HYDROGEN POTENTIAL AS A VEHICULAR FUEL
By Dr. Azhar M. Khan
May 22 - 28, 2000
In view of technological developments, a reassessment of alternate fuel studies becomes necessary.
Hydrogen's potential as a vehicular fuel for transportation has recently received much attention. During the eighteenth century, as technology was being evolved to harness power from internal combustion engines, hydrogen was considered as a possible fuel. H.R. Ricardo, a British scientist was able to achieve high thermal efficiencies with a hydrogen powered internal combustion engine. There were problems associated with using this fuel in the spark-ignited engine, like poor engine operation caused by the detonation of the hydrogen-air mixture and of even more significance was the little knowledge available concerning the storage, generation, and handling of this recently discovered element. Thereafter, a German engineer, became interested in the possibility of using the fuel to power large airship-balloons. Injecting hydrogen directly into the combustion chamber alleviated the problem of carburetor backfiring.
Lately, the interest in the use of hydrogen as a vehicular fuel has been revived due to awareness about environmental degradation and the desire to control urban pollution resulting from the ever-increasing number of internal combustion engines in our major cities. Another reason could also be the depleting oil reserves and their ever-surging prices. Thus, replacing the ICE (internal combustion engine) with a non-petroleum-based alternative system will have at least two beneficial effects. First, it will lessen or eliminate need for imported oil to reduce trade deficit of certain countries and will also reduce the pollution in urban areas. Inasmuch as high levels of carbon monoxide can result in heart attacks, strokes, and death; sulfur-based pollutants irritate respiratory epithelium and aggravate asthma. Total suspended particulate cause inflammation of respiratory epithelium and can lead to death. Oxides of nitrogen can cause chronic obstructive pulmonary disease and decrease normal breathing capacity. Lead pollution is permanently absorbed by the body to cause neurological problems.
The most abundant fossil-energy resource available in the world is coal. But it is an undesirable form of fuel due to resultant air pollution. In order to utilise this abundant resource, efforts have been made at several laboratories to develop a method for converting it into a form suitable for widespread transportation applications. Consequently, coal can be used to dissociate water into hydrogen.
In fact, the difficulties of storing hydrogen for its subsequent use in an internal combustion engine have caused many researchers to overlook this fuel as a potential source of "synthetic" energy. Hydrogen was initially stored in high-pressure gas cylinders, but these containers become extremely heavy and bulky. Hydrogen, which is nature's best example of an ideal gas, is very difficult to compress, inasmuch as low-viscosity hydrogen gas leaks past conventional compressor seals.
As the need for hydrogen became apparent to the space industry, millions of dollars were spent to develop cryogenic, or low-temperature, storage techniques. Although cryogenic hydrogen storage was ideal for the aerospace industry, its applications for everyday transportation seemed very limited. In the first place, liquefied hydrogen costs two to three times as much as hydrogen, since hydrogen liquefaction takes place only at temperatures below -400¡F (33 K). Furthermore, the cryogenic or super-insulated container, which is used to store hydrogen at this remarkably low temperature, must be of a very sophisticated construction, resulting in an expensive initial investment. Other major factors hindering the utilization of cryogenics in ground vehicular transportation are the phenomena of "flashoff' and "boil-off". When a hydrogen vessel is initially filled with hydrogen, a large volume of the gas is "flashed offs during a process in which the inner parts of the tank are cooled to the very cold temperature of the liquid hydrogen. Later, after the tank has been charged, heat leaks through the "super insulation," "boiling off" the hydrogen at a rate which is proportional to the quality, and hence to the cost, of the container. Although this problem can be overcome when using hydrogen as an aircraft fuel, the prospects of using cryogenic hydrogen storage for on-the-ground vehicular transportation presently do not hold great promise.
Metallic compounds, known as the metal hydrides were discovered in sixties to store hydrogen for vehicular use. These metal hydrides react with a large volume of hydrogen gas and hold it through weak bonds in a solid, or powdery form. As the pressure temperature equilibrium is reversed, the hydrides dissociate, giving off the hydrogen gas. The metallic materials are then ready to take on a new charge of hydrogen by supplying the gas at a medium-low pressure and by dissipating heat from the tank. This process has been run through thousands of cycles with no apparent sign of deterioration. Although development of metal hydrides is still in infancy, this technique appears to hold great promise as a hydrogen storage method.
Vehicles have already been operated using the metal hydride storage method. The first, a 1973 Chevrolet Monte Carlo, was operated using a dual hydrogen storage system: (a) a cryogenic liquid container, and (b) an iron-titanium hydride tank. After the vehicle had been operated on hydrogen for more than 5,000 miles (8045 km), it was determined that either system was capable of adequate engine performance, with the exception that the hydride prototype required further refinement. The cryogenic system, of course, suffered from the previously mentioned problems, and the less-expensive gaseous-charging hydride system appeared to be far superior for ground vehicle utilization. A more highly refined metal hydride system, still employing the iron-titanium material, has been installed in a 1975 Pontiac Grand Ville. The new hydride system employs exhaust heat to support the metal hydride dissociation process. The refined metal hydride storage system easily sustains the vehicle's large 450-cubic inch [7375.5 m3 (cubic meters)] engine up to speeds of 70-90 miles per hour [112.6- 144.8 km (kilometers)]. A similar hydride storage system is presently being installed in a minibus, where similar results are anticipated.
Advancements in engine technology have resulted in the virtual elimination of pollution from hydrogen-powered automobiles. Since no carbon is present in hydrogen fuel system, hydrocarbon and carbon monoxide pollution do not exist. However, when the air, consisting of nitrogen and oxygen, is heated inside the engine, nitric oxide pollution is formed. Using the Billings Energy Research Corporation water induction technique, peak combustion temperatures inside the hydrogen engine can be maintained at levels below the threshold for nitric oxide formation. This results in a substantial decrease in nitric oxide formation.
The recent observation of improved engine operating efficiencies, the development of successful methods for virtually eliminating nitric oxide formation, and development and refinement of metal hydride storage systems, have all enhanced hydrogen's potential as an alternate fuel for vehicular transportation. In view of these technological developments, a reassessment of alternate fuel studies becomes necessary.
The first phase of implementation appears to be high-density urban mass transit systems. Hydrogen buses could do much to alleviate the critical pollution problems of cities and at the same time would begin to eliminate the enormous drain on petroleum related resources. Such a project is currently being considered for the cities of Provo and Orem, Utah; for Riverside, California; and for the Idai Falls National Laboratory. Should these pilot projects be successful, they could be expanded to include cities around the country. Since urban bus systems refuel at a centralized location, the vast distribution problems associated with the implementation of a new fuel would be greatly reduced. From the springboard of a mass transit system, hydrogen implementation could then encompass fleet vehicles and trucking lines and could have an eventual widespread usage. With continued support, dedicated research, and a national commitment, hydrogen will have a significant impact on this nation's energy economy.
Although the environmental and societal benefits associated with conversion to a hydrogen-based transportation are substantial, the cost of operating a hydrogen-fuelled vehicle along with the penalties of hydrogen storage have, so far, made widespread implementation of hydrogen vehicle systems impractical. As the promises of a hydrogen energy economy create increasing incentives for a clean energy system based on renewable fuels without environmental deterioration, the ability to convert hydrogen into useful energy on board a vehicle will become increasingly important.
Unless researchers are able to make magnificent breakthroughs in the economics of producing hydrogen, and in the size and weight of hydrogen storage systems, the practical application of hydrogen as a fuel for transportation vehicles may remain intricate.B
The writer is a faculty member at the National University of Sciences and Technology (NUST).