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The use of LNG and LH2 as fuels in heavy duty vehicles is increasing steadily because cryogenic liquids provides superior volumetric and gravimetric energy densities compared to other means of on-board storage. Although several sizes and types of tanks exist, a typical vehicular storage tank has a volume of ∼400 liters (∼100 gallons). The pressure in the ullage space of a tank freshly filled is usually ∼0.25 MPa but may vary during use from ∼0.25 MPa (∼20 psig) to ∼0.92 MPa (∼120 psig). Cryogenic vehicular tanks are typically dual-walled, stainless steel vessels with vacuum and superinsulation isolation between the inner and outer vessel walls. The heat leaks into such tanks are measured as a percentage boil-off per day. For a storage tank of vehicular size range, the boil-off may be ∼ 1 % day, depending upon the cryogen and the quality of the tank. The corresponding heat leak into the cryogenic liquid vaporizes a certain amount of liquid that in turn increases the pressure in the tank which in turn significantly influences the properties of the cryogens. We have used a novel approach to calculate the increase in pressure of LNG and LH2 in a closed cryogenic vessel with a fixed heat leak as a function of time using real equations of state for the properties of the cryogens. The method and results for the time it takes for a freshly filled tank to increase in pressure from the filling pressure of ∼0.25 MPa to a venting pressure of ∼1.73 MPa are presented. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The choice of liquefied natural gas (LNG) as a heavy-duty vehicular fuel is growing rapidly due to improved LNG economics, diesel price uncertainties caused by the dependence on imported crude oil, liabilities associated with environmental and health concerns, and governmental programs related to concerns over greenhouse gas emissions. However, vehicle owners who wish to use LNG are impeded by a lack of refueling infrastructure and reliable supply of inexpensive fuel. These barriers are being overcome by the development of innovative purifier/liquefier systems that economically convert a wide array of distributed, low cost methane gas sources into high quality LNG. This paper describes the engineering design, manufacture, installation, and initial operations of two such systems. One unit was a pilot-scale system using an innovative cryogenic freezing process to remove bulk concentrations of carbon dioxide from the landfill gas (LFG). The second unit converts stranded well gas containing ∼ 18% nitrogen gas into LNG. The paper closes with a summary of lessons learned from these two installations and directions for future improvements. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]