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Thermochemical energy transfer and storage
Thermochemical energy transfer is a concept for transferring heat over long distances with minimum heat loss. It relies on the fact that chemical reactions are typically reversible: they can be driven in either an exothermic or heat giving direction  or an endothermic or heat absorbing direction. An example is the reaction 2NH3=N2+3H2

When this reaction is forced from left to right ammonia is dissociated into nitrogen and hydrogen at the same time absorbing heat. When forced in the opposite direction ammonia is produced and heat is released. When this reaction is employed in a thermochemical energy transfer system ammonia at ambient temperature flows to the location of the energy source e.g. solar, and enters one leg of a counterflow heat exchanger. Here the fluid's temperature is raised to the temperature of a reaction chamber by absorption of heat F derived from the second leg of the counterflow heat exchanger. The hot ammonia then passes to a reaction chamber in which it undergoes an essentially isothermal reaction in which energy from an external source such as insolation is absorbed as ammonia is dissociated. The products of dissociation, viz. a mixture of nitrogen and hydrogen, cool to ambient temperature while passing through the second leg of the counterflow heat exchanger, releasing heat F.

Two pipe lines link the 'heat absorbing' location to the 'heat releasing' location separated by some distance.

The process is essentially the same at the heat releasing location except that in the reaction chamber ammonia is produced and the heat of reaction is released. This is available for use in say a heat engine coupled to an electricity generator.

In the case of the ammonia reaction, forcing in the exothermic direction requires a substantial increase in pressure and also a catalyst. In practise the nitrogen/hydrogen mix would be pressurised by a pump before entering the first leg of the heat exchanger and the exiting ammonia would be depressurised in a fluid motor after leaving the second leg. These two devices would be coupled in order to achieve a near balance of mechanical work. The optimum pipeline pressure is chosen to optimise transmission cost and is typically a few tens of atmospheres.

The endothermic reaction proceeds fairly readily at high temperature and pipeline pressure