Occupying a unique space as both an energy vector and a fuel, hydrogen has gained interest for propulsion and power generation applications over the past several years. Hydrogen has been investigated thoroughly throughout the previous decades, but with advances in hydrogen generation and infrastructure, and the increasing need to low-carbon propulsion solutions across all sectors, hydrogen propulsion may be poised for significant market penetration.
Hydrogen as a fuel offers energy and power density benefits over battery electric propulsion for high-load and high-range applications, with no local carbon emissions and little to no criteria pollutant emissions. Hydrogen can be used in an internal combustion engine or in a fuel cell, and these two applications have been widely evaluated for commercial vehicles. These propulsion methods are complementary, but each also faces its own unique barriers to widespread commercialization.
While battery electric propulsion is proving to be an effective solution for a range of applications, it can be ill-suited to the operational requirements in certain market sectors such as the heavy duty long-haul and non-road sectors. The duty cycle, payload, and refueling time requirements in these sectors are particularly challenging to meet and require a higher density energy storage solution than is offered by batteries. As a means by which to “decarbonize” these sectors, hydrogen propulsion is an attractive solution, and it has garnered research and development investment in recent years.
Fuel Cell
Fuel cells are electrochemical devices wherein each cell exploits the chemical nature of a given fuel to generate electricity in the presence of electrolytes and catalysts. Although various types of fuel cells have been developed, hydrogen-fueled proton exchange membrane (PEM) fuel cells have emerged as a preferred choice for transportation applications due to their fast start-up time, high power density, and superior load-following capability.
One of the primary advantages of PEM fuel cells is the lack of criteria pollutant generation. Because of this, fuel cells have been considered for a range of applications such as long-haul transport. Fuel cell electric vehicles (FCEVs), utilizing hydrogen as fuel, offer improved drivetrain efficiency with minimal changes in refueling time and payload capacity compared to diesel internal combustion engines (ICEs), while maintaining an acceptable vehicle range.
Despite the many advantages of FCEVs, their reliability and durability must be improved to meet the requirements of the Class 8 truck market and provide a competitive total cost of ownership (TCO) versus diesel ICE powertrains. A study by the National Renewable Energy Laboratory on fleets using FCEV buses found that the FCEV buses had the lowest availability among various powertrain options. The FCEV bus fleet also consistently fell below the target 85 percent availability rate over a 6-month period of operation.
The fuel cell stack consistently met or exceeded the target availability, with the reliability issues centering around the fuel cell balance-of-plant components. Among those sub-systems, compressors, plumbing, and air blowers have been found to have the most significant impact on downtime. This highlights the need to develop rightsized, system-level components dedicated for fuel cells that match the robustness expected for a lifetime of operation in on-road heavy duty vehicles.
This result is partially driven by the traditional fuel cell development scale which has been historically centered around smaller power applications. However, large commercial vehicles have a higher durability requirement for powertrain components than smaller passenger car or light commercial vehicle applications. Maintenance and downtime play significant roles in TCO determination. Therefore, there is a need for dedicated development of high durability, high efficiency balance-of-plant components for heavy duty FCEVs, and system-level optimization that promotes the same.
Leveraging air compression technologies commonly used in ICEs, along with rightsizing of these components, can improve both durability and efficiency. The U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office has supported this area of development for several years, with promising results.
Promoting durability within FCEVs requires a system-level approach. For example, the use of conventional lubricants for shaft bearings of the air compressor can damage the fuel cell stack and other sensitive components should there be leaks or failures. Advanced filtration is also needed to limit exposure of the fuel cell stack to ambient levels of pollutants that may damage the catalyst. Even ambient levels of nitrogen oxides and sulfur compounds found in areas such as ports can be enough to cause irreversible damage to these stacks. Filtration solutions capable of producing high purity air without significantly impacting compressor efficiency are critical future development steps.
Hydrogen ICE
While FCEVs are well-suited to some heavy-duty markets such as long-haul, non-road applications such as construction have harsh environmental conditions that may challenge FCEV effectiveness. Conditions such as extreme humidity levels, both high and low, high levels of dust, and extreme temperatures are largely incompatible with FCEVs and battery electric powertrains.
Incumbent diesel ICEs can generally accommodate all of these conditions, and so an effective low-carbon strategy is to replace diesel with a low or zero carbon-containing fuel such as hydrogen. This approach retains the benefits inherent to ICEs such as durability and existing production tooling and maintenance infrastructure, all of which promote relatively low TCO. Conventional ICEs require moderate component-level changes in order to accommodate hydrogen but generally they are minor and incorporate common technologies.
Hydrogen is a relatively easy fuel to combust, with a low auto-ignition energy. This is both a benefit for its use in ICEs and the source of a substantial limitation. Hydrogen in ICEs is highly prone to uncontrolled pre-ignition, which can damage the engine. Pre-ignition can become more prominent, and its impact more severe, at higher loads. To avoid this, hydrogen ICEs are typically operated at lower power and torque levels compared to diesel ICEs.
In the non-road sector, a single ICE family typically sees duty in a wide range of vehicle types. Because of this, there is little opportunity to combat power de-rating in hydrogen ICEs by simply increasing engine displacement. Substantial changes to engine hardware and especially vehicle packaging put upward pressure on the TCO.
Several additive engine technologies have been proposed to reduce this de-rating. These include water injection to reduce in-cylinder temperatures, high pressure direct fuel injection to eliminate backfire into the intake, tumble ports to promote fuel-air mixing, and advanced ignition technologies such as pre-chamber igniters. This latter technology promotes the use of high levels of charge dilution to keep in-cylinder temperatures low, and therefore, reduce hot-spot formation that could induce pre-ignition. A further benefit is improved cycle-to-cycle combustion stability, which can further reduce the risk of pre-ignition.
Future diversification of propulsion approaches has the potential to enable targeting of the optimal powertrain for each market sector. In some sectors that require rapid de-carbonization but where electrification is operationally prohibitive, hydrogen propulsion can serve a strong role. As with all new technologies, commercial viability depends heavily on the TCO. Addressing component durability in FCEVs and power density in hydrogen ICEs can help to improve the TCO and help make hydrogen power a practical proposition.
This article was written by Dr. Mike Bunce, Head of Research and Head of Design & Analysis, Dumarey USA (Plymouth, MI). For more information, visit here .
