(343a) Decarbonization of Long-Haul Trucking: LOHC Powertrains

Decarbonization of trucks is a necessity to achieve 2050 climate goals. Trucking has been classified as a tough to decarbonize sector by International Energy Agency (IEA). Their report forecasts the sector's emissions to be 2.5 Gigatons of CO₂ per year by 2030, a 25% increase from 2020. Long-haul trucks make up 9% of the global cargo vehicles but account for 39% of the sector's greenhouse gas (GHG) emissions. In the US, long-haul trucks presently carry 71% of the payload making them an integral part of the transportation network. The demand for long-haul trucks is expected to double between 2020 and 2050, making it critical to find renewable powertrains to replace diesel trucks. There are numerous decarbonization routes such as biodiesel, battery electric, and hydrogen-based options. However, there is no silver bullet, as each option has its own set of benefits and drawbacks. Decarbonization decisions require an accurate assessment of all trucking options which allows us to make apples-to-apples comparisons.

Previous work by our group, has developed a comprehensive framework to compare the total cost to society (TCS) and well-to-wheel emissions (WTW) associated with each powertrain option. The powertrains are evaluated on an in-house drive cycle developed from 58,000 miles of real-driving data and represents long-haul trucking in the US. The commercial software GT-Suite is used to perform all the vehicle level simulations to obtain fuel economies and tail-pipe emissions for the different powertrain options. The fuel economy is used as an input to a techno-economic model to calculate the TCS for the powertrain. While the tail-pipe emissions become an input to the detailed emissions analysis to get the WTW emissions associated with each powertrain option.

Applying this framework to hydrogen (cryogenic liquid, compressed gas) powertrains we found that the TCS is about a factor of two higher than the TCS shipping via diesel trucks in the short-term scenario. Even for the long-term scenario, we find that the TCS for hydrogen trucks is 25% more than diesel trucks. A large portion of this cost is associated with the cost of hydrogen at the retail station, which is dominated by the cost of transportation and refuelling, not H₂ production. Traditionally, hydrogen is transported either as a cryogenic liquid or a compressed gas. Liquefaction and compression are both energy-intensive processes. Additionally, existing infrastructure is unsuitable for cryogenic liquids or compressed gasses, making it a significant expense.

A promising option to address the current pain points with hydrogen powertrains is the use of liquid organic hydrogen carriers (LOHCs). LOHCs benefit from being non-reactive, stable, room temperature liquids that are easy to transport and store hydrogen within their chemical bonds. This allows considerable reductions in transportation costs as it benefits from the synergy with the existing fuel transportation infrastructure. However, liberating at H2 from LOHC at the refuelling station (to fuel hydrogen trucks) has drawbacks: First, there is a 30% energy penalty from the endothermicity of the dehydrogenation reaction. Second, once the hydrogen is released it still needs to be compressed before it can be used to power a truck causing compression losses (~15% energy penalty) and requiring expensive capital equipment.

We address both these problems by developing a powertrain design to support the on-board dehydrogenation of the LOHC using the waste heat in the engine exhaust to drive the endothermic reaction. A computational model is developed that involves four major components: an engine, the dehydrogenation reactor, the waste heat recovery loop/heating systems, and control strategies to meet variable power demand. This model is simulated over the in-house drive-cycle developed for long-haul trucks. Unlike traditional powertrain simulations, our objective is to maximize the sum of the motive force and the waste heat recovered from the exhaust. We consider several design choices to optimize the powertrain design: reactor, heat exchangers, intermediate batteries, turbochargers/superchargers, exhaust after-treatment position, and more. A major focus of this work has been modelling the two-phase flow dehydrogenation reactor necessary for scale-up from bench scale systems reported in literature. Accurate estimates of heat transfer coefficients, and hydrodynamics become particularly important for scale-up where several bench scale assumptions such as uniform bed temperature tend to breakdown. We also perform a detailed study to optimize the engine parameters such as valve timings, spark timings, air-to-fuel ratio, throttle, and more. These variables allow us to control the exhaust temperature and mass flowrate for better heat integration in the overall system. Using the optimized powertrain design, we perform vehicle-level simulations similar to the hydrogen powertrains to obtain TCS and WTW emissions for the LOHC-powered long-haul trucks. Preliminary analyses suggest a trucking system based on this LOHC concept has several practical advantages over competing concepts and would be only modestly more expensive than the current system based on diesel.