Exergy Analysis

The use of closed-loop thermochemical energy storage systems for the storage of solar energy places fundamental limits on the amount of work that can be extracted from the recovered energy. These arise because of thermodynamic irreversibilities associated with the storage system itself and because of the need to degrade collected solar energy to the characteristic temperature of the reaction system chosen.

The assumption that the irreversibility is entirely due to the reaction processes has allowed useful general expressions for exergetic and work recovery efficiencies to be derived. The exergetic efficiency of such systems is limited only by how close reaction paths can be made to follow the equilibrium line. The work recovery efficiency on the other hand has a maximum value which is characteristic of the reaction chosen and reflects the loss in exergy associated with storing energy at the characteristic temperature of that reaction. The spontaneous separation attribute of the ammonia-based thermochemical system results in thermodynamically reversible paths having constant work recovery efficiencies irrespective of their reaction extent endpoint. This is in contrast to the case of systems where all reactants remain in the gas phase. The transition between the two situations can be examined by calculating constant efficiency contours for the ammonia-based system for various assumed sink temperatures.

The analyses undertaken represent an essential prerequisite for the theoretical development of reactor configurations which optimise efficiencies.

Further reading: "Thermodynamic Limits on the Performance of a Solar Thermochemical Energy Storage System", K. Lovegrove, Int. Journal of Energy Research, 17, 817 (1993).