Editing IPCC:AR6/WGIII/Chapter-6 (section)

==== 6.4.5.1 Hydrogen: Low-carbon Energy Fuel ====

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Hydrogen is a promising energy carrier for a decarbonised world (Box 6.9). It can be utilised for electricity, heat, transport, industrial demand, and energy storage ( [[#Abdin--2020|Abdin et al. 2020]] ). In low-carbon energy systems, hydrogen is expected to be utilised in applications that are not as amenable to electrification, such as a fuel for heavy-duty road transport and shipping, or as a chemical feedstock ( [[#Schemme--2017|Schemme et al. 2017]] ; [[#Griffiths--2021|Griffiths et al. 2021]] ). Hydrogen could also provide low-carbon heat for industrial processes or be utilised for direct reduction of iron ore ( [[#Vogl--2018|Vogl et al. 2018]] ). Hydrogen could replace natural gas-based electricity generation ( [[#do%20Sacramento--2013|do Sacramento et al. 2013]] ) in certain regions and support the integration of variable renewables into electricity systems by providing a means of long-term electricity storage. Hydrogen-based carriers, such as ammonia and synthetic hydrocarbons, can likewise be used in energy-intensive industries and the transport sector ( [[#Schemme--2017|Schemme et al. 2017]] ; [[#IRENA--2019b|IRENA 2019b]] ) (e.g., synthetic fuels for aviation). These hydrogen-based energy carriers are easier to store than hydrogen. At present hydrogen has limited applications – mainly being produced onsite for the creation of methanol and ammonia ( [[#IEA--2019c|IEA 2019c]] ), as well as in refineries.

Low- or zero-carbon produced hydrogen is not currently competitive for large-scale applications, but it is likely to have a significant role in future energy systems, due to its wide-range of applications ( ''high confidence'' ). Key challenges for hydrogen are: (i) cost-effective low/zero carbon production; (ii) delivery infrastructure cost; (iii) land area (i.e., ‘footprint’) requirements of hydrogen pipelines, compressor stations, and other infrastructure; (iv) challenges in using existing pipeline infrastructure; (v) maintaining hydrogen purity; (vi) minimising hydrogen leakage; and (vii) the cost and performance of end uses. Furthermore, it is necessary to consider the public perception and social acceptance of hydrogen technologies and their related infrastructure requirements ( [[#Iribarren--2016|Iribarren et al. 2016]] ; [[#Scott--2020|Scott and Powells 2020]] ).

'''Hydrogen production.''' Low- or zero-carbon hydrogen can be produced from multiple sources. While there is no consensus on the hydrogen production spectrum, ‘blue’ hydrogen ( [[#Goldmann--2018|Goldmann and Dinkelacker 2018]] ) generally refers to hydrogen produced from natural gas combined with CCS through processes such as steam methane reforming (SMR) ( [[#Sanusi--2019|Sanusi and Mokheimer 2019]] ) and advanced gas reforming ( [[#Zhou--2020|Zhou et al. 2020]] ). Low-carbon hydrogen could also be produced from coal coupled with CCS ( [[#Hu--2020|Hu et al. 2020]] ) (Table 6.7). Current estimates are that adding CCS to produce hydrogen from SMR will add on average 50% on the capital cost, 10% to fuel, and 100% to operating costs. For coal gasification, CCS will add 5% to the capital and fuel costs and 130% to operating costs ( [[#Staffell--2018|Staffell et al. 2018]] ; [[#IEA--2019d|IEA 2019d]] ). Further, biomass gasification could produce renewable hydrogen, and when joined with CCS could provide negative carbon emissions. ‘Green’ hydrogen ( [[#Jaszczur--2016|Jaszczur et al. 2016]] ) is most often referred to as hydrogen produced from zero-carbon electricity sources such as solar power and wind power ( [[#Schmidt--2017|Schmidt et al. 2017]] ) (Table 6.8). Nuclear power could also provide clean hydrogen, via electrolysis or thermochemical water splitting ( [[#EERE--2020|EERE 2020]] ). Hydrogen can even be produced by pyrolysis of methane (Sánchez-Bastardo et al. 2020) – sometimes called ‘turquoise’ hydrogen, solar thermochemical water splitting, biological hydrogen production (cyanobacteria) ( [[#Velazquez%20Abad--2017|Velazquez Abad and Dodds 2017]] ) – and microbes that use light to make hydrogen (under research) ( [[#EIA--2020|EIA 2020]] ).

'''Table 6.7 | Key performance and cost characteristics of different non-electric hydrogen production technologies, including carbon capture and storage (CCS).'''

{| class="wikitable"
|-
| rowspan="2"| Technology

| colspan="2"| LHV efficiency (%)

| rowspan="2"| Carbon intensity (kgCO 2 (kgH 2 ) –1 )

| colspan="2"| Cost estimates * (USD (kgH 2 ) –1 )

|-
| Current

| Long-term

| Current

| Long-term

|-
| Steam methane reforming (SMR)

| 65 e

| 74 e,f

| 1.0–3.6 e,i

| 1.0–2.7 a,b,c,d,e

| 1.5–2.6 e

|-
| Advanced gas reforming

| –

| 81–84 e,f

| 0.9–2.9 e

| 1.3–2.1 e

| 1.2–3.4 e,f

|-
| Hydrogen from coal gasification

| 54 e

| 54 (5)

| 2.1–5.5 e,i

| 1.8–3.1 a,b,c,d,e

| 2.4–3.3 e

|-
| Hydrogen from biomass gasification

| 53.6 g

| 40–60 e

| Potential to achieve negative emission e,h

| 4.9 e

| 2.9–5.9 e,f

|}

Source: a CSIRO 2021; b IEA 2020; c IRENA 2019; d Hydrogen Council 2020; e CCC 2018; f [[#BEIS--2021|BEIS 2021]] ; g Ishaq et al. 2021; h Al-Mahtani et al. 2021; i IEA 2019.

\* USD per GBP exchange rate: 0.72 (August 2021); LHV: Lower Heating Values; Long-term refers to 2040 and 2050 according to different references.

'''Table 6.8 | Efficiency and cost characteristics of electrolysis technologies for hydrogen production.'''

{| class="wikitable"
|-
| rowspan="2"| Technology

| colspan="2"| LHV efficiency (%)

| colspan="2"| CAPEX (USD kW e –1 )

| colspan="2"| Cost estimates *,† (USD (kgH 2 ) –1 )

|-
| Current

| Long-term b,e,f,h

| Current g

| Long-term g

| Current

| Long-term

|-
| Alkaline Electrolysers

| 58–77 a,b,e,f,h

| 70–82

| 500–1400

| 200–700

| 2.3–6.9 a,b,c,e

| 0.9–3.9 c,e

|-
| Polymer electrolyte membrane (PEM)

| 54–72 a,b,e,f,h

| 67–82

| 1100–1800

| 200–900

| 3.5–9.3 a,d,e,f

| 2.2–7.2 e,f

|-
| Solid oxide electrolyser cell (SOEC)

| 74–81 b,f,h

| 77–92

| 2800–5600

| 500–1000

| 4.2 e

| 2.6–3.6 e

|}

Source: a CSIRO 2021; b IEA 2020; c IRENA 2019; d Hydrogen Council 2020; e CCC 2018; f [[#BEIS--2021|BEIS 2021]] ; g IEA 2019; h [[#Christensen--2020|Christensen 2020]] .

\* USD per GBP exchange rate: 0.72 (August 2021); † The cost of hydrogen production from electrolysers is highly dependent on the technology, source of electricity, and operating hours, and some values provided are based on the assumptions made in the references.

'''Hydrogen energy carriers.''' Hydrogen can be both an energy carrier itself, be converted further into other energy carriers (such as synthetic fuels) and be a means of transporting other sources of energy. For example, hydrogen could be transported in its native gaseous form or liquefied. Hydrogen can also be combined with carbon and transported as a synthetic hydrocarbons ( [[#Gumber--2018|Gumber and Gurumoorthy 2018]] ) ( [[#IRENA--2019d|IRENA 2019d]] ) as well as be transported via liquid organic hydrogen carriers (LOHCs) or ammonia ( [[#IRENA--2019d|IRENA 2019d]] ). For synthetic hydrocarbons such as methane or methanol to be considered zero carbon, the CO 2 used to produce them would need to come from the atmosphere either directly through DACCS or indirectly through BECCS ( [[#IRENA--2019b|IRENA 2019b]] ). LOHCs are organic substances in liquid or semi-solid states, which store hydrogen based on reversible catalytic hydrogenation and de-hydrogenation of carbon double bounds ( [[#Niermann--2019|Niermann et al. 2019]] ; [[#Rao--2020|Rao and Yoon 2020]] ). Hydrogen produced from electrolysis could also be seen as an electricity energy carrier. This is an example of the PtX processes ( [[#6.4.4|Section 6.4.4]] ), entailing the conversion of electricity to other energy carriers for subsequent use.

Ammonia is a promising cost-effective hydrogen carrier ( [[#Creutzig--2019|Creutzig et al. 2019]] ). Onsite generation of hydrogen for the production of ammonia already occurs today, and the ammonia (NH 3 ) could be subsequently ‘cracked’ (with a 15–25% energy loss) to reproduce hydrogen ( [[#Hansgen--2010|Hansgen et al. 2010]] ; [[#Montoya--2015|Montoya et al. 2015]] ; [[#Bell--2016|Bell and Torrente-Murciano 2016]] ). Because the energy density of ammonia is 38% higher than liquid hydrogen ( [[#Osman--2018|Osman and Sgouridis 2018]] ), it is potentially a suitable energy carrier for long-distance transport and storage ( [[#Salmon--2021|Salmon et al. 2021]] ). Moreover, ammonia is more easily condensable (liquefied at 0.8 MPa, 20°C), which provides economically viable hydrogen storage and supply systems. Ammonia production and transport are also established industrial processes (about 180 MMT yr –1 ) ( [[#Valera-Medina--2017|Valera-Medina et al. 2017]] ), and hence ammonia is considered to be a scalable and cost-effective hydrogen-based energy carrier. At present, most ammonia is used in fertilisers (about 80%), followed by many industrial processes, such as the manufacturing of mining explosives and petrochemicals ( [[#Jiao--2018|Jiao and Xu 2018]] ). In contrast to hydrogen, ammonia can be used directly as a fuel without any phase change for internal combustion engines, gas turbines, and industrial furnaces ( [[#Kobayashi--2019|Kobayashi et al. 2019]] ). Ammonia can also be used in low- and high-temperature fuel cells ( [[#Lan--2014|Lan and Tao 2014]] ), whereby both electricity and hydrogen can be produced without any nitrogen oxide (NO x ) emissions. Furthermore, ammonia provides the flexibility to be dehydrogenated for hydrogen-use purposes. Ammonia is considered a carbon-free sustainable fuel for electricity generation, since in a complete combustion, only water and nitrogen are produced ( [[#Valera-Medina--2017|Valera-Medina et al. 2017]] ). Like hydrogen, ammonia could facilitate management of VRE, due to its cost-effective grid-scale energy storage capabilities. In this regard, production of ammonia via hydrogen from low- or zero-carbon generation technologies along with ammonia energy recovery technologies ( [[#Afif--2016|Afif et al. 2016]] ) could play a major role in forming a hydrogen and/or ammonia economy to support decarbonisation. However, there are serious concerns regarding the ability to safely use ammonia for all these purposes, given its toxicity – whereas hydrogen is not considered toxic.

In general, challenges around hydrogen-based energy carriers – including safety issues around flammability, toxicity, storage, and consumption – require new devices and techniques to facilitate their large-scale use. Relatively high capital costs and large electricity requirements are also challenges for technologies that produce hydrogen energy carriers. Yet, these energy carriers could become economically viable through the availability of low-cost electricity generation and excess of renewable energy production ( [[#Daiyan--2020|Daiyan et al. 2020]] ). A key challenge in use of ammonia is related to the significant amount of NO x emissions, which is released from nitrogen and oxygen combustion, and unburned ammonia. Both have substantial air pollution risks, which can result in lung and other injuries, and can reduce visibility ( [[#EPA--2001|EPA 2001]] ). Due to the low flammability of hydrogen energy carriers such as liquefied hydrogen ( [[#Nilsson--2016|Nilsson et al. 2016]] ) and ammonia (Li et al. 2018), a stable combustion ( [[#Lamas--2019|Lamas and Rodriguez 2019]] ; [[#Zengel--2020|Zengel et al. 2020]] ) in the existing gas turbines is not currently feasible. In recent developments, however, the proportion of hydrogen in gas turbines has been successfully increased, and further development of gas turbines may enable them to operate on 100% hydrogen by 2030 ( [[#Pflug--2019|Pflug et al. 2019]] ).

'''Long-distance hydrogen transport.''' Hydrogen can allow regional integration and better utilisation of low- or zero-carbon energy sources (Boxes 6.9 and 6.10). Hydrogen produced from renewables or other low-carbon sources in one location could be transported for use elsewhere ( [[#Philibert--2017|Philibert 2017]] ; [[#Ameli--2020|Ameli et al. 2020]] ). Depending on the distance to the user and specific energy carrier utilised (e.g., gaseous hydrogen or LOHC), various hydrogen transport infrastructures, distribution systems, and storage facilities would be required ( [[#Hansen--2020|Hansen 2020]] ; [[#Schönauer--2021|Schönauer and Glanz 2021]] ) (Figure 6.17).

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[[File:72ef3135851f13d316c17d144338f3e3 IPCC_AR6_WGIII_Figure_6_17.png]]

'''Figure 6.17 | Hydroge''' '''n value chain.''' '''Hydrogen can be produced by various means and input and fuel sources.''' These processes have different emissions implications. Hydrogen can be transported by various means and in various forms, and it can be stored in bulk for longer-term use. It also has multiple potential end uses. CHP: Combined heat and power.

Hydrogen can be liquefied and transported at volume over the ocean without pressurisation. This requires a temperature of –253°C and is therefore energy-intensive and costly ( [[#Niermann--2021|Niermann et al. 2021]] ). Once it reaches its destination, the hydrogen needs to be re-gasified, adding further cost. A demonstration project is under development exporting liquid hydrogen from Australia to Japan ( [[#Yamashita--2019|Yamashita et al. 2019]] ). Hydrogen could also be transported as ammonia by ocean in liquid form. Ammonia is advantageous because it is easier to store than hydrogen ( [[#Zamfirescu--2008|Zamfirescu and Dincer 2008]] ; [[#Soloveichik--2016|Soloveichik 2016]] ; [[#Nam--2018|Nam et al. 2018]] ). Liquid ammonia requires temperatures below –33°C and is therefore more straightforward and less costly to transport than liquefied hydrogen and even liquefied natural gas ( [[#Singh--2018|Singh and Sahu 2018]] ). A project exporting ammonia from Saudi Arabia to Japan is under consideration (Nagashima 2018). LOHCs could also be used to transport hydrogen at ambient temperature and pressure. This advantageous property of LOHCs makes them similar to oil products, meaning they can be transported in existing oil infrastructure including oil tankers and tanks (IEA 2019; [[#Niermann--2019|Niermann et al. 2019]] ). A project is under development to export hydrogen from Brunei to Japan using LOHCs ( [[#Kurosaki--2018|Kurosaki 2018]] ).

'''Intra-regional hydrogen transportation.''' Within a country or region, hydrogen would likely be pressurised and delivered as compressed gas. About three times as much compressed hydrogen by volume is required to supply the same amount of energy as natural gas. Security of supply is therefore more challenging in hydrogen networks than in natural gas networks. Storing hydrogen in pipelines (linepack) would be important to maintaining security of supply ( [[#Ameli--2017|Ameli et al. 2017]] , 2019). Due to the physics of hydrogen, in most cases exiting gas infrastructure would need to be upgraded to transport hydrogen. Transporting hydrogen in medium- or high-pressure networks most often would require reinforcements in compressor stations and pipeline construction routes ( [[#Dohi--2016|Dohi et al. 2016]] ). There are several recent examples of efforts to transport hydrogen by pipeline. For example, in the Iron Mains Replacement Programme in the UK, the existing low-pressure gas distribution pipes are being converted from iron to plastic (Committee on Climate Change 2018). In the Netherlands, an existing low-pressure 12 km natural gas pipeline has been used for transporting hydrogen ( [[#Dohi--2016|Dohi et al. 2016]] ).

To bypass gas infrastructure in transporting hydrogen, methane can be transported using the existing gas infrastructure, while hydrogen can be produced close to the demand centres. This approach will only make sense if the methane is produced in a manner that captures carbon from the atmosphere and/or if CCS is used when the methane is used to produce hydrogen.

'''Bulk hydrogen storage''' . Currently, hydrogen is stored in bulk in chemical processes such as metal and chemical hydrides as well as in geologic caverns ( [[#Andersson--2019|Andersson and Grönkvist 2019]] ; [[#Caglayan--2019|Caglayan et al. 2019]] ) (e.g., salt caverns operate in Sweden) ( [[#Elberry--2021|Elberry et al. 2021]] ). There are still many challenges, however, due to salt or hard rock geologies, large size, and minimum pressure requirements of the sites ( [[#IEA--2019c|IEA 2019c]] ). Consequently, alternative carbon-free energy carriers, which store hydrogen, may become more attractive ( [[#Lan--2012|Lan et al. 2012]] ; [[#Kobayashi--2019|Kobayashi et al. 2019]] ).

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