• Energy Research

In this work an overview of the TESIS:store thermocline test facility and its current construction status will be given. Based on this, the TESIS:store facility using sensible solid filler material is modelled with a fully transient model, implemented in MATLAB®. Results in terms of the impact of filler site and operation strategies will be presented. While low porosity and small particle diameters for the filler material are beneficial, operation strategy is one key element with potential for optimization. It is shown that plant operators have to ponder between utilization and exergetic efficiency. Different durations of the charging and discharging period enable further potential for optimizations.

Abstract In this work, an extensive parametric study of the molten salt thermocline storage concept with filler is presented. The parametric study is coupled with an optimization routine, allowing a better comparison, since it finds only those storage configurations, which can directly substitute the two-tank system in a given power plant. Results show that, compared to the two-tank molten salt system, the thermocline technology achieves high exergetic efficiency at only slightly increased storage volume size and a huge decrease in salt inventory.

Abstract Molten salts are widely used as thermal energy storage media in the Concentrating Solar Power (CSP) technologies. The melting point and the thermal stability of the salt determine the applicable temperature range of the storage system. The focus of this paper is to evaluate the effect of different gas atmospheres on the thermal stability of binary Solar Salt and two ternary salt mixtures containing lithium nitrate (Lithium mixture) and sodium nitrite (Hitec). The isothermal stability experiments were carried out at 550 °C during 500 h and the results show that the initial decomposition reaction of nitrate to nitrite depends strongly on the gas atmosphere. It is observed that changes in the nitrate-nitrite-ratio are the key parameters influencing the melting temperatures of the salt mixtures. For example, for experiments with oxygen in the atmosphere a large increase of the liquidus temperature of the Hitec mixture was observed. Metal oxides are formed during the irreversible decomposition of nitrite-ions but are found to affect the solidus and liquidus temperature of the salt mixtures only marginally. No carbonate formation was detected according to titration analysis of the salt mixtures in our experiments due to the absence of carbon dioxide in the atmosphere.

Abstract The present publication introduces a novel three-equation model for the simulation of bidisperse packed beds, typically utilized in thermocline thermal energy storages with filler. These systems aim for the substitution of a large fraction of cost intensive liquid storage material such as molten salts by a cost effective solid storage material. A higher packing density can be achieved by a combination of large and small particles, i.e. a bidisperse packing. For the simulation of such systems, the currently most widely applied Schumann and continuous solid phase models consider the solid phase as single sized spheres with a mean particle diameter derived from the bidisperse packing. The present work introduces a novel approach, where, besides the differential equation for the fluid, two differential equations for the small and for the large particles are applied. The model is validated with bidisperse experimental data from the literature. For comparison a reference case of a 100 MW electric solar thermal power plant is defined and the bidisperse model is compared to the Schumann model in its outcome for four different particle size combinations. In the single blow operation, where the storage volume is charged from uniform temperature, the temperature curves show differences of up to 6.6 Kelvin. If the thermocline is given time to develop over several consecutive cycles, the difference in charging and discharging time of one period reaches up to 3.5%. With the model presented in this work, accuracy of long duration or annual simulations can be significantly increased.