ECENTLY, the Clinton Administration's Partnership for a New Generation of Vehicles (PNGV) set an automobile fuel efficiency goal of 80 mpg (34 km/L) to achieve responsible energy and environmental conservation. Even before the goal was announced, researchers at LLNL had joined investigators at Los Alamos National Laboratory and Sandia National Laboratories, California, to design and test a hydrogen hybrid concept vehicle that will meet or exceed PNGV guidelines. The hydrogen piston engine they have designed gets mileage equivalent to the 80 mpg of a gasoline-powered vehicle on the combined city-highway driving cycle.
Our conceptual design of a hydrogen hybrid vehicle features a large fuel tank for pressurized hydrogen. It has a gasoline-equivalent fuel efficiency of 80 mpg and a driving range of 380 mi (608 km).
Why Hydrogen Fuel?|
Hydrogen has several features that make it a serious contender as an alternative fuel. It can be produced from various domestic sources, including renewable sources; it can reduce emissions to near zero while maintaining performance; and it can now be safely stored and transported. An immediate motive for moving to hydrogen is its potential to improve urban air quality. In the longer term, such a transition would also benefit the balance of payments and the energy security of the U.S. by reducing dependence on foreign oil.
Because hydrogen is a manufactured fuel, it is likely to cost more than fossil fuels for at least several decades. The cost issue means that researchers need to exploit the use of hydrogen fuel in those applications that have the highest leverage or payoff. One obvious application is in transportation. The energy efficiency of today's automobiles is only about18%.
Despite its many advantages, hydrogen has yet to become a significant transportation fuel, even in advanced countries. Several factors hinder a transition from gasoline to hydrogen, including the absence of available vehicles with engines that can use this resource efficiently and the lack of an adequate distribution infrastructure.
Hydrogen Fuel Efficiency|
Current engine designs have low energy efficiency. Small piston engines (in the range of about 40 kW or 54 horsepower) have not been optimized specifically for hydrogen fuel. The unique combustion properties of hydrogen allow engines to run leaner and at a higher compression ratio than they do with hydrocarbon fuels. Energy efficiency is a serious problem if consumers want a driving range comparable to that of today's gasoline-powered vehicles. Thus, what we need are high-efficiency drive trains if we are to consider hydrogen seriously as an alternative fuel. Researchers at LLNL are showing that such drive trains are feasible and that hydrogen has a genuine opportunity to compete for the first time in the transportation sector.
Our studies demonstrate that considerable improvement over conventional automobile efficiency can be achieved through a hybrid-electric drive train. In this concept, all the chemical energy of the fuel is converted to electrical energy by means of a piston engine coupled to an electrical generator. The electrical energy can be stored in various ways, including an advanced battery, an ultracapacitor, or an electromechanical battery (EMB), also known as a flywheel battery. Of these three technologies, the EMB is closest to full-scale demonstration. The flywheel battery, which will be the subject of a forthcoming article in Science and Technology Review, has an energy recovery efficiency of more than 90% and a long lifetime. Compared to the EMB, today's electrochemical batteries have an energy recovery efficiency of about 70%.
How the Hybrid Hydrogen Vehicle Works|
In the hybrid concept vehicle we are developing (see the illustration), stored electrical energy is extracted as needed by the power demands for accelerating, cruising, and accessories. The engine does not idle; rather, it shuts down each time the energy-storage device is fully charged. To complete the power train, an electric motor is coupled to the wheels by a single-speed transmission. By turning the electric motor into a generator during braking, our concept vehicle includes the feature of regenerative braking. Thus, kinetic energy returns to the storage device when the brakes are applied.
If the engine/generator in a hydrogen-powered vehicle supplies enough power for a fully loaded vehicle to climb hills at cruising speeds, then it performs much like today's gasoline-powered automobiles. However, if the engine/generator supplies just enough power for average energy consumption, then it can serve as a range extender. The difference in power required for cruising versus hill climbing is about a factor of four. We are designing a fully capable concept car that can cruise and climb hills.
The Design Team's Challenges|
LLNL researchers are working on the technical details of a new hydrogen piston engine with investigators at Los Alamos National Laboratory and Sandia National Laboratories, California. Essentially, LLNL is responsible for the initial system studies, engine design, and combustion kinetics. Los Alamos investigators perform the computational fluid-dynamics modeling (combustion modeling) and integrate this information into our vehicle simulation codes. Researchers at Sandia's Combustion Research Facility then do the engine-performance and emissions testing.
The need for a highly efficient vehicle and power train is driven by the associated problem of onboard storage of hydrogen fuel. Onboard fuel storage is perhaps the single most difficult task associated with our project. Table 1 shows two options we are considering for fuel storage: a cryogenic tank for liquid hydrogen or a high-pressure tank for hydrogen gas. Without increased efficiency, the onboard fuel tank would need to be about three times the volume listed in Table 1 and three times the size shown in the illustration; that is, the tank would become so large as to be impractical. We are applying the hybrid vehicle evaluation code (HVEC) developed at LLNL as a guide to select components that maximize efficiency and thus reduce fuel-tank volume and weight. HVEC incorporates a wide range of details and complexity. The code calculates power-train dimensions, fuel economy, time to accelerate to 60 mph (96 km/h), hill-climbing performance, and emissions. Our basic premise is that we need to generate electrical energy at efficiencies of about 42%, based on a generator that is 95% efficient and an engine efficiency of about 46%.
Our calculations show that an empty vehicle weighing 2508 lb (1140 kg) (see Table 1 for additional specifications) would have a combined EPA urban/highway mileage of about 80 mpg (expressed as gasoline-equivalent fuel efficiency). Such a vehicle would require only about 10.45 lb (4.75 kg) of hydrogen for a driving range of 380 miles (608 km). For perspective, a kilogram of hydrogen has nearly the same energy content as a gallon of gasoline. Thus, our hydrogen-powered vehicle is extremely energy-efficient and has emissions equivalent to those of electric vehicles when the emissions from power plants are included. And its gasoline-equivalent fuel efficiency of 80 mpg meets the goal set by PNGV.
With current technology, we believe that a general-purpose, low-emission, long-range vehicle that uses a hydrogen internal combustion engine is now possible. Such a vehicle could become competitive in the marketplace if hydrogen production and distribution issues are addressed. These issues are being studied at the Laboratory and will be the subjects of Science and Technology Review articles in the future.