Experimental and Numerical Solutions of Transient Thermal Problems in Granular Media Used for Storing Hydrogen in Solid State

Abstract

Hydrogen is an attractive and very e cient fuel as well as an excellent medium for energy storage. According to energy researchers, hydrogen could be one of the possible primary energy carriers (fuel) when oil is exhausted. As a result of diminishing fossil fuel (petroleum, natural gas and coal) supplies, hydrogen technology can be practically advantageous and it can be universally employed for household, industrial and automotive purposes in a similar way to petroleum technology. Hydrogen production and conversion are already technologically feasible, but its delivery and storage face serious challenges. Hydrogen has been traditionally stored, transported and used as compressed gas or cryogenic liquid. The problems encountered with gas cylinder storage are the high pressure, large volume, weight and safety risks; whereas with liquid hydrogen, the factors are the high consumption of primary energy, extremely low temperatures, high evaporation rates, and safety risks. But, there is an alternative, namely rechargeable metal hydrides. The reversible reaction of hydrogen with metals, alloys or compositions is considered as a convenient way for storing hydrogen. Besides these advantages, unit volume of a metal hydride holds more hydrogen than gaseous/liquid hydrogen. Metal hydrides, which store hydrogen safely at densities higher than liquid or solid hydrogen, represent the most potentially attractive hydrogen energy system. Magnesium and magnesium based alloys/compositions are looked upon as promising for hydrogen storage due to their high theoretical storage capacity (7.66 wt %), light weight and low cost. With hydrogen, magnesium can form a hydride MgH2 with nominal 7.66 wt% of hydrogen. However, high operating temperature and slow kinetics prevent them from being employed in practical applications. Magnesium hydride is a promising approach for stationary power system application, due to high hydrogen storage capacity by weight. The fast absorption/desorption kinetics of the Mg based metal hydride with high hydrogen storage capacity has been investigated at ERDA. Magnesium hydride based reactor design is more complex due to high heat of formation during hydriding/ dehydriding reaction. This work presents a state-of-the-art report on metal hydride based absorptiondesorption process. Modeling and simulation of the hydriding reactor is carried out for 15 gm capacity of magnesium based alloy/composition. Temperature pro le in reactor for capacity of 15 gm is computed by FEM analysis using ANSYS software for hydriding and dehydriding process. FEM analysis for this system indicates that during hydriding process, temperature will increase from 200 oC to 242 oC in the metal hydride region within 20 minutes. On the outer surface of the reactor maximum temperature rise ( T) is 6 oC. For dehydriding process, temperature will decrease from 350 oC to 304 oC in the metal hydride region in 20 minutes. Outer surface of the reactor maximum temperature drop ( T) is 29 oC. From the experimental results, during hydriding process, maximum temperature rise in the outer surface of the reactor is 3.6 oC and during dehydriding process, maximum temperature drop in the outer surface of the reactor is 34.5 oC. Experimental results are validated with FEM analysis using ANSYS software for hydriding process and dehydriding process. Design of the reactor is presented having a capacity of 1.5 kg of metal hydride. Modeling and simulation of the hydriding reactor is also presented for 1.5 kg capacity of Magnesium based alloy/composition. Temperature pro le in reactor is computed by FEM analysis using ANSYS software for hydriding and dehydriding reactions. FEM analysis indicates that the temperature is rise from 200 oC to 445 oC in the rst 20 minutes during hydriding process. At 350 oC metal hydride will start discharging. Hence, the cooling system is required for proper hydriding process. Air ow cooling design of 3.75 kW capacity is proposed for e ective hydriding process. During the dehydriding process, maximum temperature drop occurs from 350 oC to 189 oC in 20 minutes. Metal hydride requires minimum 350 oC temperature for desorbing hydrogen from the metal hydride. Therefore, 2 kW additional heat is required for continuous hydrogen ow. In this work, using FEM tool requirement of capacity of heating and cooling system was predicted.