U.S. Department of Energy’s System Targets for On-Board Vehicular Hydrogen Storage
Dr. Carole Read,, Dr. George Thomas,, Ms. Grace Ordaz, and, Dr. Sunita Satyapal
Material Matters Volume 2 Article 2
Introduction
The performance of hydrogen fuel cell vehicles must be comparable or superior to today’s gasoline vehicles in order to achieve widespread commercial success. In the North American market, an on-board hydrogen storage technology that allows a driving range of more than 300 miles is critical to meet consumer requirements for most light-duty vehicles. By translating vehicle performance requirements into storage system needs, DOE has described technical targets for 2010 and 2015. These targets are based on equivalency to current gasoline storage systems in terms of weight, volume, cost, and other operating parameters. The DOE hydrogen storage system targets help guide researchers by defining system requirements in order to achieve commercially viable hydrogen storage technologies.
On-Board Vehicular Hydrogen Storage Targets
On-board vehicular hydrogen storage system performance targets were developed by the DOE through the FreedomCAR & Fuel Partnership.1 These performance targets are application driven, based on achieving similar performance and cost specifications as commercially available gasoline storage systems for light-duty vehicles. The storage system includes all of the hardware (e.g. tank, valves, regulators, piping, mounting brackets, insulation, added cooling capacity, thermal management and any other balance-of-plant components) in addition to any storage media and a full charge of hydrogen.
Table 1 shows a subset of the DOE hydrogen storage system targets for 2010.2–4 The 2010 targets would allow some vehicles to achieve a driving range of 300 miles for early market penetration. The system volumetric capacity target includes a 20% penalty for storage systems that are not conformable to the existing packaging space currently utilized by conventional gasoline tanks. Even more challenging targets have been identified for the 2015 timeframe to enable the required driving range for the full range of light-duty vehicles in the North American market.
Table 1.Excerpt of U.S. DOE hydrogen storage system performance targets.
The DOE storage targets assume a factor of 2.5 to 3 in improved efficiency for the vehicle’s fuel cell power plant over today’s gasoline ICE vehicle. Assuming the efficiency gains, it is estimated that on-board capacities in the range of 5–13 kg of hydrogen (1 kg hydrogen ~1 gallon of gasoline energy equivalent) would satisfy the needs of the range of today’s light-duty fuel cell vehicles.
Much emphasis has been placed on meeting the volumetric and gravimetric targets. While this is central, it must be noted that transient performance must also be achieved by the storage system from “full” tank to “almost depleted.” Two key targets noted in Table 1 are the system fill time and the minimum full flow rate. The system fill time, for all methods that involve an exotherm upon hydrogen filling, is strongly dependent upon the thermodynamic properties of the materials (e.g. hydride formation enthalpy or heat of adsorption) and the efficiency of the thermal management system for heat removal and rejection. For example, in order to fill a tank with 8 kg of hydrogen in 5 minutes, a material with a heat of adsorption of 30 kJ/mol will generate heat at a rate of 400 kW. This heat will need to be removed and rejected between the vehicle and the filling station. Conversely, an 80 kW power demand by the fuel cell corresponds to a minimum full flow rate of hydrogen of 1.6 g/s, based on the DOE target of 0.02 g/s/kW. For materials1 approaches, this hydrogen release rate must be achieved (ideally) at temperatures that can use the waste heat from the PEM fuel cell power plant (e.g. less than 80 °C) over the entire composition range of the material.
In summary, all targets are application driven and not based upon a particular method or technology for storing hydrogen. For commercially acceptable system performance, the targets must be attained simultaneously. For material approaches, it is important to remember that in order to achieve system-level capacities the gravimetric and volumetric capacities of the material alone must clearly be higher than the system-level targets. Recent system developments suggest that, depending on the material and on the system design, material capacities may need to be a factor of up to 2 times higher than system capacity targets.5
Storage System Technology Status
DOE hydrogen storage research is focused on, but not limited to, materials-based technologies that have the potential to meet DOE’s 2010 targets and eventually 2015 targets. Currently, research is focused on achieving the volumetric and gravimetric capacity targets in Table 1, while also meeting the energy and temperature requirements for hydrogen release and the kinetics of hydrogen charging and discharging. In Figure 1, the current status of vehicular hydrogen storage systems is shown in comparison to the gravimetric, volumetric and system cost targets. This figure includes R&D data and modeled projections provided by developers, and will be periodically updated by DOE as more data becomes available. The figure also includes a range of tank data from 63 vehicles validated through DOE’s “Learning Demonstration” project which includes about 70 hydrogen-fueled vehicles and 10 fueling stations operating to date. Based on operation of these vehicles under “real world” conditions, a driving range of 103 to 190 miles has been achieved thus far (assuming an EPA drive cycle). These vehicles had a hydrogen capacity of about 2 to 4.5 kg. It is evident from Figure 1 that none of the current vehicular hydrogen storage systems meet the combined gravimetric, volumetric, and cost targets for either 2010 or 2015.
Figure 1.Status of hydrogen storage systems.
Notes on Figure 1: Costs exclude regeneration/processing. Data based on R&D projections and independent analysis (FY05–FY06). To be periodically updated. *Learning Demo data shows range across 63 vehicles.
Research Challenges
On-board hydrogen storage approaches presently being examined by developers include compressed hydrogen gas, cryogenic gas and liquid hydrogen, metal hydrides, high surface area adsorbents, and chemical hydrogen storage media. Compressed and cryogenic hydrogen, adsorbents and some metal hydrides are categorized as “reversible” on-board because these materials may be refilled/recharged with gaseous hydrogen on-board the vehicle, similar to gasoline refueling today. In contrast, chemical hydrogen storage materials generally require a chemical reaction pathway to be regenerated and so the storage system cannot be directly refueled with hydrogen on-board the vehicle. Such systems are referred to as “off-board regenerable,” requiring the spent media to be removed from the vehicle and then regenerated with hydrogen either at the fueling station or at a centralized processing facility. Chemical hydrogen storage approaches may also serve as a hydrogen delivery carrier by providing an alternative to transporting hydrogen as a gas or cryogenic liquid.
The various hydrogen storage options under consideration have both common and differing issues. For compressed gas and cryogenic tanks, volume and cost are the primary concerns. Cost and thermal management are issues for all material approaches. For chemical hydrogen storage approaches, the cost and energy efficiency for off-board regeneration are key issues. Research is also needed on improving hydrogen discharge kinetics and simplifying the reactor required for discharging hydrogen on-board the vehicle (e.g. the volume, weight, and operation). For metal hydrides, weight, system volume and refueling time are the primary issues. Volumetric capacity and operating temperature are prime issues for adsorbents that are inherently low density materials and have low hydrogen binding energies thus requiring cryogenic temperature. Finally, for all materials approaches, transient performance and its control for hydrogen discharge is relatively unexplored from a “full” tank to the “nearly depleted” tank.
Summary
The DOE hydrogen storage system targets were developed by translating vehicle performance requirements into storage system needs. The targets are not based upon a particular method or technology for storing hydrogen but for commercially acceptable system performance; they must be attained simultaneously. High pressure or cryogenic hydrogen storage systems are already available in prototype vehicles. DOE-sponsored research is being directed at materials-based approaches with the potential to meet the long-term targets. Details on the DOE’s “National Hydrogen Storage Project” and progress made by DOE-funded researchers are available in references 2 through 4. In addition to gravimetric capacity, the R&D community should focus on understanding the volumetric capacity, thermodynamics, kinetics and potential durability/cyclability of materials. Material and system safety is obviously a foremost requirement. The ultimate goal is for storage systems to be integrated with PEM fuel cell power plants, utilizing available waste heat as effectively as possible. Finally, the performance of the storage system must be comparable from a “full tank” to when the owner and vehicle arrive at the filling station with a nearly empty tank. While many promising new approaches have been developed in the last two years, technical challenges remain in order to achieve the 2010 and eventually the 2015 system targets.
References
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