Although fuel cells are receiving a great deal of developmental attention

Although fuel cells are receiving a great deal of developmental attention, a major impediment to their widespread adoption is that they use hydrogen as a fuel and the low density of hydrogen makes it difficult to store and transport. Consequently, alternative ways of providing storing or providing a hydrogen source are being investigated. One alternative is to use liquid ammonia. Liquid anhydrous ammonia has a relatively high heating value, and products of combustion, in at least hydrogen transferring fuel cells, are only water and elemental nitrogen. Ammonia of course has several major drawbacks, but it is worth investigating. The molar mass of ammonia is MW = 17.03. The enthalpy of formation is h¯ f orm = −45,937 kJ/kmol and the absolute entropy at 25◦C is ¯s = 192.8 kJ/K-kmol.

a) What are the lower and higher heating values of liquid ammonia, in units of kJ/kg?

b) What pressure would the ammonia need to be maintained at in the fuel tank? Provide a short description of how you arrive at this answer.

c) What is the ratio of the volume of the ammonia tank to volume of a gasoline tank that is necessary to provide the same theoretical driving distance?


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