Sand-propylene glycol-water nanofluids for improved solar energy collection

• Sand-propylene glycol-water nanofluids prepared from milled sand nanoparticles.
• 16.3% enhancement in thermal conductivity for 2 vol % nanofluid.
• 47% reduction in viscosity for 2 vol % nanofluid.
• Collection efficiency increases with nanoparticle concentration.
• 16.5% enhancement in solar energy collection for 2 vol% nanofluid.

Experiments were carried out on the preparation, thermophysical properties' measurement and application of surfactant-free, sand-propylene glycol-water nanofluids for solar energy collection. Thermal conductivity enhancement of 16.3% and viscosity reduction of 47% were observed for 2 vol % sand-PG-water nanofluid at 28 °C. Microconvection caused by Brownian motion was identified as one of the mechanisms of thermal conductivity enhancement. The relative viscosity of sand-propylene glycol-water nanofluid was less than 1 over a temperature range of 10–60 °C and sand nanoparticle concentration range of 0.5–2 vol %. The lower viscosity of sand-PG-water nanofluids in comparison to that of propylene glycol-water mixture is attributed to non-covalent interactions between sand nanoparticles and propylene glycol, leading to perturbation of hydrogen bonding network. The use of 2 vol % sand-PG-water nanofluid resulted in enhancement of heat transfer and solar energy collection by 16.5%. Our results demonstrate that sand-PG-water nanofluids are suitable for use as heat transfer fluid in solar collectors.