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The pressure vessels needed to store hydrogen for next-generation hydrogen fuel cell vehicles are expected to be a substantial portion of the total system mass, volume, and cost. Gravimetric capacity, volumetric capacity, and cost per kilogram of usable hydrogen are key performance metrics that the U.S. Department of Energy (DOE) uses to determine the viability of hydrogen fuel cell systems. Research and development related to hydrogen storage systems covers a wide range of potential operating conditions, from cryogenic temperatures to high temperatures (above ambient) and low pressure to high pressure. Researchers at PNNL have developed methods for estimating these key pressure vessel characteristics to support on-board hydrogen storage system design and performance evaluation and to support decision-making about DOE hydrogen storage system research investments. This article describes the pressure tank estimation methodology that has been used as a stand-alone calculation and has been incorporated into larger system evaluation tools. The methodology estimates the geometry, mass, and material cost of type I, type III, and type IV pressure vessels based on operating pressure and material strength at the system's operating temperature, using classical thin-wall and thick-wall pressure vessel stress calculations. The geometry, mass, and material cost requirements of the pressure vessel have significant impacts on the total system performance. For example, hydrogen storage materials that can separately achieve a very high hydrogen density can be deemed impractical for use in fuel cell vehicle hydrogen storage systems because the pressure tank containing them is too large, heavy, or expensive. This article describes the design philosophy and analytical process of the tank characteristic estimation methodology, which has been implemented in spreadsheet calculation tools and system-level analysis tools used by DOE researchers. Each of the three tank types (type I, type III, and type IV) uses a different analysis methodology with some common elements. This article also provides examples of implementing the methodology to perform parametric studies of all three pressure vessel types. The goal of this article is to present the methodology in sufficient detail so it can be implemented in other hydrogen fuel cell vehicle design and analysis tools.

Formic acid (53 g H2/L) is a promising liquid storage and delivery option for hydrogen for fuel cell power applications. In this work we compare and evaluate several process options using formic acid for energy storage. Each process requires different steps, which contribute to the overall energy demand. The first step, i.e. production of formic acid, is thermodynamically unfavorable. However, the energy demand can be reduced if a formate salt is produced via a bicarbonate route instead of forming the free acid from hydrogen and carbon dioxide. This bicarbonate/formateapproach also turns out to be comparatively more efficient in terms of hydrogen release than the formic acid route even though less energy efficient, catalytic decomposition of formic acid has the advantage of reaching higher volumetric power densities during hydrogen release. Efficiencies of all process options involve aqueous media and are dependent on concentration. Heating water leads to additional energy demand for hydrogen release and thus lowers the overall efficiency. Separation and purification of hydrogen contribute a minor impact to the overall energy demand. However, its effect on efficiency is not negligible. Other process options like thermal decomposition of formic acid or direct formic acid fuel cells thus far do not appear competitive.

Hydrogen fueling stations require multiple stages of compression to achieve the pressure needed to refuel hydrogen fuel cell electric vehicles at 700 bar. The physical compression equipment constitutes a large share of the total investment cost of hydrogen fueling stations. Hydrogen carriers, i.e., materials that carry either physisorbed or chemisorbed H2, provide an alternate approach to transport and deliver higher densities of hydrogen to the fueling station at lower pressures. Additionally, some liquid phase hydrogen carriers (LPHCs) are defined by thermodynamic properties that allow H2release at elevated pressure, thus providing an opportunity to reduce the number of compressors at the fueling station. This study compares a series of LPHCs and evaluates the approach of using aqueous solutions of formic acid (FA) to deliver high volumetric densities of H2to fueling stations and provide a first step of compression. While hydrogen release from most liquid carriers will provide hydrogen slightly above ambient pressure at high temperatures, hydrogen release from the decomposition of FA can provide hydrogen at pressures of several hundred bars at moderate temperatures. A challenge of formic acid is that the high pressure hydrogen is accompanied by 1 equivalent of carbon dioxide and thus requires subsequent separation and purification operations. Nevertheless, formic acid has the advantage of being liquid, which simplifies its handling and provides a continuous supply to a release unit. Furthermore, the energy demand for hydrogen release from FA is lower than for most alternative hydrogen carrier materials.

Research on ammonia borane (AB, NH3BH3) has shown it to be a promising material for chemical hydrogen storage in PEM fuel cell applications. AB was selected by DOE's Hydrogen Storage Engineering Center of Excellence (HSECoE) as the initial chemical hydride of study because of its high hydrogen storage capacity (up to 19.6% by weight for the release of three molar equivalents of hydrogen gas) and its stability under typical ambient conditions. A model of a bead reactor system which includes feed and product tanks, hot and cold augers, a ballast tank/reactor, a H2 burner and a radiator was developed to study AB system performance in an automotive application and estimate the energy, mass, and volume requirements for this off-board regenerable hydrogen storage material. Preliminary system simulation results for a start-up case and for a transient drive cycle indicate appropriate trends in the reactor system dynamics. A new controller was developed and validated in simulation for a couple of H2 demand cases.

Hydrogen storage is a key technology for enabling the decarbonization of future energy systems, but is subject to low volumetric density in the case of compressed hydrogen. Chemical conversion to a liquid hydrogen carrier can overcome this challenging issue. For example, the reversible reaction of 1,4-butanediol (BDO) to γ-butyrolactone releasing two moles of hydrogen could be an attractive option for storing and transporting hydrogen. Herein, the thermodynamics of both, the reaction and the process, are evaluated. An important question that should be answered is whether there is a difference in energy efficiency between this reaction being performed in the liquid and in the gas phase. The thermodynamic analysis showed that heat demand for evaporation from the liquid to the gas phase is a major drawback of gas-phase reactions even though the enthalpy of reaction is somewhat smaller in the gas than in the liquid phase. However, due to the vapor pressure of BDO, there is some evaporation occurring in the case of liquid-phase reactions at reaction temperatures required to release hydrogen. This evaporation reduces the energy benefits of the liquid-phase reaction. As a consequence, the round-trip efficiencies of both options (liquid- and gas-phase reaction) are similar (varying by <5%). This is an advantageous finding as it allows selection of reaction conditions (liquid- or gas-phase reaction) solely based on catalytic considerations, without major restrictions due to concerns from process engineering.

A fixed bed reactor was designed, modeled and simulated for hydrogen storage on-board the vehicle for PEM fuel cell applications. Ammonia borane was selected by DOE’s Hydrogen Storage Engineering Center of Excellence as the initial chemical hydride of study because of its high hydrogen storage capacity (up to ~16% by weight for the release of ~2.5 molar equivalents of hydrogen gas) and its stability under typical ambient conditions. The design evaluated consisted of a tank with eight thermally isolated sections in which H2 flows freely between sections to provide ballast. Heating elements are used to initiate reactions in each section when pressure drops below a specified level in the tank. Reactor models in Excel and COMSOL were developed to demonstrate the proof-of-concept, which was then used to develop systems models in Matlab/Simulink. Experiments and drive cycle simulations showed that the storage system meets thirteen 2010 DOE targets in entirety and the remaining four at greater than 60% of the target.