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

=== Box 6.8 | 100% Renewables in Net-zeroEnergy Systems ===

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The decreasing cost and increasing performance of renewable energy has generated interest in the feasibility of providing nearly all energy services with renewables. Renewable energy includes wind power, solar power, hydroelectric power, bioenergy, geothermal energy, tidal power, and ocean power. There are two primary frames around which 100% renewable energy systems are discussed: 100% renewable electricity systems and 100% renewable energy systems, considering not only electricity but all aspects of the energy system.

It is technically feasible to use very high renewable shares (e.g., above 75% of annual regional generation) to meet hourly electricity demand under a range of conditions, especially when VRE options, notably wind and solar, are complemented by other resources ( ''high confidence'' ). There are currently many grids with high renewable shares and large anticipated roles for VRE sources, in particular wind and solar ( [[#6.4|Section 6.4]] ), in future low-carbon electricity systems. An increasingly large set of studies examines the feasibility of high renewable penetration and economic drivers under different policy, technology, and market scenarios ( [[#Cochran--2014|Cochran et al. 2014]] ; [[#Deason--2018|Deason 2018]] ; [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Bistline--2019|Bistline et al. 2019]] ; [[#Hansen--2019|Hansen et al. 2019]] ; [[#Dowling--2020|Dowling et al. 2020]] ; [[#Blanford--2021|Blanford et al. 2021]] ; [[#Denholm--2021|Denholm et al. 2021]] ). High wind and solar penetration involves technical and economic challenges due to their unique characteristics such as spatial and temporal variability, short- and long-term uncertainty, and non-synchronous generation ( [[#Cole--2017|Cole et al. 2017]] ). These challenges become increasingly important as renewable shares approach 100% (Sections 6.6.2.2 and 6.4.3).

There are many balancing options in systems with very high renewables ( [[#Milligan--2015|Milligan et al. 2015]] ; [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Mai--2018|Mai et al. 2018]] ; [[#Bistline--2021a|Bistline 2021a]] ; [[#Denholm--2021|Denholm et al. 2021]] ).

'''•''' '''Energy storage.''' Energy storage technologies like batteries, pumped hydro, and hydrogen can provide a range of system services ( [[#Balducci--2018|Balducci et al. 2018]] ; [[#Bistline--2020a|Bistline et al. 2020a]] ) ( [[#6.4.4|Section 6.4.4]] ). Lithium-ion batteries have received attention as costs fall and installations increase, but very high renewable shares typically entail either dispatchable generation or long-duration storage in addition to short-duration options ( [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Arbabzadeh--2019|Arbabzadeh et al. 2019]] ; [[#Schill--2020|Schill 2020]] ). Energy storage technologies are part of a broad set of options (including synchronous condensers, demand-side measures, and even inverter-based technologies themselves) for providing grid services ( [[#Castillo--2014|Castillo and Gayme 2014]] ; [[#EPRI--2019a|EPRI 2019a]] ).

'''•''' '''Transmission and trade.''' To balance differences in resource availability, high renewable systems will very likely entail investments in transmission capacity ( [[#Mai--2014|Mai et al. 2014]] ; Macdonald et al. 2016; [[#Pleßmann--2017|Pleßmann and Blechinger 2017]] ; [[#Zappa--2019|Zappa et al. 2019]] ) ( [[#6.4.5|Section 6.4.5]] ) and changes in trade ( [[#Abrell--2016|Abrell and Rausch 2016]] ; [[#Bistline--2019|Bistline et al. 2019]] ). These increases will likely be accompanied by expanded balancing regions to take advantage of geographical smoothing.

'''•''' '''Dispatchable (‘on-demand’) generation.''' Dispatchable generation could include flexible fossil units or low-carbon fuels such as hydrogen with lower minimum load levels ( [[#Denholm--2018|Denholm et al. 2018]] ; [[#Bistline--2019|Bistline 2019]] ), renewables like hydropower, geothermal, or biomass ( [[#Hirth--2016|Hirth 2016]] ; [[#Hansen--2019|Hansen et al. 2019]] ), or flexible nuclear ( [[#Jenkins--2018a|Jenkins et al. 2018a]] ). The composition depends on costs and other policy goals, though in all cases, capacity factors are low for these resources ( [[#Mills--2020|Mills et al. 2020]] ).

'''•''' '''Demand management:''' Many low-emitting and high-renewables systems also utilise increased load flexibility in the forms of energy efficiency, demand response, and demand flexibility, utilising newly electrified end uses such as electric vehicles to shape demand profiles to better match supply ( [[#Ameli--2017|Ameli et al. 2017]] ; Hale 2017; [[#Brown--2018|Brown et al. 2018]] ; [[#Imelda--2018|Imelda et al. 2018]] a; [[#Bistline--2021a|Bistline 2021a]] ).

'''•''' '''Sector coupling:''' Sector coupling includes increased end-use electrification and PtX electricity conversion pathways, which may entail using electricity to create synthetic fuels such as hydrogen ( [[#Davis--2018|Davis et al. 2018]] ; [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ) (Sections 6.4.3, 6.4., 6.4.5, 6.6.4.3, and 6.6.4.6).

Deployment of integration options depends on their relative costs and value, regulations, and electricity market design. There is considerable uncertainty about future technology costs, performance, availability, scalability, and public acceptance ( [[#Kondziella--2016|Kondziella and Bruckner 2016]] ; [[#Bistline--2019|Bistline et al. 2019]] ). Deploying balanced resources likely requires operational, market design, and other institutional changes, as well as technological changes in some cases ( [[#Denholm--2021|Denholm et al. 2021]] ; [[#Cochran--2014|Cochran et al. 2014]] ). Mixes will differ based on resources, system size, flexibility, and whether grids are isolated or interconnected.

Box 6.8

Although there are no technical upper bounds on renewable electricity penetration, the economic value of additional wind and solar capacity typically decreases as their penetration rises, creating economic challenges at higher deployment levels ( [[#Hirth--2013|Hirth 2013]] ; [[#Gowrisankaran--2016|Gowrisankaran et al. 2016]] ; [[#Cole--2021|Cole et al. 2021]] ; [[#Denholm--2021|Denholm et al. 2021]] ; [[#Millstein--2021|Millstein et al. 2021]] ). The integration options above, as well as changes to market design, can mitigate these challenges but likely will not solve them, especially since these options can exhibit declining value themselves (De Sisternes et al. 2016; [[#Bistline--2017|Bistline 2017]] ; [[#Denholm--2019|Denholm and Mai 2019]] ) and may be complements or substitutes to each other.

Energy systems that are 100% renewable (including all parts of the energy sector, and not only electricity generation) raise a range of technological, regulatory, market, and operational challenges that make their competitiveness uncertain ( ''high confidence'' ). These systems require decarbonising all electricity, using this zero-carbon electricity broadly, and then utilising zero-carbon energy carriers for all end uses not served by electricity, for example, air travel, long-distance transport, and high-temperature process heat. Broader questions emerge regarding the attractiveness of supplying all energy, and not just electricity, with renewables (Figure 6.22). Integrated assessment and energy systems research suggest large roles for renewables, but energy and electricity shares are far from 100%, even with stringent emissions reductions targets and optimistic assumptions about future cost reductions ( [[#Bauer--2018|Bauer et al. 2018]] ; [[#Bistline--2018|Bistline et al. 2018]] ; [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Huntington--2020|Huntington et al. 2020]] ) ( [[#6.7.1|Section 6.7.1]] ). Scenarios with 100% renewable energy systems are an emerging subset in the decarbonisation literature, especially at regional levels ( [[#Hansen--2019|Hansen et al. 2019]] ; [[#Denholm--2021|Denholm et al. 2021]] ). Many 100% renewables studies focus more heavily on electrification for decarbonising end uses, and include less biofuels and hydrogen than the broader literature on deep decarbonisation ( [[#Bauer--2018|Bauer et al. 2018]] a). These studies typically assume a constrained set of available technologies to demonstrate the technical feasibility of very high renewable systems and do not optimise to find least-cost, technology-neutral decarbonisation pathways, and many 100% renewables studies focus on the electricity sector or a limited number of sectors ( [[#Jenkins--2018a|Jenkins et al. 2018a]] ; [[#Hansen--2019|Hansen et al. 2019]] ). In addition to renewables, studies broadly agree that including additional low-carbon options – including not only low-carbon electricity but also targeted use of fossil fuels with and without CCS ( [[#6.6.2.1|Section 6.6.2.1]] ) and alternative fuels for sectors that are difficult to electrify ( [[#6.6.2.4|Section 6.6.2.4]] ) – can lower the cost of decarbonisation, even with very high shares of renewables (Figure 6.22). However, there is disagreement about the magnitude of cost savings from larger portfolios, which depend on context- and scenario-specific assumptions about technologies, markets, and policies.

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==== 6.6.2.3 Widespread Electrification of End Uses ====

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Net-zero energy systems will rely more heavily on increased use of electricity (electrification) in end uses ( ''high confidence'' ). The literature on net-zero energy systems almost universally calls for increased electrification ( [[#Sugiyama--2012|Sugiyama 2012]] ; [[#Williams--2012|Williams et al. 2012]] ; [[#Kriegler--2014a|Kriegler et al. 2014a]] ; [[#Williams--2014|Williams et al. 2014]] ; [[#Rogelj--2015a|Rogelj et al. 2015a]] ; [[#Sachs--2016|Sachs et al. 2016]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Sven--2018|Sven et al. 2018]] ; [[#Schreyer--2020|Schreyer et al. 2020]] ). At least 30% of the global final energy needs are expected to be served by electricity, with some estimates suggesting upwards of 80% of total energy use being electrified (Figure 6.22, panel c). Increased electrification is especially valuable in net-zero energy systems in tandem with decarbonised electricity generation or net-negative emissions electricity generation ( [[#6.5.4|Section 6.5.4]] .2). Flexible electric loads (electric vehicles, smart appliances) can in turn facilitate incorporation of VRE electricity options, increase system flexibility, and reduce needs for grid storage ( [[#6.4.3|Section 6.4.3]] ) ( [[#Mathiesen--2015|Mathiesen et al. 2015]] ; Lund et al. 2018).

Several end uses, such as passenger transportation (light-duty electric vehicles, two and three wheelers, buses, rail) as well as building energy uses (lighting, cooling) are likely to be electrified in net-zero energy systems ( ''high confidence'' ). Variations in projections of electrification largely result from differences in expectations about the ability and cost-competitiveness of electricity to serve other end uses such as non-rail freight transport, aviation, and heavy industry ( [[#McCollum--2014|McCollum et al. 2014]] ; [[#Bataille--2016|Bataille et al. 2016]] ; [[#EPRI--2018|EPRI 2018]] ; [[#Breyer--2019|Breyer et al. 2019]] ) ( [[#6.5.4|Section 6.5.4]] .4), especially relative to biofuels and hydrogen (‘low-carbon fuels’) ( [[#McCollum--2014|McCollum et al. 2014]] ; [[#Sachs--2016|Sachs et al. 2016]] ; [[#Rockström--2017|Rockström et al. 2017]] ), the prospects for which are still quite uncertain ( [[#6.4|Section 6.4]] ). The emergence of CDR technologies and the extent to which they allow for residual emissions as an alternative to electrification will also affect the overall share of energy served by electricity ( [[#6.6.2.7|Section 6.6.2.7]] ).

Regions endowed with cheap and plentiful low-carbon electricity resources (wind, solar, hydropower) are likely to emphasise electrification, while those with substantial bioenergy resources or availability of other liquid fuels might put less emphasis on electrification, particularly in hard-to-electrify end uses ( ''medium confidence'' ). For example, among a group of Latin American countries, relative assumptions about liquid fuels and electricity result in an electrification range of 28–82% for achieving a net-zero energy system ( [[#Bataille--2020|Bataille et al. 2020]] ). Similarly, the level of penetration of biofuels that can substitute for electrification will depend on regional circumstances such as land-use constraints, competition with food, and sustainability of biomass production ( [[#6.6.2.4|Section 6.6.2.4]] ).

Electrification of most buildings services, with the possible exception of space heating in extreme climates, is expected in net-zero energy systems ( ''high confidence'' ) (Chapter 9). Space cooling and water heating are expected to be largely electrified. Building electrification is expected to rely substantially on heat pumps, which will help lower emissions both through reduced thermal requirements and higher efficiencies ( [[#Mathiesen--2015|Mathiesen et al. 2015]] ; [[#Sven--2018|Sven et al. 2018]] ; [[#Rissman--2020|Rissman et al. 2020]] ). The level of electrification for heating will depend on the trade-offs between building or household level heat pumps versus more centralised district heating network options ( [[#Mathiesen--2015|Mathiesen et al. 2015]] ; [[#Brown--2018|Brown et al. 2018]] ), as well as the cost and performance of heat pumps in more extreme climates and regional grid infrastructure ( [[#EPRI--2018|EPRI 2018]] ; [[#Waite--2020|Waite and Modi 2020]] ).

A significant share of transportation, especially road transportation, is expected to be electrified in net-zero energy systems ( ''high confidence'' ). In road transportation, two- and three-wheelers, light-duty vehicles (LDVs), and buses, are especially amenable to electrification, with more than half of passenger LDVs expected to be electrified globally in net-zero energy systems ( ''medium confidence'' ) ( [[#Fulton--2015|Fulton et al. 2015]] ; [[#Sven--2018|Sven et al. 2018]] ; [[#Khalili--2019|Khalili et al. 2019]] ; [[#Bataille--2020|Bataille et al. 2020]] ). Long-haul trucks, large ships, and aircraft are expected to be harder to electrify without technological breakthroughs ( [[#Fulton--2015|Fulton et al. 2015]] ; [[#Mathiesen--2015|Mathiesen et al. 2015]] ), although continued improvements in battery technology may enable electrification of long-haul trucks ( [[#Nykvist--2021|Nykvist and Olsson 2021]] ) (Chapter 10). Due to the relative ease of rail electrification, near complete electrification of rail and a shift of air and truck freight to rail is expected in net-zero energy systems ( [[#Fulton--2015|Fulton et al. 2015]] ; [[#Rockström--2017|Rockström et al. 2017]] ; [[#Sven--2018|Sven et al. 2018]] ; [[#Khalili--2019|Khalili et al. 2019]] ). The degree of modal shifts and electrification will depend on local factors such as infrastructure availability and location accessibility. Due to the challenges associated with electrification of some transport modes, net-zero energy systems may include some residual emissions associated with the freight sector that are offset through CDR technologies ( [[#Muratori--2017b|Muratori et al. 2017b]] ), or reliance on low and zero-carbon fuels instead of electrification.

A non-trivial number of industry applications could be electrified as a part of a net-zero energy system, but direct electrification of heavy industry applications such as cement, primary steel manufacturing, and chemical feedstocks is expected to be challenging ( ''medium confidence'' ) ( [[#Davis--2018|Davis et al. 2018]] ; [[#Philibert--2019|Philibert 2019]] ; [[#Madeddu--2020|Madeddu et al. 2020]] ; van Sluisveld et al. 2021). Process and boiler heating in industrial facilities are anticipated to be electrified in net-zero energy systems. Emissions intensity reductions for cement and concrete production can be achieved through the use of electrified cement kilns, while emissions associated with steel production can be reduced through the use of an electric arc furnace (EAF) powered by decarbonised electricity ( [[#Rissman--2020|Rissman et al. 2020]] ). Electricity can also be used to replace thermalheat such as resistive heating, EAFs, and laser sintering ( [[#Madeddu--2020|Madeddu et al. 2020]] ; [[#Rissman--2020|Rissman et al. 2020]] ). One study found that as much as 60% of the energy end-use in European industry could be met with direct electrification using existing and emerging technologies ( [[#Madeddu--2020|Madeddu et al. 2020]] ). Industry electrification for different regions will depend on the economics and availability of alternative emissions mitigation strategies such as carbon neutral fuels and CCS ( [[#Davis--2018|Davis et al. 2018]] ; [[#Madeddu--2020|Madeddu et al. 2020]] ).

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==== 6.6.2.4 Alternative Fuels in Sectors not Amenable to Electrification ====

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Net-zero energy systems will need to rely on alternative fuels – notably hydrogen or biofuels – in several sectors that are not amenable to electricity and otherwise hard to decarbonise ( ''medium confidence'' ). Useful carbon-based fuels (e.g., methane, petroleum, methanol), hydrogen, ammonia, or alcohols can be produced with net-zero CO 2 emissions and without fossil fuel inputs (Sections 6.4.4 and 6.4.5). For example, liquid hydrocarbons can be synthesised via hydrogenation of non-fossil carbon by processes such as Fischer-Tropsch (MacDowell et al. 2017) or by conversion of biomass ( [[#Tilman--2009|Tilman et al. 2009]] ). The resulting energy-dense fuels can serve applications that are difficult to electrify, but it is not clear if and when the combined costs of obtaining necessary feedstocks and producing these fuels without fossil inputs will be less than continuing to use fossil fuels and managing the related carbon through, for example, CCS or CDR ( [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ).

CO 2 emissions from some energy services are expected to be particularly difficult to cost-effectively avoid, among them: aviation; long-distance freight by ships; process emissions from cement and steel production; high-temperature heat (e.g., &gt;1000°C); and electricity reliability in systems with high penetration of variable renewable energy sources (NAS) ( [[#Davis--2018|Davis et al. 2018]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Sepulveda--2018|Sepulveda et al. 2018]] ; [[#Chiaramonti--2019|Chiaramonti 2019]] ; [[#Bataille--2020|Bataille 2020]] ; [[#Madeddu--2020|Madeddu et al. 2020]] ; [[#Rissman--2020|Rissman et al. 2020]] ; [[#Thiel--2021|Thiel and Stark 2021]] ). The literature focused on these services and sectors is growing, but remains limited, and provides minimal guidance on the most promising or attractive technological options and systems for avoiding these sectors’ emissions. Technological solutions do exist, but those mentioned in the literature are prohibitively expensive, exist only at an early stage, and/or are subject to much broader concerns about sustainability (e.g., biofuels) ( [[#Davis--2018|Davis et al. 2018]] ).

Liquid biofuels today supply about 4% of transportation energy worldwide, mostly as ethanol from grain and sugar cane and biodiesel from oil seeds and waste oils ( [[#Davis--2018|Davis et al. 2018]] ). These biofuels could conceivably be targeted to difficult-to-electrify sectors, but face substantial challenges related to their lifecycle carbon emissions, cost, and further scalability ( [[#Tilman--2009|Tilman et al. 2009]] ; [[#Staples--2018|Staples et al. 2018]] ), ( [[#6.4.2|Section 6.4.2]] ). The extent to which biomass will supply liquid fuels or high temperature heat for industry in a future net-zero energy system will thus depend on advances in conversion technology that enable use of feedstocks such as woody crops, agricultural residues, algae, and wastes, as well as competing demands for bioenergy and land, the feasibility of other sources of carbon-neutral fuels, and integration of bioenergy production with other objectives, including CDR, economic development, food security, ecological conservation, and air quality ( [[#Fargione--2010|Fargione 2010]] ; [[#Williams--2010|Williams and Laurens 2010]] ; [[#Creutzig--2015|Creutzig et al. 2015]] ; [[#Chatziaras--2016|Chatziaras et al. 2016]] ; [[#Laurens--2017|Laurens 2017]] ; [[#Lynd--2017|Lynd 2017]] ; [[#Bauer--2018|Bauer et al. 2018]] a, b; [[#Strefler--2018|Strefler et al. 2018]] ; [[#Muratori--2020b|Muratori et al. 2020b]] ; [[#Fennell--2021|Fennell et al. 2021]] ) ( [[#6.4.2.6|Section 6.4.2.6]] ).

Costs are the main barrier to synthesis of net-zero emissions fuels ( ''high confidence'' ), particularly costs of hydrogen (a constituent of hydrocarbons, ammonia, and alcohols) ( [[#6.4.5|Section 6.4.5]] ). Today, most hydrogen is supplied by steam reformation of fossil methane (CH 4 into CO 2 and H 2 ) at a cost of 1.30– USD1.50 kg –1 ( [[#Sherwin--2021|Sherwin 2021]] ). Non-fossil hydrogen can be obtained by electrolysis of water, at current costs of USD5–7 kgH 2 –1 (assuming relatively low electricity costs and high utilisation rates) ( [[#Graves--2011|Graves et al. 2011]] ; [[#DOE--2020a|DOE 2020a]] ; [[#Newborough--2020|Newborough and Cooley 2020]] ; [[#Peterson--2020|Peterson et al. 2020]] ). At these costs for electrolytic hydrogen, synthesised net-zero emissions fuels would cost at least USD1.6 per litre of diesel equivalent (or USD6 gallon –1 and USD46 GJ –1 , assuming non-fossil carbon feedstock costs of USD100 per tonne of CO 2 and low process costs of USD0.05 litre –1 or USD1.5 GJ –1 ). Similar calculations suggest that synthetic hydrocarbon fuels could currently avoid CO 2 emissions at a cost of USD936–1404 tonne –1 ( [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ). However, economies of scale are expected to bring these costs down substantially in the future ( [[#IRENA--2020c|IRENA 2020c]] ; [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ), and R&amp;D efforts are targeting 60–80% reductions in costs (to less than USD2 kg –1 (H 2 ) –1 ) possibly by use of less mature but promising technologies such as high-temperature electrolysis and thermochemical water splitting ( [[#Kuckshinrichs--2017|Kuckshinrichs et al. 2017]] ; [[#Pes--2017|Pes et al. 2017]] ; [[#Schmidt--2017|Schmidt et al. 2017]] ; [[#Saba--2018|Saba et al. 2018]] ; [[#DOE--2018|DOE, 2018]] , 2020b). Technologies capable of producing hydrogen directly from water and sunlight (photoelectrochemical cells or photocatalysts) are also under development, but are at an early stage ( [[#Nielander--2015|Nielander et al. 2015]] ; [[#DOE--2020a|DOE 2020a]] ). High hydrogen production efficiencies have been demonstrated, but costs, capacity factors, and lifetimes need to be improved in order to make such technologies feasible for net-zero emissions fuel production at scale ( [[#McKone--2014|McKone et al. 2014]] ; [[#DOE--2020a|DOE 2020a]] ; [[#Newborough--2020|Newborough and Cooley 2020]] ).

The carbon contained in net-zero emissions hydrocarbons must have been removed from the atmosphere either through DAC, or, in the case of biofuels, by photosynthesis (which could include CO 2 captured from the exhaust of biomass or biogas combustion) ( [[#Zeman--2008|Zeman and Keith 2008]] ; [[#Graves--2011|Graves et al. 2011]] ). A number of different groups are now developing DAC technologies, targeting costs of USD100 per tonne of CO 2 or less ( [[#Darton--2018|Darton and Yang 2018]] ; [[#Keith--2018|Keith et al. 2018]] ; [[#Fasihi--2019|Fasihi et al. 2019]] ).

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'''Figure 6.23 | Schematic of net-zero emissions energy system, including methods to address difficult-to-electrify sectors.''' Source: with permission from [[#Davis--2018|Davis et al. (2018)]] .

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