In this paper, a thermodynamic procedure is presented to analyse energy and exergy balances of a rock bed thermal storage system. The thermal behaviour is described by means of a control volume that includes three subsystems: the solar collectors, the fluid distribution system and the storage chamber. Solar-to-thermal energy conversion was obtained by means of a solar collector at a fixed airflow rate. The final purpose of the method is to determine how well the thermodynamic modelling fits the real data obtained experimentally from the prototype under normal operating conditions.