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Thermochemical Storage

Solar energy is abundant and virtually inexhaustible, however, its availability is not continuous and can be unreliable in some locations. Therefore, a practical method for storing solar energy is needed to meet demands for energy that occur when the sun is not shining. Stored solar energy may also be delivered at locations remote from the collection point. So far, photovoltaically generated electricity (from solar cell arrays) stored in banks of lead acid batteries is the most common method for solar energy storage. The aim of this project is evaluate an alternative means of storage based on a reversible chemical reaction.  Computer simulations will be utilized to establish the thermodynamic performance of the cycle in a solar application, and its economic and practical advantages and disadvantages will be compared to battery storage and other storage mechanisms.

Methane reformation was chosen as a promising thermochemical storage method mainly due its proven record in other applications, the relative ease in handling the reactants (methane and water) and the end result (syngas) as well as the relatively low temperature required. The process may be described by the following chemical reaction,

Chemical equation for the endothermic reaction of methane and water to produce carbon monoxide and hydrogen.

where the heat to drive the endothermic reaction is supplied by a parabolic dish concentrator (Fig. 1).  The reaction products, syngas, can be stored indefinitely. When electricity is needed, heat from reverse reaction operates a Stirling engine-powered generator.

 

Schematic of the cycle for producing and storing syngas for delayed production of electricity. Solar collectors heat methane and water in a reformer to produce syngas, which is stored in a tank. When needed, electricity is produced by the exothermic reaction of syngas back to methane and water. The heat from this reaction drives a Stirling cycle engine, which is connected to a electrical generator.

 

Figure 1. Schematic of a reversible solar storage cycle.

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