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Magnesium-nickel Hydride. The magnesium-nickel hydrides contain 

 the largest percentage by weight of hydrogen of any hydride so far 

 studied. An alloy of 94Z oagnesium and 52 nickel can hold as much as 

 6.97 percent by weight of hydrogen when in its hydride fona. This 

 hydride was first investigated in the late 1960*s by Drs. J. J. Reilly 

 and R. H. Wiswall, Jr. [32], who have reported that the kinetics of this 

 hydride are satisfactory and inprove with temperature. As in most 

 cases, though, there is no reported data cz. the rates of absorption and 

 generation. 



This hydride has several good points and several drawbacks; the 

 drawbacks do not appear insuperable, however. The benefits are that a 

 large aaount of hydrogen can be stored at low pressure, at low tempera- 

 ture, with a low weight (compared to all other known intersetallic 

 hydrides) and with a relatively low volusse. Table 3 shows that 21.2 

 pounds of hydrogen, which can be burned to produce the sase amount of 

 energy as 10 gallons of gasoline, can be stored in 317 pounds and 3.17 

 cubic feet of taagnesiua-n J Jtel hydride. It can also be seen that this 

 volume is less than the volume required to store liquid hydrogen, 4.8 

 cubic feet. In actuality, the volume required in the hydride system 

 would be greater due to spaces left between the granules of the hydride 

 and due to the spaces required In the heat exchanger. These spaces 

 determine the void fraction. However, even if the actual volume were 

 twice as large, it would only be a little greater than liquid hydrogen. 



The aajor drawback of this hydride is that a temperature of about 

 600 F must be maintained to release the hydrogen from trie hydride and to 

 maintain a satisfactory rate of absorption and generation. This mediua 

 temperature heat is frequently available in combustion engines and 

 combustion exhausts, but there generally is not enough of this high 

 quality energy. One possible solution is to burn a part of the hydrogen 

 produced to heat the exhaust products and thus to provide the necessary 

 extra energy. Another possibility is to design for a higher exhaust 

 temperature. Though these methods may at first appear to be intensive 

 energy users, a further look will reveal that metal-hydride systems 

 allow the energy available in hydrogen to be more completely used. In a 

 pressurized hydrogen storage system the hydrogen can be burned in an 

 engine, but the exhaust products are released to the atmosphere, carry- 

 ing with them a great deal of usable energy. A magnesiun-nickel-hydride 

 system, on the other hand, will produce hydrogen which can similarly be 

 burned but the exhaust products can then be reheated slightly and passed 

 through the hydride heat exchanger. Thus, some of the excess energy 

 normally lost can be used to release more hydrogen. When the hydrogen 

 is replenished this heat is given off by the hydride and can be used to 

 provide the energy for an absorption cooling system or for heating 

 systems. 



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