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

=== 6.6.2 Configurations of Net-zero Energy Systems ===

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Net-zero energy systems entail trade-offs across economic, environmental, and social dimensions ( [[#Davis--2018|Davis et al. 2018]] ). Many socio-economic, policy, and market uncertainties will also influence the configuration of net-zero energy systems ( [[#Smith--2015|Smith et al. 2015]] ; [[#van%20Vuuren--2018|van Vuuren et al. 2018]] ; [[#Bistline--2019|Bistline et al. 2019]] ; [[#Krey--2019|Krey et al. 2019]] ; [[#Azevedo--2021|Azevedo et al. 2021]] , [[#Pye--2021|Pye et al. 2021]] ). There are reasons that countries might focus on one system configuration versus another, including cost, resource endowments, related industrial bases, existing infrastructure, geography, governance, public acceptance, and other policy priorities ( [[#6.6.4|Section 6.6.4]] and Chapter 18 of WGII).

Explorations of net-zero energy systems have been emerging in the detailed systems modelling literature ( [[#Azevedo--2021|Azevedo et al. 2021]] ; [[#Bistline--2021b|Bistline 2021b]] ). Reports associated with net-zero economy-wide targets for countries and sub-national entities typically do not provide detailed roadmaps or modelling but discuss high-level guiding principles, though more detailed studies are emerging at national levels ( [[#Capros--2019|Capros et al. 2019]] ; [[#Wei--2020|Wei et al. 2020]] ; [[#Duan--2021|Duan et al. 2021]] ; [[#Williams--2021a|Williams et al. 2021a]] ). Most analysis has focused on identifying potential decarbonisation technologies and pathways for different sectors, enumerating opportunities and barriers for each, their costs, highlighting robust insights, and characterising key uncertainties ( [[#Davis--2018|Davis et al. 2018]] ; [[#Hepburn--2019|Hepburn et al. 2019]] ).

The literature on the configuration of net-zero energy systems is limited in a few respects. On the one hand, there is a robust integrated assessment literature that provides characterisations of these systems in broad strokes (the AR6 database), offering internally consistent global scenarios to link global warming targets to regional/national goals. All integrated assessment scenarios that discuss net-zero energy system CO 2 emissions provide high-level characterisations of net-zero systems. Because these characterisations have less temporal, spatial, technological, regulatory, and societal detail, however, they may not consider the complexities that could ultimately influence regional, national, or local pathways. High-fidelity models and analyses are needed to assess the economic and environmental characteristics and the feasibility of many aspects of net-zero or net-negative emissions energy systems ( ''high confidence'' ) ( [[#Blanford--2018|Blanford et al. 2018]] ; [[#Bistline--2020|Bistline and Blanford 2020]] ). For example, evaluating the competitiveness of electricity sector technologies requires temporal, spatial, and technological detail to accurately represent system investments and operations ( [[#Collins--2017|Collins et al. 2017]] ; [[#Santen--2017|Santen et al. 2017]] ; Helistoe et al. 2019; [[#Bistline--2021c|Bistline 2021c]] ; [[#Victoria--2021|Victoria et al. 2021]] ).

Configurations of net-zero energy systems will vary by region but are likely to share several common characteristics ( ''high confidence'' ) (Figure 6.22). We focus on seven of those common characteristics in the remainder of this subsection.

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[[File:b4f15a3c586b00584ada54b2cfbb6621 IPCC_AR6_WGIII_Figure_6_22.png]]

'''Figure 6.22 | Characteristics of global net-zero energy systems when global energy and industrial CO''' 2 '''emissions reach net-zero.'''

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==== 6.6.2.1 Limited and/or Targeted Use of Fossil Fuels ====

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Net-zero energy systems will use far less fossil fuel than today ( ''high confidence'' ). The precise quantity of fossil fuels will largely depend on the relative costs of such fuels, electrification, alternative fuels, and CDR ( [[#6.6.2.4|Section 6.6.2.4]] ) in the energy system ( ''high confidence'' ). All of these are affected by regional differences in resources ( [[#McGlade--2015|McGlade and Ekins 2015]] ), existing energy infrastructure ( [[#Tong--2019|Tong et al. 2019]] ), demand for energy services, and climate and energy policies. Fossil fuel use may persist, for example, if and where the costs of such fuels and the compensating carbon management (e.g., CDR, CCS) are less than non-fossil energy. For most applications, however, it is likely that electrification ( [[#McCollum--2014|McCollum et al. 2014]] ; [[#Madeddu--2020|Madeddu et al. 2020]] ; [[#Zhang--2020|Zhang and Fujimori 2020]] ) or use of non-fossil alternative fuels ( [[#Zeman--2008|Zeman and Keith 2008]] ; [[#Graves--2011|Graves et al. 2011]] ; [[#Hänggi--2019|Hänggi et al. 2019]] ; [[#Ueckerdt--2021|Ueckerdt et al. 2021]] ) will prove to be the cheapest options. Most residual demand for fossil fuels is likely to predominantly be petroleum and natural gas given their high energy density ( [[#Davis--2018|Davis et al. 2018]] ), while demand for coal in net-zero energy systems is likely to be very low ( [[#Luderer--2018|Luderer et al. 2018]] ; [[#Jakob--2020|Jakob et al. 2020]] , [[#6.7.4|Section 6.7.4]] ) ( ''high confidence'' ).

There is considerable flexibility regarding the overall quantity of liquid and gaseous fuels that will be required in net-zero energy systems ( ''high confidence'' ) (Figure 6.22 and [[#6.7.4|Section 6.7.4]] ). This will be determined by the relative value of such fuels as compared to systems which rely more or less heavily on zero-emissions electricity. In turn, the share of any fuels that are fossil or fossil-derived is uncertain and will depend on the feasibility of CCS and CDR technologies and long-term sequestration as compared to alternative, carbon-neutral fuels. Moreover, to the extent that physical, biological, and/or socio-political factors limit the availability of CDR ( [[#Smith--2015|Smith et al. 2015]] ; [[#Field--2017|Field and Mach 2017]] ), carbon management efforts may prioritise residual emissions related to land use and other non-energy sources.

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==== 6.6.2.2 Zero or Negative CO 2 Emissions from Electricity ====

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Net-zero energy systems will rely on decarbonised or net-negative CO 2 emissions electricity systems, due to the many lower-cost options for producing zero-carbon electricity and the important role of end-use electrification in decarbonising other sectors ( ''high confidence'' ).

There are many possible configurations and technologies for zero- or net-negative-emissions electricity systems ( ''high confidence'' ). These systems could entail a mix of variable renewables, dispatchable renewables (e.g., biomass, hydropower), other firm, dispatchable (‘on-demand’) low-carbon generation (e.g., nuclear, CCS-equipped capacity), energy storage, transmission, carbon removal options (e.g., BECCS, DACCS), and demand management ( [[#Luderer--2017|Luderer et al. 2017]] ; [[#Bistline--2018|Bistline et al. 2018]] ; [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Bistline--2021b|Bistline and Blanford 2021b]] ). The marginal cost of deploying electricity sector mitigation options increases as electricity emissions approach zero; in addition, the most cost-effective mix of system resources changes as emissions approach zero and, therefore, so do the implications of electricity sector mitigation for sustainability and other societal goals ( [[#Mileva--2016|Mileva et al. 2016]] ; [[#Bistline--2018|Bistline et al. 2018]] ; [[#Sepulveda--2018|Sepulveda et al. 2018]] ; [[#Jayadev--2020|Jayadev et al. 2020]] ; [[#Cole--2021|Cole et al. 2021]] ). Key factors influencing the electricity mix include relative costs and system benefits, local resource bases, infrastructure availability, regional integration and trade, co-benefits, societal preferences and other policy priorities, all of which vary by country and region ( [[#6.6.4|Section 6.6.4]] ). Many of these factors depend on when the net-zero point is reached (Figure 6.22).

Based on their increasing economic competitiveness, VRE technologies, especially wind and solar power, will likely comprise large shares of many regional generation mixes ( ''high confidence'' ) (Figure 6.22). While wind and solar will likely be prominent electricity resources, this does not imply that 100% renewable energy systems will be pursued under all circumstances, since economic and operational challenges increase nonlinearly as shares approach 100% (Box 6.8) ( [[#Frew--2016|Frew et al. 2016]] ; [[#Imelda--2018|Imelda et al. 2018]] b; [[#Shaner--2018|Shaner et al. 2018]] ; [[#Bistline--2021a|Bistline and Blanford 2021a]] ; [[#Cole--2021|Cole et al. 2021]] ). Real-world experience planning and operating regional electricity systems with high instantaneous and annual shares of renewable generation is accumulating, but debates continue about how much wind and solar should be included in different systems, and the cost-effectiveness of mechanisms for managing variability (Box 6.8). Either firm, dispatchable generation (including nuclear, CCS-equipped capacity, dispatchable renewables such as geothermal, and fossil units run with low capacity factors and CDR to balance emissions) or seasonal energy storage (alongside other balancing resources discussed in Box 6.8) will be needed to ensure reliability and resource adequacy with high percentages of wind and solar ( [[#Jenkins--2018b|Jenkins et al. 2018b]] ; [[#Dowling--2020|Dowling et al. 2020]] ; [[#Denholm--2021|Denholm et al. 2021]] ) though each option involves uncertainty about costs, timing, and public acceptance ( [[#Albertus--2020|Albertus et al. 2020]] ).

Electricity systems require a range of different functional roles – for example, providing energy, capacity, or ancillary services. As a result, a range of different types of generation, energy storage, and transmission resources may be deployed in these systems ( [[#Baik--2021|Baik et al. 2021]] ). There are many options for each of these roles, each with their strengths and weaknesses (Sections 6.4.3 and 6.4.4), and deployment of these options will be influenced by the evolution of technological costs, system benefits, and local resources ( [[#Fell--2013|Fell and Linn 2013]] ; [[#Hirth--2015|Hirth 2015]] ; [[#Bistline--2018|Bistline et al. 2018]] ; [[#Mai--2018|Mai et al. 2018]] ; [[#Veers--2019|Veers et al. 2019]] ).

System management is critical for zero- or negative-emissions electricity systems. Maintaining reliability will increasingly entail system planning and operations that account for characteristics of supply- and demand-side resources ( [[#Hu--2018|Hu et al. 2018]] ). Coordinated planning and operations will likely become more prevalent across portions of the electricity system (e.g., integrated generation, transmission, and distribution planning), across sectors, and across geographies ( [[#EPRI--2017|EPRI 2017]] ; [[#Konstantelos--2017|Konstantelos et al. 2017]] ; [[#Chan--2018|Chan et al. 2018]] ; [[#Bistline--2019|Bistline and Young 2019]] ) ( [[#6.4.3|Section 6.4.3]] ).

Energy storage will be increasingly important in net-zero energy systems, especially in systems with shares of VRE ( ''high confidence'' ). Deployment of energy storage will vary based on the system benefits and values of different options ( [[#Arbabzadeh--2019|Arbabzadeh et al. 2019]] ; [[#Denholm--2019|Denholm and Mai 2019]] ). Diurnal storage options like lithium-ion batteries have different value than storing and discharging electricity over longer periods through long-duration energy storage with less frequent cycling, which require different technologies, supporting policies, and business models ( [[#Gallo--2016|Gallo et al. 2016]] ; Blanco and Faaij 2017; [[#Albertus--2020|Albertus et al. 2020]] ; [[#Dowling--2020|Dowling et al. 2020]] ; [[#Sepulveda--2021|Sepulveda et al. 2021]] ) ( [[#6.4.4|Section 6.4.4]] ). The value of energy storage varies with the level of deployment and on the competitiveness of economic complements such as VRE options ( [[#Mileva--2016|Mileva et al. 2016]] ; [[#Bistline--2020|Bistline and Young 2020]] ) and substitutes such as flexible demand ( [[#Brown--2018|Brown et al. 2018]] ; [[#Merrick--2018|Merrick et al. 2018]] ), transmission ( [[#Schlachtberger--2017|Schlachtberger et al. 2017]] ; [[#Brown--2018|Brown et al. 2018]] ; [[#Merrick--2018|Merrick et al. 2018]] ; [[#Bistline--2019|Bistline and Young 2019]] ), trade ( [[#Bistline--2020b|Bistline et al. 2020b]] ), dispatchable generators ( [[#Hittinger--2015|Hittinger and Lueken 2015]] ; [[#Gils--2017|Gils et al. 2017]] ; [[#Arbabzadeh--2019|Arbabzadeh et al. 2019]] ), direct air capture (DAC) ( [[#Daggash--2019|Daggash et al. 2019]] ), and efficiencies in system operations (Tuohy et al. 2015).

The approach to other sectors could impact on electricity sector planning, and the role of some technologies (e.g., hydrogen, batteries, CCS) could depend on deployment in other sectors. CCS offers opportunities for CO 2 removal when fuelled with syngas or biomass containing carbon captured from the atmosphere ( [[#Hepburn--2019|Hepburn et al. 2019]] ); however, concerns about lifecycle environmental impacts, uncertain costs, and public acceptance are potential barriers to widespread deployment ( [[#6.4.2|Section 6.4.2]] ). It is unclear whether CDR options like BECCS will be included in the electricity mix to offset continued emissions in other parts of the energy system or beyond (MacDowell et al. 2017; [[#Bauer--2018|Bauer et al. 2018]] a; [[#Luderer--2018|Luderer et al. 2018]] ). Some applications may also rely on power to fuels (PtX) electricity conversion to create low-emissions synthetic fuels (Sections 6.6.2.6, 6.4.4, and 6.4.5), which could impact on electricity system planning and operations. Additionally, if DAC technologies are used, electricity and heat requirements to operate DAC could impact electricity system investments and operations ( [[#Realmonte--2019|Realmonte et al. 2019]] ; [[#Bistline--2021a|Bistline and Blanford 2021a]] ).

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