by Peter Kushnir
Hydrogen is the ideal alternative fuel for Army After Next (AAN) platforms. However, while hydrogen offers many benefits, there are two drawbacks to using it as a fuel with current technology. Liquid hydrogen, the preferred form of hydrogen, requires four times the storage space of conventional petroleum-based fuels. The other problem is that hydrogen production depends on the availability of a nonrenewable resource, petroleum. Currently, hydrogen is produced from raw petroleum for industrial use, but petroleum supplies may become limited in the near future.
Liquid hydrogen is the best alternative fuel for AAN platforms; however, further research is needed to move the hydrogen fuel technologies from prototypes to usable military hardware and to optimize power outputs from internal combustion engines (ICE's), gas turbine engines, and fuel cells.
Petroleum production is expected to decrease significantly by 2025, the year that AAN concepts and force structures are scheduled to be operational. Current oil production is 25 billion barrels of oil per year; by 2025, annual oil production most likely will be between 18 and 19 billion barrelsless than the annual production during the oil shortages of the 1970's. The predicted decrease, as well as possible interruption of imported oil due to political instability in the Middle East, will result in increased petroleum prices.
On the other hand, high speed and high mobility will characterize the AAN battle force, and speed and mobility mean high fuel consumption. The 1998 AAN Annual Report states, "An absolute imperative exists to develop alternative fuels (nonfossil) . . . for AAN-era forces." The report goes on to say that there are numerous alternatives to fossil fuels but does not specify what those fuels are. In the January-February 1999 issue of Army Logistician, Lieutenant Colonel Allen Forte recommends " . . . new systems [ought] to examine alternatives to fossil fuels as their first option for a power source." Other writers have recommended that AAN planners develop hydrogen as the fuel for AAN platforms; one unequivocally states, "The development of hydrogen-based vehicles is a national imperative."
The Attributes of Hydrogen
Hydrogen is considered an alternative fuel for two reasons: It is renewable, and it is the most abundant element on the earth. Hydrogen comprises more than 75 percent of the environment; so if it became a primary fuel, dependence on foreign sources of fuel would be eliminated. However, hydrogen in nature exists primarily in combination with other elements. For hydrogen to be useful as a fuel, it must exist as free hydrogen (H2). One common source of hydrogen is water, which is 11.2 percent hydrogen by weight. Hydrogen also can be produced from biomass. Biomass is essentially plant matter, so the vast agricultural resources of the United States could be used to "grow" the fuel required by AAN platforms.
Hydrogen's physical and chemical properties make it a good candidate for a fuel. At normal atmospheric conditions, hydrogen is a colorless and odorless gas. It is stable and coexists harmlessly with free oxygen until an input of energy drives the exothermic (heat releasing) reaction that forms water. Fuel cells also may use hydrogen as a fuel. A fuel cell is an electrochemical engine that converts the chemical energy contained in the hydrogen molecule into electrical energy. Hydrogen can react with oxygen to produce electricity in a fuel cell.
Hydrogen is the lightest element occurring in nature and contains a large amount of energy in its chemical bond. Because of its low density, liquid hydrogen weighs less than petroleum-based fuels. The density of gaseous hydrogen is 0.0899 grams per liter (g/l). (Air is 1.4 times as dense.) Liquid hydrogen boils at -252.77 degrees Celsius, and it has a density of 70.99 g/l. With these properties, hydrogen has the highest energy-to-weight ratio of all fuels: 1 kilogram (kg) of hydrogen has the same amount of energy as 2.1 kg of natural gas or 2.8 kg of gasoline. Hydrogen burns in air at concentrations in the range of 4 to 75 percent by volume (methane burns at 5.3 to 15 percent concentrations by volume). The highest burning temperature of hydrogen is 2,318 degrees Celsius and is reached at 29-percent concentration by volume in air.
These data give hydrogen both advantages and disadvantages. The major advantage is that hydrogen stores approximately 2.8 times the energy per unit mass as gasoline. The disadvantage is that it needs four times the volume for a given amount of energy. For example, a 15-gallon tank of gasoline contains 90 pounds of gasoline; a 60-gallon tank of gaseous hydrogen would weigh only 34 pounds. Hydrogen has the potential to reduce the amount of fuel consumed by AAN platforms, but the size of the storage container would increase.
Extraction and Use of Hydrogen Energy
There are two ways to extract the energy contained in hydrogen: by simple combustion in ICE's or turbine engines or by converting it to electricity in a fuel cell.
Daimler-Benz AG (now DaimlerChrysler), BMW, and Mazda have developed and tested ICE's fueled with hydrogen and have concluded that hydrogen can be used successfully as a vehicle fuel. Hydrogen also can be used to power aircraft gas turbines. In 1988, a triple-jet-powered, modified Tupolev-154 airliner was flown in the former Soviet Union using liquid hydrogen as a fuel. Daimler-Benz Aerospace Airbus (DASA), in cooperation with Russia, is developing a liquid-hydrogen-powered aircraft. The only drawback is that adjustments in manufactured parts and components will be necessary to handle the cryogenic liquid hydrogen. The cryogenic temperature range is from -150 degrees Celsius (-238 degrees Fahrenheit) to -273 degrees Celsius (-460 degrees Fahrenheit).
Fuel cell drive concepts with highly efficient electric drive systems can provide fuel-efficient solutions for vehicle propulsion that are two to three times as efficient as ICE's with mechanical transmission systems. Fuel cells convert chemical energy directly to electricity, so they lose less energy to waste heat than ICE's. The electrical output of fuel cells can power an electric motor, and vehicles with fuel cells are being developed and tested.
Several types of fuel cells are being developed. The proton-exchange membrane (PEM) fuel cell generally is considered the most promising fuel cell for automotive use, such as light trucks. The PEM fuel cell has a low operating temperature, which enables quick starts, and the amount of power it generates for its weight and size (power density) is high enough for light-duty trucks. Several experiments are being conducted in Germany using PEM-fuel-cell-powered buses. The fuel cells, coupled with electric drive motors, are able to move 18-metric-ton buses efficiently and reliably.
Unlike fossil fuels that can be mined or extracted, hydrogen must be produced. Hydrogen can be produced from a variety of feedstocks, including oil, coal, natural gas, biomass, and water.
The main feedstock for hydrogen is natural gas, because the efficiency is high and the production cost is relatively low. Other feedstocks that are used to produce hydrogen are coal and residual oil from the treatment of crude oil. However, any process producing hydrogen from petrochemical-based feedstock does not reduce dependence on foreign oil.
Hydrogen production from biomass, though promising, is still in the early research and development phase. Basically, biomass includes all organic substances, such as plants, wood chips, bales of straw, liquid manure, and organic wastes. Currently, there is no commercially available process for producing hydrogen from biomass, but the method is to use a high-temperature process to convert biomass into hydrogen and carbon dioxide.
Electrolysis can be used to separate water into its basic constituents, hydrogen and oxygen. In electrolysis, a current is passed through water. Although any power source can be used to produce the electric current, hydroelectric resources offer the lowest price for hydrogen production.
Hydrogen may be stored on platforms using a variety of technologies. At room temperature, hydrogen is a gas that can be stored in compressed gas cylinders similar to those used on natural-gas-powered vehicles. Gaseous fuels contain comparatively little energy per unit volume, so platforms using gaseous hydrogen may have a somewhat reduced range compared to platforms using liquid fuels such as gasoline or diesel. Hydrogen also may be stored in liquid form, but it becomes a liquid only at very low temperatures, so special fuel tanks are necessary to keep the hydrogen cold and prevent losses.
Compressed-gas cylinders made of stainless steel are being used for storing fuel aboard natural-gas-powered automobiles. These cylinders have a pressure level of 20 megapascals (MPa), or 2,900 pounds per square inch (psi). The pressure levels desired for on-board storage range from 20 to 30 MPa, or 4,350 psi. Under development are high-pressure cylinders made of plastic composite structural materials with steel or aluminum liners, to be used for liquid hydrogen.
Liquid hydrogen storage is preferred to compressed gas storage since more hydrogen can be stored in the liquid state than in the gaseous state. Tanks for cars and buses are available as individually manufactured items. Small vacuum tanks with a 100-liter capacity are available with a super insulation consisting of some 30 aluminum foil layers separated by plastic foil. Larger tanks consist of three elliptical cross-section tanks, each with a 190-liter capacity. The tanks are constructed with 200 to 300 layers of insulating foil. Evaporation rates (evaporation of liquid hydrogen into gaseous hydrogen) for both tanks are on the order of 1 percent per day.
Both compressed gaseous hydrogen and liquid hydrogen can be transported by trucks or rail. Liquid hydrogen can be transported in pressurized tanks by truck, rail, barge, or ship. Insulation of the storage tanks is of utmost importance. Due to the very low boiling point of hydrogen, losses resulting from boil-off can be considerable.
Pressurized hydrogen gas can be transported via pipelines. In Germany, there are two large hydrogen distribution networks that have more than 50 kilometers of pipeline with pressures of 2 MPa, or 290 psi. There have been no accidents in more than 50 years.
The safety of any energy source is always a concern. AAN platforms must be engineered properly to minimize risks to their crews. Although hydrogen has different characteristics from petroleum-based fuels, it is as safe as gasoline, diesel, or kerosene.
Hydrogen's explosive range is a 13- to 79-percent concentration in air. It is colorless and odorless and burns with a nearly invisible flame. Hydrogen's wide explosive range, coupled with its very low ignition energy, give it a potential disadvantage since an accumulation of hydrogen in a poorly ventilated vehicle interior may explode easily.
The minimum ignition energy required to ignite a hydrogen mixture is 0.02 millijoules, which is equal to the energy of a static electric discharge from the arcing of a spark. However, the vapors of petroleum-based fuels ignite just as easily.
The diffusion coefficient for hydrogen is 0.61 cubic centimeters per second (cm3/sec), which means that hydrogen mixes with air faster than does gasoline vapor. Hydrogen's low vapor density and high diffusion coefficient cause it to rise quickly, so that in the open, hydrogen mixes with air and disperses rapidly with no pooling on the groundunlike petroleum-based fuels.
Since there is a possibility that hydrogen might leak into the crew compartment, hydrogen detectors must be used aboard platforms to detect explosive concentrations of hydrogen. A ventilation system could be used to exhaust the explosive mixture to the atmosphere. Also, since hydrogen's ignition energy is extremely low, a sparkless environment must be provided. The sparkless environment should include an extremely well-insulated electrical system and some form of grounding for the crew so they do not build up a static charge during platform operation.
Hydrogen is the cleanest fuel available. Hydrogen-fueled ICE's and gas turbine engines have negligible emissions of air pollutants. Hydrogen-powered-fuel-cell vehicles have zero emissions. On the other hand, platforms powered by petroleum-based fuels emit significant amounts of air pollutants (hydrocarbons, carbon monoxide, nitrogen oxides, sulfur oxides, and particulate matter), air toxics (either confirmed or suspected human carcinogens, including benzene, formaldehyde, 1,3-butadiene, and acetaldehyde), and carbon dioxide. The health effects of these pollutants range from headaches to serious respiratory damage such as lung cancer.
Burning hydrogen with air under appropriate conditions in ICE's or gas turbines results in very low emissions. Trace hydrocarbon and carbon monoxide emissions, if generated at all, can result only from the combustion of motor oil in the combustion chamber of ICE's. Nitrogen oxides (NOx) emissions increase exponentially with the combustion temperature. Therefore, these can be influenced through appropriate process control. Particulate and sulfur emissions are limited to small quantities of lubricant remnants. Aircraft gas turbine engines fueled with hydrogen produce no carbon dioxide emissions and cut nitrogen emissions up to 80 percent.
Using hydrogen in fuel cell propulsion systems with low temperature fuel cells completely eliminates all polluting emissions. The only byproduct resulting from the generation of electricity from hydrogen and atmospheric oxygen is water.
Hydrogen has a higher energy density than petroleum-based fuels. It supplies more energy per unit volume than gasoline, diesel, or kerosene. Hydrogen is extremely abundant, thus eliminating U.S. dependence on foreign sources of supply. Research and development projects have demonstrated that using compressed hydrogen or liquid hydrogen as a fuel for ICE's, gas turbine engines, or fuel cells is feasible today. Further research is needed to increase the power outputs from the ICE's and gas turbine engines. Despite a few remaining limitations, liquid hydrogen shows much promise for the future. ALOG
Peter Kushnir is an instructor in the Environmental Management Department, School of Logistics Science, Army Logistics Management College, Fort Lee, Virginia. He holds a bachelor's degree in chemistry from Adelphi University, New York, and is a graduate of the Logistics Executive Development Course, for which he prepared this article.