Connect, no.63, January 1995, p.1
Natural gas is finding increasing use worldwide as an alternative fuel for motor vehicles
British Gas plc has been working with TWI to develop an alternative gas storage system for natural gas vehicles (NGVs).
Conventional natural gas vehicles run on compressed natural gas (CNG) stored in cylinders at a pressure of around 200bar. The amount of useful space occupied by these cylinders is a distinct drawback - particularly in smallervehicles. British Gas has produced a new system where gas is adsorbed within a microporous carbon matrix, providing 2/ 3 the storage capacity of CNG at only 1/ 6 the storage pressure.
Reduced storage pressure provides the opportunity to design and build specially shaped non-cylindrical storage tanks giving greater packaging flexibility to onboard storage systems. TWI designed and manufactured experimental tankswhich were rectangular rather than cylindrical in section - a 'briefcase' size and shape of tank was produced.
To achieve the desired shape with adequate pressure-containment capability, a cellular tank was considered essential. This led to the idea of forming the complete body of the tank from a single multicell extrusion in aluminiumalloy. Despite the complexity of the design, extrusion was successfully achieved by Alcan Speciality Extrusions Ltd in Workington.
Various end closure designs and joining options were considered. The project concentrated on individual end caps for each cell, attached by welding. Initial trials were carried out using simple cylinders with circular end caps, fourjoint designs and four welding processes. Sixteen closed cylinders were manufactured and subjected to pressure fatigue and burst tests, and on the basis of the results obtained a single end cap design and welding process was chosen forthe ten-cell tanks.
Four ten-cell tanks were manufactured and tested: a burst strength of around three times the design pressure of 41 bar and a fatigue life of 13 times the design life of 40 000 full pressure cycles were achieved.
The project demonstrated that the concept of a cellular extruded aluminium tank with welded end closures can be realised in practice with the required levels of pressure containment and fatigue performance. The successful outcomepaves the way for further work to refine and improve the design and performance of the multicell tank, and carry out vehicle trials.
