A method for the design and sizing of a packed bed thermal storage unit for a hot air solar heating system is presented. A controlled bypass of the heat storage unit during the retrieval process is used in order to maintain the fluid temperature leaving the system at a constant value. The use of a bypass arrangement with a closed air circulation loop for the collector enables the storage unit to operate at a higher mean temperature and the system to utilize the maximum allowable pressure drop. This results in a greater heat storage per unit volume of storage material. The complete solar system is simulated numerically during both the heating and retrieval modes of operation to obtain a set of design charts for sizing the thermal storage unit. In the simulation, consideration is given to geographical location, composition of the storage bed and the maximum allowable pressure drop across the bed. A set of design curves for State College, Pennsylvania, is presented.

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