Electric vehicle
Electric Vehicle Technology Explained, Second EditionTable 6.5
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| 132 Electric Vehicle Technology Explained, Second Edition Table 6.5 Data for comparing methods of storing hydrogen fuel Method Gravimetric storage Volumetric mass Energy efficiency of efficiency, % mass (in kg) of hydrogen storage (energy in hydrogen per litre stored H 2 )/(energy in H 2 + energy to store H 2 ) Pressurised gas 0.015 0.9 at 200 bar Reversible metal hydride 0.65 Cryogenic liquid 0.040 0.75 for large systems now. Obviously these figures cannot be used in isolation – they do not include cost, for example. Safety aspects do not appear in this table either. The cryogenic storage method has the best figures. The storage efficiency of compressing hydrogen to 200 bar is at best around The efficiency of liquefying hydrogen is typically 45% for small systems and 75% for larger systems. 6.6 Hydrogen Storage II – Chemical Methods 6.6.1 Introduction None of the methods for storing hydrogen outlined in Section 6.5 is entirely satisfactory. Other approaches that are being developed rely on the use of chemical hydrogen carriers’. These could also be described as ‘human-made fuels. There are many compounds that can be manufactured that hold, for their mass, quite large quantities of hydrogen. To be useful these compounds must pass three tests. It must be possible to make these compounds give up their hydrogen very easily, otherwise there is no advantage overusing a reformed fuel in one of the ways already outlined in Section 6.2. 2. The manufacturing process must be simple and use little energy – in other words, the energy and financial costs of putting the hydrogen into the compound must below. They must be safe to handle. A large number of chemicals that show promise have been suggested or tried. Some of these, together with their key properties, are listed in Table 6.6. Some of them do not warrant a great deal of consideration, as they easily fail one or more of the three tests above. Hydrazine is a good example. It passes the first test very well, and it has been used in demonstration fuel cells with some success. However, hydrazine is both highly toxic and very energy intensive to manufacture, and so fails the second and third tests. Nevertheless, several of the compounds of Table 6.6 are being considered for practical applications, and will be described in more detail here. |