COMPARISON OF NUMERIC MODELS FOR THE SIMULATION OF MOLTEN SALT THERMOCLINE THERMAL ENERGY STORAGE WITH FILLER UNDER DIFFERENT APPLICATIONS

To date, two-tank molten salt systems mark the current state-of-the-art for large scale high-temperature thermal energy storage applications. Single-tank thermocline systems with filler (TCF) promise significant cost reductions, but are still under development. Available publications have utilized various models with different simplifications or assumptions. Most of these models have been developed for gas through-flow regenerator storage systems, such as Cowper stoves. The applications range from simple sizing calculations up to annual simulations. So far, only a few publications deal with the different outcome of those models. Particularly for molten salt applications, there has been no comparative work. To resolve this shortcoming, a comparative study of four one-dimensional models suitable for packed bed molten salt storage systems is presented. The investigated models are the single phase model, the Schumann model, the continuous solid phase model and a recently developed bidisperse model by the own group. Based on practical considerations, different representative outer boundary conditions are defined for the comparison In the first case, the storage volume is charged beginning from uniform temperature (single blow) followed by a standby period (hold). This scenario is often used for model validations since it causes harsh gradients. In the second case, the storage volume is charged and discharged for 14 consecutive cycles, to unveil slowly developing inaccuracies during operation. The outcome of the simulations shows significant differences depending on those outer boundary conditions. It is shown that the single blow operation must be evaluated carefully as it might lead to a false validation of a model. Furthermore, it is shown that the investigated models differ by up to 14 % from the chosen reference model. The outcome of this work provides an overall contribution to a better understanding of single tank molten salt storage simulation with filler.