If solar energy is to become a major contributor to our energy supply, means to store it have to be found. A high energy storage density, by volume and mass. The reactions are easy to control and to reverse and there are no unwanted side reactions.
All constituents involved are environmentally benign.
There exists a history of industrial application with the associated available expertise and hardware. All reactants for transport and handling are in the fluid phase, which provides a convenient means of energy transport without thermal loss. This is an important point, particularly if large arrays of paraboloidal dishes are being considered as the method for solar energy collection. At ambient temperature the ammonia component of reactant mixtures condenses to form a liquid, whilst the nitrogen and hydrogen remains as a gas. This means that only one storage vessel is required for reactants and products.