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

=== Box 6.9 | The Hydrogen Economy ===

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The phrase ‘hydrogen economy’ is often used to describe future energy systems in which hydrogen plays a prominent role. These future energy systems would not use hydrogen for all end uses; they would use hydrogen to complement other energy carriers, mainly electricity, where hydrogen might have advantages. Hydrogen could provide long-term electricity storage to support high-penetration of intermittent renewables and could enable trading and storage of electricity between different regions to overcome seasonal or production capability differences ( [[#Dowling--2020|Dowling et al. 2020]] ; [[#Sepulveda--2021|Sepulveda et al. 2021]] ). It could also be used in lieu of natural gas for peaking generation, provide process heat for industrial needs, or be used in the metal sector via direct reduction of iron ore (Chapter 11). Clean hydrogen could be used as a feedstock in the production of various chemicals and synthetic hydrocarbons. Finally, hydrogen-based fuel cells could power vehicles. Recent advances in battery storage make electric vehicles the most attractive alternative for light-duty transport. However, fuel cell technology could complement electric vehicles in supporting the decarbonisation of heavy-duty transport segments (e.g., trucks, buses, ships, and trains) (Chapter 10).

Hydrogen production costs have historically been prohibitive, but recent technological developments are bringing costs down. These developments include improvements in hydrogen production technologies in terms of efficiency and capital costs (e.g., steam methane reforming) ( [[#Alrashed--2021|Alrashed and Zahid 2021]] ; [[#Boretti--2021|Boretti and Banik 2021]] ) and the emergence of alternative production technologies such as electrolysers ( [[#Dawood--2020|Dawood et al. 2020]] ). These technological changes, along with decreasing costs of renewable power, are increasing the viability of hydrogen. Other improvements in hydrogen-based technologies are also emerging quickly. Gas turbines now run on blended fuels containing 5–95% hydrogen by volume ( [[#GE--2020|GE 2020]] ) and could operate entirely on hydrogen by 2030 ( [[#Pflug--2019|Pflug et al. 2019]] ). Fuel cell costs have decreased by 80–95% since the early 2000s, while power density and durability have improved ( [[#Jouin--2016|Jouin et al. 2016]] ; [[#IEA--2019e|IEA 2019e]] ; [[#Kurtz--2019|Kurtz et al. 2019]] ).

For hydrogen to support decarbonisation, it will need to be produced from zero-carbon or extremely low-carbon energy sources. One such production category is ‘green hydrogen’. While there is no unified definition for green hydrogen, it can be produced by the electrolysis of water using electricity generated without carbon emissions (such as renewables). Hydrogen can also be produced through biomass gasification with carbon capture and storage (BECCS), leading to negative carbon emissions ( [[#Arnaiz%20del%20Pozo--2021|Arnaiz del Pozo et al. 2021]] ). Additionally, ‘blue hydrogen’ can be produced from natural gas through the process of auto-thermal reforming (ATR) or steam methane reforming, combined with CCS technology that would absorb most of the resulting CO 2 (80–90%).

However, the potential role of hydrogen in future energy systems depends on more than just production methods and costs. For some applications, the competitiveness of hydrogen also depends on the availability of the infrastructure needed to transport and deliver it at relevant scales ( [[#Lee--2021|Lee et al. 2021]] ). Transporting hydrogen through existing gas pipelines is generally not feasible without changes to the infrastructure itself ( [[#Gumber--2018|Gumber and Gurumoorthy 2018]] ; [[#Muratori--2018|Muratori et al. 2018]] ). Existing physical barriers, such as steel embrittlement and degradation of seals, reinforcements in compressor stations, and valves, require retrofitting during the conversion to H 2 distribution or new dedicated pipelines to be constructed ( [[#Dohi--2016|Dohi et al. 2016]] ). The capacity to leverage and convert existing gas infrastructure to transport hydrogen will vary regionally, but in many cases could be the most economically viable pathway ( [[#Cerniauskas--2020|Cerniauskas et al. 2020]] ; [[#Brändle--2021|Brändle et al. 2021]] ; Brooks 2021; Wettengel 2021). Hydrogen could also be transported as liquid gas or as liquid organic hydrogen carriers such as ammonia, for which industry knowledge exists (Demir et al. 2018; Wulf et al. 2018; Hong et al. 2021). Additionally, improvements in fuel cell technologies are needed to make hydrogen-based transport economically viable. There are also safety concerns associated with the flammability ( [[#Nilsson--2017|Nilsson et al. 2017]] ) and storage ( [[#Andersson--2019|Andersson and Grönkvist 2019]] ; [[#Caglayan--2019|Caglayan et al. 2019]] ) of hydrogen which will need to be considered.

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==== 6.6.2.5 Using Less Energy and Using It More Efficiently ====

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Demand-side or demand reduction strategies include technology efficiency improvements, strategies that reduce energy consumption or demand for energy services (such as reducing the use of personal transportation, often called ‘conservation’) ( [[#Creutzig--2018|Creutzig et al. 2018]] ), and strategies such as load curtailment.

Net-zero energy systems will use energy more efficiently than those of today ( ''high confidence'' ). Energy efficiency and energy use reduction strategies are generally identified as being flexible and cost-effective, with the potential for large-scale deployment (Chapters 5, 9, 10, and 11). For this reason, existing studies find that energy efficiency and demand reduction strategies will be important contributors to net-zero energy systems ( [[#Creutzig--2018|Creutzig et al. 2018]] ; [[#Davis--2018|Davis et al. 2018]] ; [[#DeAngelo--2021|DeAngelo et al. 2021]] ). Lower demand reduces the need for low-carbon energy or alternative fuel sources.

Characterising efficiency of net-zero energy systems is problematic due to measurement challenges ( ''high confidence'' ). Efficiency itself is difficult to define and measure across full economies ( [[#Saunders--2021|Saunders et al. 2021]] ). There is no single definition of energy efficiency and the definition understandably depends on the context used ( [[#Patterson--1996|Patterson 1996]] ), which ranges from device-level efficiency all the way to the efficient use of energy throughout an economy. Broadly, energy-efficient strategies allow for the same level of services or output while using less energy. At the level of the entire economy, measures such as primary or final energy per capita or per GDP are often used as a proxy for energy efficiency; these measures reflect not only efficiency, but also many other factors such as industrial structure, endowed natural resources, consumer preferences, policies, and regulations. Energy efficiency and other demand-side strategies represent such a large set of technologies, strategies, policies, market and consumers’ responses and policies that aggregate measures can be difficult to define ( [[#Saunders--2021|Saunders et al. 2021]] ).

Measurement issues notwithstanding, virtually all studies that address net-zero energy systems assume improved energy intensity in the future ( ''high confidence'' ). The overall efficiency outcomes and the access to such improvements across different nations, however, are not clear. Energy consumption will increase over time – despite energy efficiency improvements – due to population growth and development ( [[#DeAngelo--2021|DeAngelo et al. 2021]] ).

A study ( [[#DeAngelo--2021|DeAngelo et al. 2021]] ) reviewed 153 integrated asset management scenarios that attain net-zero energy sector CO 2 emissions and found that, under a scenario with net-zero emissions: global final energy per capita lies between 21–109 GJ per person (median: 57), in comparison to 2018 global final energy use of 55 GJ per person; many countries use far more energy per capita than today as their incomes increase; global final energy use per unit of economic output ranges from 0.7–2.2 EJ per trillion USD (median: 1.5), in comparison to 5 EJ per trillion USD in 2018; and the median final energy consumption is 529 EJ. By comparison, final energy consumption would be 550 EJ if current energy consumption per capita continued under a future population of 10 billion people. Across all scenarios, total final energy consumption is higher today than in the year in which net-zero emissions are attained, and regionally, only the OECD+EU and Eurasia have lower median total final energy than in 2010.

Net-zero energy systems will be characterised by greater efficiency and more efficient use of energy across all sectors ( ''high confidence'' ). Road transportation efficiency improvements will require a shift from liquid fuels (Chapters 5 and 10). Emissions reductions will come from a transition to electricity, hydrogen, or synthetic fuels produced with low-carbon energy sources or processes. Vehicle automation, ride-hailing services, online shopping with door delivery services, and new solutions like last mile delivery with drones may result in increased service share. Lighter vehicles, a shift to public transit, and incorporation of two- and three-wheelers will be features of a net-zero energy system (Chapter 10). Teleworking and automation of work may provide reductions in driving needs. Other sectors, such as air travel and marine transportation may rely on alternative fuels such as biofuels, synthetic fuels, ammonia, produced with zero carbon energy source ( [[#6.6.2.4|Section 6.6.2.4]] ).

Under net-zero energy systems, buildings would by characterised by improved construction materials, an increase in multi-family dwellings, early retirement of inefficient buildings, smaller floor areas, and smart controls to optimise energy use in the building, namely for heating, cooling, LED lighting, and water heating (Chapter 9). End uses would utilise electricity, or potentially hydrogen, produced from zero-carbon sources. The use of electricity for heating and cooking may often be a less efficient process at converting primary energy to energy services than using natural gas, but using natural gas would require CDR in order to be considered net-zero emissions. Changes in behaviour may modestly lower demand. Most economies would have buildings with more efficient technologies powered by zero-carbon electricity, and developing economics would shift from biomass to electricity, raising their energy consumption as population and wealth increase under net-zero energy systems.

Industry has seen major efficiency improvements in the past, but many processes are now close to their thermodynamic limits. Electrification and breakthrough processes (such as producing steel with electricity and hydrogen), using recycled materials, using heat more efficiently by improving thermal insulation, and using waste heat for heat pumps, as well using advanced sensors, monitoring, and visualisation and communication technologies may provide further efficiency improvements (Chapter 11).

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==== 6.6.2.6 Greater Reliance on Integrated Energy System Approaches ====

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Energy systems integration refers to connected planning and operations across energy carriers, including electricity, fuels, and thermal resources. Coordinated planning could be important in lowering system costs, increasing reliability, minimising environmental impacts, and ensuring that costs of R&amp;D and infrastructure account for not just current needs but also for those of future energy systems ( [[#6.4.3|Section 6.4.3]] ). Integration includes not only the physical energy systems themselves but also simultaneous societal objectives (e.g., sustainable development goals), innovation processes (e.g., coordinating R&amp;D to increase the likelihood of beneficial technological spillovers), and other institutional and infrastructural transformations ( [[#Sachs--2019|Sachs et al. 2019]] ). Given system variability and differences in regional resources, there are economic and technical advantages to greater coordination of investments and policies across jurisdictions, sectors, and levels of government ( [[#Schmalensee--2017|Schmalensee and Stavins 2017]] ). Coordinated planning and operations can improve system economics by sharing resources, increasing the utilisation of capital-intensive assets, enhancing the geographical diversity of resource bases, and smoothing demand. But integration could require regulatory and market frameworks to facilitate and appropriate price signals to align incentives and to coordinate investments and operations.

Carbon-neutral energy systems are likely to be more interconnected than those of today ( ''high confidence'' ). The many possible feedstocks, energy carriers, and interconversion processes imply a greater need for the integration of production, transport, storage, and consumption of different fuels ( [[#Davis--2018|Davis et al. 2018]] ). For instance, electrification is expected to play an important role in decarbonising light-duty vehicles (Chapter 10, [[#6.4.3|Section 6.4.3]] ), yet the electricity and transport sectors have few direct interactions today. Systems integration and sectoral coupling are increasingly relevant to ensure that net-zero energy systems are reliable, resilient, and affordable ( [[#EPRI--2017|EPRI 2017]] ; Martin et al. 2017; [[#Buttler--2018|Buttler and Spliethoff 2018]] ; [[#O’Malley--2020|O’Malley et al. 2020]] ). Deep decarbonisation offers new opportunities and challenges for integrating different sectors as well as supply- and demand-side options. For instance, increasing electrification will change daily and seasonal load shapes, and end-use flexibilities and constraints could impact the desirability of different supply-side technologies ( [[#Brown--2018|Brown et al. 2018]] ; [[#EPRI--2019b|EPRI 2019b]] ). The feasibility of net-zero energy system configurations could depend on demonstrating cross-sector benefits like balancing VRE sources in the electricity sector, and on offering the flexibility to produce multiple products. For instance, low-emissions synthetic fuels could help to bridge stationary and mobile applications, since fuel markets have more flexibility than instantaneously balanced electricity markets due to the comparative ease and cost of large-scale, long-term storage of chemical fuels ( [[#Davis--2018|Davis et al. 2018]] ).

There are few detailed archetypes of integrated energy systems that provide services with zero- or net-negative CO 2 emissions (such as [[#Jacobson--2019|Jacobson et al. 2019]] ), so there is considerable uncertainty about integration and interactions across parts of the system. Although alternate configurations, trade-offs, and pathways are still being identified, common elements include fuels and processes like zero- or negative-CO 2 electricity generation and transmission, hydrogen production and transport, synthetic hydrocarbon production and transport, ammonia production and transport, and carbon management, where linkages across pathways could include the use of electricity to produce hydrogen via electrolysis ( [[#Smith--2016|Smith et al. 2016]] ; [[#Moore--2017|Moore 2017]] ; [[#Davis--2018|Davis et al. 2018]] ; [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Shih--2018|Shih et al. 2018]] ; [[#van%20Vuuren--2018|van Vuuren et al. 2018]] ). Linked analytical frameworks are increasing being used to understand the potential role for system coupling with greater temporal resolution, spatial resolution, and heterogeneity of consumer and firm decisions (Bohringer and Rutherford 2008; [[#Bistline--2017|Bistline and de la Chesnaye 2017]] ; [[#Collins--2017|Collins et al. 2017]] ; [[#Gerboni--2017|Gerboni et al. 2017]] ; [[#Santen--2017|Santen et al. 2017]] ; [[#Pye--2021|Pye et al. 2021]] ).

Challenges associated with integrating net-zero energy systems include rapid technological change, the importance of behavioural dimensions in domains with limited experience and data, policy changes and interactions, and path dependence. Technological cost and public acceptance will influence the degree of integration. Sectoral pathways will likely be adaptive and adjust based on the resolution of uncertainties over time, and the relative competitiveness will evolve as the technological frontier evolves, which is a complex and path-dependent function of deployment, R&amp;D, and inter-industry spillovers. Supply-side options interact with demand-side measures in increasingly integrated energy systems ( [[#Sorrell--2015|Sorrell 2015]] ; [[#van%20Vuuren--2018|van Vuuren et al. 2018]] ).

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==== 6.6.2.7 Carbon Dioxide Removal ====

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While CDR is likely necessary for net-zero energy systems, the scale and mix of strategies is unclear –nonetheless some combination of BECCS and DACCS are likely to be part of net-zero energy systems ( ''high confidence'' ). Studies indicate that energy-sector CDR may potentially remove 5–12 GtCO 2 annually globally in net-zero energy systems ( [[#Fuss--2018|Fuss et al. 2018]] ) (Figure 6.22; [[#6.7|Section 6.7]] ; Chapter 12). CDR is not intended as a replacement for emissions reduction, but rather as a complementary effort to offset residual emissions from sectors that are not decarbonised and from other low-carbon technologies such as fossil CCS ( [[#McLaren--2019|McLaren et al. 2019]] ; [[#Gaffney--2020|Gaffney et al. 2020]] ; [[#Iyer--2021|Iyer et al. 2021]] ).

CDR covers a broad set of methods and implementation options (Chapters 7 and 12). The two CDR methods most relevant to the energy sector are BECCS, which is used to produce energy carriers, and DACCS which is an energy user ( [[#Smith--2016|Smith et al. 2016]] ; [[#Singh--2021|Singh and Colosi 2021]] ). BECCS has value as an electricity generation technology, providing firm, dispatchable power to support electricity grids with large amounts of VRE sources, and reducing the reliance on other means to manage these grids, including electricity storage ( [[#Mac%20Dowell--2017|Mac Dowell et al. 2017]] ; [[#Bistline--2021a|Bistline and Blanford 2021a]] ). BECCS may also be used to produce liquid fuels or gaseous fuels, including hydrogen ( [[#6.4.2.6|Section 6.4.2.6]] ) ( [[#Muratori--2020b|Muratori et al. 2020b]] ). For instance, CO 2 from bio-refineries could be captured at &lt;USD45 tCO 2 –1 ( [[#Sanchez--2018|Sanchez et al. 2018]] ). Similarly, while CO 2 capture is expensive in the electricity sector, its integration with hydrogen via biomass gasification can be achieved at an incremental capital cost of 3–35% ( [[#Muratori--2020b|Muratori et al. 2020b]] ) ( [[#6.4|Section 6.4]] ). As with all uses of bioenergy, linkages to broad sustainability concerns may limit the viable development, as will the presence of high-quality geologic sinks in close proximity ( [[#Melara--2020|Melara et al. 2020]] ).

DACCS offers a modular approach to CDR ( [[#Creutzig--2019|Creutzig et al. 2019]] ), but it could be a significant consumer of energy. DAC could also interact with other elements of the energy systems as the captured CO 2 could be reused to produce low-carbon methanol and other fuels ( [[#Hoppe--2018|Hoppe et al. 2018]] ; [[#Realmonte--2019|Realmonte et al. 2019]] ; [[#Zhang--2020|Zhang and Fujimori 2020]] ). DACCS might also offer an alternative for use of excess electricity produced by variable renewables ( [[#Wohland--2018|Wohland et al. 2018]] ), though there are uncertainties about the economic performance of this integrated approach.

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