Multicriteria Comparison of two Different-Nature Fillers for High Temperature Sensible Heat Storage

The most suitable materials for thermal energy storage should combine low cost, high thermal capacity, high charging and discharging velocities, compatibility with the plant components in which they are integrated and, long-term durability. The use of solids to fill packed beds where the heat enters and leaves through a heat transfer fluid (HTF), is a solution that provides satisfactory amounts of energy density at low cost. In this work, two solid materials of different nature were comparatively tested at device level with air as HTF and according to various criteria. Material A is manufactured from recycled waste and tailored to improve its thermal properties, while material B is a natural, cheap and highly available in nature (pebbles). Material B showed significant mechanical strength problems at temperatures above 500 °C. Material A, instead, withstood well temperatures up to 850 °C under operating conditions. The comparative KPI revealed that material B has a slightly higher thermal capacity and a 4% higher energy efficiency than material A. However, these results lost validity after the first-round experiment with material B, since a strong degradation was detected affecting its physical properties and hindering its ability to store heat.