Study of Thermocline Tank Performance in Dynamic Processes and Stand-by Periods with an Analytical Function

The behavior of an Andasol-like thermocline tank with and without solid filler has been predicted with an analytical Logistic Cumulative Distribution Function not only during dynamic processes of charge/discharge but also during stand-by periods of no operation. The evolution of thermocline thickness with dwelling time in stand-by periods has been studied under the condition of energy conservation when thermocline zone is total or partially inside the tank. Thermocline degradation is faster for the tank with molten salt and solid filler than for the tank with only molten salt because thermal diffusivity of the molten salt is lower than thermal diffusivity of the solid material. The strong decrease in tank efficiency with consecutive charge/discharge cycles when thermocline zone is left inside the tank can be relieved if a certain percentage of thermocline zone is extracted. Actually, if 50% thermocline is extracted, efficiency after a second process can be as high as 96% comparing with the 77% obtained if thermocline is not extracted. However, extracting thermocline zone implies a decrease of outlet temperature in discharge or an increase of outlet temperature if the process is a charge. Therefore, temperature limits and operation strategies should be defined for the solar thermal power plant that includes a thermocline storage tank in order to establish which amount of thermocline zone can be extracted.