Thermo-physical properties and thermal energy storage performance of two vegetable oils

Abstract

Sensible heat storage materials are cheaper than latent heat storage materials for small storage volumes. Vegetable oils are cheap and readily available sensible heat storage materials produced locally in most countries in Sub-Saharan Africa. In this paper, the refractive indices, sound velocities, densities, specific heat capacities, and thermal conductivities of Sunflower oil and Roki oil (a blend of Palm oil and Sunflower oil) are determined experimentally. These two vegetable oils are locally produced in Uganda. Isentropic compressibility and thermal expansion coefficient are derived from the physical properties for the first time. The results show the refractive index, sound velocity, density, and thermal conductivity decrease with the increase in temperature, whereas isentropic compressibility, thermal expansion coefficient, and specific heat capacity increase with the increase in temperature for both oils. Roki oil shows higher thermal conductivity values than Sunflower oil at various temperatures. Additionally, an experimental setup to compare the performance of Roki oil and Sunflower oil as sensible heat storage materials for domestic medium temperature applications is presented. Roki oil and Sunflower oil are experimentally compared using charging and discharging experiments. The charging/heating cycles are first simulated with electrical energy followed by experimental tests with solar energy using parabolic dish solar cookers. The thermal comparison is made in terms of the temperature profiles during charging, discharging and heat utilization processes. During charging, the results show that Roki oil attains higher maximum temperatures (170–233 °C) compared to Sunflower oil (160–210 °C). Roki oil shows higher temperatures (71–78 °C) during cooling compared to Sunflower oil (67–76 °C). Roki oil shows higher heat utilization temperatures compared to Sunflower oil. The results provide insights on the utilization of locally available materials in Uganda as sensible heat thermal energy storage which are cheaper than commercial solar heat transfer oils.