BEHAVIOR AND PERFORMANCE OF ADSORPTIVE NATURAL-GAS STORAGE CYLINDERS DURING DISCHARGE

Adsorbed natural gas (ANG) has the potential to replace compressed nat ural gas in mobile storage applications, such as in vehicles. Although a substantial research effort has been devoted to ANG, very few studi es evaluate the impact of heat of adsorption on system performance. Th is paper concentrates on the impact of heat of adsorption on ANG perfo rmance during discharge, while the gas outflow rate is dictated by the energy demand of the application. The temperature drop and performanc e loss was measured with commercially available ANG cylinders under re alistic conditions. Data show as high as a 37 degrees C temperature dr op at high discharge rate, with a performance loss approaching 25% of isothermal capacity. The performance loss is expected to be 15-20% at moderate discharge rates. Analysis of data and predictions of a simple model indicate that the ANG system is neither adiabatic nor isotherma l during discharge; the thermal capacity of the vessel wall and extern al heat transfer conditions have a significant effect on system behavi or. The poor thermal conductivity of packed adsorbent is a major obsta cle for the utilization of these energy sources. Changing the flow dir ection during discharge from axial to radial by a perforated tube inse rted at the center of the cylinder significantly reduces the performan ce loss by increasing the heat transfer from the wall to the central r egion. At intermediate discharge rates, where the inserted tube has th e largest impact, the performance loss is reduced to 12% from 22% with out the tube under identical conditions.