Heat transfer characterization of cryoadsorbents for hydrogen storage
Abstract
New materials are being considered to facilitate the transportation of energy in the form of stored hydrogen. Some of these storage materials include metal hydrides, chemical hydrides, activated carbons (ACs) and metal organic frameworks (MOFs). The present study focuses primarily on the hydrogen adsorption characteristics of various AC and MOF materials from theoretical and experimental approaches. This work also includes a brief comparison of and contrast between the chemisorption and physisorption processes, both of which could be present in hydrogen storage although one process is more dominant than the other depending on the material being considered. Numerical models are determined and explored using the parameters and characteristics found experimentally. General MOF characteristics including structure, density, and heat of adsorption are presented. A theoretical model for the adsorption equilibrium is necessary for determining the amount and rate of hydrogen adsorbed. Options for the adsorption equation of state including the Langmuir model and the Dubinin-Astakhov equation are discussed. The progression from a simplified, lumped parameter, model to a 1-dimensional transient model is thoroughly explained. The basis for these changes in the models is the relaxation of assumptions such as uniform distribution of temperature, pressure, and heat generation. The experimental setup requirements are discussed with respect to the assumptions made in the modeling portion of the project. Assumptions regarding the boundary and initial conditions from the models are related to the physical system. For example, the system design must include precise temperature control from ambient to cryogenic conditions for both the jacket surrounding the test article as well as hydrogen inlet gas. The results from the parametric study at room temperature conditions indicate that the maximum pressure has the most significant impact on the change in temperature of the cryoadsorbent material while the mass flow rate at room temperature seems to have the least effect. Empty pressure vessel tests were used to determine whether compression heating was the primary source of temperature increase during pressurization. While compression heating is significant, the large change in maximum temperature difference signifies that heat is probably generated from the adsorption process.
Degree
M.S.M.E.
Advisors
Fisher, Purdue University.
Subject Area
Mechanical engineering
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