Pressure build-up in LNG and LH2 vehicular cryogenic storage tanks

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 similar to400 liters (similar to100 gallons). The pressure in the ullage space of a tank freshly filled is usually similar to0.25 MPa but may vary during use from similar to0.25 MPa (similar to20 psig) to similar to0.92 MPa (similar to120 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 similar to 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 similar to0.25 MPa to a venting pressure of similar to1.73 MPa are presented.