In the ammonia-based thermal energy storage system (Fig. 1), liquid ammonia (NH3) is dissociated in an energy storing (endothermic) chemical reactor as it absorbs solar thermal energy. At a later time, the gaseous products hydrogen (H2) and nitrogen (N2) are reacted on demand in an energy releasing (exothermic) reactor to resynthesise ammonia and recover the stored solar energy.
Because the solar energy is stored in a chemical form at ambient temperature, there are no energy losses in the store regardless of the length of time that the reactants remain in storage. The reactors are packed with standard commercial catalyst materials to promote both reactions. Counter-flow heat exchangers transfer heat between in-going and out-going reactants at each reactor to use the energy most effectively. Apart from the ability of the ammonia system to allow for solar energy storage, other advantages, that are not necessarily shared by other solar thermochemical or photochemical systems, make this process unique:
- High energy storage density, by volume and mass.
- The reactions are easy to control and to reverse and there are no unwanted side reactions.
- There exists a history of industrial application with the associated available expertise and hardware.
- A readily achievable turning temperature of 400–500°C, depending on the pressure. This helps to reduce thermal losses from dish receivers, avoids some high temperature materials limitations, and allows lower quality (and hence cheaper) dish optics to be used.
- At ambient temperature the ammonia component of reactant mixtures condenses to form a liquid, whilst the nitrogen and hydrogen remains\ as a gas. Thus, only one storage vessel is required for reactants and products.
- Ammonia re-synthesis could occur at a site some distance from the solar plant, by transporting the ammonia, nitrogen and hydrogen in high-pressure pipelines to and from the plant. In this configuration the pipelines would be forming part of the storage volume.
- The ammonia storage process was originally developed for arrays of large dishes, but could also be implemented on central tower or trough systems.