Compatibility of vegetable oils with solid filler materials for thermocline thermal energy storage systems

Compatibility tests involving four vegetable oils and three different solid materials were performed, with a focus on rapeseed oil and quartzite rock. The objective was to verify the relevance of using vegetable oils as heat transfer fluid in concentrated solar power plants with direct thermocline thermal energy storage. First tests consisted in using small crucibles to put in contact rapeseed oil respectively with quartzite, blast furnace slags and alumina for 2160 h and at 210 °C. On a chemical point of view, the oil composition was modified by ageing and contact with solids. However, thermal properties that drive heat transfer (density, specific heat, thermal conductivity and dynamic viscosity) were not modified. Thermal stability temperature and flash point decreased and a correlation with the index of acidity of the samples has been found. Second set of tests consisted in putting in contact quartzite with rapeseed, palm, soybean and jatropha oils for 720 h and at 210 °C. Results confirmed that index of acidity can be a good indicator of the vegetable oil ageing. From the whole study, no incompatibility has been concluded, but one may recommend a limit value of 25 mg KOH.g−1 for the index of acidity to consider partial replacement of the vegetable heat transfer.