Not-So-Quick and Not-So-Dirty Solutions to Decarbonize Off- Road Vehicles

IF 18.2 1区 材料科学 Q1 CHEMISTRY, PHYSICAL
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.

Abstract Image

越野车脱碳的不那么快速和不那么肮脏的解决方案
交通运输排放了大量的温室气体(GHG),例如,在2022年,它占美国温室气体排放量的33%。(1,2)尽管近年来通过电动汽车实现交通脱碳的努力加快了,(3)我们仍然面临着采用电动汽车的障碍。此外,如图1所示,它们只代表了解决方案的一半。剩下的一半是由于其他交通方式,如公共汽车、卡车、飞机、火车和轮船。每种运输模式都有其独特的需求,因此,在许多情况下,设计良好的解决方案并不适合部署。例如,燃料电池技术在轻型汽车上已经足够成熟,但在重型卡车上还需要进一步的研究和发展,需要像百万英里燃料电池卡车联盟(5)这样的努力来弥补这一差距。另外,航空、铁路和海洋应用的更严格要求要求我们明智地将新技术开发(6,7)(例如,超越锂离子电池的能量密度)与核动力船舶等专业解决方案(8 - 10)的部署结合起来。图1所示。2022年美国不同运输方式的交通相关温室气体排放分解,(1)以及大规模脱碳的进展。越野车是指工程机械、矿用车辆、农用设备等远离道路作业的车辆。相比之下,越野车,即建筑、采矿、农业和其他远离道路的机器,在2022年排放的温室气体量与航空、铁路和海洋运输的总和相同(1),但科学文献中对这种运输方式脱碳的讨论很少(参见图S5和相关文本,了解越野车排放的更详细详细情况)。随着世界人口的增加和生活水平的提高,用于建造更多的建筑物、开采更多的矿物、种植更多的粮食、经营仓库和工业,以及支持航空、铁路和海洋作业的越野车将会增加。最近的地缘政治紧张局势进一步凸显了支持这些对我们的材料供应链至关重要的行业的重要性。尽管越野车辆具有如此深远的重要性,但大多数讨论都忽略了它们的脱碳潜力,更不用说承认它们对环境的影响了。虽然很少有现有的讨论承诺使这一领域脱碳,但(15−23)相应的预测报告了一个不确定的未来,因为他们假设当前的技术,例如昂贵的锂离子电池,将被用于使越野车脱碳(2,16,17,23−25)(图1中越野车旁边的问号象征着这种不确定性,而不是集中努力推动其他模式的脱碳)。相反,我们在开发新技术方面重新定义了对越野车脱碳的追求,这些新技术既满足运营要求,又具有可持续性和可大规模部署性。对于乘用车(EV)的采用来说,第一个重要的里程碑是实现与发动机汽车相同的行驶里程,即里程焦虑挑战。(26)本着同样的精神,任何潜在的越野技术都必须与现有的柴油车辆性能相当。我们在此考虑了三种潜在的解决方案((i)氢动力内燃机,(ii)氢燃料电池和(iii)电池),它们的尾气排放可以忽略不计。从最近发布的越野车数据库(2)开始,我们分析了来自两家领先制造商(图2中的卡特彼勒和图S1中的利勃海尔)的H2发动机、H2燃料电池和电池性能要求,以使越野车脱碳。同等的脱碳汽车在不超过柴油发动机和燃料箱总质量的情况下,提供与柴油汽车相同的动力和运行时间。第S1节概述了与这种分析相对应的数学步骤。请注意,我们只考虑接近室温的聚合物电解质膜燃料电池,因为它们已经在运输应用中进行了大量的研究,同样,与本分析相关的信息是可用的。(27)相比之下,固体氧化物燃料电池(sofc)长期以来被认为不适合运输应用,因为它们的高温要求和材料挑战,以快速加热燃料电池组。(28,29)一些正在进行的工作旨在使SOFC用于运输,(30)并且随着足够的系统级演示数据的出现,可以预测越野车辆的等效SOFC性能目标。图2。 (a)、(b) H2发动机(c)、(d) H2燃料电池和(e)、(f)电池动力系统的性能指标,以使卡特彼勒目前的柴油动力汽车脱碳。不同的符号表示不同的越野车辆类型,如图右上角所示。(a)中褪色的符号标识了相应的柴油机特性。(a)、(d)中的虚线为恒定比电力线,(b)、(c)中的虚线为恒定油耗线。图2 (a)和(b)分别表示了氢能发动机和储氢(即H2燃料箱)目标。对于相同的发动机质量,任何发动机提供的功率大于(a)中对应的数据点,并且在相同数量的氢气储存下,比(b)的运行时间更长,都是有希望的解决方案。虽然最近H2发动机的演示似乎满足了动力需求,但我们必须承认这些发动机的质量限制。为便于比较,等效柴油机数据点也在图2(a)中以褪色符号表示。尽管氢的重量热值(~ 120 [MJ/kg])比柴油(~ 44 [MJ/kg])更高,但考虑到其低温和压缩状态,氢的储存重量是柴油的~ 2.93倍(31,32)(参见表S1了解更多细节)。因此,与柴油发动机相比,H2发动机必须更轻,才能成为可行的替代方案。相比之下,图2(c)中燃料电池的储氢量比H2发动机轻(约1.16倍),因为燃料电池传动系统的效率略高。(33)燃料电池系统的预计功率密度(27)为900 [W/kg],与图2(a)中功率密度更高的H2发动机相当。因此,燃料电池系统的总质量(≡燃料堆,其热管理和工厂平衡)和H2存储比同等的H2发动机和存储组合要小。由于燃料电池动力系统通常是燃料电池-电池混合动力系统,因此这些大量的节省允许额外的电池重量。图2(d)显示了相应的电池功率和质量目标(根据最近的一项研究,(22)所需的电池功率是根据燃料电池的空闲功率额定值)。图2(d)还显示了恒定的特定电力线,以评估支持燃料电池运行的电池技术的准备情况。目前的电动汽车电池可以在短时间内提供200-500 [W/kg],因此可以支持大多数这些燃料电池动力系统。图2 (b)和(c)分别规定了H2发动机和H2燃料电池驱动的越野车的H2存储指标。相比之下,为行驶750[英里]的8级重型卡车(32辆)设计的氢气存储系统可存储65 [kg]氢气,消耗率为~ 16.6[氢气/小时]。这种燃料系统似乎足以满足图2(b)和(c)中所确定的大约一半的车辆。另一半则需要更大的储罐,能够以足够快的速度输送氢气,以匹配更高的燃料消耗率。与燃料电池汽车中间歇性运行的电池不同,电池是电池驱动汽车的主要能源,并且是连续运行的。因此,虽然图2(e)中的特定功率在~ 300 [W/kg]达到峰值,并且似乎与前面提到的现代电动汽车电池相当,(34)该功率必须交付数天而不是数秒,并且代表了非常不同的操作。因此,与目前的锂离子电池(34)或不久的将来的锂金属电池相比,用于越野车辆的电池需要更高的能量密度(图2(f))。(35)所需的电池能量密度甚至比最近推出的Propel 1k计划(6,7)建造1000 [Wh/kg]电池组的要求更高。从图S1分析利勃海尔车辆可以得出类似的结论。遗憾的是,仅仅开发与现有越野柴油车性能相匹配的技术是不够的。表1确定了脱碳技术需要满足的无数操作、可持续性和可部署性要求,才能成为有意义的大规模解决方案。性能只是操作要求的一个方面,但我们还必须考虑耐久性、安全性、加油/充电和全温操作。另一方面,可持续性要求确保所提出的解决方案在从摇篮到坟墓的意义上是可持续的,而不仅仅是没有尾气排放。我们承认,文献在如何评估不同技术的可持续性方面存在相当大的分歧。更糟糕的是,随着相关技术变得更加可持续,同一技术的可持续性确实会随着时间的推移而改变。例如,几年前,尽管没有尾气排放(由于发电和原材料开采产生的排放),但乘用电动汽车的净排放量为正。 (36)为了在实现2050年净零排放目标方面取得有意义的进展,我们认为应根据未来潜力而不是当前可行性来评估可持续性。除了前面提到的操作和可持续性要求之外,可部署性是另一个重要的要求,即解决方案必须可扩展到未来的需求,并为消费者所接受(与乘用电动汽车不同,消费者不是越野车的个人)。2050年的目标要求我们快速开发脱碳解决方案,然而,正如操作性、可持续性和可部署性需求的结合所揭示的那样,在一个需求方面表现更好的技术不一定是事实上的解决方案。例如,生物燃料是一种临时替代品,最新的预测显示,到2050年,我们有足够的原料为越野车生产燃料。然而,该研究也认识到,生产这些燃料的生物精炼厂的相关成本、研究、开发、商业化和政策支持的需要以及生产大量生物质的长期社会和环境后果都是限制因素。(25)基于H2发动机的动力系统(38,39)是另一个原型发动机似乎满足动力需求的例子,但它们需要进一步研究以提高能量密度(如图2(a)和图S1(a)所示)以及发动机材料的耐用性。虽然根据氢源,它们不产生CO或CO2,但会产生SOx等其他污染物。氢气与空气燃烧总是会产生NOx,这就需要新的催化转化器技术。考虑到越野车辆多尘的运行环境,燃料电池面临的一个意想不到的挑战是避免性能损失和更快退化所需的氧气纯度水平(相比之下,内燃机对进气口的污染有更高的容忍度)。希望能够设计出专门的空气过滤器来解决这个问题,因为在不牺牲性能、安全性和经济性的情况下,在车上安装纯氧储存装置似乎是不可实现的。虽然有些人认为,越野车的热管理面临着意想不到的挑战(25),因为它不能像高速行驶的公路车辆那样进行风冷,但热管理可能是可控的。例如,H2发动机可以借鉴现有越野车的发动机冷却设计;氢气罐的冷却方式与其他燃料电池汽车类似;虽然所需的电池相对更大,但它们的运行速度要慢得多,而且预计不会像乘用电动汽车电池那样产生那么多热量。25年时间框架的另一个必然结果是,我们需要在多个方面同时取得进展。例如,氢动力系统还需要大规模的无碳生产、运输、储存和加氢。幸运的是,人们正在努力解决其中的一些挑战,而地质上的氢气可能会极大地影响这一探索。(43)相比之下,在锂离子以外的各种潜在能量密度化学物质中,(2)电池研究应侧重于使用更便宜的材料和更多样化的供应链的子集。与其他动力系统相比,电池的模块化特性可以在车辆设计、安全性和充电方面提供意想不到的优势。根据车辆运行情况,电池交换(2,44)可以提供额外的灵活性。请注意,本分析概述了与现有越野车辆相同操作的各种动力系统的性能指标。然而,电气化动力系统,即燃料电池和电池,提供了独特的优势,如快速启动和停止。相比之下,对于一些车辆,如自卸卡车,柴油发动机(可能还有H2发动机)在运行过程中会闲置相当长的时间。在这种情况下,电动动力系统的性能指标可以放松。根据车辆的运行情况,人们可以通过利用再生制动和其他等效技术进一步放宽这一限制。另外,正如许多越野车制造商在过去几年中所展示的那样,传统柴油越野车的减程版本可以使用现有的锂离子电池技术。(2,45)除了这些技术之外,还应该为一些越野车探索直接用电的创造性解决方案。例如,接触网充电或电气化轨道可用于为沿着明确路径运行的地下采矿车辆提供动力(有趣的是,接触网是全球部署最多的电动轨道解决方案(9))。同样,系绳动力也可用于在操作过程中不经常移动的机器,例如挖掘机和起重机。 请注意,这些解决方案已经在有限的能力下实施,例如电动绳铲、电动起重机和隧道掘进机。(48)基于以上讨论,现有的动力总成技术都不可能在一夜之间使越野车脱碳。考虑到这些车辆严格的性能要求,为其他运输方式开发的动力系统也不能用于脱碳越野车。因此,我们需要专门的材料开发、科学突破和工程进步来设计有意义的解决方案,为越野车辆提供动力。例如,我们需要开发更轻、更耐用的氢发动机缸体材料,比其他地方开发的能量密度更高的电池,以及为燃料电池设计高纯度的氢和氧原料。除了满足这些性能目标之外,任何潜在的脱碳技术都应该根据表1中确定的可操作性、可持续性和可部署性需求进行开发,这样它就代表了大规模的有意义的解决方案。如此严格的要求为变革性研究提供了一个很好的机会,这也将有助于许多其他应用。在我们努力实现越野车行业完全脱碳的同时,其中一些越野车──特别是穿越明确道路的车辆──应该通过直接从电网供电来实现电气化。支持信息可在https://pubs.acs.org/doi/10.1021/acsenergylett.5c02459免费获取。总结研究的关键理论细节的清单(49);命名法;S1。各种脱碳解决方案性能指标的识别分析S2。借鉴文献的关键参数与目标指标探讨S3。与本分析相关的潜在不确定性;S4。各种越野车的排放贡献。(PDF)大多数电子支持信息文件无需订阅ACS Web版本即可获得。这些文件可以通过文章下载用于研究用途(如果相关文章有公共使用许可链接,该许可可以允许其他用途)。如有其他用途,可通过RightsLink权限系统http://pubs.acs.org/page/copyright/permissions.html向ACS申请。作者代表了一个自发的工作组,专注于评估不同技术对越野车辆脱碳的潜力,作为指导研究工作的第一步,以开发有意义的解决方案。A.N.M.感谢与Brian Ingram(阿贡国家实验室)、James Pikul(威斯康星大学)、Lyle Pickett(桑迪亚国家实验室)、Paul Gasper(国家可再生能源实验室)和Stephen J. Harris(劳伦斯伯克利国家实验室)的讨论。本文引用了其他49个出版物。这篇文章尚未被其他出版物引用。
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来源期刊
ACS Energy Letters
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. ACS Energy Letters is ranked as the number one journal in the Web of Science Electrochemistry category. It also ranks within the top 10 journals for Physical Chemistry, Energy & Fuels, and Nanoscience & Nanotechnology. The journal offers several types of articles, including Letters, Energy Express, Perspectives, Reviews, Editorials, Viewpoints and Energy Focus. Additionally, authors have the option to submit videos that summarize or support the information presented in a Perspective or Review article, which can be highlighted on the journal's website. ACS Energy Letters is abstracted and indexed in Chemical Abstracts Service/SciFinder, EBSCO-summon, PubMed, Web of Science, Scopus and Portico.
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