is  not  instantaneous,  as  the  hydrogen  molecule  does  not  directly  absorb 
infrared radiation. The change happens when hydrogen is oxidised, indirectly affecting the radiative forcing 
of methane.  

8 

 
 
                                           
Earth by 0.13 mW/m2. This additional energy flux will lead to a variation in the Earth’s temperature to reach a 
new  radiative  equilibrium  (i.e.,  incoming  radiation  equal  to  the  outgoing  radiation  at  the  top  of  the 
atmosphere).  Due  to  the  lifetimes  of  the  different  species  involved  in  the  process  (i.e.,  hydrogen,  methane, 
water vapour, ozone), it takes at least a decade for the atmospheric radiative forcing to equilibrate after the 
perturbation  caused  by  hydrogen  emissions.  Different  processes  related  to  hydrogen  oxidation  in  the 
atmosphere  contribute  to  the  ERF.  In  the  model  developed  by  Paulot  et  al.,  46%  of  hydrogen’s  radiative 
forcing is due to longer lifetime of methane caused by the hydrogen emission. Overall, the reaction of H2 with 
OH leads to a feedback mechanism where less OH is available to react with methane. Since the reaction with 
the  hydroxyl  radical  is  the  primary  sink  for  methane  (CH4  +  OH  (+O2)  →  CH3O2  +  H2O),  its  lifespan  in  the 
atmosphere  is  extended  and  its  concentration  increases  when  OH  concentrations  decline.  The  remaining 
radiative forcing of hydrogen is attributed by Paulot et al. to the increased production of water vapour in the 
stratosphere (28%), and to the increased ozone in the troposphere (21%) and in the stratosphere (5%). The 
increased  production  of  ozone  is  due  to  the  fact  that  both  the  hydroperoxyl  radicals  (HO2)  formed  via 
oxidation and the extra methane available in the atmosphere are tropospheric ozone precursors. 

In the following sections, metrics to estimate the potential global warming impact of hydrogen emissions are 
presented. First, the Global Warming Potential over 100 and 20 years of a pulse emissions is presented (2.2.1 
and  2.2.2,  respectively).  Then,  the  potential  global  warming  impact  arising  from  continuous  hydrogen 
emissions  is  discussed  (2.2.3).  Finally,  some  discussion  regarding  the  metric  to  adopt  when  assessing  the 
climate impact of hydrogen emissions are provided (2.2.4).   

2.2.1  GWP100 

To  better  grasp  the  climate  effects  of  hydrogen,  its  radiative  forcing  is  compared  against  CO2  (i.e.,  the 
reference climate forcing agent). A widely used approach to appreciate the effect on climate of emissions is 
to integrate the radiative forcing over time, taking into account the lifetime of the species involved. The most 
adopted  metric  based  on  this  approach  is  the  global  warming  potential  (GWP).  This  metric  compares  the 
effects of an idealised pulse over 1 year of 1 kg of some climate forcing agent (e.g., H2) against the effects of 
a pulse of 1 kg of CO2. This allows one to describe the different greenhouse gases in terms of CO2 equivalent 
emissions (CO2e), simplifying the design of climate policies addressing multiple gases. The GWP of the forcing 
agent depends on the timespan over which the potential is calculated: gases which are quickly removed from 
the  atmosphere  may  initially  have  a  large  effect,  but  for  longer  time  periods  become  less  important.  To 
implement  the  multi-gas  approach  embedded  in  the  United  Nations  Framework  Convention  on  Climate 
Change  (e.g.,  Kyoto  Protocol  and  COP21),  a  time  horizon  of  100  years  for  the  GWP  (i.e.,  GWP100)  has  been 
adopted as the standard metric. 

There are currently only three model estimates for the GWP of hydrogen: the estimate based on the radiative 
forcing  calculated  by  Paulot  et  al.  (2021),  the  estimate  proposed  by  Derwent  et  al.  (2020)  based  on  the 
chemistry-transport  model  STOCHEM-CRI,  and  the  recently  published  estimate  from  Warwick  et  al.  (2022). 
Models can have either an emission- or concentration-driven approach. Concentration-based models typically 
prescribe  a  surface  concentration  for  the  different  species  (e.g.,  H2)  at  each  latitude  band,  based  on  sparse 
observations. Since the real world distribution of the gases is unlikely to be perfectly zonally symmetric (i.e., 
invariant around a band of latitude), concentration-based models likely add sources and sinks in places where 
there  might  not  be  sources/sinks.  On  the  other  hand,  emission-driven  models  allow  the  different  species  to 
freely evolve in the atmosphere, implementing sources and sinks of H2 in places where exchanges are known 
to happen. The downside of this approach is that concentrations at the different latitudes are inevitably not 
effectively reproduced. 

Derwent et al. (2020) uses an emission-driven approach  for all species (i.e., H2, CH4,  O3),  while Paulot et al. 
(2021) uses an emission-driven approach for H2 and O3, but prescribes CH4 as a lower boundary condition5. In 
their  models,  the  effect  that  an  extra  pulse  of  hydrogen  has  on  the  atmospheric  chemistry  over  a  certain 
period of time is monitored (e.g., the impact on methane and ozone concentrations), allowing an estimate of 
the  variation  in  the  radiative  forcing.  In  Figure  4,  the  decay  rate  of  the  different  substances  affected  by  a 
pulse  emission  of  hydrogen  estimated  by  Stevenson  and  colleagues  is  schematically  shown.  The  scheme  is 

5 Although Paulot et al. use a concentration-driven approach for methane in their model, its lifetime is affected 
by the freely evolving OH. This effect can be used to indirectly calculate how CH4 would respond if it were 
not prescribed (Paulot et al., 2021). 

9 

                                           
hand-drawn, but the actual model results for CH4 and O3 for 5 years following a pulse of H2 emissions from 
Derwent et al. (2020) are superimposed. The decay rate of the substances is affected both by the initial H2 
pulse  emission  (1  Tg)  and  the  methane  perturbation:  H2,  CH4,  and  O3  concentrations  increase  following  the 
hydrogen  emission,  then  decay  according  to  the  lifetime  of  H2  (~2  years)  and  CH4  (~12  years)  in  the 
atmosphere.  Methane  concentration  increases  over  approximately  3  years  following  the  pulse  H2  emission, 
and then  decays  with an e-folding time 6 of 12 years (Derwent et al., 2020).  Model results superimposed in 
Figure 4 clearly show the faster response of ozone than methane to the hydrogen emission (i.e., the peak of 
ozone concentration is reached earlier than methane’s peak). Conversely to methane and ozone, the hydroxyl 
radical concentration initially decreases  due to the  H2 emission and the extra CH4 produced, then returns to 
the almost original concentration. The negative OH anomaly induced by the H2 pulse initially decays with the 
H2 lifetime, and later with the CH4 lifetime. 

Figure 4. Hand-drawn schematic of the global burden perturbations relative to control run after adding 1 Tg 
instantaneous pulse of H2. In the case of methane and ozone, the actual model results for 5 years following a pulse of H2 
emissions are superimposed on the hand-drawn schematic. 

Source: adapted from presentation by D. Stevenson 

To obtain the GWP100, the variation in the radiative forcing estimated by the models are extrapolated out to 
100 years, integrated, and normalised to a pulse of the same mass of CO2. The resulting GWPs vary from 5±1 
kg CO2e/kg H2 based on Derwent et al.’s model (2020) to 11±5 kg CO2e/kg H2 based on Paulot’s et al. (2021) 
and Warwick et al. (2022) models. The discrepancy in the values arises at least partially from the attribution 
of the climate effect to different phenomena: while Paulot et al. (2021) and Warwick et al. (2022) attributed 
approximately  one  third  of  the  forcing  to  the  increased  production  of  water  vapour  and  ozone  in  the 
stratosphere, Derwent et al. did not include these phenomena in their model and attributed half of the climate 
effect  to  the  methane  route  and  half  to  the  tropospheric  ozone  (whereas  the  attribution  in  Paulot  et  al. 
indicated  the  contribution  from  methane  was  more  than  double  that  from  tropospheric  ozone).  Different 
tropospheric  lifetimes  are  obtained  for  hydrogen  and  methane  in  the  two  models:  while  Derwent  et  al. 

6 Timescale for a quantity to decrease to 1/e of its previous value 

10 

 
 
                                           
obtained a shorter lifetime (1.5 years for H2 and 7.2 years7 for CH4), Paulot et al. obtained a lifetime of 2.1 
and 8.5 years for hydrogen and methane, respectively. Hence different representations of chemistry, as well 
as soil deposition, influence the derived GWP values. 

The GWP figures are affected by a high level of uncertainty. In addition to the aforementioned uncertainties 
regarding hydrogen sources and sinks (e.g., the hydrogen deposition velocity), a source of uncertainty in the 
model presented by Stevenson is the limited timescale of the simulation (i.e., 5 years). Assumptions need to 
be  made  to  extrapolate  the  curves  over  100  years;  this  is  a  problem  in  particular  for  ozone,  given  the 
uncertainty  on  the  exact  point  in  time  where  its  decay  kinetic  switches  from  the  relatively  short  lifetime  of 
hydrogen to the longer one of methane. Another source of uncertainty is the dependence of the lifetime on 
the  emission  latitude.  Concentration-based  models  likely  result  in  higher  H2  emissions  in  the  southern 
hemisphere, where H2 has a longer lifetime, thus generating a larger GWP (Field and Derwent, 2021). Figure 5 
highlights the dependence of the GWP estimates by Derwent et al. to the emission latitude (left panel) and to 
the hydrogen deposition velocity (right panel).  

Figure 5. GWP variation depending on emission latitude (left) and deposition velocity (right) 

Source: presentation by D. Stevenson, produced by D. Derwent using the model described in Field and Derwent (2021) 

2.2.2  GWP100 vs. GWP20 

To assess the climate effect of anthropogenic emissions, different timescales can be considered. In addition 
to GWP100, a metric widely adopted to present the global warming impact of short-lived gases is the GWP20: 
i.e., the global warming potential over 20 years. The GWP20 for hydrogen based on the model from Derwent et 
al. can be seen for instance in the left panel of Figure 5, with a value (~12 kgCO2e/kg H2) that is more than 
double  the  GWP100.  Warwick  et  al.  (2022)  obtained  a  GWP20  for  hydrogen  of  33  kgCO2e/kg  H2,  with  an 
uncertainty  range  of  20  to  44.  To  avoid  any  misunderstanding,  the  GWP  refers  to  the  amount  of  hydrogen 
released to the atmosphere, not to the amount of hydrogen supplied. For instance, if the hydrogen lost to the 
atmosphere  was  1%  of  the  hydrogen  supplied,  the  GWP20  for  1  kg  of  H2  supplied  would  be  0.33  kg  CO2e 
(using the values proposed by Warwick et al. (2022)). 

7  The  atmospheric  lifetime  (or  e-folding  lifetime)  differs  from  the  perturbation  lifetime  (i.e.,  12  years  for 
methane according to the best IPCC estimates) since the atmospheric lifetime do not include the feedback 
effects.  Methane  emissions  poison  their  own  sink,  which  significantly  lengthens  the  residence  time  of  a 
methane  pulse  added  to  the  atmosphere.  This  feedback  factor  varies  a  little  between  models  (range 
approximately 1.19-1.45), with a typical value of ~1.3 (Stevenson et al., 2020). 

11 

 
 
 
                                           
The difference between the GWP100 and GWP20 values can be explained by the different lifetimes of CO2 and 
the  species  involved  with  the  hydrogen  perturbation.  Hydrogen  and  methane  have  shorter  lifetimes  in  the 
atmosphere  (approximately  2  and  12  years,  respectively)  than  CO2,  for  which  it  takes  several  hundreds  of 
years to be removed from the atmosphere by natural processes. In Figure 6 the atmospheric mass increment 
following a pulse emission (1 Tg) of hydrogen and its radiative forcing is compared to the one of 1 Tg of CO2. 
In the left panel it can be seen how the hydrogen mass in the atmosphere disappears almost completely after 
20 years, while approximately 40% of the CO2 is still present after 100 years. In the same panel, it can be 
noticed also the effect on methane’s concentration: the hydrogen emission causes the presence of extra mass 
of methane in the atmosphere, which almost entirely disappears after 70 years. To avoid any confusion, the 
model takes into account the pulse of H2, but not the potential displacement of methane emissions. The right 
panel  of  highlights  how  the  mass  variation  in  the  atmosphere  affects  the  radiative  balance:  while  the 
radiative  effect  of  hydrogen  (and  methane)  ends  long  before  the  100-year  timescale,  CO2  affects  the 
radiative balance throughout the time. Given that the GWP is the ratio of the integral of the radiative forcing 
caused by the hydrogen emission with the one caused by the CO2 emission, it is clear how the GWP20 results 
in a larger value for hydrogen than GWP100.  

Figure 6. Comparison of the atmospheric mass increment (left) and radiative forcing with time (right) of pulse emissions 
of hydrogen and carbon dioxide. The scale of the y-axis in the right panel is logarithmic. 

Source: presentation by D. Hauglustaine 

2.2.3  Pulse vs. continuous emissions  

Another  method  to  assess  the  global  warming  impact  of  a  gas  is  to  account  for  the  effect  of  continuous 
(sustained) emissions rather than pulse emissions. This allows to capture some aspects of short-lived climate 
forcers  overlooked  in  the  GWP:  i.e.,  the  temporal  dynamics  are  more  realistically  reflected  in  the  resulting 
estimates of net radiative forcing impacts (Alvarez et al., 2012). In Figure 7, the GWP (left panel) is compared 
to the cumulative radiative forcing of hydrogen relative to that from equal CO2 emissions (right panel). When 
sustained  emissions  are  considered,  the  cumulative  radiative  forcing  of  hydrogen  over  100  years  is 
significantly higher (approximately double) than the GWP100 assessed from a 1 year pulse of emissions. When 
the Paulot et al. model is considered, the cumulative radiative forcing of hydrogen emissions is approximately 
20 times the one of CO2. Although the short lifetime of hydrogen would eventually lead to a stable higher H2 
concentration in the atmosphere (as opposed to an increasing CO2 concentration), the higher indirect radiative 
forcing caused by H2 oxidation leads to a larger warming effect relative to CO2 than when a metric based on 
pulse emissions is used (i.e., GWP).  

12 

 
 
 
 
Figure 7. Cumulative radiative forcing of hydrogen relative to carbon dioxide for equal emissions over time. The unit of 
the x-axes (time horizon) is years. 

Source: Ocko and Hamburg (2022) 

2.2.4  Which metric to use? 

The choice of the metric to adopt when assessing the climate impact of short-lived pollutants (e.g., hydrogen) 
is often a source of controversy. Every metric has flaws, and the choice must depend on the purpose of the 
analysis  (e.g.,  which  aspect  of  climate  change  is  most  relevant  for  the  application  and  over  which  time 
horizon). For this reason, the latest IPCC reports do not recommend an emission metric, and report emissions 
and mitigation options for individual gases rather than aggregating them as CO2e (IPCC, 2021; IPCC, 2022). In 
line with the IPCC, this JRC report recommends always providing the amount of hydrogen emitted rather than 
the CO2e alone.  

When the global warming impact of hydrogen needs to be aggregated to other GHGs for policy purposes, the 
metric that aligns  best  with  the goal of the policy  should be  used. The latest  IPCC report  chose the GWP100 
metric  to  aggregate  emissions  in  order  to  align  with  decisions  under  the  Paris  Agreement  Rulebook  (IPCC, 
2022). Nevertheless, the use of other metrics is useful to have a more complete understanding of the climate 
impact of  hydrogen emissions. For instance, the GWP20 allows to assess  potential unintended consequences 
(e.g., creating near-term problems while aiming for long-term solutions). It has been suggested that reporting 
both  GWP20  and  GWP100  together  is  helpful  for  policy  makers  to  be  able  to  better  understand  any  temporal 
trade-offs, which are relatively large for hydrogen given its very short residence time in the atmosphere (Ocko 
et  al.,  2017).  The  use  of  end-point  metrics  such  as  the  global  temperature  change  potential  (GTP)  allow  to 
assess the temperature change in a specific year and not the integrated effect. For H2, the GTP100 is estimated 
to  be  7  times  smaller  than  the  GWP100,  further  highlighting  the  smaller  effect  that  a  pulse  of  hydrogen 
emission  has  on  long-term  climate  change  compared  to  shorter  timescales.  On  the  other  hand,  a  climate 
model that considers continuous emissions allows assessing the impact that a sustained or growing hydrogen 
economy might have on the climate. 

13 

 
 
 
3  Global warming impact of a hydrogen economy 

In this section, considerations regarding the potential global warming impact from the deployment at scale of 
hydrogen are presented. In Section 3.1, a brief introduction to the hydrogen economy and the potential future 
hydrogen demand are presented. In Section 3.2 the potential hydrogen emissions from the hydrogen economy 
are  discussed  and  estimated.  A  hydrogen  economy  may  alter  the  radiative  balance  of  the  atmosphere  not 
only via hydrogen emissions, but also via all the direct and indirect greenhouse gases emitted along the value 
chain. Considerations and examples of non-hydrogen emissions that also result from the transition towards a 
hydrogen economy are presented in Section 3.3. In Section 3.4 some potential implications that a hydrogen 
economy  would  have  on  natural  gas  uses  and  methane  concentration  in  the  atmosphere  are  discussed. 
Finally, Section 3.5 states the overall climate consequences from the transition towards a hydrogen economy. 

3.1  H2 demand 

Hydrogen  is  considered  as  a  key  energy  carrier  in  many  future  energy  scenarios,  especially  related  to  the 
increased  usage of renewable energy sources,  due to its  versatility. Hydrogen offers  ways to decarbonise a 
range of sectors for which it is proving difficult to significantly reduce emissions using renewable electricity 
(e.g.,  long-distance  transport,  chemicals,  steel).  The  demand  for  hydrogen,  which  has  grown  more  than 
threefold since 1975, continues to rise (see Figure 8). This growth has been driven to a large extent by the US 
and  European  mandates  to  produce  cleaner  fuels  (e.g.  hydrogenation  /  hydro-treatment  for  the 
desulphurisation of diesel). 

Figure 8. Global annual demand for hydrogen since 1975 

Source: International Energy Agency (2019) 

Hydrogen  is  the  third  most-abundant  element  on  the  Earth’s  surface,  although  it  is  usually  combined  with 
other  elements  forming  molecules  from  which  it  must  be  extracted.  Owing  to  its  high  reactivity,  most 
hydrogen exists combined with oxygen as water or bound to carbon forming liquid or gaseous hydrocarbons. 
Thus, hydrogen is typically not a primary energy source like coal or natural gas, but an energy carrier that is 
produced  by  using  energy  from  a  range  of  primary  feedstocks.  As  can  be  seen  in  Figure  8,  in  2018  the 
demand  for  hydrogen  was  more  than  70  million  tonnes  per  year.  This  hydrogen  is  almost  entirely  supplied 
from  fossil  fuels  (95  %),  most  of  it  is  produced  from  natural  gas,  some  from  coal,  and  a  small  fraction  is 
produced  from  water  electrolysis.  Figure  9  shows  the  main  applications  for  hydrogen  today  which  are  oil 
refining  and  ammonia  production,  mainly  for  fertilisers.  A  further  42  million  tonnes  of  demand  exists  for 
hydrogen as part of a mixture of gases, such as synthesis gas, for fuel or feedstock. The main applications for 

14 

 
hydrogen as part of a  mixture of gases are  methanol  production and  steel production (International Energy 
Agency, 2019). 

Figure 9. Hydrogen current value chains 

Source: International Energy Agency (2019) 

Different  scenarios  are  available  for  future  global  hydrogen  demand.  Presenters  at  the  workshop  mainly 
referred to two scenario studies from the Hydrogen Council for their analyses. In 2017, the Hydrogen Council 
estimated  that  by  2050  hydrogen  could  account  for  approximately  20%  of  the  final  energy  demand 
(Hydrogen Council, 2017). This would imply a ten-fold increase of the current hydrogen demand (i.e., from 8 
EJ in 2015 to almost 80 EJ in 2050). In terms of mass, the demand would increase from approximately 90 Mt 
in  2020  (IEA,  2021)  to  550  Mt  in  2050.  This  is  in  line  with  the  International  Energy  Agency’s  net  zero 
emissions  scenario,  where  200  Mt  of  hydrogen  are  assumed  to  be  needed  in  2030  and  more  than  500  in 
2050  (IEA,  2021).  The  most  recent  study  from  the  Hydrogen  Council  is  even  more  ambitious,  estimating  a 
demand of hydrogen in 2050 of 660 Mt, equivalent to 22% of global final energy demand (Hydrogen Council 
and McKinsey & Company, 2021). To assess the theoretical maximum environmental impact from a hydrogen 
economy, Hamburg and colleagues investigated also a scenario where all the final energy demand was met 
by hydrogen (see Section 3.5), corresponding to 400 EJ and 2,750 Mt H2. 

3.2  H2 losses 

3.2.1  Sources of hydrogen losses across the value chain 

Hydrogen  can  be  produced  from  a  wide  variety  of  sources  and  used  in  a  wide  variety  of  applications,  with 
value  chains  containing  different  combinations  of  supply,  handling  and  demand  technologies.  The  different 
production  processes,  the  transport  and  distribution  options,  as  well  as  the  variety  of  end-uses,  have 
associated some hydrogen losses which constitute potential additional hydrogen sources to the atmosphere. 
Hydrogen  emissions  can  either  result  from  an  unintended  release  (e.g.,  out  of  normal  events  such  as  an 
unplanned safety venting, breaches or component failure), from operational releases such as depressurisation 
events or from design features which typically result in very small releases (e.g., permeation through vessel 
walls, seals, etc.). Hydrogen releases can happen at any stage of the hydrogen value chain. Guy de Reals from 
Air  Liquide  and  Pere  Margalef  and  Davide  Scrocchi  from  SNAM  provided  estimates  for  credible  losses  of 
hydrogen along the chain. 

For  hydrogen  production,  losses  are  currently  estimated  around  0.2%  when  produced  via  electrolysis.  In 
addition to accidental leakage, hydrogen losses are mainly linked to hydrogen and oxygen crossover through 
the membrane, and to  processes  such as dryers’ regeneration. By  2030,  hydrogen losses  during electrolytic 
production are expected to drop to 0.03% thanks to the maturing of the technology (e.g., reduced crossover 
through the membrane). On the other hand, hydrogen losses are very limited (0.0001%) when produced via 

15 

 
 
steam methane reforming (SMR). Losses from SMR are currently typically flared, not causing an increase of 
hydrogen  concentration  in  the  atmosphere.  Hydrogen  losses  in  the  delivery  step  varies  with  the  form  of 
hydrogen. In Figure 10 a summary of the current and future hydrogen losses along the hydrogen supply chain 
estimated by Air Liquide is presented. Figures are based on operation feedback and know-how.  

Figure 10. Hydrogen release fractions to the atmosphere along the hydrogen supply chain and design goals for 2030 

Source: presentation by G. de Reals (Air Liquide) 

Filling and transportation of  compressed  hydrogen via tube trailers is a mature technology with low losses, 
estimated as less than  1% of the  delivered hydrogen. These losses are mainly linked to the purging of the 
trailer  hose,  and  to  leakages  from  fittings  and  valves.  On  the  other  hand,  the  delivery  chain  for  liquefied 
hydrogen is still under development and currently more prone to leakages. Air Liquide estimates an average 
loss  of  10%  of  the  delivered  hydrogen,  related  mainly  to  the  liquefaction  process  and  the  boil-off  during 
transfer operations. Due to the high economic value of hydrogen and the low maturity level of liquid hydrogen 
delivery for widespread fuelling applications, these supply chain losses will likely reduce in the coming years. 
The reduction is expected to arise from improvements in the supply chain and from the recycling of the boil-
off.  Moreover,  some of the  hydrogen that is currently vented is expected to be flared  in the future, so that 
only  water  vapour  will  be  emitted  to  the  atmosphere.  Air  Liquide  estimate  for  2030  is  a  2%  loss  of  the 
delivered liquefied hydrogen at the hydrogen refuelling stations and an additional 2% loss due to dispensing. 
Finally, international transportation via pipelines in Europe has a current estimated leakage of approximately 
1% of the delivered hydrogen, which is expected to reduce to less than 0.7% by 2030.  

Additional  losses  happen  at  the  end-use  site:  refuelling  stations  for  instance  lose  from  3%  to  8.5%  of  the 
delivered  hydrogen,  depending  on  the  form  of  hydrogen  (i.e.,  gaseous  vs.  liquid).  Losses  at  the  refuelling 
station are mainly due to purging and, in the case of liquefied hydrogen, boil-off. Since refuelling technologies 
are  still  under  development,  the  margin  for  improvement  in  reducing  losses  is  high.  Air  Liquide  target  for 
2030 is 2% losses of the delivered hydrogen.  

In total, the current compressed hydrogen supply chain is estimated by Air Liquide to lose to the atmosphere 
approximately  4.2%  of  the  hydrogen  delivered  to  the  gas  stations,  while  the  current  losses  for  liquefied 
hydrogen  can  reach  up  to  20%  of  the  hydrogen  delivered  to  liquid  stations.  Losses  are  expected  to  reduce 
significantly  in  2030,  with  average  losses  to  the  atmosphere  of  3%  and  4-5%  for  gaseous  and  liquefied 
hydrogen, respectively. The delivery pathway with lower losses is via pipeline, with expected losses in 2030 of 
less than 1% of the delivered hydrogen.  

Hydrogen losses via pipelines were also estimated by SNAM, the main Italian operator for the transport and 
dispatching of natural gas in Italy. Based on their experience with methane losses, they calculated how much 
hydrogen  could  leak  if  their  pipelines  were  converted  for  hydrogen  transport.  According  to  ASME  B31.12 
(ASME, 2020), all of their existing pipelines are ready to transport 100% hydrogen, most of which (70%) with 
no or limited reductions on the maximum operating pressure. Four types of losses currently occur in SNAM’s 
natural gas pipelines: fugitive emissions due to tightness failure (50%), intentional and unintentional vented 
emissions  (26%),  pneumatic  emissions  resulting  from  gas  operated  devices  such  as  controllers,  positioners, 

16 

 
 
and actuators (23%), and unburned methane in the exhaust gases from natural gas combustion devices (1%). 
Overall, these losses represent 0.038% of the total volume of natural gas injected in the SNAM’s grid. When 
repurposed  to  hydrogen  pipelines,  pneumatic  emissions  will  cease  to  exist  thanks  to  the  replacement  of 
pneumatic actuators by electrical actuators. On the other hand, fugitive hydrogen emissions are expected to 
be 1.25 to 3 times the methane ones (depending on the dynamics of the flow in the pipeline, whether laminar 
or turbulent)  due to the different gas characteristics (Kiwa Technology,  2019). Finally, vented emissions are 
not  expected  to  change  due  to  the  same  volume  displacements,  and  incomplete  combustion  emissions  are 
considered to be negligible. 

3.2.2  Overall hydrogen losses 

Air  Liquide  estimated  the  potential  overall  hydrogen  losses  to  the  atmosphere  by  combining  the  hydrogen 
losses data with the expected hydrogen demand. Results are presented in Table 3. For hydrogen demand, 140 
Mt  are  assumed  to  be  needed  in  2030,  and  660  Mt  in  2050  (Hydrogen  Council  and  McKinsey  &  Company, 
2021). The figures in Table 3 are calculated assuming that all the end uses except for mobility are supplied 
by  pipelines,  while  transportation  is  covered  for  70%  by  liquefied  hydrogen  and  30%  by  compressed 
hydrogen. The share of hydrogen lost out of the total hydrogen produced and its contribution to the overall 
hydrogen sources to the atmosphere are also reported in Table 3. For the hydrogen sources, a value for 2020 
including  both natural and anthropogenic sources was considered (i.e., ~90  Mt). Results show that by 2050, 
hydrogen lost to the atmosphere would represent approximately 2% (i.e., 13 Mt) of all the hydrogen produced. 
This  means  an  increase  of  approximately  10%  of  the  overall  hydrogen  sources  to  the  atmosphere.  The 
increase from 2030 to 2050 of the hydrogen losses is due to the expected larger use of liquefied hydrogen 
after  2030.  The  average  losses  estimated  by  AirLiquide  for  2050  are  significantly  lower  than  the  ones 
estimated in a recent report by Center on Global Energy Policy of the Columbia University (Fan et al., 2022), 
where a range from 2.9% (low-risk case) and  5.6% (high-risk  case) loss  rate is estimated for the  hydrogen 
economy in 2050. Most of the losses (54%) are assumed to happen in the production phase, while delivery 
and  end-use  losses  are  assumed  to  account  for  22%  and  24%,  respectively.  Assuming  a  total  hydrogen 
demand of 660 Mt, total losses would reach a value of 37 Mt in the high-risk case.  

Table 3. Potential hydrogen losses from the deployment of hydrogen at scale. 

End-use 

Unit 

2020 

2030 

2050 

Power generation buffering 

Transportation 

Building heat and power 

Industry feedstock 

Total losses 

Share losses of hydrogen produced 

Share industry losses of hydrogen sources 

Mt 

Mt 

Mt 

Mt 

Mt 

% 

% 

0 

0.18 

0.47 

0.02 

0.20 

10.6 

0 

0.07 

0.80 

1.08 

0.73 

1.46 

1.10 

1.18 

13.3 

1.2 

1 

0.8 

1 

2 

10 

Source: adapted from presentation by G. de Reals (Air Liquide) 

17 

 
 
3.3  A life cycle view on H2 economy emissions 

The deployment of hydrogen at scale affects the planet’s temperature not only because of hydrogen losses, 
but also because it implies the increase or reduction of  other direct and indirect  greenhouse  gas emissions 
(e.g.,  CO2,  CH4,  NOx).  To  investigate  the  overall  climate  implications  of  a  hydrogen  economy,  life  cycle 
assessments  (LCAs),  taking  into  account  all  the  changes  in  emissions  due  to  hydrogen  production  and  use, 
should be performed (European Commission, 2003). Replacing fossil fuels with hydrogen is expected to lead 
to  significant  GHG  emission  reductions  because  it  does  not  emit  CO2  when  used.  However,  the  life  cycle  of 
hydrogen is not free of emissions: from the energy to produce hydrogen, through the losses along the supply 
chain, to the emissions when used. Even when the hydrogen is produced by renewable sources, emissions are 
generated  to  build  the  renewable  energy  infrastructure  or  when  the  hydrogen  is  transported  and  used.  For 
instance, hydrogen produced via electrolysis powered by  solar power is expected to  generate approximately 
1.0  kg  CO2e/kg  H2  in  2030,  and  0.6  kg  CO2e/kg  H2  in  2050  due  to  production  of  the  materials  used  in  the 
infrastructure  (Hydrogen  Council,  2021).  Depending  on  the  leakage  rate,  these  emissions  could  generate  a 
larger  impact  on  climate  than  the  hydrogen  lost  along  the  supply  chain.  Hydrogen  end-use  can  generate 
indirect  greenhouse  gases  too:  when  hydrogen  is  combusted  in  high-temperature  engines,  it  produces  NOx 
which  warm  the  planet  by  increasing  the  amount  of  ozone  in  the  troposphere  (Lammel  and  Graßl,  1995). 
Although  an  LCA  of  the  overall  climate  impact  of  a  hydrogen  economy  is  outside  the  scope  of  the  present 
report,  a  couple  of  examples  provided  during  the  workshop  on  how  the  use  of  renewable  hydrogen  could 
affect global warming via non-H2 emissions are summarised in this section: hydrogen delivered via ammonia 
(3.3.1), and hydrogen used as fuel for aviation (3.3.2).  

3.3.1  Hydrogen delivery: ammonia 

Hydrogen delivery is expected to play an important role in the global decarbonisation efforts. The European 
Hydrogen  Strategy  for  instance  recognises  the  important  role  that  the  transport  of  hydrogen  will  play  in 
enabling  the  market  penetration  of  renewable  hydrogen  in  Europe  (European  Commission,  2020).  One 
attractive  option  to  transport  and  deliver  hydrogen  is  to  convert  it  into  ammonia  and  use  the  existing 
infrastructure  to  deliver  it  where  needed  (Ortiz  Cebolla,  Dolci,  and  Weidner,  2021).  Despite  the  clear 
advantage of transporting hydrogen via a carrier with a higher volumetric energy density using a consolidated 
infrastructure, the transformation and shipping processes could generate unintended consequences in terms 
of  environmental  impact.  Ammonia  is  not  a  direct  GHG,  but  when  emitted  to  the  atmosphere  it  leads  to 
particulate  matter  formation,  acidification,  and  eutrophication,  affecting  air  quality  and  biodiversity  (Van 
Damme  et  al.,  2018).  Moreover,  ammonia  and  ammonium  deposited  on  soils  can  be  converted  by  bacteria 
into  nitrous  oxide,  indirectly  contributing  to  global  warming  (Wrage-Mönnig  et  al.,  2018).  Although  these 
impacts could be avoided if ammonia was not lost throughout the supply chain, ammonia users for fertiliser 
production  and  other  industrial  uses,  were  identified  by  infrared  satellite  images  as  hotspots  for  ammonia 
emissions(Van  Damme  et  al.,  2018).  Moreover,  direct  (e.g.,  CO2)  and  indirect  (e.g.,  NOx)  greenhouse  gases 
could be emitted in the conversion process of hydrogen to ammonia and during ammonia combustion (if used 
directly as fuel), respectively. If ammonia was reconverted to hydrogen at the delivery site, a recent JRC study 
showed  that  the  life  cycle  GHG  emissions  of  the  H2  delivery  would  be  higher  in  the  case  of  ammonia 
compared to deliver it as compressed or liquefied H2 (Arrigoni et al., 2022).  

3.3.2  Hydrogen use: fuel for aviation 

A  particularly  relevant  case  for  the  global  warming  consequences  of  a  hydrogen  economy  is  aviation. 
Liquefied hydrogen is considered a potential solution to decarbonise the aviation sector, but research is still 
underway  on  the  effects  that  hydrogen-powered  aircraft  would  have  on  climate.  In  addition  to  the  CO2 
emitted  when  producing  and  using  the  fuel,  non-CO2  emissions  during  flights  (i.e.,  water  vapour,  nitrogen 
oxides,  soot  particles,  and  oxidised  sulphur  species)  currently  alter  the  chemical  composition  of  the 
atmosphere and the cloudiness, perturbing the radiative balance. It is estimated that these non-CO2 impacts 
could currently account for two thirds of the overall aviation net positive radiative forcing, though with quite 
an  uncertain  range  (EASA,  2020).  It  is  therefore  fundamental  to  understand  how  fuel  replacement  would 
affect  the  overall  aviation  impact.  Grewe  presented  the  results  of  the  AHEAD  EU-project  (Advanced  Hybrid 
Engines  for  Aircraft  Development)  during  the  workshop,  on  the  potential  global  warming  consequences  of 
replacing aviation’s traditional fuel (kerosene) with liquefied hydrogen (Gangoli Rao, Yin, and P. van Buijtenen, 
2014;  Grewe  et  al.,  2017).  Results  show  that  renewable  hydrogen  could  significantly  reduce  the  global 
warming impact of aviation from a life cycle perspective. Moreover, NOx emissions could be reduced as well 

18 

by optimising the combustion chamber. On the other hand, using hydrogen would produce roughly 3 times the 
water  vapour  produced  with  traditional  fuels.  The  increase  of  atmospheric  water  vapour  increases  global 
warming (currently around 3% of the aviation’s global warming impact is due to water vapour emissions (Lee 
et al., 2021)), though largely depending on the altitude and location where the water  vapour is released. In 
Figure 11 the variation of the water vapour lifetime with the latitude and the pressure altitude (expressed in 
hecto-Pascal) is presented. Most air traffic takes place in the northern hemisphere between 9 and 13 km from 
the  Earth’s  surface.  At  the  lower  boundary  of  the  interval,  water  vapour  emissions  will  remain  in  the 
atmosphere on average for 2 weeks. The situation however becomes critical at higher altitudes, where water 
vapour molecules can linger in the atmosphere for months (for approximately 10 weeks at 13 km).  

Figure 11. Water vapour lifetime 

Source: presentation by V. Grewe, adapted from Grewe and Stenke (2008). 

The  probability  to  form  contrails  will  increase  using  hydrogen,  but  their  lifetime  is  expected  to  decrease. 
Contrails have an average lifetime of a few hours (2-5 hours), but they can occasionally reach a maximum of 
roughly a day. The use of hydrogen is expected to reduce the number of ice particles in the contrail since no 
soot is emitted, leading to lower lifetimes of the contrails and lower temperature changes. Nevertheless, this 
topic needs to be investigated further.  

Based  on  these  assumptions,  the  investigated  AHEAD  aircraft  propelled  with  liquefied  hydrogen  and 
sustainable alternative fuels are expected to reduce the global warming potential by 8 to 12% compared to 
reference  aircraft  using  conventional  fuels  (including  future  enhancements  of  the  overall  propulsion 
efficiency).  The  results  are  very  much  dependent  on  the  investigated  aircraft  type  that  is  operated  at  the 
upper  end  of  current  cruise  altitudes.  However,  the  use  of  hydrogen  as  an  aviation  fuel  very  likely  will  still 
contribute to a net warming of the planet through water vapour, contrails, and NOx effects.  

3.4  Methane 

As  presented  in  Section  2,  the  global  warming  potential  of  hydrogen  emissions  is  partly  linked  to  methane 
concentration in the atmosphere. Current atmospheric models, however, do not take into account that the use 
of  hydrogen  could  reduce  the  demand  for  fossil  fuels  and  therefore  methane  emissions.  A  reduction  in 
methane  emissions  in  the  atmosphere  will  certainly  reduce  global  warming,  both  from  the  reduction  of  its 
concentration  and  the  reduction  in  the  stratospheric  water  vapour.  In  this  section,  the  effect  that  replacing 
natural  gas  with  hydrogen  might  have  on  the  climate  is  explored.  In  Section  3.4.1  the  consequences  on 
methane concentration in the atmosphere when natural gas is substituted with renewable hydrogen or fossil 
hydrogen  with  carbon  capture  are  presented,  while  in  Section  3.4.2  the  climate  consequences  of  replacing 
natural gas with hydrogen in the existing gas pipelines are discussed. 

19 

 
 
3.4.1  Hydrogen production: implications for methane sources to the atmosphere 

In  a  recent  work  (not  yet  peer-reviewed)  presented  by  Paulot,  Bertagni  et  al.  estimated  the  maximum 
hydrogen loss rate that would avoid an increase in methane concentration in the atmosphere via a simple box 
model. Results are shown in Figure 12, where CLR (%) on the y-axis indicates the critical loss rate and EOH on 
the x-axis indicates the amount of OH reacting with compounds other than H2, CO, and CH4 normalised by the 
amount reacting with H2, CO, and CH4. Assuming a hydrogen demand by 2050 of 550 Tg yr-1 (17% lower than 
the  assumption  by  Air  Liquide),  if  all  the  hydrogen  was  produced  via  renewable  sources  (i.e.,  “green  H2”  in 
Figure 12) and no loss occurred, methane sources to the atmosphere would reduce by approximately 18 Tg yr-
1. This reduction is directly linked with the reduction in natural gas use, assuming that anthropogenic methane 
emissions  are  reduced  in  proportion  to  the  reduction  in  fossil-fuel  energy  (~16%  of  110  Tg  yr-1).  However, 
savings would reduce if hydrogen losses occurred or if hydrogen was produced via steam methane reforming 
with carbon capture (i.e., “blue H2”). If all the hydrogen produced was renewable, the maximum hydrogen loss 
rate that would avoid an increase in methane concentration in the atmosphere would be in the range of 5 to 
11%, accounting for uncertainty in OH and the rate of hydrogen uptake from the soil. Under the most realistic 
scenario  (represented  by  the  triangles  in  Figure  12),  the  critical  loss  rate  would  be  7%.  If  the  hydrogen 
produced  was  “blue”  and  no  hydrogen  losses  occurred,  a  methane  loss  rate  of  0.5%  would  lead  to  a  net 
reduction  of  methane  sources  to  the  atmosphere  of  12  Tg  yr-1.  Net  reductions  would  drop  to  6  Tg  yr-1  if 
methane losses were 1%, and there would be no net reduction if methane losses were 1.5%. Even in the best 
case scenario for “blue” hydrogen considered by Bertagni et al. (i.e., 0.5% methane loss rate), hydrogen loss 
rate  should  be  limited  to  5%  to  avoid  an  increase  of  methane  sources  to  the  atmosphere.  The  critical 
hydrogen  loss  rate  would  drop  to  3%  and  0%  in  the  cases  of  methane  loss  rates  of  1%  and  1.5%, 
respectively. The error range in the curves indicate the effect that a variation of 20% in the biological removal 
would  have  on  the  results,  highlighting  the  importance  of  better  understanding  the  processes  governing 
hydrogen concentration in the atmosphere. It is worth highlighting that this study investigates only the effect 
on  methane  concentration,  and  not  the  overall  climate  effect  of  replacing  natural  gas  with  hydrogen.  In 
addition to the potential savings of methane sources, replacing natural gas with hydrogen would lead to a net 
change in the carbon dioxide emissions too. 

Figure 12. Loss rate for hydrogen causing an increase rate in atmospheric methane. CLR: critical loss rate; EOH: amount of 
OH reacting with compounds other than H2, CO, and CH4 normalised by the amount reacting with H2, CO, and CH4. 

Source: presentation by F. Paulot from Bertagni et al. (in review). 

20 

 
3.4.2  Hydrogen delivery: repurposing of natural gas pipelines 

Pere Margalef and Davide Scrocchi presented the potential global warming impact from losses when natural 
gas is replaced with hydrogen in SNAM pipelines. The analysis considers the emissions taking place during gas 
transmission,  storage,  and  regasification;  emission  variations  in  natural  gas  production  and  distribution  are 
not included. Both the global warming potential over 20 and 100 years were considered in the assessment. 
The GWP20 and GWP100 values considered are 84 and 28 for methane, and 70.5 and 21 for hydrogen. Global 
warming  potential  values  for  hydrogen  were  mistakenly  derived  from  the  preprint  of  Ocko  and  Hamburg 
(2022),  and  are  significantly  higher  than  those  reported  in  section  2.2.  Nevertheless,  results  show  that  a 
switch from natural gas to hydrogen in the SNAM pipelines would lead to a reduction of the global warming 
impact from losses of 81% in a 20 year timeframe, and 83% in a 100 year timeframe (Figure 13). In fact, 
notwithstanding  the  higher  fugitive  rate  of  hydrogen  (see  section  3.2),  the  lower  volumetric  density  of 
hydrogen  compared  to  natural  gas  leads  to  lower  CO2e  emissions  when  the  gases  are  transported  at  the 
same pressure (70-75 bar). Given the lower volumetric energy density of hydrogen, the flow rate of hydrogen 
needs  to  be  increased  3  times  to  deliver  the  same  amount  of  energy  delivered  with  natural  gas.  This, 
however, is achieved thanks to the different fluid-dynamics characteristics of natural gas and hydrogen (i.e., 
density, kinematic viscosity, and compressibility). At the same design conditions (i.e., pipeline length, diameter, 
and  pressure  difference  between  starting  and  end  points),  hydrogen  travels  in  fact  three  times  faster  than 
natural gas.  

Figure 13. Global warming implications of substituting natural gas with hydrogen in SNAM pipelines. 

Source: presentation by P. Margalef and D. Scrocchi (SNAM). 

3.5  Overall climate consequences 

In  this  section,  the  first  attempts  to  estimate  the  overall  climate  consequences  of  replacing  fossil  fuel 
systems with hydrogen are presented. This is a developing area of research, and initial models may conflict 
due to different underlying assumptions and extrapolations. This report presents statements given directly by 
the  presenters  at  the  workshop  and  is  not  intended  to  support  or  refute  the  opinions  of  the  presenters.  In 
Figure  14,  the  climate  impact  results  from  the  technology  switch  presented  by  Hamburg  is  shown.  In  their 
analysis, a metric considering sustained emissions was used and an average avoided emission value (11 kg 
CO2/kg  H2)  for  the  replacement  of  fossil  fuel  with  hydrogen  technologies  was  assumed.  Two  hydrogen 
production  pathways  (i.e.,  renewable  (“green”  in  Figure  14)  and  fossil  with  carbon  capture  (“blue”)),  and  a 
range  of  loss  rates  from  1%  to  10%  per  unit  H2  deployed  was  considered.  In  the  case  of  “blue”  hydrogen, 

21 

 
 
methane  loss  rates  from  1%  to  3%  per  unit  methane  consumed8  were  also  considered.  For  all  scenarios 
considered, the climate benefits of replacing fossil fuel with hydrogen are clear in the long term. However, the 
study  shows  the  potentially  deleterious  consequences  for  the  global  temperatures  in  the  short  term  when 
switching to “blue”  hydrogen. The authors estimated that  a switch from  fossil fuel to “blue” hydrogen could 
lead  to  up  to  a  40%  increase  in  warming  over  the  first  10  years,  considering  the  worst  case  scenario  for 
losses  (i.e.,  10%  for  hydrogen  and  3%  for  methane).  On  the  other  hand,  a  switch  to  renewable  hydrogen 
would guarantee climate benefits from the very beginning, with a reduction from 60% to more than 95% of 
the fossil fuel climate impact over the first 10 years. Over 100 years, switching to renewable hydrogen would 
guarantee a climate impact reduction larger than 80% even in the worst case scenario for hydrogen losses 
(10%). Nevertheless, a 100% reduction (i.e., “no H2 climate impact” in Figure 14) would not be possible unless 
all the hydrogen was renewable and there were no losses. 

Figure 14. Relative climate impact over time from replacing fossil fuel systems with renewable (“green”) or fossil with 
carbon capture (“blue”) hydrogen alternatives for a generic case. Error bars represent a 20% uncertainty in hydrogen’s 
radiative efficiency. Corresponding GWP100-derived climate impacts are shown using the “X” and “O” markers, for worst-
case and best-case loss rates, respectively. [The figure was updated by the author with respect to the one presented at 
the workshop]. 

Source: Ocko and Hamburg (2022) 

Hamburg presented also the potential global-mean surface air temperature responses to different hydrogen 
emission scenarios depending on loss rate and extent of deployment. Results show that a hydrogen-intensive 
economy (20% final energy demand, corresponding to a H2 demand of 600 Tg) with high losses (10%) could 
contribute 0.05 °C additional warming in mid-century. It is important to remark that this warming would cease 
to  exist  if  hydrogen  losses  were  prevented,  given  the  short  lifetime  of  the  species  affected  by  hydrogen 
emissions  in  the  atmosphere.  Moreover,  the  analysis  did  not  take  into  account  the  potential  cooling  effect 
generated by a reduction in methane concentration in the atmosphere thanks to fossil fuel replacement with 
hydrogen.   

8 Range for methane losses in the assessment are from Alvarez et al (2018), which assumes an average value 

of 2.3% along the supply chain.  

22 

 
 
 
                                           
Hauglustaine  further  stressed  during  the  workshop  the  clear  climate  benefits  arising  from  a  transition  to  a 
renewable  hydrogen  economy.  Based  on  the  estimations  from  Hauglustaine  and  colleagues  (not  yet  peer-
reviewed), assuming a hydrogen demand by 2050 of 549 Mt/year and a loss rate for hydrogen of 1-3%, the 
CO2  equivalent  emissions  from  using  renewable  hydrogen  would  represent  only  1-4%  of  the  CO2  emission 
avoided (GWP100 based) by replacing fossil fuels. The percentage would drop to 0.2-0.7% if the GTP100 metric 
was  used  instead.  In  other  words,  this  means  that  the  climate  impact  would  be  reduced  by  96  to  99.8%  if 
fossil fuels were replaced with renewable hydrogen. This is in line with what presented by Hamburg (Figure 
14) for the renewable hydrogen scenario with low leakage rate. If “blue” hydrogen is included in the equation 
(assuming a 30% share of the total H2 supply being “blue” H2), the climate impact reduction would be limited 
to 70-80% (i.e., the CO2 equivalent emissions would represent 20-30% of the CO2 emission avoided (GWP100 
based)).  

Although  further  research  is  needed  to  improve  the  accuracy  of  the  estimates  presented  in  this  section, 
results clearly shows the importance of preventing and minimising hydrogen losses along the hydrogen value 
chain  to  minimize  the  climate  impact  of  a  hydrogen  economy.  Better  monitoring  and  quantification  of 
hydrogen concentration in the atmosphere would allow a better assessment of the environmental impact of 
the hydrogen economy, and the prevention of future emissions. In the next chapter (Section 4), the state of 
the art and the main challenges on hydrogen leakage prevention, detection and quantification are presented.    

23 

 
 
 
4  Prevention and monitoring of hydrogen losses 

Chapter  2  and  3  clearly  show  the  importance  of  preventing  hydrogen  losses  to  reduce  the  global  warming 
impact from a full-fledged hydrogen economy. In addition to the potential environmental concerns, there are 
safety concerns when dealing with hydrogen releases. Hydrogen has a wide flammability range (4% to 75% 
by volume), about seven times wider than methane, and the amount of energy required for ignition is small, 
see  Figure  15.  Special  handling  is  required  to  prevent  fires  or  explosions  that  could  result  from  mixing 
hydrogen  with  air.  At  atmospheric  pressures  and  temperatures,  hydrogen  is  a  highly  diffusive,  buoyant, 
colourless,  odourless,  tasteless,  non−irritating,  and  nontoxic  gas.  Additionally,  it  does  not  present  the  same 
optical/thermal  signatures  as  hydrocarbon  leaks,  which  means  different  methods  may  be  needed  for 
detection, as it will be further discussed in Section 4.2. For all these reasons, the prevention and monitoring of 
hydrogen  emissions  is  paramount  in  systems  in  use  today  and  is  reflected  in  the  industry  standards  (Fuel 
Cells  and  Hydrogen  Observatory),  (Kotchourko  and  Jordan,  2022),  European  Industrial  Gases  Association 
(EIGA)  and  US  Compressed  Gases  Association  (CGA).  Many  of  these  standards  have  been  adopted  or 
referenced  by  CEN,  ISO  and  ADR  and  by  regulators.  Section  4.1  details  prevention  measures  to  minimise 
hydrogen releases in hydrogen production facilities and across the supply chain. Section 4.2 expands on the 
detection  and  monitoring  tools  available  at  different  length  scales  and  the  remaining  technological 
challenges. 

Figure 15. The flammability range of hydrogen compared to methane and gasoline 

Source: EIGA presentation 

4.1  Leakage prevention: design considerations 

Hydrogen production is a  highly regulated process. Permits under the industrial emission directive (Directive 
2010/75/EU Industrial Emissions, 2010) are required in Europe, irrespective of the plant size. This corresponds 
to Title V permits in the US (EPA), the highest level of permitting. In addition, ATEX legislation on hazardous 
atmospheres/areas  applies  to  these  facilities  (Directive  2014/34/EU  (ATEX),  2014).  Further,  as  hydrogen 
production is cross sectoral the EIGA has a best available techniques (BAT) guidance document available for 
hydrogen facilities (EIGA Document 155).  

Hydrogen point sources are typically minimised by reusing the fuel gas in the process. For a typical hydrogen 
plant  the  gas  is  purified,  the  pure  gas  is  the  product  and  the  impure  gas  stream  which  contains  some 
hydrogen is returned to the process to be used as fuel. This is part of the permit requirements and represents 
BAT. In emergency cases, hydrogen is usually flared, however, direct releases go through relief valve systems 
at  times,  especially  further  down  the  supply  chain  where  it  is  not  possible  to  install  a  flare.  The  direct 
hydrogen releases often result in a ‘light off’ where the hydrogen is consumed due to low ignition energy. For 
safety reasons vents are elevated due to possible thermal radiation.  

24 

 
The  hydrogen  supply  chain  is  also  regulated,  e.g.  via  the  European  Agreement  concerning  the  International 
Carriage  of  Dangerous  Goods  by  Road  (ADR)  for  road  transport  which  is  the  basis  for  EU  rules  (Directive 
2008/68/EC, 2008) and for many countries globally that have adopted it via the UN, and via the Department 
of  Transportation  (DoT)  in  the  US  and  around  the  world.  These  have  specifications  and  rules  for  container 
design, operation, maintenance and methods to minimise leakage in transit. 

In terms of fugitive emissions control, the plants and pipelines are designed to minimise these emissions by 
reducing  the  number  of  pipe  connections  (potential  leak  points),  by  using  welded  connections  rather  than 
flanges,  and  by  ensuring  that  the  valves  are  leak  tight  (e.g.  the  valves  in  hydrogen  service  are  typically 
designed in accordance with API 602, API 600, API 594 highest specifications). Welded pipes are continuous, 
minimising  leak  points,  whereas  flanged  connections  can  leak  at  the  flanged  connection.  Leak  tight  valves 
have additional packing in the valve to minimise the leaks for the valve stem. Further, the hydrogen facilities 
use plant-wide automatic flammable gas detection systems (see more details on sensors in Section 4.2).  

In addition to the design considerations, there are operational measures to control fugitive emissions that are 
put in place. Some of these operational measures are: 1) proactive leak detection and repairs (leak patrols), 2) 
stringent control during maintenance and line breaking activities using ‘permits to work’ and making sure lines 
are not containing products, 3) periodic maintenance and good operational practices in place so not to set off 
relief valves, and 4) pipeline monitoring systems (e.g. sectioning of the pipeline so sections can be sealed off 
without having to empty the whole pipeline, pipeline patrols), see EIGA document 235/21 on pipeline integrity 
management. 

4.2  Leakage monitoring: detection and quantification methods 

The  main  drivers  for  the  detection  of  hydrogen  releases  have  been  traditionally  associated  with  safety 
concerns and to a lesser extent the maximisation of product throughput in specific applications. Accordingly, 
sensor  development  for  hydrogen  releases  has  been  primarily  oriented  to  detect  hydrogen  on  the  order  of 
1,000 - 10,000 ppm (0.1 to 10 vol %), mainly due to the flammable and explosive behaviour of hydrogen gas. 
Detection  is  currently  intended  to  identify  hydrogen  leakages  and  avoid  accumulation  beyond  the  lower 
flammability  limit  of  4  vol.  %  in  confined  spaces.  Alternatively,  within  the  context  of  the  potential 
environmental  concerns  stated  in  Chapter  2,  the  main  driver  is  the  need  to  monitor  and  quantify  hydrogen 
losses across the whole value chain, resulting from the transition towards a  hydrogen economy. This  would 
require detecting hydrogen a distance away from the source/leak, as is the case for the methane value chain. 
Environmental  impacts  of  hydrogen  may  arise  from  cumulative  effect  of  sub-ppm  levels  of  released 
hydrogen across the value chain. Consequently, the desired monitoring range of interest for hydrogen release 
quantification needs to be extended at the lower end by several orders of magnitude, to about 0.01 to 1 ppm 
(0.001 – 0.1 vol. %). The lower the sensor detection limits, the lower the concentration of hydrogen that can 
be  monitored  and  quantified,  thus  allowing  wide  area  monitoring  at  concentration  levels  relevant  for 
atmospheric  and  environmental  modelling.  The  ultimate  goal  is  to  be  able  to  quantify  more  accurately  the 
hydrogen concentrations in the atmosphere and therefore reliably detect and quantify hydrogen losses along 
the value chain arising from the hydrogen economy. 

Hydrogen  gas  is  colourless  and  odourless,  similarly  to  methane  gas.  However,  it  presents  signature 
differences  with  respect  to  methane,  making  hydrogen  leaks  more  difficult  to  detect.  This  means  that  the 
techniques applied to detect methane and other hydrocarbon leaks are not directly transferable to hydrogen 
in  many  cases.  For  example,  the  optical  detection  of  hydrogen  is  really  challenging.  There  are  many 
commercial products for imaging methane leaks at a wide range of length scales, including increasing precise 
measurements  from  space,  e.g.  aerial  imaging  to  detect  medium  to  large  leaks  and  high  precision  fast 
response gas analysers capable of detecting and quantifying small leaks. These devices provide quantitative 
concentration data using the infrared absorption features of methane, which in turn can be converted to flux 
rates. However, these absorption features are not present for hydrogen, and therefore these devices are not 
applicable (see the comparison of the absorption bands for hydrogen and methane in Figure 16. 

25 

 
 
 
Figure 16. Comparison of absorption spectra of hydrogen and methane 

Source: Sandia National Laboratories  

Gas sensors/detectors are the most common strategy for the direct detection and empirical characterisation 
of  hydrogen  releases.  There  are  numerous  different  platforms  for  the  direct  detection  of  hydrogen.  These 
include  thermal  conductivity  (TC),  combustible  gas  sensors  (CGS),  metal  oxide  sensors  (MOX),  platinum  thin 
films and electrochemical sensors. However, not all these sensors are able to specifically detect hydrogen. For 
example, a particular property of hydrogen that can be measured is its low density but that is also a feature 
of  helium  gas.  Similar  cross-sensitivity  occurs  with  hydrogen  combustion  properties.  Selective  hydrogen 
detection  is  quite  challenging  due  to  cross-sensitivity  issues,  but  is  achievable  with  palladium  thin  film 
sensors. Cross-sensitivity is a phenomenon that occurs when a gas other than the gas being monitored causes 
the gas sensor to show a reading, even when the target gas is not present.  

An  overview  of  the  sensor  technologies  for  hydrogen  detection  is  given  in  Section  4.2.1.  Optical  detection 
techniques  are  being  explored  for  the  detection  of  hydrogen  despite  the  many  challenges  associated  with 
hydrogen  signatures.  An  overview  of  these  techniques  is  given  in  Section  4.2.2.  Other  strategies  exist  to 
support  quantification  of  hydrogen  losses  from  a  facility,  e.g.,  leak  rates  through  system  components,  etc. 
These  strategies  and  other  industry  compatible  techniques  are  discussed  in  Section  4.2.3.  Atmospheric 
hydrogen monitoring approaches are detailed in Section 4.2.4. It is also important to remark that hydrogen-
methane  blends  could  serve  as  a  bridge  towards  the  hydrogen  economy,  especially  in  specific  industrial 
sectors  so  not  only  hydrogen  leaks  have  to  be  assessed,  also  the  behaviour  of  hydrogen-methane  blends 
needs  to  be  evaluated.  Sections  4.2.1.1  and  4.2.2.1  are  dedicated  to  the  detection  and  quantification  of 
hydrogen-methane blend leaks by point sensors and optical techniques, respectively.  

4.2.1  Overview of sensor technologies for hydrogen detection 

Sensors  represent  one  strategy  for  detecting  and  quantifying  hydrogen  releases.  Hydrogen  gas  sensing 
technology uses various methods where a specific property, usually electrical in nature, of a sensing element 
changes in presence of hydrogen gas. This specific property can be thermal based, resistance based, optically 
assisted, acoustically assisted, mechanical or electrochemical change based. Instrumental techniques such as 
gas chromatography (GS) and mass spectrometry (MS) can also be employed for direct hydrogen detection. 
These  instruments  have  shown  good  response  over  a  wide  concentration  range  of  detection,  however  they 
suffer from several disadvantages such as the large size and weight, portability issues, slow response times, 
high  cost,  time  consuming  process,  requirement  of  trained  personnel  to  operate  them  and  dedicated 
maintenance. These limitations restrict the continuous operation of such instruments and thus the ability to 
quantify emissions in the open. 

Ranging  from  fundamental  elements  to  whole  instrument  systems,  the  term  “sensor”  can  have  different 
meanings among stakeholders within the hydrogen community. We can use the following terminology based 
on terminology used in ISO 26142 for different aspects of what constitutes a “sensor” (ISO 26142, 2010), see 
Figure 17: 

26 

 
Sensing element: device dealing with the interaction between stimuli and transduction into electrical signal. 
As  above  mentioned,  different  platforms  represent  different  types  of  sensing  element,  for  example: 
combustible gas sensor (CGS), thermal conductivity (TC), electrical conductivity (EC), metal oxide sensor (MOX), 
etc. 

Sensor:  device  that  provides  quantitative  information.  It  is  composed  of  a  sensing  element(s)  and  support 
electronics that converts the electrical response from a sensing element into a useful signal. 

Detection  apparatus  /  analyser:  equipment  providing  quantification,  alarm,  and  control  functions 
capabilities. 

Figure 17. Schematic of sensing element, sensor and analyser 

Source: (Hübert, Boon-Brett, and Buttner, 2016) 

Taken into account the above mentioned terminology, a practical definition for a hydrogen sensor would be 
the  one  describing  it  as  a  device  that  provides  quantitative  information  on  the  presence  and  amounts  of 
hydrogen. An overview of the various hydrogen sensors, commercially available or in development state, and 
its performance (response metrics) is presented in Table 4: 

Table 4. Overview of hydrogen sensors / detectors and metrological performance (nominal metrics): response time (RT), 
lower detection limits (LDL), measurement range (MR), selectivity, and sampling frequency. 

Source: NREL 

27 

 
 
Many of the sensors metrics presented in Table 4 represent nominal response time (RT), lower detection limits 
(LDL),  measurement  rage  (MR),  selectivity  rating  and  sampling  frequency  values  for  the  indicated  sensor 
platform type. Most met the concentration range for safety applications (measurement range 0.1 to 10 vol. % 
H2).  However,  most  of  the  sensors  do  not  have  an  appropriate  detection  limit  for  applications  requiring 
hydrogen  detection  below  100  ppm  let  alone  less  than  1  ppm.  Only  few  types  of  sensors  are  available  for 
these applications,  such as  miniaturised  combustible gas sensors  whose operation has been optimised with 
specialised  electronic  circuitry  coupled  with  controlled  sampling  and  operation  methodology.  With  proper 
configurations,  some  CSG  sensor  elements  are  able  to  measure  low  ppm  levels  of  methane  and  would  be 
expected to respond similarly to hydrogen or to hydrogen-methane mixtures, albeit without the possibility to 
differentiate between hydrogen and methane constituents. 

Improved  detection  limits  of  hydrogen  detectors  are  feasible  with  further  technology  development.  An 
emerging technology which has significantly improved lower detection limits is plasmonic hydrogen detection, 
based  on  palladium  nanoparticles.  Plasmonic  sensors  rely  on  optical  resonances  in  metal  nanoparticles. 
Hydrogen  is  typically  absorbed  inside  optically-lossy  palladium  nanoparticles  achieving  detection  limits  that 
are at the low ppm range (Nugroho et al., 2019; Nugroho et al., 2022). This technology is advancing quickly 
with  commercial  developers  producing  nanoplasmonic  sensor  prototypes  with  detection  limits  below  1  ppm 
and exceptional selectivity in pre-production models. (Insplorion)  

Another  emerging  approach  to  achieve  a  rapid  and  sensitive  detection  of  molecular  hydrogen  relies  into 
transforming  hydrogen  from  an  infrared  transparent  molecule  through  a  gas  phase  reaction  to  produce  a 
product with an infrared signature proportional to the hydrogen concentration. Aerodyne Hydrogen analysers 
work  by  chemically  converting  ambient  hydrogen  to  an  infrared  active  molecule  that  is  quantified  using  an 
Aerodyne  Tuneable  Infrared  Laser  Differential  Absorption  Spectrometer  (TILDAS),  providing  high  sensitivity, 
selectivity, and speed. Ongoing instrument development effort is supported by the US Department of Energy 
SBIR  program, through which  Aerodyne plans to commercialise three  versions of the analyser ranging from 
highly portable to highly sensitive (<0.5 ppb at 1 Hz, 10 Hz operation), (US DOE SBIR). The advancement of 
this  technology  allow  the  development  of  remote  monitoring  methods  similar  to  those  developed  for 
ammonia monitoring, looking at infra-red absorption profiling from satellites (Van Damme et al., 2018). 

Other emerging developments rely in the improvement of advanced materials and manufacturing approaches 
to produce hydrogen sensing element prototypes with exceptional analytical performance metrics. Most of the 
research advances are focused on semiconductor thin-films (Zhou et al., 2021), nanotubes (Xiao et al., 2018) 
and  nanostructures  (Liang  et  al.,  2022).  However,  there  is  still  uncertainty  on  the  chemical  interference  for 
possible  cross-sensitivity  and  more  data  is  required  on  the  impact  of  environmental  parameters  such  as 
temperature, pressure and humidity in the sensitivity of these novel sensors. 

Most  of  the  sensors  discussed  above  are  point  sensors  in  that  hydrogen  must  physically  interact  with  the 
sensing  element.  However,  point  sensors  can  be  arranged  in  a  distributed  array  to  perform  wide  area 
monitoring, allowing spatial and temporal detection of hydrogen releases. NREL Hydrogen Wide Area Monitor 
(HyWAM)  research  program  is  following  this  approach,  developing  a  distributed  network  of  hydrogen  point 
sensors supplemented by additional parameter measurements, including wind speed, wind direction and other 
environmental conditions to guide monitoring strategies. This technique is helping to elucidate the hydrogen 
dispersion behaviour serving as a validation tool of modelling approaches (e.g., computational fluid dynamics 
simulations).  Correlating  sensor  measurements  profiles  to  source  hydrogen  releases  requires  modelling. 
Therefore, research activities should also be focused at advancing the development of inverse models to use 
concentration  measurements  from  the  sensors  to  quantify  source  releases.  NREL  is  also  investigating  the 
development  of  advanced  methodologies  for  wide  area  monitoring  including  fiber  optic  sensors  specifically 
designed for hydrogen leak detection signatures, ultrasonic leak detection methods, Schlieren imaging, Raman 
methods and wireless systems. A commercial wrap has been developed to fit around pneumatic components, 
such as valves and fitting. A hydrogen leak occurs if the fitting fails, and the wrap changes colour. At present 
the wrap is visually interrogated to identify leak sources. The wrap is now being configured with RFID (Radio 
Frequency Identification) tags to allow for remote interrogation of the exposed wrap. Research advances on 
Schlieren imaging, Raman methods and other optical techniques are further developed in Section 4.2.2.  

4.2.1.1  Hydrogen-methane blends 

It is known that hydrogen disperses differently from methane due to the fact that hydrogen is more diffusive 
and buoyant than methane. Point sensors have been applied to study the dispersion behaviour of hydrogen-

28 

methane blends in recent years. By using extractive measurements at different points within enclosures and 
then running those extractive measurements across oxygen sensors it is possible to detect the displacement 
of oxygen in the enclosed test cell and elucidate what the concentrations are over time, see (Lowesmith et al., 
2009;  Marangon  and  Carcassi,  2014).  However,  it  is  worth  mentioning  that  the  use  of  oxygen  sensors  to 
quantify  hydrogen  releases  in  enclosures  can  induce  significant  errors  (Buttner  et  al.,  2014).  Handheld 
combustible gas sensors were also used to inspect test sections for leaks prior to testing.  

Figure 18. Experimental setup using oxygen sensors to evaluate hydrogen-methane mixtures. 

Source: (Marangon and Carcassi, 2014) 

4.2.2  Overview of optical techniques for hydrogen detection  

There is a wide range of optical detection techniques available to visualise gas flows, however not all of them 
work  for  hydrogen  gas.  A  summary  of  the  most  common  optical  techniques  and  its  suitability  to  monitor 
hydrogen is shown in Table 5. 

Table 5. Overview of optical detection techniques and its suitability for hydrogen detection 

Source: Sandia National Laboratories 

Shadowgraphy and Schlieren techniques, which rely in the principle of light rays bending as they pass through 
density  variations,  work  with  hydrogen  but  they  are  not  hydrogen  specific.  This  means  that  if  there  was  a 
hydrogen  leak  these  techniques  will  be  able  to  detect  a  change  in  density.  However,  they  are  not  able  to 
differentiate that the detection signal comes from hydrogen itself. These techniques are based on evaluating 
index of refraction differences and these differences can also be caused by temperature gradients as well as 
by  the  specific  molecule  (hydrogen,  or  other  density  gases)  itself.  Additionally,  they  present  further 
complications if  quantification is attempted. Shadowgraphy  does  not allow  hydrogen  quantification but it is 

29 

 
 
 
possible  for  specific  Schlieren  techniques.  However,  a  patterned  background  and  imaging  processing 
(tomography)  are  required  to  perform  hydrogen  quantification,  assuming  that  hydrogen  is  the  only  gas 
present (except air). 

Fluorescence detection works by molecules absorbing light at a resonant transition and then re-emitting at a 
shifted  wavelength.  This  technique  is  gas  specific  and  quantitative,  but  it  does  not  detect  pure  hydrogen. 
Using  fluorescence  would  require  mixing  the  hydrogen  with  another  molecule.  Infrared  absorption  does  not 
work with hydrogen either because of the lack of absorption features, as previously discussed. Transforming 
hydrogen  from  an  infrared  transparent  molecule  to  a  chemically  reacted  molecule  producing  an  infrared 
signature proportional to the hydrogen concentration is a way around to apply this technique that is currently 
under investigation.(US DOE SBIR)  

Rayleigh  scattering  is  a  technique  based  on  elastic  scattering  of  light  that  has  been  used  to  measure 
hydrogen leaks in the lab. However, its performance presents many challenges when attempting to measure 
cryogenic  hydrogen  leaks.  The  moisture  from  the  entrained  air  condenses  in  the  flow,  presenting  an 
overwhelming Mie scattering signal at the same wavelength as the Rayleigh signal. Furthermore, quantitative 
Rayleigh scattering relies on knowledge of the temperature of the gas flow, which can be managed for room 
temperature releases, but not for cryogenic releases. Rayleigh scattering is also not specific to hydrogen; the 
technique  is  suitable  for  laboratory  experiments,  but  not  for  a  diagnostic  measurement  in  environments 
where other gases might be present. 

Raman  scattering,  which  is  inelastic  light  scattering,  is  the  only  common  optical  technique  suitable  for 
hydrogen, as it is specific to hydrogen and quantitative. The visualisation of room temperature and cryogenic 
hydrogen  releases  has  been  demonstrated  applying  Raman.  Figure  19  describes  how  this  technique  is 
performed  at  Sandia  National  Laboratories  (Hecht  and  Panda,  2019).  Hydrogen  is  projected  through  a  high 
powered  laser  sheet  and  the  Raman  scattering  from  the  hydrogen  molecules  is  detected  by  a  camera  with 
filters to only detect light at a specific wavelength (the Raman shifted light). This setup also requires a second 
camera detecting the light that scatters off the nitrogen molecules. A small portion of the hydrogen plume is 
imaged at a time and from the combination of images planes a full image of the hydrogen mole fraction is 
obtained, capturing the turbulent fluctuations. This has allowed the validation of hydrogen dispersion models. 

Figure 19. Raman imaging of room temperature and cryogenic hydrogen releases: a) Schematic of the experimental 
setup, b) Instantaneous full images of the hydrogen and nitrogen mole fractions from a combination of multiple image 
planes, c) Comparison of average hydrogen dispersion 2D image measured (thick, dashed lines and shading) and model-
predicted (thick, solid lines). 

Source: Sandia National Laboratories 

Despite  the  challenges  associated  with  Raman  technique  (high  power  illumination  and  sensitive  detectors 
required due to the low signal strength) several research groups are working on miniaturising Raman based 
detection systems. (Ninomiya, 2007; Liméry et al., 2017; Shiina, 2018; Choi et al., 2019).  

Raman scattering technique  has also been demonstrated at a larger scale by Sandia  researchers, detecting 
hydrogen  from  10  meters  away  (Hecht  and  Killingsworth,  2021).  Figure  20  shows  how  the  technique  is 
performed to evaluate the hydrogen dispersion from venting. Similar to lab-scale experiments, the technique 

30 

 
requires high-powered laser illumination and presents some challenges when attempting quantification with 
small signals. 

Figure 20. Demonstration of Raman scattering applied at large scale to monitor hydrogen from venting: a) Experimental 
setup, b) Raman signals and validation of dispersion model 

Source: Sandia National Laboratories 

4.2.2.1  Hydrogen-methane blends 

Raman  scattering  is  currently  applied  to  measure  the  dispersion  behaviour  of  hydrogen-methane  blends  at 
lab scale by Sandia researchers. The experimental setup includes an additional camera looking for the Raman 
scatter off methane, as can be seen in Figure 21. These experiments will elucidate the dispersion evolution of 
blends, whether the hydrogen and methane molecules stay well-mixed as they disperse or whether they tend 
to separate.  

Figure 21. Schematic of the experimental setup of Raman imaging for hydrogen-methane blends 

Source: Sandia National Laboratories 

To  understand  better  the  behaviour  of  hydrogen-methane  blends,  experimental  approaches  must  be 
combined with modelling activities. The relationships between pressure, flow rate, rate of pressure drop, and 
the Reynolds number have been evaluated to analyse fixed simulated leaks (equivalent leak of hydrogen and 
of methane) by Sandia researchers. The volumetric flowrate of hydrogen is higher than for methane at fixed 
pressure.  In  contrast,  mass  and  energy  flow  rate  of  methane  are  higher  than  hydrogen  at  fixed  pressure. 
Figure 22 a) shows that volumetric flow rate of hydrogen, depicted in blue, is about three times that of the 
methane for an equivalent leak. However, the mass flow of hydrogen is significantly lower. This means that 
the  leak  rate  is  substantially  lower  on  a  mass  basis  for  hydrogen  due  to  the  lower  density  of  hydrogen  vs 
methane.  

31 

 
 
Figure 22. a) Comparison of volumetric flow rate, mass flow rate and energy flow rate for an equivalent leak size of 
hydrogen and methane, b) Comparison of the CFD simulations of the flammable plume of hydrogen and methane from an 
equivalent leak. 

Source: a) Sandia National Laboratories, b) Joint Research Centre 

Taking these observations into account, it is assumed that as the hydrogen content increases in blends with 
methane, the behaviour of the blend will change. Additionally, hydrogen is more diffusive and buoyant than 
methane,  as  previously  mentioned.  Figure  22  b)  shows  the  results  from  computational  fluids  dynamics 
simulations on equivalent leaks of hydrogen and methane (Wilkening and Baraldi, 2007). It can be observed 
that  for  a  given  pressure  and  leak  size,  the  flammable  plume  of  hydrogen  will  likely  be  larger  than  the 
flammable plume of methane. This means that as the percentage of hydrogen increases in a methane blend 
the  dispersion  behaviour  changes,  the  diffusion  and  buoyancy  of  the  resulting  blend  would  increase  and 
consequently  the  flammable  plume.  Conversely,  for  a  given  pressure  and  leak  size,  the  energy  and  mass 
released will be less for hydrogen than for methane. Therefore, as the percentage of hydrogen increases in a 
methane blend the energy and mass released will be lower. 

4.2.3  Other leak quantification methods: industry compatible techniques  

Gas  sensing  techniques  constitute  the  most  common  approach  to  detect  and  quantify  leaks  in  industrial 
settings. However, there are other strategies to support the quantification of hydrogen releases from a facility 
in  addition  to  sensors.  One  such  strategy  consists  of  leak  rate  quantification  through  system  components. 
Leak  rate  quantification  research  has  been  performed  at  NREL  with  the  purpose  of  getting  a  better 
understanding of leak behaviour and leak sizes for different components and failure modes. This research is 
intended to help industry to understand leaks better and quantify these leaks in the field. To do this NREL has 
developed a test apparatus to measure the mass flow  rate of leaks from failed  hydrogen components, see 
Figure 23. 

32 

 
 
 
 
 
 
 
 
Figure 23. Leak rate quantification apparatus setup at NREL 

Source: NREL 

This Leak Rate Quantification  Apparatus (LRQA)  can  nowadays test a  variety of components up to  85  MPa. 
The  test  pressure  is  determined  by  the  pressure  rating  and  the  expected  leak  rate  of  the  installed  failed 
component. The LRQA methodology consists of four steps. First, the failed component is pressurised using the 
storage vessel of known volume of gas from the LQRA. Then the pressure and temperature are measured to 
calculate  the  mass  at  each  time  step.  The  mass  flow  rate  (dm/dt)  is  then  determined  from  those 
measurements. Finally, an equivalent orifice diameter is  calculated from the obtained mass  flow rate  using 
standard equations (ISO 9300). In this way, similar failed components can be compared.  

Testing  using  the  LRQA  has  been  performed  on  a  variety  of  components,  including  orifices,  air  operated 
valves, manual valves, check valves, and a thermocouple using N2, H2, and He gas. Component failure modes 
can include internal or external leakage. The testing will be expanded to a greater number and other types of 
components  at  varying  pressures  using  different  types  of  gases  (including  hydrogen-methane  blends)  to 
quantify  leak  rates.  Component  failure  is  one  potential  source  of  hydrogen  emissions.  The  data  and  results 
from the LRQA testing would help understand the different types of component failures and the associated 
hazard. Additionally, the data obtained will feed into hydrogen leak and plume modelling.  

Similar approaches are adopted to characterise the leakages that could possibly appear through the lifetime 
of  hydrogen  dispensers  in  the  WP2  of  MultHyfuel  project.(FCH  JU2  MultHyFuel)  Project  partner  INERIS 
investigates  the  quantification  of  large  leakages  from  storages  and  reservoirs  by  establishing  tests  and 
validated methods to evaluate mass flow rates. For example, they experimentally determined the mass flow 
rate of a leak in a reservoir from the slope of the mass loss vs time trendline, measuring the mass loss using 
load cells. This method may be used to provide an instantaneous flowrate value, with an update rate and an 
accuracy, depending significantly on the sampling rate. Another test method validated by INERIS to evaluate 
leak flow rate in large reservoirs is similar to the NREL approach, measuring pressure and temperature and 
using equations of state. INERIS performed this method to evaluate the flow rate of a leak in a 50L cylinder 
at  700  bar  and  observed  that  the  accuracy  strongly  depends  on  the  temperature  measurement  conditions. 
From the testing  performed  by INERIS it can  be concluded that large leaks from dynamic processes can be 
detected through mass, pressure and temperature measurements.  

33 

 
INERIS  is  also  evaluating  different  methods  for  detection  and  quantification  of  small  leaks  in  system 
components (e.g., fittings, valves, manifolds, etc.). One way of doing this is calculating the leak mass flow rate 
of a component from the pressure increase measured in a closed tight volume. The accuracy of this method is 
a  function  of  the  pressure,  temperature,  time  and  acquisition  rate.  It  is  possible  to  measure  very  small 
pressure  increments  accurately,  in  the  order  of  magnitude  of  1  Pa.  However,  the  difficulty  resides  in  the 
temperature management. Keeping the leak upstream and downstream temperatures the same (ΔTmax < 0.1 
°C) is the key to obtain a good accuracy as temperature variations have a huge impact in the pressure slope. 
Another experimental method performed by INERIS to detect and quantify small leakages from components 
relies on mass spectrometry, see Figure 24. The failed component is enclosed in a tight volume and the gas 
concentration  inside  that  volume  is  monitored  by  a  mass  spectrometer,  measuring  the  hydrogen 
concentration  in  a  calibrated  stream  of  a  carrier  gas.  A  gas  vector  (e.g.,  nitrogen  on  air)  is  needed  to 
continuously flush the volume. This experimental technique is very sensitive since the accuracy only depends 
on the sensitivity of the mass spectrometer and the gas vector flow (perfect mixture with the leakage). The 
gas  spectrometer  could  be  replaced  by  any  other  gas  analyser  or  sensor  (electrochemical  cells,  catalytic 
sensors, katharometric sensors, etc.).  

Figure 24. Experimental setup based on mass spectrometry. 

Source: INERIS 

These  experimental  methods  for  hydrogen  leak  detection  and  quantification  in  components  have  been 
demonstrated  in  the  lab  but  they  are  not  easily  transferable  to  industrial/larger  scales,  especially  for  small 
leaks  involving  small  ΔP  measurements.  Gas  sensing  techniques  could  be  better  suited  for  detecting  these 
types  of  leaks  and  even  provide  a  mean  to  estimate  the  flow  rate,  but  they  need  to  be  placed  where  the 
hydrogen  is  leaking  and  the  leak  should  be  trapped  in  an  enclosed  volume.  Since  the  hydrogen  flows  up, 
collecting the gas in the upper part of a casing or canopy could be a good strategy but the dynamic of the 
accumulation must be known (transfer function). Additionally, these sensors must be maintained periodically 
and present other limiting issues (calibration, sensitivity, response time, sampling rate, etc.). 

To further support industry with validated methods and compatible detection techniques, the WP4 of EMPIR 
Metrology  for Decarbonising  the Gas  Grid (DECARB)  project led by NPL is focusing on the development and 
application of leak detection equipment for use with hydrogen and hydrogen-enriched natural gas (EURAMET 
Decarb, 2022). The objectives of the task 4.1 which started in June 2021 and will expand until May 2024 are: 
1) Perform an assessment of the available techniques, 2) Perform an assessment of the required metrology 
infrastructure to support the calibration and traceability of hydrogen detectors, 3) Develop suitable traceable 
reference  hydrogen  leaks  providing  industry  with  the  metrology  infrastructure  to  calibrate  leak  detection 
equipment,  4)  Assess  the  spectroscopic  issues  of  measurement  of  hydrogen-enriched  natural  gas,  and  5) 
Assess down-selected sensors in the laboratory, and then test them at Enagas’s hydrogen leak facility. 

34 

 
 
4.2.4  Monitoring tools for hydrogen detection in the atmosphere  

As previously mentioned, the hydrogen molecule lacks a dipole moment which makes it infrared transparent 
and  renders  infrared  absorption  monitoring  techniques  employed  for  the  detection  of  other  gases  in  the 
atmosphere not applicable. Hydrogen is also present at a low amount fraction of around 500 nmol mol-1 in 
the  atmospheric  background.  Instruments  based  on  gas  chromatography  separation  with  a  gas  reduction 
detector have been deployed at atmospheric monitoring stations for many decades. The time series recorded 
at  Mace  Head  on  the  West  Coast  of  Ireland  extends  to  1990  and  is  representative  of  the  Northern 
Hemisphere background. Measurements are made on mainland UK at Weybourne Atmospheric Observatory. 

Such atmospheric measurements are already used to verify emissions inventory for other greenhouse gases, 
such  as  methane,  nitrous  oxide,  and  sulphur  hexafluoride.  The  University  of  Bristol  and  NPL  maintain  a 
network  of  tall-tower  sites  in  mainland  UK,  and  at  Mace  Head,  measuring  these  gases  at  hourly  time 
resolution.  An  atmospheric  transport  model  is  used  to  interpret  the  data  and  provide  estimates  of  the 
regionally-resolved  annual  emissions  (Manning  et  al.,  2011;  Lunt  et  al.,  2021).  Hydrogen  is  not  currently 
measured  at  these  sites,  and  interpretation  in  terms  of  regional  emissions  is  difficult  due  to  the  poorly 
constrained local emissions and sinks. However, extending the capability of this UK tall tower network in the 
near term will help in the baseline estimation of natural hydrogen emissions before increased anthropogenic 
emissions associated with deployment of hydrogen as an energy gas. 

Research  measurements  are  performed  aiming  to  improve  the  understanding  of  emission  scenarios  to 
improve the impact of models. Other observatory networks such as the ones from CSIRO (Australia), AGAGE 
(US) operate H2 monitoring throughout the world. Currently there are only 11 observing stations for hydrogen 
worldwide, eight of which are in the southern hemisphere (World Data Centre for Greenhouse Gases, 2022). 
Ongoing work as NOAA (US) is expected to greatly expand the number of H2 monitoring stations.  

Moreover,  Patterson  et  al.  recently  managed  to  reconstruct  hydrogen  concentration  in  Antarctica  from  ice 
cores  (Patterson  et  al.,  2020;  Patterson  et  al.,  2021).  The  historical  hydrogen  concentration  estimated  by 
Patterson  et  al.  is  presented  in  Figure  25  (firn  reconstruction),  showing  a  good  agreement  with  the  model 
results provided by Paulot et al. (GFDL in the figure). 

Figure 25. Historical hydrogen concentration in Antarctica. 

Source: presentation by F. Paulot, courtesy of James Patterson (UCI) 

35 

 
5  Conclusions 

The  Clean  Hydrogen  JU  organised  a  two-day  workshop  aimed  at  uncovering  the  current  knowledge  on  the 
potential hydrogen emissions from a hydrogen economy and their indirect global warming impact. The main 
conclusions from the workshop are summarised in the following points: 

•  Hydrogen  emissions  to  the  atmosphere  impact  the  lifetime  of  other  greenhouse  gases, 
namely  methane,  ozone,  and  water  vapour,  indirectly  increasing  the  Earth’s  surface 
temperature. Current  best  estimates for the Global Warming Potential of hydrogen  emissions are 
5±1 and 11±5 kg CO2e/kg H2 over a 100-year time horizon (GWP100), and 12-33 kg CO2e/kg H2 over 
20  years  (GWP20).  The  average  lifetimes  of  hydrogen  (~2  years)  and  the  species  affected  by 
hydrogen emissions responsible for global warming are much shorter than the one of CO2, reducing 
the  global  warming  effect  of  hydrogen  emissions  (compared  to  an  equivalent  mass  of  CO2)  over 
longer time frames. 

•  Hydrogen emissions to the atmosphere from the hydrogen value chain are expected from 
gas providers to be in the range of 1-5% of the hydrogen delivered, with an average value 
of 2% by 2050. Assuming a hydrogen demand of 660 Mt/year by 2050 and a GWP100 value of 12.8, 
the CO2 equivalent emissions arising from hydrogen leaks would be approximately 170 Mt CO2e/year 
(i.e., 0.3% of current global CO2e emissions). 

•  Although it will not be climate neutral, a low-carbon and, in particular, a renewable hydrogen 
economy would significantly reduce the global warming impact compared to a fossil fuel 
economy.  Estimates  as  to  the  scale  of  that  reductions  depends  on  several  uncertain  parameters, 
such  as  the  evolution  of  energy  demand,  the  fraction  of  it  that  will  be  covered  by  hydrogen  (and 
what  technology  hydrogen  will  substitute),  the  hydrogen  production  pathway,  and  loss  rates. 
Nevertheless, switching technologies would guarantee a lower climate impact in the long term even 
under  high  hydrogen  loss  rate  scenarios.  However,  high  loss  rates  could  significantly  reduce  the 
climate benefits in the near-term. 

• 

To  reach  climate  neutrality,  the  additional  warming  caused  by  the  hydrogen  economy 
needs to be minimised and residual emissions compensated for (e.g., increase hydrogen sinks, 
reduce methane emissions, or carbon capture). Hydrogen supply (renewable versus fossil with carbon 
capture) and losses appear as the key parameters to minimise the climate impact of an H2 economy 
transition. Reducing hydrogen leakage would also increase the safety of using hydrogen. 

•  Estimates  of  industrial  hydrogen  emissions  and  their  global  warming  impact  are  subject 
to  a  high  level  of  uncertainty.  Uncertainties  are  mainly  due  to  lack  of  monitoring  data,  lack  of 
industry data, and knowledge gaps. 

• 

• 

The  physical  characteristics  of  hydrogen  (highly  buoyant,  colourless,  odourless,  tasteless,  non-
irritating and IR inactive) make it difficult to detect. Sensors are the most common strategy for 
the  direct  detection  and  empirical  characterisation  of  hydrogen  releases  with  conventional  point 
sensors the most used.  

The  optical  detection  of  hydrogen  is  challenging  as  it  does  not  present  common  optical 
signatures  for  its  monitoring  such  as  infrared  absorption  bands.  Only  Raman  scattering  can  be 
specific to hydrogen and quantitative but requires high power illumination and sensitive detectors. 

•  Other wide area monitoring techniques for hydrogen detection and quantification that work well 
at  lab  scale  exist  but  further  development  is  needed  to  demonstrate  effectiveness  in  situ 
industrial/larger scales. 

To improve the accuracy of the estimates of the global warming impact of a future hydrogen economy, and 
to prevent unintended consequences we advocate for the following: 

36 

•  More  data:  more  monitoring  stations  of  hydrogen  in  the  atmosphere  and  more  transparent, 
accurate,  and  precise  data  from  the  industry  regarding  emission  rates  are  necessary  to  improve 
value  chain  emission  estimates  as  well  as  global  hydrogen  budget  models,  and  therefore  global 
warming impact estimates. 

•  More research: there is still a limited understanding of some natural and anthropogenic phenomena 
affecting the global warming impact of a  hydrogen economy. Topics emerged during the workshop 
that need to be further investigated are: 

o  Natural  hydrogen  sources  and  sinks.  In  particular,  more  research  is  needed  to 
understand  the  behaviour  of  hydrogen  consuming  organisms  in  soil.  How  they  are 
distributed and how they will behave with an increase in hydrogen emissions in a changing 
climate (i.e.,  different temperatures, land  uses, and  hydrology). Moreover,  more  research is 
needed regarding natural hydrogen sources (e.g., geological sources).  

o  Anthropogenic  hydrogen  sources  and  impacts.  More  research  (and  transparency)  is 
needed  to  precisely  quantify  the  hydrogen  uses  (e.g.,  how  much  hydrogen  will  be  burned 
compared to how much will be used in fuel cells) and emissions from the industry. Moreover, 
more research is needed to understand the climate effect of specific hydrogen applications: 
e.g., the effect on contrails from hydrogen use for aviation. 

o  Life  cycle  assessments.  More  research  is  needed  regarding  the  life  cycle  impacts  of 
hydrogen  systems:  from  the  production  of  the 
infrastructure,  to  other  potential 
environmental  impacts  caused  by  hydrogen  use  (e.g.,  ammonia  emissions  when  ammonia 
used as hydrogen carrier). 

o  Quantification of hydrogen losses. Sensor development has traditionally focused on the 
1,000  -  10,000  ppm  measurement  range  for  safety  purposes.  However,  to  evaluate  the 
environmental  impacts  arising  from  the  cumulative  effect  of  sub-ppm  levels  of  released 
hydrogen  across  the  value  chain  there  is  a  need  to  expand  towards  lower  detection  limits 
and quantification capabilities. Deployment of fast response instruments with five orders of 
magnitude greater sensibility using emerging technologies such as nanoplasmonic detection, 
miniaturising  Raman  scattering  and  infrared  adaptation  among  others,  is  needed  to 
accurately measure (detect and quantify) emissions in situ. 

o  Dispersion  mechanisms.  Owing  to  its  high  diffusivity  and  buoyancy,  hydrogen  does  not 
behave  like  methane  when  emitted  to  the  atmosphere,  it  disperses  differently  and  this 
should  be  taken  into  account  when  assessing  hydrogen  and  hydrogen-methane  blend 
releases.  Experimental  and  modelling  approaches  should  be  combined  to  elucidate  the 
dispersion of hydrogen and hydrogen-methane blends. Novel sensor applications should be 
investigated to help in the dispersion model validation. 

o  Scale-up of novel detection and quantification techniques. Further research into how 
to scale-up experimental detection and quantification techniques which work well at the lab 
scale  is  needed  to  further  support  an  emerging  hydrogen  industry  with  validated  methods 
and effective detection and quantification techniques. 

•  Always reporting the amount and location of hydrogen emissions throughout the life cycle 
when assessing the environmental impact of an activity, not only the CO2 equivalents. The 
climate impact depends on the latitude and altitude of the emission. If the global warming impact of 
hydrogen  needs  to  be  aggregated  to  other  greenhouse  gases  for  policy  purposes,  the  metric  that 
aligns  best  with  the  goal  of  the  policy  should  be  used  (e.g.,  GWP100  in  the  framework  of  UNFCCC 
agreements).  We  also  advocate  for  standardised  experimental  methodologies  to  increase  the 
consistency and comparability of the GWP estimations from different models. 

• 

Inclusion  of  hydrogen  emissions  in  the  IPCC  models.  Hydrogen  emissions  from  a  hydrogen 
economy could represent approximately 10% of the overall hydrogen sources to the atmosphere; it is 
therefore important to include them in the hydrogen budget and models to estimate the GWP. 

37 

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41 

 
List of abbreviations and definitions 

ADR 

BAT 

CEN 

CGA 

CGS 

European Agreement concerning the International Carriage of Dangerous Goods by Road 

Best available techniques 

European Committee for Standardisation 

Compressed gas association 

Combustible gas sensor 

CO2e 

CO2 equivalents 

EC 

Electrical conductivity 

EIGA 

European Industrial Gases Association 

EPA 

ERF 

GC 

GHG 

GTP 

US Environmental Protection Agency 

Effective radiative forcing 

Gas chromatography 

Greenhouse gas 

Global temperature change potential 

GWP 

Global warming potential 

GWP20 

Global warming potential over 20 years 

GWP100  Global warming potential over 100 years 

ISO 

JU 

LDL 

International Committee for Standardisation 

Joint undertaking 

Lower detection limit 

LRQA  

Leak rate quantification apparatus 

MOX 

Metal oxide sensors 

MR 

MS 

Measurement range 

Mass Spectrometry 

NDIR 

Non-Dispersive Infrared 

NMHC 

Non-methane hydrocarbons 

NOAA 

National oceanic and atmospheric administration 

Ppbv 

Parts per billion by volume 

Ppmv 

Parts per million by volume 

RFID 

Radio Frequency Identification Detection 

RT 

Response time 

SMR 

Steam methane reforming 

TC 

Thermal conductivity 

42 

 
 
List of figures 

Figure 1. Global hydrogen budget .................................................................................... 6 

Figure 2. Historical changes of hydrogen sources. Chemical production refers to the natural chemical 
reactions in the atmosphere producing hydrogen. ................................................................... 7 

Figure 3. Hydrogen concentration. Diamonds in the figure show the observation points. ....................... 8 

Figure 4. Hand-drawn schematic of the global burden perturbations relative to control run after adding 1 Tg 
instantaneous pulse of H2. In the case of methane and ozone, the actual model results for 5 years following a 
pulse of H2 emissions are superimposed on the hand-drawn schematic. ........................................10 

Figure 5. GWP variation depending on emission latitude (left) and deposition velocity (right) .................11 

Figure 6. Comparison of the atmospheric mass increment (left) and radiative forcing with time (right) of 
pulse emissions of hydrogen and carbon dioxide. The scale of the y-axis in the right panel is logarithmic. ...12 

Figure 7. Cumulative radiative forcing of hydrogen relative to carbon dioxide for equal emissions over time. 
The unit of the x-axes (time horizon) is years. ......................................................................13 

Figure 8. Global annual demand for hydrogen since 1975 .......................................................14 

Figure 9. Hydrogen current value chains ............................................................................15 

Figure 10. Hydrogen release fractions to the atmosphere along the hydrogen supply chain and design goals 
for 2030 ................................................................................................................16 

Figure 11. Water vapour lifetime ....................................................................................19 

Figure 12. Loss rate for hydrogen causing an increase rate in atmospheric methane. CLR: critical loss rate; 
EOH: amount of OH reacting with compounds other than H2, CO, and CH4 normalised by the amount reacting 
with H2, CO, and CH4. ..................................................................................................20 

Figure 13. Global warming implications of substituting natural gas with hydrogen in SNAM pipelines. .......21 

Figure 14. Relative climate impact over time from replacing fossil fuel systems with renewable (“green”) or 
fossil with carbon capture (“blue”) hydrogen alternatives for a generic case. Error bars represent a 20% 
uncertainty in hydrogen’s radiative efficiency. Corresponding GWP100-derived climate impacts are shown 
using the “X” and “O” markers, for worst-case and best-case loss rates, respectively. [The figure was updated 
by the author with respect to the one presented at the workshop]. ...............................................22 

Figure 15. The flammability range of hydrogen compared to methane and gasoline ..........................24 

Figure 16. Comparison of absorption spectra of hydrogen and methane ........................................26 

Figure 17. Schematic of sensing element, sensor and analyser ..................................................27 

Figure 18. Experimental setup using oxygen sensors to evaluate hydrogen-methane mixtures. ..............29 

Figure 19. Raman imaging of room temperature and cryogenic hydrogen releases: a) Schematic of the 
experimental setup, b) Instantaneous full images of the hydrogen and nitrogen mole fractions from a 
combination of multiple image planes, c) Comparison of average hydrogen dispersion 2D image measured 
(thick, dashed lines and shading) and model-predicted (thick, solid lines). .......................................30 

Figure 20. Demonstration of Raman scattering applied at large scale to monitor hydrogen from venting: a) 
Experimental setup, b) Raman signals and validation of dispersion model .......................................31 

Figure 21. Schematic of the experimental setup of Raman imaging for hydrogen-methane blends ..........31 

Figure 22. a) Comparison of volumetric flow rate, mass flow rate and energy flow rate for an equivalent leak 
size of hydrogen and methane, b) Comparison of the CFD simulations of the flammable plume of hydrogen 
and methane from an equivalent leak. ..............................................................................32 

Figure 23. Leak rate quantification apparatus setup at NREL ....................................................33 

Figure 24. Experimental setup based on mass spectrometry. ....................................................34 

43 

Figure 25. Historical hydrogen concentration in Antarctica. .......................................................35 

44 

 
List of tables 

Table 1. Day 1 presentations ......................................................................................... 4 

Table 2. Day 2 presentations ......................................................................................... 4 

Table 3. Potential hydrogen losses from the deployment of hydrogen at scale. ................................17 

Table 4. Overview of hydrogen sensors / detectors and metrological performance (nominal metrics): response 
time (RT), lower detection limits (LDL), measurement range (MR), selectivity, and sampling frequency. .......27 

Table 5. Overview of optical detection techniques and its suitability for hydrogen detection ..................29 

45 

 
 
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doi:10.2760/065589 

ISBN 978-92-76-55848-4