Analytical Description of Thermocline Tank Performance in Dynamic Processes and Stand-by Periods

In this paper a Logistic Cumulative Distribution Function has been used as approximate analytical expression for predicting the behaviour of thermocline tanks when consecutive charge and discharge processes are performed and also during stand-by periods of no operation. This Logistic-CDF is expressed in dimensionless coordinates, parameterised in terms of working conditions and has been validated with experimental data reported in the literature. For the case of dynamic process, even if a thermocline tank is operated under optimum conditions, efficiency decreases with charge/discharge cycles leading to a tank delivering only about half of the ideal total energy after 5 days of operation. As for stand-by period, the degradation velocity and the full degradation time depends on the thermal diffusivity of the storage materials