Discharge effectiveness of thermal energy storage systems

•Increasing Re number in the discharge allows for more of the stored energy to be used.•Higher porosities improve heat exchange and are more efficient in the discharge.•Less permeable systems are more efficient due to increased heat exchange.•Lower thermal conductivity results in higher temperatures at the outlet of the discharge.•Higher thermal capacity ratios lead to more efficient discharges as long as flow velocities are high enough in the system. The use of air as heat transfer fluid and a packed bed of rocks as storage medium for a thermal energy system (TES) can be a cost-effective alternative for thermal applications. Here, a porous media turbulent flow (standard k-ε) and heat transfer (local thermal non-equilibrium) model is used to simulate the discharge cycle of such system. Temperature fields of corresponding charging cycles are used as initial conditions. Effects of varying mass flow rates (Re number), porosity, permeability (Da number), thermal conductivity ratio and thermal capacity ratio on the effectiveness of the discharge are compared. The examination of these effects indicated that increasing the mass flow rate improved the effectiveness of the discharge, which was not seen for the charging cycle. Also, increasing porosity improved discharge efficiency more significantly than it did in the charging cycle. In both charge and discharge cycles the effect of permeability is significant and reducing Da number improved temperature stratification and efficiencies. The effect of the thermal conductivity ratio was mostly seen on the outlet temperatures, where lower ratios allowed for higher temperature values. Increasing the thermal capacity ratio improved charging effectiveness but, on the discharge cycle, cycle this effect was reduced. Moreover, for lower Re number flows, increasing this ratio reduced efficiency indicating that the mass flow rate should be matched carefully with the thermal capacity of the system. All these effects have important implications which should be taken into consideration when designing an effective thermal energy storage system.