Analysis of packed-bed thermocline storage tank performance by means of a new analytical function

In this work we have compared the experimental results from the thermocline tank of STONE installation of CEA with an analytical model which considers an algebraic sigmoid as solution for the energy balance equation. The parameters of this function have been obtained by fitting its mathematical expression to the experimental temperature-time data for different tank heights. From that fitting physical parameters like the velocity of thermocline zone inside the tank and the effective heat capacity of the storage medium could be determined. These values depend on operating conditions like superficial oil velocity and temperature range but they deviate from the theoretical ones due to the specific STONE tank design in which metallic wall shares 10 % tank volume. In this way, considering or not the tank wall in the effective heat capacity leads to 6.6 % error when predicting the thermocline zone position with the model. However, for thermocline tanks with larger volumes the effect of walls is expected to be negligible and hence the analytical model will be able to predict more accurately their performance.