Comparison of Thermocline Molten Salt Storage Performances to Commercial Two-tank Configuration

Abstract

This work deals with the assessment of thermocline heat storage performances when applied to solar thermal plants. The considered thermocline is based on molten salt heat transfer fluid (Solar Salts between 300°C and 550°C) and filled with quartzite. A 2-D finite element heat transfer model is developed to determine the temperatures inside the vessel with mass flows input/output. The model includes heat conductivity of molten salt and quartzite rocks, heat transfer between the molten salts and the quartzite, as well as heat loss to the environment. Results of the model are compared to available experimental data as well as analytic results showing good agreement. Then, the thermocline storage with the performances predicted by the 2-D code was integrated in a CSP plant previously modelled with the two-tank TES system. Plant management is kept equal to the two-tank configuration. A performance index is introduced to make a consistent comparison between the thermocline and the two-tank system: storage efficiency is defined as the heat withdrawn from the storage above 545°C divided by the overall input in the storage. The defined index is equal to 100% for the two tank system as thermal losses have a negligible impact. On the contrary, in thermocline storage, part of heat stored in the molten salt is in the thermocline region and this molten salt is not accounted as useful. The thickness of the thermocline is about 4 to 6 meter height out of 14 meters making the storage performances in the range of 65%, hence significantly lower than in two-tank configuration. A sensitivity analysis on tank size and tank shape factor is performed to assess the optimal configuration for the thermocline.