Hydrogen storage in glass capillary arrays for portable and mobile systems

A crucial problem of new hydrogen technologies is the lightweight and also safe storage of acceptable amounts of hydrogen for portable or mobile applications. A new and innovative technology based on capillary arrays has been developed. These systems ensure safe infusion, storage, and controlled release of hydrogen gas, although storage pressures up to 1200 bar are applied. This technology enables the storage of a significantly greater amount of hydrogen than other approaches. In storage tests with first capillary arrays a gravimetric storage capacity of about 33% and a volumetric capacity of 28% was determined at a comparative low pressure of only 400 bar. This is much more than the actual published storage capacities which are to find for other storage systems. This result already surpassed the US Department of Energy's 2010 target, and it is expected to meet the DOE's 2015 target in the near future. Different safety aspects have been evaluated. On the one hand experiments with single capillaries or arrays of them have been carried out. The capillaries are made of quartz and other glasses. Especially quartz has a three times higher strength than steel. At the same time the density is about three times lower which means that much less material is necessary to reach the same pressure resistance. The pressure resistance of single capillaries has been determined in dependence of capillary materials and dimensions, wall thickness etc. in order to find out optimal parameters for the “final” capillaries. In these tests also the sudden release of hydrogen was tested in order to observe possible spontaneous ignitions. On the other hand a theoretical evaluation of explosion hazards was done. Different situations were analyzed e.g. release of hydrogen by diffusion or sudden rupture. 1.0 INTRODUCTION A crucial problem of new hydrogen technologies is the lightweight and also safe storage of acceptable amounts of hydrogen for portable or mobile applications. A new and innovative technology based on capillary arrays has been developed for safe infusion, storage and controlled release of hydrogen gas [1]. Using single glass capillaries it is possible to store hydrogen as pressurized gas up to pressures of 1200 bar. The capillaries are molten at one end, the other one will be closed in a specialized storing procedure with an alloy. For the hydrogen release this end of capillaries has to be heated up to the melting point of the alloy. This principle is not only valid for single capillaries. It is also possible to bundled hundreds of them to capillary arrays. There is no limitation for the number of capillaries or their length and therefore every dimension of those arrays is possible. This enables one to produce such hydrogen storage systems for any kind of portable or mobile application. In preliminary tests it was proven that the storage of great amounts of hydrogen is possible. A gravimetric storage capacity of about 33% and a volumetric capacity of 28% was determined at comparative low pressures of only 400 bar [2,3]. This is much more than the actual published storage capacities which are to find for other storage systems. This result already surpassed the US Department of Energy's 2010 target. It is expected to meet the DOE's 2015 target in the near future. First calculations showed that gravimetric storage capacities of about 50 % are possible at pressures of about 1000 bar. The great advantage of glass capillaries especially those made from quartz is the higher strength compared to steel while having a lower density. Each capillary can be treated as a single pressure resistant vessel. Compared to steel vessels only a low wall thickness is necessary to reach the same pressures resistance. That means less material is needed and a lightweight storage system results. Especially for mobile and portable applications the safety aspects are of great interest. No hazardous situations should occur due to unwanted release of hydrogen. Such release can be a result of diffusion processes or mechanical impacts. Furthermore strong heat impacts are critical for each storage system as it is possible for example in a fire. All these situations have been examined or theoretically evaluated for this new trendsetting storage system. 2.0 STORAGE PRINCIPLE Single glass capillaries or already bundled capillaries (arrays) are installed in a larger pressure resistant vessel made from steel. The set-up is pressure resistant up to pressures of more than 1500 bar. The glass capillaries are closed by melting at one end, the other one will be closed with an alloy after adding hydrogen to the steel vessel. In a first step the set-up is evacuated. Afterwards hydrogen is filled into the vessel till the storage pressure is reached. This storage pressure is a flexible parameter and can be changed. Applying heat to one area of the set-up will melt the alloy which is closing the open end of the capillaries. In a specialized procedure the alloy will be slightly pressed into the capillaries so that after cooling the alloy respectively its solidification hydrogen is stored in the capillaries. The hydrogen release is realized by carrying out the storage procedure vice versa. Heat is applied to the end of capillaries which is closed with the alloy. It will melt again and due to the high pressure in the capillaries hydrogen is released. Pressure control will be realized on the one hand by opening of specified areas of the arrays respectively by volume specification. 3.0 PRESSURE RESISTANCE In the literature many properties for glasses are to find [4,5,6]. But no property is valid to describe sufficiently the resistance of glasses when there is a pressure inside of small structures. Therefore basic examinations have been carried out in order to determine the pressure resistance of round capillaries made from different types of glass. The outer respectively inner diameter and therefore the wall thickness are varied as well as the length of the capillaries (Table 1). Table 1. Glass capillaries used for pressure resistance tests. No. Material outer diameter [m] inner diameter [m] wall thickness [m] length [mm] 1 soda 400 300 50 100 2 soda 340 300 20 100 3 borosilicate 400 360 20 100 4 borosilicate 340 300 20 100 5 borosilicate 400 360 20 200 6 borosilicate 340 300 20 200 7 alumosilicate 400 300 50 100 8 alumosilicate 340 300 20 100 9 alumosilicate 400 300 50 200 10 alumosilicate 340 300 20 200 11 quartz 400 300 50 100 12 quartz 340 300 20 100 13 quartz 400 300 50 200 14 quartz 340 300 20 200