A number of physical and materials-based hydrogen storage options are being developed to simultaneously meet the vehicular targets for gravimetric and volumetric capacity, cost, efficiency and greenhouse gas emissions, durability and operability, fuel purity, and environmental health and safety. The purpose of this paper is to evaluate the currently available physical, complex metal hydride, sorbent, and chemical hydrogen storage methods for their potential to meet these targets.
Our analyses show that hydrogen stored as a compressed gas (cH2) at 350 bar in Type III or Type IV tanks cannot approach the near-term (2010) volumetric target of 28 g/L. Increasing the storage pressure to 700 bar improves the volumetric capacity (without reaching the target value) but degrades the gravimetric capacity. Dormancy and hydrogen loss are major concerns if hydrogen is stored as a cryogenic liquid (LH2) in insulated low-pressure tanks, even if advanced vapor-cooled heat shield concepts are deployed. These problems can be mitigated by storing cryogenic hydrogen in pressure-bearing insulated tanks (the cryo-compressed option, CcH2). Our analyses confirm that CcH2 can also meet the near-term and intermediate-term (2015) targets for gravimetric and volumetric capacities. However, the cost of the cryo-compressed system is projected to be 3-4 times the target cost of $4/kWh, and the well-to-tank efficiency for the common pathway of producing hydrogen by central steam methane reforming (SMR) and subsequent hydrogen liquefaction is only about 41%, significantly lower than the efficiency target of 60%.
We have evaluated an on-board regenerable complex metal hydride (NaAlH4, sodium alanate) system and an off-board regenerable metal hydride (AlH3, alane) system. Our analyses show that, because of the large heat of reaction (
We have analyzed two sorbent-based hydrogen storage systems, one using AX-21, a high surface-area superactivated carbon, and the other using MOF-177, a metal organic framework material, Zn4O(1,3,5-benzenetribenzoate) crystals. We found that moderate pressures and cryogenic temperatures are needed for reasonably high storage capacities, and that a temperature swing must be imposed in addition to the pressure swing to desorb hydrogen. Using off-board liquid N2 for removing the heat of sorption is not very attractive because of the poor thermal conductivity of the sorbent bed and the unfavorable fuel cycle (well-to-tank) efficiency. We also evaluated the adiabatic refueling option, in which the sorbent bed is cooled evaporatively by charging it with liquid hydrogen, and warmed hydrogen is recirculated during discharge to supply the heat of desorption. The results from our analyses indicate higher storage capacities with MOF-177 than with AX-21, but these capacities are lower than those available with the cryo-compressed option.
We have also evaluated several exothermic and endothermic chemical methods of storing hydrogen. Releasing hydrogen by hydrolysis of sodium borohydride (NaBH4, SBH) has been demonstrated in the laboratory and on-board an experimental vehicle. Our analysis of the SBH system pointed to a number of shortcomings, including the limited solubility of the product of reaction (NaBO2) in the aqueous alkaline medium, difficulty in rejecting the large amount of heat that is liberated in the exothermic reaction (
Recently, there has been much interest in ammonia borane (NH3BH3, AB) as a hydrogen carrier because it has a high H2 content (19.6 wt%), it undergoes exothermic thermolysis to release the H2 (
Figure 1 summarizes the volumetric and gravimetric capacities of the different on-board hydrogen storage options evaluated to date and compares them with the near-term, intermediate-term and ultimate targets. Figure 1 also compares the well-to-engine (WTE) efficiencies of these options for the pathway in which H2 is produced in a central SMR plant. The paper will discuss others aspects of the storage options as well, including high-volume manufacturing cost projections, greenhouse gas emissions, durability and operability, and environmental health and safety issues (dormancy and hydrogen loss).
Figure 1 System capacities and fuel cycle efficiencies of different hydrogen storage options
See more of this Group/Topical: Topical 8: Hydrogen Production and Storage