Hydrogen Storage Options: Technologies and Comparisons for Light-Duty Vehicle Applications

This report is concerned with the characterization and comparison of various technologies for hydrogen storage for light-duty vehicle applications. The storage technologies considered are compressed gas, cryogenic liquid, metallic and chemical hydrides, and activated carbon at 77 K. The technologies were evaluated in terms of weight and volume metrics - %wt H2/ system kg and gm H2/system and an energy intensity metric kJ/kg H2 for preparing the hydrogen fuel and placing it into storage for use on-board the vehicle. It was found that these metrics varied widely for the various hydrogen storage systems studied. The United States Department of Energy has presented a series of design targets/goals for hydrogen storage system development. Technologies that meet these design goals for hydrogen storage would permit the development of fuel cell powered vehicles that would meet consumer needs for vehicle performance, range, cost, and utility. The results of the present study were compared with the DOE goals in order to assess the present and projected state-of-the-art of the various hydrogen storage technologies. Special attention was given to systems using activated carbon as the storage medium as those systems have not been included in detail in past studies. The near-term (2005-2010) DOE hydrogen storage goals are 6%wt/kg system and 45 gmH2/L system. The long-term (2010-2015) goals are 9% wt./kg system and 80 gm H2/ L system. Only liquid hydrogen (LH2) and high temperature hydrides (HTH) appear to have the potential to meet the combined near-term goals and none of the hydrogen storage technologies currently being developed seem to have the potential to meet the combined long-term goals. Both the LH2 and HTH technologies are energy intensive having energy intensities of 25-35 MJ/kg H2. Activated carbon storage has weight and volume metrics and an energy intensity close to those of compressed hydrogen (5%wt, 25 gm/L, and10 MJ/kg H2), but much less convenient from an operational point-of-view as the carbon must be maintained near 77 K and cooled and heated while the hydrogen is put into and removed from storage. The objectives of the DOE hydrogen storage goals are to achieve the same range between refueling with fuel cell powered vehicle using hydrogen as is presently achieved in a conventional ICE vehicle using gasoline and not to reduce the utility of the vehicle due to the increased weight and volume of the H2 storage system. The results of the study indicate that using available, known hydrogen storage technologies some reduction in vehicle range on hydrogen will likely be necessary in order to package the storage unit on board the vehicle. Volume constraints appear to be the most restrictive and could result in a reduction in the range to about 50% of that of the conventional ICE vehicle even if the effective fuel economy of the fuel cell vehicle is twice that of the conventional vehicle. This appears to be the case for all classes of vehicles from compact cars to standard size pickup trucks. High pressure (10kpsi) compressed gas seems to be the most satisfactory near term technology when all factors are considered including operational and energy intensity factors. The high temperature hydride materials using a catalyst to reduce the temperature required appear to offer some potential for achieving fuel cell vehicle ranges of about 75% of that of a conventional ICE gasoline vehicle. None of the hydrogen storage technologies appear to have the potential to reach less than $100 per kgH2 stored.