If solar energy is to become a major contributor to our energy supply, means to store it have to be found. One promising method is “closed loop thermochemical energy storage using ammonia”.

In this system, ammonia (NH3) is dissociated in an energy storing (endothermic) chemical reactor as it absorbs solar thermal energy. At a later time and place, the reaction products hydrogen (H2) and nitrogen (N2) react in an energy releasing (exothermic) reactor to resynthesise ammonia.

2 NH₃< + head N₂ + 3 H₂

A fixed amount of reactants (ammonia, nitrogen and hydrogen) are contained in a closed loop, and pass alternately between energy storing and energy releasing reactors with provision for storage of reactants in between. Because the solar energy is stored in a chemical form at ambient temperature, there are zero 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.

Feeding the reactors with pure reactants is possible through the natural separation of reactants and products in the storage system: at the pressures applied, ammonia condenses.

By ensuring that the stuff leaving each reactor transfers its own thermal energy (sensible heat) to the stuff going in – using heat exchangers – most of the solar energy is stored in the change in composition of the chemicals which are kept at ambient temperature.

Advantages

Apart from the ability of the ammonia system to allow for continuous energy supply on a 24-hour basis, other advantages, that are not necessarily shared by other solar thermochemical or photochemical systems, make this process unique:

• 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.
• A readily achievable turning temperature of 400 °C to 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.
• 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.

Production of ammonia is one of the world’s largest chemical process industries, with in excess of 100 million tonnes produced annually and about 90,500 tonnes produced in Australia in 2004, the bulk of which is used for fertilizers (~90%). The industry has a 100 year history of operation.

Images from the Incitec ammonia plant in Brisbane