reader is referred to the presentations from the Partnership to the committee on the various technical areas: these can all be found in the project’s public access file, available through the National Academies Public Access Records Office. Chapter 4 will address issues associated with hydrogen and biomass-based fuels.
Steady progress is being made in the advancement of power plants that rely on energy carriers other than liquid hydrocarbon (HC) fuels. However, one unique characteristic of mobility applications is that the energy being supplied to the power plant needs to be carried around with the vehicle. As weight and volume are important parameters in vehicle design and function, it is critical to have the highest possible energy per unit of mass and per unit of volume within the vehicle’s fuel system. Here, the fuel system includes all aspects of carrying the energy on the vehicle—that is, the fuel tank or containment system (battery pack, or hydride material) and supporting structures are included in this weight and volume assessment. On this basis, liquid HC fuels are very effective energy carriers for mobility systems.
Using the metrics of energy density (watt-hour per liter [Wh/L]) and specific energy (watt-hour per kilogram [Wh/kg]) of a vehicle’s complete fuel system highlights differences compared to conventional vehicles and the challenges of implementing alternative energy carriers to mobility systems. When one makes these comparisons, it is important to consider not only the energy density of the vehicle’s fuel system but also the efficiency of converting the energy carried on the vehicle to motive power (power that causes motion) at the wheels of the vehicle.
Liquid HC fuels have very high energy density and specific energy relative to batteries and hydrogen systems, but the efficiency of the ICE is typically lower than that of systems using electric motors and power electronics and fuel cell systems. Thus the concentrated effort to improve the engine and power-train efficiency is easily understood. However, the energy density and specific energy of liquid HC fuels is so great that even considering these efficiency differences, a typical vehicle carrying a liquid HC will have significantly higher capability than that of an electric or hydrogen-powered vehicle in terms of deliverable work to the wheels per unit of mass and volume of vehicle energy storage onboard the vehicle.
For example, comparing an ICE with an efficiency of 40 percent to a hydrogen fuel cell vehicle (HFCV)1 with an overall power-train efficiency of 65 percent results in a work capacity of the liquid-fueled ICE vehicle that is approximately