Aashutosh N. Mistry, Polina Brodsky, Andrea J. Brickey, Adam Duran, Eve Mozur, Rajavasanth Rajasegar, Gregory Jackson, Steven C. Decaluwe, Robert J. Kee, Ryan O’Hayre, Alexandra Newman, Robert Braun, Morgan Bazilian, Neal P. Sullivan
{"title":"Not-So-Quick and Not-So-Dirty Solutions to Decarbonize Off- Road Vehicles","authors":"Aashutosh N. Mistry, Polina Brodsky, Andrea J. Brickey, Adam Duran, Eve Mozur, Rajavasanth Rajasegar, Gregory Jackson, Steven C. Decaluwe, Robert J. Kee, Ryan O’Hayre, Alexandra Newman, Robert Braun, Morgan Bazilian, Neal P. Sullivan","doi":"10.1021/acsenergylett.5c02459","DOIUrl":null,"url":null,"abstract":"Transportation emits a considerable amount of greenhouse gases (GHG), for example, in 2022, it was responsible for about 33% of the United States GHG emissions. (1,2) While the transportation decarbonization efforts through electric vehicles have sped up in recent years, (3) we still face barriers to their adoption. Additionally, as Figure 1 shows, they only represent half of the solution. The remaining half is due to other transportation modes like buses, trucks, planes, trains, and ships. Each of these transportation modes has unique requirements and accordingly, in many cases, a well-engineered solution is not ready for deployment. For example, the fuel cell technology is mature enough for light duty vehicles but requires further research and development for heavy duty trucks, (4) and efforts like the Million Mile Fuel Cell Truck consortium (5) are required to bridge this gap. Alternatively, more stringent requirements of aviation, rail, and marine applications demand that we judiciously combine new technology development (6,7) (e.g., energy-dense beyond lithium-ion batteries) with deployment of specialized solutions (8−10) like nuclear-powered ships. Figure 1. Breakdown of 2022 US transportation-related GHG emissions by different transportation modes, (1) along with the progress toward decarbonization at scale. The off-road vehicles collective represent vehicles operating away from roads such as construction machines, mining vehicles, agricultural equipment, etc. In comparison, off-road vehicles, i.e., construction, mining, agricultural and other machines operating away from roads, emitted the same GHG amount in 2022 as aviation, rail, and marine transportation combined, (1) but there is minimal discussion of decarbonizing this transportation mode in the scientific literature (refer to Figure S5 and related text for a more detailed breakdown of the off-road vehicle emissions). As the global population rises and the living standards improve, off-road vehicle use will increase for constructing more buildings, mining more minerals, (11) growing more food, operating warehouses and industries, and supporting aviation, rail and marine operations. Recent geopolitical tensions have further underscored the importance of bolstering these sectors critical to our material supply chains. (12) Despite such profound importance of off-road vehicles, most discussions omit their <i>decarbonization potential</i>, (13,14) let alone acknowledge their environmental impact. While few existing discussions pledge to decarbonize this sector, (15−23) corresponding projections report an uncertain future since they assume that the present-day technologies, e.g., costly lithium-ion batteries, will be used to decarbonize the off-road vehicles (2,16,17,23−25) (the question mark next to the off-road vehicles in Figure 1 symbolizes such uncertainty as opposed to focused efforts driving decarbonization of other modes). We instead <i>rephrase this quest of decarbonizing off-road vehicles in terms of developing new technologies</i> that meet the operational requirements as well as are sustainable and deployable at scale. For the passenger electric vehicle (EV) adoption, the first significant milestone was achieving the same driving range as their engine counterparts, i.e., the range anxiety challenge. (26) In the same spirit, any potential off-road technology has to perform comparably to the existing diesel vehicles. We herein consider three potential solutions ((i) hydrogen-powered internal combustion engines, (ii) hydrogen fuel cells, and (iii) batteries) exhibiting negligible tailpipe emissions. Starting with the off-road vehicles database (2) published recently, we analyze H<sub>2</sub> engine, H<sub>2</sub> fuel cell, and battery performance requirements to decarbonize off-road vehicles from two leading manufacturers─Caterpillar in Figure 2 and Liebherr in Figure S1. The equivalent decarbonized vehicle <i>offers the same power and operation time as the diesel vehicle it is meant to replace without</i> exceeding the combined mass of the diesel engine and the fuel tank. The mathematical steps corresponding to such an analysis are outlined in section S1. Note that we only consider near-room-temperature polymer electrolyte membrane fuel cells, as they have been considerably examined for transportation applications, and equivalently, information relevant for the present analysis is available. (27) In comparison, the solid oxide fuel cells (SOFCs) have long been considered unsuitable for transportation application given their high temperature requirements and materials challenges to rapidly heat up the fuel cell stacks. (28,29) Some ongoing efforts are targeted at enabling SOFCs for transportation, (30) and as sufficient system-level demonstration data becomes available, equivalent SOFC performance targets for the off-road vehicles can be predicted. Figure 2. Performance metrics for (a), (b) H<sub>2</sub> engine (c), (d) H<sub>2</sub> fuel cell and (e), (f) battery-based powertrains to decarbonize present-day Caterpillar diesel-powered vehicles. Different symbols indicate different off-road vehicle types as schematically shown in the top right corner of the figure. The faded symbols in (a) identify the corresponding diesel engine characteristics. The dotted lines in (a), (d) are constant specific power lines, and in (b), (c) are constant fuel consumption lines. Figure 2 (a) and (b) respectively express the hydrogen engine and storage (i.e., H<sub>2</sub> fuel tank) targets. For the same engine mass, any engine delivering more power than the corresponding data point in (a) and higher operation time than (b) for the same amount of stored hydrogen are promising solutions. While recent H<sub>2</sub> engine demonstrations appear to meet the power demands, we must acknowledge the mass constraint for these engines. For comparison, equivalent diesel engine data points are also shown in Figure 2(a) as faded symbols. Despite a higher gravimetric heating value of hydrogen (∼120 [MJ/kg]) compared to diesel (∼44 [MJ/kg]), hydrogen storage is ∼2.93 times heavier given its cryogenic temperatures and compressed state (31,32) (refer to Table S1 for additional details). Hence, the H<sub>2</sub> engine has to be lighter compared to its diesel counterpart to be a viable alternative. In comparison, the hydrogen storage for the fuel cells in Figure 2(c) is somewhat (∼1.16 times) lighter than the H<sub>2</sub> engine given the slightly higher efficiency of the fuel cell drivetrain. (33) The projected fuel cell system power density (27) of 900 [W/kg] is comparable to more power-dense H<sub>2</sub> engines in Figure 2(a). Consequently, the combined mass of the fuel cell system (≡ stack, its thermal management, and balance of plant) and H<sub>2</sub> storage is smaller than an equivalent H<sub>2</sub> engine and storage combination. Since fuel cell powertrains are typically fuel cell–battery hybrids, these mass savings permit an additional battery weight. Figure 2(d) shows the corresponding battery power and mass targets (following a recent study, (22) the required battery power is rated based on the fuel cell idle power). Figure 2(d) also shows constant specific power lines to assess the readiness of the battery technology for supporting fuel cell operation. The present-day EV batteries can deliver 200–500 [W/kg] for short durations (34) and can accordingly support most of these fuel cell powertrains. Figure 2 (b) and (c) prescribe H<sub>2</sub> storage metrics, respectively for the H<sub>2</sub> engine- and H<sub>2</sub> fuel cell-powered off-road vehicles. For comparison, the H<sub>2</sub> storage systems being designed for Class 8 heavy duty trucks (32) driving 750 [mi] store 65 [kg] of hydrogen with a consumption rate of ∼16.6 [kg<sub>hydrogen</sub>/h]. This fuel system seems sufficient for about half of the vehicles identified in Figure 2(b) and (c). The other half requires larger tanks that can potentially deliver hydrogen fast enough to match higher fuel consumption rates. Unlike the intermittently operating batteries in the fuel cell vehicles, batteries are the primary energy source for the battery-powered vehicles and operate continuously. Accordingly, while the specific power in Figure 2(e) peaks at ∼300 [W/kg], and seems comparable to the present-day EV batteries mentioned earlier, (34) this power has to be delivered for days instead of seconds and represents a very different operation. Hence, the batteries for off-road vehicles demand much higher energy densities (Figure 2(f)) than the present-day lithium-ion batteries (34) or the near-future lithium metal variants. (35) The requisite battery energy densities are even more demanding than the recently launched Propel 1k program (6,7) building 1000 [Wh/kg] battery packs. Similar conclusions can be drawn from Figure S1 analyzing the Liebherr vehicles. Unfortunately, it is not sufficient to develop technologies that only match the performance of existing off-road diesel vehicles. Table 1 identifies myriad operational, sustainability, and deployability requirements a decarbonizing technology needs to meet to be a meaningful solution at scale. Performance is just one aspect of the <i>operational requirements</i>, but we also have to consider durability, safety, refueling/recharging, and all-temperature operation. On the other hand, the <i>sustainability requirements</i> ensure that the proposed solution is sustainable in a cradle-to-grave sense, and not just free of tailpipe emissions. We acknowledge that <i>literature is fairly divided on how sustainability is assessed for different technologies</i>. To make matters worse, the sustainability of the same technology does change over time as related technologies are made more sustainable. For example, a few years ago, passenger EVs had net positive emissions despite the absence of tailpipe emissions (due to the emissions from electricity generation and mining of raw materials). (36) To make meaningful progress toward the <i>net-zero emissions by 2050 goal, we believe sustainability should be assessed based on future potential, instead of current feasibility</i>. Beyond the aforementioned operational and sustainability requirements, deployability is yet another important requirement, i.e., the solution has to be scalable to future demands and acceptable to the consumers (unlike passenger EVs, the consumers are not individuals for the off-road vehicles). The 2050 target demands that we quickly develop decarbonization solutions, however, as the combination of operational, sustainability, and deployability requirements reveals, <i>a technology performing better in terms of one requirement is not necessarily the de facto solution</i>. For example, biofuels are a drop-in replacement, and the latest projections show that we have enough feedstock to produce fuels for the off-road vehicles in 2050. (37) However, the study (37) also recognizes that the associated cost, the need for research, development, commercialization, and policy support for biorefineries producing these fuels as well as the long-term social and environmental consequences of producing vast quantities of biomass are limiting factors. (25) H<sub>2</sub> engine-based powertrain (38,39) is another example where the prototype engines appear to meet the power demands, but they require further research to improve energy density (as identified in Figure 2(a) and Figure S1(a)) as well as the durability of engine materials. While depending on the hydrogen source, they do not generate CO or CO<sub>2</sub>,other pollutants like SO<sub><i>x</i></sub> are generated. And H<sub>2</sub> combustion with air invariably generates NO<sub><i>x</i></sub>, requiring new catalytic converter technologies. (40) Given the dusty operating environment of off-road vehicles, an unexpected challenge for the fuel cells is the oxygen purity levels required to avoid performance loss and faster degradation (41) (the combustion engines in comparison have a higher tolerance for contamination at the air intake). Hopefully, specialized air filters can be designed to solve this problem since installing onboard pure oxygen storage seems infeasible without sacrificing performance, safety and economics. While some argue that thermal management of off-road vehicles possesses unexpected challenges (25) since it cannot be air-cooled similar to faster moving on-road vehicles, the thermal management is likely manageable. For example, H<sub>2</sub> engines can borrow the engine cooling designs from existing off-road vehicles; H<sub>2</sub> tanks can be cooled in a similar fashion as other fuel cell vehicles; and while the required batteries are comparatively bigger, they operate at much slower rates and are not expected to generate as much heat (2) as passenger EV batteries. Another corollary of the short 25-year time frame is that we need simultaneous progress on multiple fronts. For example, H<sub>2</sub> powertrains also require scalable carbon-free production, transportation, storage and refueling of hydrogen. Fortunately, dedicated efforts are underway to solve some of these challenges, (42) and the geological hydrogen may dramatically influence this quest. (43) In comparison, out of various potential energy-dense beyond lithium-ion chemistries, (2) battery research should focus on the subset <i>using cheaper materials with a more diversified supply chain</i>. The modular nature of batteries can offer unexpected advantages (compared to other powertrains) in terms of vehicle design, safety, and recharging. Depending on the vehicle operation, battery swapping (2,44) can offer additional flexibility. Note that the present analysis outlines the performance metrics of various powertrains for identical operations as the existing off-road vehicles. However, electrified powertrains, i.e., fuel cells and batteries, offer unique advantages such as rapid start and stop. In comparison, diesel engines (and likely H<sub>2</sub> engines) idle for an appreciable duration of their operation for some vehicles, e.g., dump trucks. In such instances, the performance metrics for the electrified powertrains can be relaxed. Depending on the operational profile of the vehicle, one may be able to further relax this constraint by leveraging regenerative breaking and other equivalent technologies. Alternatively, as various off-road vehicle manufacturers have demonstrated over the past couple of years, reduced range variants of the conventional diesel off-road vehicles can be deployed using the existing Li-ion battery technology. (2,45) Beyond these technologies, creative solutions directly using electricity should be explored for some off-road vehicles. For example, catenary charging or electrified rails can be used to power underground mining vehicles that operate along well-defined paths (interestingly, the catenary is the most deployed solution for electric rail across the globe (9)). Similarly, tethered power can be used for machines that do not move around much during operation, e.g., excavators and cranes. Note that such solutions have already been implemented in limited capacity, e.g., electric rope shovels, (46) electric cranes, (47) and tunnel boring machines. (48) Based on this discussion, none of the existing powertrain technologies can decarbonize the off-road vehicles overnight. Given the stringent performance requirements of these vehicles, powertrains being developed for other transportation modes also cannot be used to decarbonize the off-road vehicles. We accordingly <i>require dedicated materials development, scientific breakthroughs, and engineering advances to design meaningful solutions for powering off-road vehicles</i>. For example, we need materials development for lighter and more durable hydrogen engine blocks, more energy-dense batteries than are being developed elsewhere, and engineering high-purity hydrogen and oxygen feeds for the fuel cells. Beyond satisfying such performance targets, any potential decarbonized technology should be developed with operational, sustainability, and deployability requirements identified in Table 1, such that it represents a meaningful solution at scale. Such stringent requirements are a <i>great opportunity for transformative research</i> that will also help many other applications. And while we invest efforts in enabling a fully decarbonized off-road vehicles sector, some of them─especially, vehicles traversing well-defined paths─should be electrified by directly powering them from the grid. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.5c02459. A checklist (49) summarizing key theoretical details of the study; nomenclature; S1. Analysis Identifying Performance Metrics of Various Decarbonization Solutions; S2. Discussion of Key Parameters and Target Metrics borrowed from Literature; S3. Potential Uncertainties Associated with the Present Analysis; S4. Emission Contributions from Various Off-road Vehicles. (PDF) Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html. The authors represent a self-initiated working group focused on assessing the potential of different technologies for decarbonizing off-road vehicles as a first step to direct research efforts for developing meaningful solutions. A.N.M. appreciates discussions with Brian Ingram (Argonne National Laboratory), James Pikul (University of Wisconsin), Lyle Pickett (Sandia National Laboratories), Paul Gasper (National Renewable Energy Laboratory), and Stephen J. Harris (Lawrence Berkeley National Laboratory). This article references 49 other publications. 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引用次数: 0
Abstract
Transportation emits a considerable amount of greenhouse gases (GHG), for example, in 2022, it was responsible for about 33% of the United States GHG emissions. (1,2) While the transportation decarbonization efforts through electric vehicles have sped up in recent years, (3) we still face barriers to their adoption. Additionally, as Figure 1 shows, they only represent half of the solution. The remaining half is due to other transportation modes like buses, trucks, planes, trains, and ships. Each of these transportation modes has unique requirements and accordingly, in many cases, a well-engineered solution is not ready for deployment. For example, the fuel cell technology is mature enough for light duty vehicles but requires further research and development for heavy duty trucks, (4) and efforts like the Million Mile Fuel Cell Truck consortium (5) are required to bridge this gap. Alternatively, more stringent requirements of aviation, rail, and marine applications demand that we judiciously combine new technology development (6,7) (e.g., energy-dense beyond lithium-ion batteries) with deployment of specialized solutions (8−10) like nuclear-powered ships. Figure 1. Breakdown of 2022 US transportation-related GHG emissions by different transportation modes, (1) along with the progress toward decarbonization at scale. The off-road vehicles collective represent vehicles operating away from roads such as construction machines, mining vehicles, agricultural equipment, etc. In comparison, off-road vehicles, i.e., construction, mining, agricultural and other machines operating away from roads, emitted the same GHG amount in 2022 as aviation, rail, and marine transportation combined, (1) but there is minimal discussion of decarbonizing this transportation mode in the scientific literature (refer to Figure S5 and related text for a more detailed breakdown of the off-road vehicle emissions). As the global population rises and the living standards improve, off-road vehicle use will increase for constructing more buildings, mining more minerals, (11) growing more food, operating warehouses and industries, and supporting aviation, rail and marine operations. Recent geopolitical tensions have further underscored the importance of bolstering these sectors critical to our material supply chains. (12) Despite such profound importance of off-road vehicles, most discussions omit their decarbonization potential, (13,14) let alone acknowledge their environmental impact. While few existing discussions pledge to decarbonize this sector, (15−23) corresponding projections report an uncertain future since they assume that the present-day technologies, e.g., costly lithium-ion batteries, will be used to decarbonize the off-road vehicles (2,16,17,23−25) (the question mark next to the off-road vehicles in Figure 1 symbolizes such uncertainty as opposed to focused efforts driving decarbonization of other modes). We instead rephrase this quest of decarbonizing off-road vehicles in terms of developing new technologies that meet the operational requirements as well as are sustainable and deployable at scale. For the passenger electric vehicle (EV) adoption, the first significant milestone was achieving the same driving range as their engine counterparts, i.e., the range anxiety challenge. (26) In the same spirit, any potential off-road technology has to perform comparably to the existing diesel vehicles. We herein consider three potential solutions ((i) hydrogen-powered internal combustion engines, (ii) hydrogen fuel cells, and (iii) batteries) exhibiting negligible tailpipe emissions. Starting with the off-road vehicles database (2) published recently, we analyze H2 engine, H2 fuel cell, and battery performance requirements to decarbonize off-road vehicles from two leading manufacturers─Caterpillar in Figure 2 and Liebherr in Figure S1. The equivalent decarbonized vehicle offers the same power and operation time as the diesel vehicle it is meant to replace without exceeding the combined mass of the diesel engine and the fuel tank. The mathematical steps corresponding to such an analysis are outlined in section S1. Note that we only consider near-room-temperature polymer electrolyte membrane fuel cells, as they have been considerably examined for transportation applications, and equivalently, information relevant for the present analysis is available. (27) In comparison, the solid oxide fuel cells (SOFCs) have long been considered unsuitable for transportation application given their high temperature requirements and materials challenges to rapidly heat up the fuel cell stacks. (28,29) Some ongoing efforts are targeted at enabling SOFCs for transportation, (30) and as sufficient system-level demonstration data becomes available, equivalent SOFC performance targets for the off-road vehicles can be predicted. Figure 2. Performance metrics for (a), (b) H2 engine (c), (d) H2 fuel cell and (e), (f) battery-based powertrains to decarbonize present-day Caterpillar diesel-powered vehicles. Different symbols indicate different off-road vehicle types as schematically shown in the top right corner of the figure. The faded symbols in (a) identify the corresponding diesel engine characteristics. The dotted lines in (a), (d) are constant specific power lines, and in (b), (c) are constant fuel consumption lines. Figure 2 (a) and (b) respectively express the hydrogen engine and storage (i.e., H2 fuel tank) targets. For the same engine mass, any engine delivering more power than the corresponding data point in (a) and higher operation time than (b) for the same amount of stored hydrogen are promising solutions. While recent H2 engine demonstrations appear to meet the power demands, we must acknowledge the mass constraint for these engines. For comparison, equivalent diesel engine data points are also shown in Figure 2(a) as faded symbols. Despite a higher gravimetric heating value of hydrogen (∼120 [MJ/kg]) compared to diesel (∼44 [MJ/kg]), hydrogen storage is ∼2.93 times heavier given its cryogenic temperatures and compressed state (31,32) (refer to Table S1 for additional details). Hence, the H2 engine has to be lighter compared to its diesel counterpart to be a viable alternative. In comparison, the hydrogen storage for the fuel cells in Figure 2(c) is somewhat (∼1.16 times) lighter than the H2 engine given the slightly higher efficiency of the fuel cell drivetrain. (33) The projected fuel cell system power density (27) of 900 [W/kg] is comparable to more power-dense H2 engines in Figure 2(a). Consequently, the combined mass of the fuel cell system (≡ stack, its thermal management, and balance of plant) and H2 storage is smaller than an equivalent H2 engine and storage combination. Since fuel cell powertrains are typically fuel cell–battery hybrids, these mass savings permit an additional battery weight. Figure 2(d) shows the corresponding battery power and mass targets (following a recent study, (22) the required battery power is rated based on the fuel cell idle power). Figure 2(d) also shows constant specific power lines to assess the readiness of the battery technology for supporting fuel cell operation. The present-day EV batteries can deliver 200–500 [W/kg] for short durations (34) and can accordingly support most of these fuel cell powertrains. Figure 2 (b) and (c) prescribe H2 storage metrics, respectively for the H2 engine- and H2 fuel cell-powered off-road vehicles. For comparison, the H2 storage systems being designed for Class 8 heavy duty trucks (32) driving 750 [mi] store 65 [kg] of hydrogen with a consumption rate of ∼16.6 [kghydrogen/h]. This fuel system seems sufficient for about half of the vehicles identified in Figure 2(b) and (c). The other half requires larger tanks that can potentially deliver hydrogen fast enough to match higher fuel consumption rates. Unlike the intermittently operating batteries in the fuel cell vehicles, batteries are the primary energy source for the battery-powered vehicles and operate continuously. Accordingly, while the specific power in Figure 2(e) peaks at ∼300 [W/kg], and seems comparable to the present-day EV batteries mentioned earlier, (34) this power has to be delivered for days instead of seconds and represents a very different operation. Hence, the batteries for off-road vehicles demand much higher energy densities (Figure 2(f)) than the present-day lithium-ion batteries (34) or the near-future lithium metal variants. (35) The requisite battery energy densities are even more demanding than the recently launched Propel 1k program (6,7) building 1000 [Wh/kg] battery packs. Similar conclusions can be drawn from Figure S1 analyzing the Liebherr vehicles. Unfortunately, it is not sufficient to develop technologies that only match the performance of existing off-road diesel vehicles. Table 1 identifies myriad operational, sustainability, and deployability requirements a decarbonizing technology needs to meet to be a meaningful solution at scale. Performance is just one aspect of the operational requirements, but we also have to consider durability, safety, refueling/recharging, and all-temperature operation. On the other hand, the sustainability requirements ensure that the proposed solution is sustainable in a cradle-to-grave sense, and not just free of tailpipe emissions. We acknowledge that literature is fairly divided on how sustainability is assessed for different technologies. To make matters worse, the sustainability of the same technology does change over time as related technologies are made more sustainable. For example, a few years ago, passenger EVs had net positive emissions despite the absence of tailpipe emissions (due to the emissions from electricity generation and mining of raw materials). (36) To make meaningful progress toward the net-zero emissions by 2050 goal, we believe sustainability should be assessed based on future potential, instead of current feasibility. Beyond the aforementioned operational and sustainability requirements, deployability is yet another important requirement, i.e., the solution has to be scalable to future demands and acceptable to the consumers (unlike passenger EVs, the consumers are not individuals for the off-road vehicles). The 2050 target demands that we quickly develop decarbonization solutions, however, as the combination of operational, sustainability, and deployability requirements reveals, a technology performing better in terms of one requirement is not necessarily the de facto solution. For example, biofuels are a drop-in replacement, and the latest projections show that we have enough feedstock to produce fuels for the off-road vehicles in 2050. (37) However, the study (37) also recognizes that the associated cost, the need for research, development, commercialization, and policy support for biorefineries producing these fuels as well as the long-term social and environmental consequences of producing vast quantities of biomass are limiting factors. (25) H2 engine-based powertrain (38,39) is another example where the prototype engines appear to meet the power demands, but they require further research to improve energy density (as identified in Figure 2(a) and Figure S1(a)) as well as the durability of engine materials. While depending on the hydrogen source, they do not generate CO or CO2,other pollutants like SOx are generated. And H2 combustion with air invariably generates NOx, requiring new catalytic converter technologies. (40) Given the dusty operating environment of off-road vehicles, an unexpected challenge for the fuel cells is the oxygen purity levels required to avoid performance loss and faster degradation (41) (the combustion engines in comparison have a higher tolerance for contamination at the air intake). Hopefully, specialized air filters can be designed to solve this problem since installing onboard pure oxygen storage seems infeasible without sacrificing performance, safety and economics. While some argue that thermal management of off-road vehicles possesses unexpected challenges (25) since it cannot be air-cooled similar to faster moving on-road vehicles, the thermal management is likely manageable. For example, H2 engines can borrow the engine cooling designs from existing off-road vehicles; H2 tanks can be cooled in a similar fashion as other fuel cell vehicles; and while the required batteries are comparatively bigger, they operate at much slower rates and are not expected to generate as much heat (2) as passenger EV batteries. Another corollary of the short 25-year time frame is that we need simultaneous progress on multiple fronts. For example, H2 powertrains also require scalable carbon-free production, transportation, storage and refueling of hydrogen. Fortunately, dedicated efforts are underway to solve some of these challenges, (42) and the geological hydrogen may dramatically influence this quest. (43) In comparison, out of various potential energy-dense beyond lithium-ion chemistries, (2) battery research should focus on the subset using cheaper materials with a more diversified supply chain. The modular nature of batteries can offer unexpected advantages (compared to other powertrains) in terms of vehicle design, safety, and recharging. Depending on the vehicle operation, battery swapping (2,44) can offer additional flexibility. Note that the present analysis outlines the performance metrics of various powertrains for identical operations as the existing off-road vehicles. However, electrified powertrains, i.e., fuel cells and batteries, offer unique advantages such as rapid start and stop. In comparison, diesel engines (and likely H2 engines) idle for an appreciable duration of their operation for some vehicles, e.g., dump trucks. In such instances, the performance metrics for the electrified powertrains can be relaxed. Depending on the operational profile of the vehicle, one may be able to further relax this constraint by leveraging regenerative breaking and other equivalent technologies. Alternatively, as various off-road vehicle manufacturers have demonstrated over the past couple of years, reduced range variants of the conventional diesel off-road vehicles can be deployed using the existing Li-ion battery technology. (2,45) Beyond these technologies, creative solutions directly using electricity should be explored for some off-road vehicles. For example, catenary charging or electrified rails can be used to power underground mining vehicles that operate along well-defined paths (interestingly, the catenary is the most deployed solution for electric rail across the globe (9)). Similarly, tethered power can be used for machines that do not move around much during operation, e.g., excavators and cranes. Note that such solutions have already been implemented in limited capacity, e.g., electric rope shovels, (46) electric cranes, (47) and tunnel boring machines. (48) Based on this discussion, none of the existing powertrain technologies can decarbonize the off-road vehicles overnight. Given the stringent performance requirements of these vehicles, powertrains being developed for other transportation modes also cannot be used to decarbonize the off-road vehicles. We accordingly require dedicated materials development, scientific breakthroughs, and engineering advances to design meaningful solutions for powering off-road vehicles. For example, we need materials development for lighter and more durable hydrogen engine blocks, more energy-dense batteries than are being developed elsewhere, and engineering high-purity hydrogen and oxygen feeds for the fuel cells. Beyond satisfying such performance targets, any potential decarbonized technology should be developed with operational, sustainability, and deployability requirements identified in Table 1, such that it represents a meaningful solution at scale. Such stringent requirements are a great opportunity for transformative research that will also help many other applications. And while we invest efforts in enabling a fully decarbonized off-road vehicles sector, some of them─especially, vehicles traversing well-defined paths─should be electrified by directly powering them from the grid. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.5c02459. A checklist (49) summarizing key theoretical details of the study; nomenclature; S1. Analysis Identifying Performance Metrics of Various Decarbonization Solutions; S2. Discussion of Key Parameters and Target Metrics borrowed from Literature; S3. Potential Uncertainties Associated with the Present Analysis; S4. Emission Contributions from Various Off-road Vehicles. (PDF) Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html. The authors represent a self-initiated working group focused on assessing the potential of different technologies for decarbonizing off-road vehicles as a first step to direct research efforts for developing meaningful solutions. A.N.M. appreciates discussions with Brian Ingram (Argonne National Laboratory), James Pikul (University of Wisconsin), Lyle Pickett (Sandia National Laboratories), Paul Gasper (National Renewable Energy Laboratory), and Stephen J. Harris (Lawrence Berkeley National Laboratory). This article references 49 other publications. This article has not yet been cited by other publications.
ACS Energy Letters Energy-Renewable Energy, Sustainability and the Environment
CiteScore
31.20
自引率
5.00%
发文量
469
审稿时长
1 months
期刊介绍:
ACS Energy Letters is a monthly journal that publishes papers reporting new scientific advances in energy research. The journal focuses on topics that are of interest to scientists working in the fundamental and applied sciences. Rapid publication is a central criterion for acceptance, and the journal is known for its quick publication times, with an average of 4-6 weeks from submission to web publication in As Soon As Publishable format.
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