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Alternative Fuels: Fuel Types
Alternative fuels, their characteristics and use in vehicles will be covered on this page. Additional information and resources are available on the US Department of Energy website [1].
Alcohol and Biodiesel: Alcohol and biodiesel are the alternative fuels that can most efficiently utilize existing infrastructure for delivery and existing vehicle technology with relatively few modifications. Mixtures of alcohol and gasoline are in common use, with some designs utilizing mixtures of 85% alcohol (ethanol) and gasoline. There are also initiatives to produce biodiesel from farm products and reduce dependence on oil resources.
Natural gas (compressed natural gas, or CNG): Vehicles may have CNG fuel storage alone, or CNG and a gasoline tank. According to the Natural Gas Vehicle coalition, there are 130,000 CNG vehicles on the road in the United States and 2 million world-wide [2]. CNG tanks may operate at pressures above 2,400-3,600 psi [2,3]. Properly designed and assembled systems will include a pressure relief device on the tank to release pressure and fuel in a controlled manner if the vehicle burns. Without the pressure relief device, overheating of the tank may result in a catastrophic failure of the tank, releasing tremendous amounts of stored mechanical energy due to compression, as well as the flammable gas. Even without the chemical energy from gas flammability, the mechanical release of pressure can cause the tank to break its restraints, pierce any containment provided by the vehicle body and fly great distances. With the pressure relief devices, the controlled release may result in an intense column of flame for a short time (until pressure in the tank is relieved), but the overall risk is likely to be reduced.
All vehicles powered by CNG, gasoline or other fuel have an inherent risk due to the flammable material carried; some rate of fire is expected for all designs. In one survey of over 8,000 CNG vehicles that traveled almost 180 million miles, there were 7 reported fires, one of which was related to the CNG fuel system [4].
Video clip of a Southwest Research Institute fire test of a
CNG tank during which a PRD failed to open. [5]If you would like more information about CNG tank types and inspection of CNG tanks for structural integrity, click here.
Propane, or liquid petroleum gas (LPG): LPG is currently used to power 350,000 single-fuel or bi-fuel vehicles in the United States (4 million world wide)[2,6]. Most are aftermarket conversions, but some are made by original equipment manufacturers.
LPG is 90% propane, with the balance made up of butane, propylene and other gases. On-board propane tanks are generally at moderate pressure (160-200 psi) as compared to hydrogen and compressed natural gas. Vehicles may have storage tanks only for propane or they may have tanks for propane and gasoline. Otherwise, these vehicles generally have traditional fuel and ignition sources.
Because LPG tanks are at a lower pressure than CNG (and hydrogen) tanks, there is less stored mechanical energy to be released if there is a tank structural failure or over-pressure due to a fire. Systems are still constructed with pressure relief devices to prevent catastrophic failure of the tank structure in a fire.
Hydrogen and hydrogen fuel cells: Prototypes of hydrogen fuel cells are currently used to power transit buses [7], and passenger cars [8].
The fuel for current hydrogen vehicles is stored as a compressed gas or in a cooled, liquefied form. Research is also being conducted on reforming hydrogen from other onboard fuels, such as ethanol, methane, and gasoline. Even more advanced methods are being explored for storage of hydrogen at lower pressures within containers that absorb and release hydrogen as a function of pressure and temperature.
In another Southwest Research Institute test of a
compressed hydrogen, the PRD was disabled so that a
catastrophic failure would ensue during a fire test.
Designs using compressed and liquefied hydrogen have some of the same design and fire considerations related to fuel storage as the CNG vehicles already described. Systems using reformers will include risks associated with storage of the relevant fuel. Including fuel storage (or reforming) the fuel cell requires:
The part of the system referred to as the “fuel cell” takes hydrogen, either from stored tanks or reformed from another fuel, chemically combines it with oxygen, producing electrical power to drive a motor and water as a byproduct. The most promising current fuel cell technologies for vehicles are:
Alkaline fuel cells (AFC): Using an alkaline electrolyte, AFCs are efficient and adaptable to the low-temperature vehicle environment, but require pure hydrogen.
Proton exchange membranes (PEM): PEMs use a solid electrolyte, are also adaptable for low temperature environments, are less efficient than AFCs but do not require a pure hydrogen source.
This video clip from UTC Power (the manufacturer of fuel cells), provides Information about how fuel cells work. [9]
Additional information is available on the website of Ballard Power Systems (another fuel cell manufacturer):
http://www.ballard.com/be_informed/fuel_cell_technology/how_the_technology_works#. The Ballard Power Systems video also includes information about the fuel cell vehicle and hydrogen distribution system.
The components of a fuel cell work together in the following way [10]:
Hydgrogen Fuel Cell Schematic
A great deal of work on hydrogen vehicle safety is ongoing [11-13].
References
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