Design and analysis of heat exchangers for high pressure metal hydride hydrogen storage
Abstract
This study explores the development of a hydrogen storage system using high-pressure metal hydride, Ti1.1CrMn. When absorbing hydrogen (filling), the metal hydride releases large amounts of heat causing the hydride temperature to rise. The reaction rate depends on the metal hydride temperature, decreasing significantly if the heat is not removed quickly. To store 5 kg hydrogen needed to drive 300 miles, about 36 MJ of heat is must be released. For a five minute fill time, this translates to 120 kW of heat generation rate. This is a formidable challenge considering the enormous amount of heat, poor thermal properties of the hydride, and the stringent limits on the heat exchanger’s weight and volume, let alone a host of manufacturing requirements. Additionally, the kinetics of the material is such that the rate of reaction (hydrogen absorption) depends on the ability to quickly dissipate the heat generated. A systematic heat exchanger design methodology is presented here, starting with a 1-D metal hydride layer distance criterion and progressing through a series of engineering decisions supported by computations of fill time. A modular tube-fin design is arrived at with a goal of achieving a fill time of less than 5 min. The prototype heat exchanger comprising of an intricate network of fins surrounding the metal hydride powder occupies 29% of the storage vessel volume. Experiments were performed to study the influence of various parameters on the hydriding reaction and a lowest fill time of 4 min 40s is successfully achieved. Coiled-tube heat exchanger is designed with a primary goal of reducing the volume while still achieving practical fill time. The heat exchanger consists of only a coolant tube strategically coiled around the metal hydride powder. The coiled-tube heat exchanger reduces fill time by 75% while occupying only 7% of the storage pressure vessel volume. Dehydriding tests were performed with each design to investigate the influences of hydrogen release rate, fluid flow rate and fluid temperature on the dehydriding process. Dehydriding reaction rate was accelerated by increasing the fluid temperature and/or the rate of pressure drop. Transient two and three dimensional numerical models are constructed that simulate the process of hydriding and dehydriding in Ti1.1CrMn. Spatial distributions of hydride temperature as a function of time over the entire duration of the hydriding reaction are determined, which agree favorably with the experimental data. The models are shown to be quite accurate at predicting the spatial and temporal variations of metal hydride temperature during both the reaction. Effect of pressure vessel size and parameters like hydriding pressure, packing density and effective thermal conductivity, on the size and volume of the storage system is studied. Operating conditions and metal hydride parameters required to meet the Department of Energy (DOE) fill time targets are presented. Various components of a high-pressure metal hydride system are discussed and the storage efficiencies of an optimized storage system using Ti1.1CrMn is presented.
Degree
Ph.D.
Advisors
Mudawar, Purdue University.
Subject Area
Mechanical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.