Electric vehicle
Electric Vehicle Technology Explained, Second EditionTable 6.2
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| 126 Electric Vehicle Technology Explained, Second Edition Table 6.2 Comparative data for two cylinders used to store hydrogen at high pressure. The first is a conventional steel cylinder, the second a larger composite tank for use on a hydrogen-powered bus l steel l composite bar bar Mass of empty cylinder (kg Mass of hydrogen stored (kg Storage efficiency (% mass H) (%) 1.2 3.1 Specific energy (kWh kg Volume of tank (approximately lii ml (0 .22 m 3 ) Mass of H 2 per litre (kg l gives it good burst behaviour, in that it rips apart rather than disintegrating into many pieces. The burst pressure is 1200 bar, though the maximum pressure used is 300 bar. The larger scale storage system is, as expected, a great deal more efficient. However, this is slightly misleading. These large tanks have to beheld in the vehicle, and the weight needed to do this should betaken into account. In the bus described by Zieger (which used hydrogen to drive an IC engine, 13 of these tanks were mounted in the roof space. The total mass of the tanks and the bus structure reinforcements is 2550 kg or kg per tank. This brings down the storage efficiency’ of the system to 1.6%, not so very different from the steel cylinder. Another point is that in both systems we have ignored the weight of the connecting valves and of any pressure-reducing regulators. For the 2 l steel cylinder system this would typically add about 2.15 kg to the mass of the system, and reduce the storage efficiency to 0.7% (Kahrom, The reason for the low mass of hydrogen stored, even at such very high pressures, is of course its low density. The density of hydrogen gas at normal temperature and pressure is 0 .084 kg m, compared with air, which is about 1 .2 kg mi Usually less than 2% of the storage system mass is actually hydrogen itself . The metal that the pressure vessel is made from needs very careful selection. Hydrogen is a very small molecule, of high velocity, and so it is capable of diffusing into materials that are impermeable to other gases. This is compounded by the fact that a very small fraction of the hydrogen gas molecules may dissociate on the surface of the material. Diffusion of atomic hydrogen into the material may then occur, which can affect the mechanical performance of materials in many ways. Gaseous hydrogen can buildup in internal blisters in the material, which can lead to crack promotion (hydrogen-induced cracking. In carbonaceous metals such as steel the hydrogen can react with carbon forming entrapped CH 4 bubbles. The gas pressure in the internal voids can generate an internal stress high enough to fissure, crack or blister the steel. The phenomenon is well known and is termed hydrogen embrittlement. Certain chromium-rich steels and Cr–Mo alloys have been found that are resistant to hydrogen embrittlement. Composite reinforced plastic materials are also used for larger tanks, as has been outlined above. As well as the problem of very high mass, there are considerable safety problems associated with storing hydrogen at high pressure. A leak from such a cylinder would generate very large forces as the gas is propelled out. It is possible for such cylinders to become Hydrogen as a Fuel – Its Production and Storage 127 essentially jet-propelled torpedoes, and to inflict considerable damage. Furthermore, vessel fracture would most likely be accompanied by autoignition of the released hydrogen and air mixture, with an ensuing fire lasting until the contents of the ruptured or accidentally opened vessel are consumed (Hord, 1978). Nevertheless, this method is widely and safely used, provided the safety problems, especially those associated with the high pressure, are avoided by correctly following the due procedures. In vehicles, for example, pressure- relief valves or rupture discs are fitted which will safely vent gas in the event of are for instance. Similarly, pressure regulators attached to hydrogen cylinders are fitted with flame-traps to prevent ignition of the hydrogen. The main advantages of storing hydrogen as a compressed gas are simplicity indefinite storage time no purity limits on the hydrogen. Designs for very high-pressure cylinders can be incorporated into vehicles of all types. In the fuel cell bus of Figure 1.17 they are in the roof. Figure 6.4 shows the design of a modern very high-pressure hydrogen storage system by General Motors, and its location in the fuel-cell-powered vehicle can be seen in the picture in the background. 6.5.4 Storage of Hydrogen as a Liquid The storage of hydrogen as a liquid (commonly called LH 2 ), at about 22 K, is currently the only widely used method of storing large quantities of hydrogen. A gas cooled to the liquid state in this way is known as a cryogenic liquid . Large quantities of cryogenic hydrogen are currently used in processes such as petroleum refining and ammonia production. Another notable user is NASA, which has huge 3200 m 000 US gal) tanks to ensure a continuous supply for the space programme. Download 3.49 Mb. Share with your friends:
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