Beyond Weight Percent - The Influence of Material Characteristics on Hydrogen Storage System Performance
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Beyond Weight Percent - The Influence of Material Characteristics on Hydrogen Storage System Performance Daniel A. Mosher and Donald L. Anton United Technologies Research Center East Hartford, CT 06108 ABSTRACT The attribute of solid state hydrogen storage materials that is most commonly the focus of evaluations is reversible hydrogen weight percent. Other material characteristics, including density, charging pressure, enthalpy and conductivity can influence the weight of storage system components and hence the overall hydrogen weight percent that is ultimately of interest. However, accounting for these effects involves some level of storage system representation that typically is not undertaken when making material assessments and comparisons. The current paper will present a simple model that represents system elements and trade-offs on a high level so that overall system performance can be estimated without the burden of detailed design studies. The model should be useful to evaluate novel materials in a more complete manner for a better assessment of their potential when implemented in a storage system. While the model has been derived based on the design of a particular NaAlH4 system, the key attributes are sufficiently general to be applicable to a range of system designs. Using this approach, the properties of materials can be related more precisely to goals for overall system performance with modest additional effort.
INTRODUCTION Often there is a chasm between the development of novel hydrogen storage materials and the estimation of the overall system performance using these materials. The reasons for this are varied including the different technical nature of the two areas, lack of material data and a wide range of possible system designs. To facilitate such preliminary material evaluations, the present work is an initial attempt to develop a model that includes important material properties and yet is generic to the system design and easy to implement. The approach developed draws upon the modeling used in the design of a NaAlH4 based storage system and includes material/system trade-offs for: 1) pressure and temperature dependence of capacity, 2) hydride density, 3) reaction enthalpy and 4) hydride thermal conductivity. The model will focus on the gravimetric performance during refueling of an in-situ rechargeable system, since this is the most demanding operation step for producing the pressure and temperature conditions that are desirable from the material’s perspective. To simplify model implementation, reaction kinetics will not be tracked in detail over time, but rather will be reduced to the hydrogen weight fraction at a specific time, which will be termed the effective capacity. The heat transfer equations will be based on steady state conduction, which will be a reasonable approximation for high capacity materials over most of the absorption reaction. To develop aspects of the model and determine ranges of constant values, Finite Element Analysis (FEA) was applied for certain heat excha
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