INTEGRATED HYDROGEN STORAGE SYSTEM MODEL
Hydrogen storage is recognized as a key technical hurdle that must be overcome for the realization of hydrogen powered vehicles. Metal hydrides and their doped variants have shown great promise as a storage material and significant advances have been made with this technology. In any practical storage system the rate of H2 uptake will be governed by all processes that affect the rate of mass transport through the bed and into the particles. These coupled processes include heat and mass transfer as well as chemical kinetics and equilibrium. However, with few exceptions, studies of metal hydrides have focused primarily on fundamental properties associated with hydrogen storage capacity and kinetics. A full understanding of the complex interplay of physical processes that occur during the charging and discharging of a practical storage system requires models that integrate the salient phenomena. For example, in the case of sodium alanate, the size of NaAlH4 crystals is on the order of 300nm and the size of polycrystalline particles may be approximately 10 times larger ({approx}3,000nm). For the bed volume to be as small as possible, it is necessary to densely pack the hydride particles. Even so, in packed beds composed of NaAlH{sub 4} particles alone, it has been observed that the void fraction is still approximately 50-60%. Because of the large void fraction and particle to particle thermal contact resistance, the thermal conductivity of the hydride is very low, on the order of 0.2 W/m-{sup o}C, Gross, Majzoub, Thomas and Sandrock [2002]. The chemical reaction for hydrogen loading is exothermic. Based on the data in Gross [2003], on the order of 10{sup 8}J of heat of is released for the uptake of 5 kg of H{sub 2}2 and complete conversion of NaH to NaAlH{sub 4}. Since the hydride reaction transitions from hydrogen loading to discharge at elevated temperatures, it is essential to control the temperature of the bed. However, the low thermal conductivity of the hydride makes it difficult to remove the heat of reaction, especially in the relatively short target refueling times, see Attachment 3. This document describes a detailed numerical model for general metal hydride beds that couples reaction kinetics with heat and mass transfer, for both hydriding and dehydriding of the bed. The detailed model is part of a comprehensive methodology for the design, evaluation and modification of hydrogen storage systems. In Hardy [2007], scoping models for reaction kinetics, bed geometry and heat removal parameters are discussed. The scoping models are used to perform a quick assessment of storage systems and identify those which have the potential to meet DOE performance targets. The operational characteristics of successful candidate systems are then evaluated with the more detailed models discussed in this document. The detailed analysis for hydrogen storage systems is modeled in either 2 or 3-dimensions, via the general purpose finite element solver COMSOL Multiphysics{reg_sign}. The two-dimensional model serves to provide rapid evaluation of bed configurations and physical processes, while the three-dimensional model, which requires a much longer run time, is used to investigate detailed effects that do not readily lend themselves to two-dimensional representations. The model is general and can be adapted to any geometry or storage media. In this document, the model is applied to a modified cylindrical shell and tube geometry with radial fins perpendicular to the axis, see Figures 4.1-1 and 4.1-2. Sodium alanate, NaAlH{sub 4}, is used as the hydrogen storage medium. The model can be run on any DOS, LINUX or Unix based system.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-96SR18500
- OSTI ID:
- 921542
- Report Number(s):
- WSRC-TR-2007-00440; TRN: US200806%%109
- Country of Publication:
- United States
- Language:
- English
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HIERARCHICAL METHODOLOGY FOR MODELING HYDROGEN STORAGE SYSTEMS PART II: DETAILED MODELS