Thermo-chemical energy storage using ammonia

When N₂ and H₂ combine to form ammonia NH₃, the reaction gives off heat.
When the reaction reverses, it takes in heat.

Heat can be stored by breaking NH₃ apart, and then recovered by allowing them to combine again.

The nitrogen, hydrogen, and ammonia is all stored in one tank. Under moderate pressure, the ammonia is liquid so can be drawn off separately  Commercial catalysts are used to convert the reactants each way.

There are several advantages:
Everything can be stored in one tank. Under moderate pressure the ammonia is a liquid.
It is stored at room temperature so heat is not lost over time.
The fluids can be transferred around the solar plant without losses, unlike hot salt or oil.
Reactants are environmentally benign
There is plenty of experience in handling ammonia.
The turning temperature is 4-500^oC. (depends on pressure) This is readily achievable by any set of solar reflectors   The materials are cheaply available and will not run out.

It is just a matter of whether it is as cheap as other options.

Ref

Sept 1998

1kW chem solar ammonia dissociation closed loop operated for the first time – a world first!    

        Ref

Ammonia as a way of storing hydrogen

Ammonia is one way to store hydrogen and has several advantages. It is non-flammable and non-explosive, it can be liquefied under mild conditions, and it has a large weight fraction of hydrogen 17.65% of the mass of ammonia. It has a volumetric hydrogen density about 45% higher than liquid hydrogen.Using a catalyst it can be cracked to produce Hydrogen.

Ammonia can be stored as metal ammine complexes of the form M(NH₃)_nXm,where M is a metal cation like Mg, Ca, Cr, Ni, and Zn, and X is an anion like Cl or SO4. For example: Mg(NH₃)₆Cl_{2 .}

It has the same volume density as liquid ammonia, but it is can be stored at room temperature and pressure.

Ammonia can power cars. More

2 NH₃ ----> 3 H₂+ N₂       H° = 46 kJ/mol

Only 16% of the energy stored in ammonia is needed
to break gaseous ammonia into nitrogen and hydrogen gases.

Ammonia production

Ammonia is produced by six producers in Australia; Incitec, Orica, Wesfarmers, BHP-Billiton,
Queensland Nitrates and Burrup fertilisers.

Ammonia is manufactured by the catalytic steam reforming of natural gas. Hydrogen from the reformed natural gas and nitrogen from air are compressed at reduced temperatures to form ammonia:
3H2  + N2  → 2NH3
The overall manufacturing process for ammonia production involves a series of stages to remove impurities such as sulphur, carbon monoxide, carbon dioxide and water from the natural gas feedstock and the generation and reaction of hydrogen and nitrogen.

Some of the  CO₂ is recovered to make urea and for use in food and drink.

If  methane is used to make ammonia then about 1.5 t of CO2 is made per tonne of ammonia produced.