Experimental investigation of thermal energy storage performance using sand-soil mixtures in a hybrid solar thermal system

The high cost and intermittency of conventional thermal-energy-storage materials limit the wider adoption of solar-assisted cooling for photovoltaic (PV) panels, particularly in hot climates such as Iraq, where excessive panel temperature reduces electrical output. This study experimentally examines whether a low-cost, locally available sand-soil storage medium, combined with a helical-coil heat exchanger operating on a passive thermosiphon principle, can simultaneously cool a PV panel and store thermal energy for later use. A cylindrical storage tank (25 cm diameter, 60 cm height) housing a copper helical coil was placed beneath a solar collector, with a 50/50 water-vegetable-oil mixture circulating as the heat-transfer fluid. Three storage media, pure sand, pure soil, and a sand-soil mixture, were tested under natural and forced convection at mass flow rates of 0.008 and 0.016 kg/s and solar irradiance up to 1000 W/m². The sand-soil mixture consistently outperformed the single-component media, achieving a thermal-storage efficiency of 77% and reducing the solar-cell temperature by approximately 37.5% under natural convection, corresponding to a 39.44% improvement in thermal-energy-storage performance relative to pure sand. These results demonstrate that an inexpensive sand-soil composite can deliver PV cooling and thermal storage performance competitive with more costly engineered storage materials, offering a practical, low-cost pathway to improving the efficiency of solar power systems in hot, arid regions.