Parametric study of filler size and properties for a liquid-metal thermal energy storage

Liquid metals are promising heat transfer fluids since they remain liquid in a wide temperature range and can transfer heat efficiently due to their high thermal conductivity. A first-of-its-kind lab-scale thermal energy storage system with filler material and with lead-bismuth as heat transfer fluid is currently tested at the Karlsruhe Institute of Technology, while a 100-kWh storage system is under construction. This numerical study aims to analyse the influence of the filler parameters on the system's efficiency when the fluid used is a liquid metal. The filler should store part of the thermal energy, be efficiently discharged during the cyclic process and buffer the degradation of the thermocline during standby. For each of these purposes different particle diameters and values of some thermophysical properties of the filler, such as thermal conductivity, specific heat capacity and density, may be advantageous. Their influence on the thermocline extension is numerically investigated using a one-dimensional concentric dispersion model. The results of the parameter study show that for an efficient discharge process in a liquid metal dual-media storage, a small filler particle size is beneficial (d< 10 mm for the reference case chosen in this work). In contrast, the standby phase is favoured by larger diameters, here an order of 10–20 mm. Furthermore, a high thermal conductivity of the filler material improves the discharge performance, due to the enhanced heat transfer, but leads to an accelerated growth of the thermocline during standby. For this case, the optimum value is 5–10W/mK. Moreover, using a filler material with a high volumetric heat capacity leads to the best overall performance. A full factorial analysis shows that the filler diameter has the strongest effect on the discharge behaviour, while, during standby, the volumetric heat capacity has the largest influence.