There are several ways that hydrogen can be used as a motor fuel. It can be used to directly replace gasoline or diesel fuel in specially designed internal combustion engines (ICEs), or it can be used to supplement these typical fuels in existing engines. In either of these cases, the vehicle drive system will be identical to those used on most gasoline-powered or diesel-powered vehicles. The engine will drive the vehicle’s wheels through a transmission, drive shaft, and front or rear axle.
Hydrogen can also be used as the fuel source for a “fuel cell engine,” in which case the vehicle’s drive system will be very different. A fuel cell directly creates electricity, which can be used to power an electric motor to drive the vehicle’s wheels. A fuel cell vehicle is, therefore, an electric vehicle, but one that creates its own electricity and does not need to be plugged in to recharge batteries. A small fuel cell can also be used to create electricity to directly power the auxiliary systems on a commercial truck (for example heating, air conditioning, and lighting in a sleeper berth), which are typically powered by the truck’s main engine. Using such a fuel cell auxiliary power unit (APU) would allow the driver to shut off the truck’s main diesel engine while resting, saving fuel and reducing pollution.
Regardless of whether the hydrogen will be used in a fuel cell main engine, a fuel cell APU, or an internal combustion engine, there are different ways that it can be stored on the vehicle. As described below, these different storage technologies can introduce significantly different potential hazards, including very high pressure (gaseous hydrogen storage), very low temperature (liquid hydrogen storage), or high temperature (liquid fuel reforming) (see Table 2).
Currently both fuel cells and hydrogen ICEs are in the early stages of commercialization. All of the major auto companies have fielded concept, prototype, or demonstration fuel cell sedans and sport utility vehicles in the last several years, with at least fifteen different models introduced since 2000 (Barnitt and Eudy, 2005; USFCC, 2006). Most of these vehicles have been operated by the companies themselves or have been fielded to government agencies and fleet customers as part of technology development or demonstration programs. The California Fuel Cell Partnership reports that its members have placed 134 light-duty fuel cell vehicles in service in California since 2000 (CAFCP, n.d.). In addition, there are currently nine fuel cell transit buses in service in the United States and Canada, and over 20 in Europe and Asia (Chandler and Eudy, 2006).
It is expected that commercial fuel cells will be introduced into government and transit bus fleets between 2010 and 2020, with sales to commercial vehicle fleets and the public sometime between 2020 and 2030 (DOE, 2002). It is also expected that the first use of hydrogen fuel in the commercial truck sector will be to power fuel cell APUs rather than to power fuel cell or hydrogen ICE main propulsion engines. At least one company has announced plans to introduce commercial fuel cell APUs as early as 2011 (Delphi, 2005).
Most current prototype fuel cell vehicles carry their hydrogen fuel as a compressed gas, and it is expected that this will continue to be the case for the earliest commercial vehicles. It may be desirable to store liquid hydrogen onboard a commercial vehicle because it has a higher energy density and would increase the range between fill-ups. However, onboard liquid hydrogen storage is more costly, and it is more likely that liquid hydrogen will be stored at fueling stations to supply gaseous hydrogen to vehicles. Other storage technologies, such as metal and chemical hydrides, are much further from commercial readiness (DOE, n.d.). Several fuel cell buses have been demonstrated that “reform,” or extract hydrogen from, liquid methanol onboard (Georgetown University, 2003), and there are fuel cell APU systems under development that will derive their hydrogen from onboard reforming of diesel fuel or gasoline (Delphi, 2005). In addition, there are several commercial “hydrogen injection” systems available for retrofit on diesel engines (CHEC, n.d.). These systems produce small amounts of hydrogen by electrolysis of water carried on the vehicle, which is injected into the diesel engine along with the diesel fuel.
The remainder of this chapter provides a brief overview of the types of systems that might be found on a vehicle to store or use hydrogen fuel.
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