INSULATED PRESSURE-VESSELS FOR HYDROGEN STORAGE ON VEHICLES

Probably the most significant hurdle for hydrogen vehicles is storing sufficient hydrogen onboard. Three viable technologies for storing hyd rogen fuel on cars are: compressed gas, metal hydride adsorption, and cryogenic liquid. However, each of these has significant disadvantages : volume, weight, boiling losses, or energy to compress or liquefy the hydrogen. Insulated pressure vessels can reduce these problems for hy drogen-fueled light-duty vehicles. Insulated pressure vessels can be f ueled with liquid hydrogen (LH2), with low-temperature (80 K) compress ed hydrogen (CH2) or with ambient-temperature CH2. In this analysis, h ydrogen venting losses are calculated for insulated pressure vessels f ueled with LH2 or with low-temperature CH2, and the results are compar ed to those obtained in low-pressure LH2 tanks. Hydrogen losses are ca lculated as a function of daily driving distance during normal operati on, as a function of time during long periods of vehicle inactivity an d as a function of initial vessel temperature during fueling. The numb er of days before any venting losses occur is also calculated as a fun ction of the daily driving distance. The results show that insulated p ressure vessels with packaging characteristics comparable to those of conventional, low-pressure LH2 tanks (low weight and volume), have gre atly improved dormancy and much lower boil-off. Insulated pressure ves sels used in a 17 km/l (40 mpg) car can hold the hydrogen indefinitely when the car is driven at least 15 km/day in average. Nearly all cars are driven for greater distances, so most cars would never need to ve nt hydrogen. Losses during long periods of parking are also relatively small. Due to their high-pressure capacity, these vessels would retai n about a third of their full charge even after a very long dormancy, so that the owner would not risk running out of fuel. If an insulated pressure vessel reaches ambient temperature, it can be cooled down ver y effectively by fueling it with LH2 with no losses during fueling. Th e vessel has good thermal performance even when inexpensive microspher e insulation is used. Finally, the vessel eases fuel availability and infrastructure requirements, since it would be compatible with both co mpressed and cryogenic hydrogen refueling. (C) 1998 International Asso ciation for Hydrogen Energy.