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Aluminium extrusions make gas storage easier

Connect, no.63, January 1995, p.1

Natural gas is finding increasing use worldwide as an alternative fuel for motor vehicles

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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 smaller vehicles. 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 tanks which 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 aluminium alloy. Despite the complexity of the design, extrusion was successfully achieved by Alcan Speciality Extrusions Ltd in Workington.

Ten-cell extrusion for tank
Ten-cell extrusion for tank
Inserting carbon briquettes
Inserting carbon briquettes
Fitting individual end caps
Fitting individual end caps
Final welded assembly
Final welded assembly

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, four joint 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 for the 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 outcome paves the way for further work to refine and improve the design and performance of the multicell tank, and carry out vehicle trials.

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