Editing IPCC:AR6/WGIII/Chapter-10 (section)

=== 10.4.2 Transit Technologies for Passenger Transport ===

<div id="h2-14-siblings" class="h2-siblings"></div>

Buses provide urban and peri-urban transport services to millions of people around the world and a growing number of transport agencies are exploring alternative-fuelled buses. Alternative technologies to conventional diesel-powered buses include buses powered with CNG, LNG, synthetic fuels, and biofuels (e.g., biodiesel, renewable diesel, dimethyl ether); diesel hybrid-electric buses; battery electric buses; electric catenary buses; and hydrogen fuel cell buses. Rail is an alternative mode of transit that could support decarbonisation of land-based passenger mobility. Electric rail systems can provide urban services (light rail and metro systems), as well as longer-distance transport. Indeed, many cities of the world already have extensive metro systems, and regions like China, Japan and Europe have a robust high-speed intercity railway network. Intercity rail transport can be powered with electricity, however, fossil fuels are still prevalent for long-distance rail passenger transport in some regions. Battery electric long-distance trains may be a future option for these areas.

Figure 10.6 shows the lifecycle GHG emissions from different powertrain and fuel technologies for buses and passenger rail. The data in each panel came from a number of relevant scientific studies ( [[#Cai--2015|Cai et al. 2015]] ; [[#Tong--2015a|Tong et al. 2015a]] ; [[#Dimoula--2016|Dimoula et al. 2016]] ; [[#de%20Bortoli--2017|de Bortoli et al. 2017]] ; [[#Valente--2017|Valente et al. 2017]] ; [[#Meynerts--2018|Meynerts et al. 2018]] ; [[#IEA--2019e|IEA 2019e]] ; [[#de%20Bortoli--2020|de Bortoli and Christoforou 2020]] ; [[#Hill--2020|Hill et al. 2020]] ; [[#Liu--2020a|Liu et al. 2020a]] ; [[#Valente--2021|Valente et al. 2021]] ). The width of the bar represents the variabilityin available estimates, which is primarily driven by variability in reported vehicle efficiency, size, or drive cycle. While some bars overlap, the Figure may not fully capture correlations between results. For example, low efficiency associated with aggressive drive cycles may drive the upper end of the emission ranges for multiple technologies; thus, an overlap does not necessarily suggest uncertainty regarding which vehicle type would have lower emissions for a comparable trip. Additionally, reported lifecycle emissions do not include embodied GHG emissions associated with infrastructure construction and maintenance. These embodied emissions are potentially a larger fraction of lifecycle emissions for rail than for other transport modes ( [[#Chester--2012|Chester and Horvath 2012]] ; [[#Chester--2013|Chester et al. 2013]] ). One study reported values ranging from 10–25 gCO 2 per passenger-kilometre ( [[#International%20Union%20of%20Railways--2016|International Union of Railways 2016]] ), although embodied emissions from rail are known to vary widely across case studies ( [[#Olugbenga--2019|Olugbenga et al. 2019]] ). These caveats are also applicable to the other figures in this section.

<div id="_idContainer030" class="Basic-Text-Frame"></div>

[[File:63378fe3140074f8a2a80581eec4c6f9 IPCC_AR6_WGIII_Figure_10_6.png]]

'''Figure 10.6 | Lifecycle greenhouse gas intensity of land-based bus and rail technologies.''' Each bar represents the range of the lifecycle estimates, bounded by minimum and maximum energy use per passenger-kilometre, as reported for each fuel/powertrain combination. The ranges are driven by differences in vehicle characteristics and operating efficiency. For energy sources with highly variable upstream emissions, low, medium and/or high representative values are shown as separate rows. The primary x-axis shows lifecycle GHG emissions, in gCO 2 -eq pkm –1 , assuming 80% occupancy; the secondary x-axis assumes 20% occupancy. The values in the figure rely on the 100-year GWP value embedded in the source data, which may differ slightly from the updated 100-year GWP values from WGI. For buses, the main bars show full lifecycle, with vertical bars disaggregating the vehicle cycle. ‘Diesel, high’ references emissions factors for diesel from oil sands. ‘advanced biofuels’, refers to the use of second-generation biofuels and their respective conversion and cultivation emissions factors. ‘IAM EMF33’ refers to emissions factors for advanced biofuels derived from simulation results from the integrated assessment models EMF33 scenarios. ‘PM’ refers to partial models, where ‘CLC’ is with constant land cover and ‘NRG’ is with natural regrowth. ‘DAC FT-Diesel, wind electricity’ refers to Fischer- Tropsch diesel produced via a CO 2 direct air capture process that uses wind electricity. ‘Hydrogen, low-carbon renewable’ refers to fuels produced via electrolysis using low-carbon electricity. ‘Hydrogen, natural gas SMR’ refers to fuels produced via steam methane reforming of natural gas. Results for ICEVs with ‘high emissions DAC FT-Diesel from natural gas’ are not included here since the lifecycle emissions are estimated to be substantially higher than petroleum diesel ICEVs.

Figure 10.6 highlights that BEV and FCV buses and passenger rail powered with low-carbon electricity or low-carbon hydrogen, could offer reductions in GHG emissions compared to diesel-powered buses or diesel-powered passenger rail. However, and not surprisingly, these technologies would offer only little emissions reductions if power generation and hydrogen production rely on fossil fuels. While buses powered with CNG and LNG could offer some reductions compared to diesel-powered buses, these reductions are unlikely to be sufficient to contribute to deep decarbonisation of the transport sector and they may slow down conversion to low- or zero-carbon options already commercially available. Biodiesel and renewable diesel fuels (from sources with low upstream emissions and low risk of induced land-use change) could offer important near-term reductions for buses and passenger rail, as these fuels can often be used with existing vehicle infrastructure. They could also be used for long haul trucks and trains, shipping and aviation as discussed below and in later sections.

There has been growing interest in the production of synthetic fuels from CO 2 produced by direct air capture (DAC) processes. Figure 10.6 includes the lifecycle GHG emissions from buses and passenger rail powered with synthetic diesel produced through a DAC system paired with a Fischer-Tropsch (FT) process, based on [[#Liu--2020a|Liu et al. (2020a)]] . This process requires the use of hydrogen (as shown in Figure 10.2), so the emissions factors of the resulting fuel depend on the emissions intensity of hydrogen production. An electricity emissions factor less than 140 gCO 2 -eq kWh –1 would be required for this pathway to achieve lower emissions than petroleum diesel ( [[#Liu--2020a|Liu et al. 2020a]] ); for example, this would be equivalent to a 75% wind and 25% natural gas electricity mix (Appendix 10.1). If the process relied on steam methane reforming for hydrogen production or fossil-based power generation, synthetic diesel from the DAC-FT process would not provide GHG emissions reductions compared to conventional diesel. DAC-FT from low-carbon energy sources appears to be promising from an emissions standpoint and could warrant the R&amp;D and demonstration attention outlined in the rest of the chapter, but it cannot be contemplated as a decarbonisation strategy without the availability of low-carbon hydrogen.

At high occupancy, both bus and rail transport offer substantial GHG reduction potential per pkm, even compared with the lowest-emitting private vehicle options. Even at 20% occupancy, bus and rail may still offer emission reductions compared to passenger cars, especially notable when comparing BEVs with low-carbon electricity (the lowest-emission option for all technologies) across the three modes. Only when comparing a fossil fuel-powered bus at low occupancy with a low-carbon powered car at high occupancy is this conclusion reversed. Use of public transit systems, especially those that rely on buses and passenger rail fuelled with the low-carbon fuels previously described, would thus support efforts to decarbonise the transport sector. Use of these public transit systems will depend on urban design and consumer preferences ( [[#10.2|Section 10.2]] , Chapters 5 and 8), which in turn depend on time, costs, and behavioural choices.

Figure 10.7 shows the results of a parametric analysis of the LCCs of transit technologies with the highest potential for GHG emissions reductions. As with Figure 10.5, the vehicle efficiency ranges are the same as those from the LCA estimates (80% occupancy). Vehicle, fuel, and maintenance costs represent ranges in the literature ( [[#Eudy--2018b|Eudy and Post 2018b]] ; [[#IEA--2019e|IEA 2019e]] ; [[#Argonne%20National%20Laboratory--2020|Argonne National Laboratory 2020]] ; [[#BNEF--2020|BNEF 2020]] ; [[#Eudy--2020|Eudy and Post 2020]] ; [[#Hydrogen%20Council--2020|Hydrogen Council 2020]] ; [[#IEA--2020b|IEA 2020b]] ; [[#IEA--2020c|IEA 2020c]] ; [[#IRENA--2020|IRENA 2020]] ; [[#Johnson--2020|Johnson et al. 2020]] ; [[#Burnham--2021|Burnham et al. 2021]] ; [[#IEA--2021c|IEA 2021c]] ; [[#IEA--2021d|IEA 2021d]] ; [[#US%20Energy%20Information%20Administration--2021|US Energy Information Administration 2021]] ), and the discount rate is 3% where applicable. Appendix 10.2 provides the details behind these estimates. The panels for the ICEV can represent buses and passenger trains powered with any form of diesel, whether derived from petroleum, synthetic hydrocarbons, or biofuels. For reference, global average automotive diesel prices from 2015–2020 fluctuated around USD1 per litre, and the 2019 world average industrial electricity price was approximately USD100 per MWh ( [[#IEA--2021d|IEA 2021d]] ). Retail hydrogen prices in excess of USD13 per kilogram have been observed( [[#Eudy--2018a|Eudy and Post 2018a]] ; [[#Argonne%20National%20Laboratory--2020|Argonne National Laboratory 2020]] ; [[#Burnham--2021|Burnham et al. 2021]] ) though current production cost estimates for hydrogen produced from electrolysis are far lower ( [[#IRENA--2020|IRENA 2020]] ) (and as reported in Chapter 6), at around USD5–7 per kg with future forecasts as low as USD1 per kg ( [[#BNEF--2020|BNEF 2020]] ; [[#Hydrogen%20Council--2020|Hydrogen Council 2020]] ; [[#IRENA--2020|IRENA 2020]] ) (and as reported in Chapter 6).

<div id="_idContainer032" class="Basic-Text-Frame"></div>

[[File:e0b77af9b26643bd31af8caba33cce7e IPCC_AR6_WGIII_Figure_10_7.png]]

'''Figure 10.7 | Lifecycle costs for internal combustion engine vehicles, battery electric vehicles, and hydrogen fuel cell vehicles for buses and passenger rail.''' The range of efficiencies for each vehicle type are consistent with the range of efficiencies in Figure 10.6 (80% occupancy). The results for the ICEV can be used to evaluate the lifecycle costs of ICE buses and passenger rail operated with any form of diesel, whether from petroleum, synthetic hydrocarbons, or biofuel, as the range of efficiencies of vehicles operating with all these fuels is similar. The secondary y-axis depicts the cost of the different energy carriers normalised in USD/GJ for easier cross-comparability.

Under most parameter combinations, rail is the most cost-effective option, followed by buses, both of which are an order of magnitude cheaper than passenger vehicles. Note that costs per pkm are strongly influenced by occupancy assumptions; at low occupancy (e.g., &lt;20% for buses and &lt;10% for rail), the cost of transit approaches the LCC for passenger cars. For diesel rail and buses, cost ranges are driven by fuel costs, whereas vehicles are both important drivers for electric or hydrogen modes due to high costs (but also large projected improvements) associated with batteries and fuel cell stacks. Whereas the current state of ICEV technologies is best represented by cheap vehicles and low fuel costs for diesel (top left of each panel), these costs are likely to rise in future due to stronger emission/efficiency regulations and rising crude oil prices. On the contrary, the current status of alternative fuels is better represented by high capital costs and mid-to-high fuel costs (right side of each panel; mid-to-bottom rows), but technology costs are anticipated to fall with increasing experience, research, and development. Thus, while electric rail is already competitive with diesel rail, and electric buses are competitive with diesel buses in the low efficiency case, improvements are still required in battery costs to compete against modern diesel buses on high efficiency routes, at current diesel costs. Similarly, improvements to both vehicle cost and fuel costs are required for hydrogen vehicles to become cost effective compared to their diesel or electric counterparts. At either the upper end of the diesel cost range (bottom row of ICEV panels), or within the 2030–2050 projections for battery costs, fuel cell costs and hydrogen costs (top left of BEV and FCV panels), both battery- and hydrogen-powered vehicles become financially attractive.

<div id="10.4.3" class="h2-container"></div>

<span id="land-based-freight-transport"></span>