Thermal Stability and Performance Testing of Oil-based CuO Nanofluids for Solar Thermal Applications

Abstract

For solar thermal systems, nanofluids have been proposed as working fluids due to their enhanced optical and thermal properties. However, nanoparticles may agglomerate over time, heating and thermal cycles. Even though pristine nanofluids have proven to enhance performance in low-temperature applications, it is still unclear if nanofluids can meet the reliability requirements of solar thermal applications. For this aim, the present study conducted experiments with several formulations of oil-based CuO nanofluids in terms of their maximum operational temperatures and their stabilities upon cyclic heating. In the samples tested, the maximum temperature ranged from 80 to 150 °C, and the number of heating cycles ranged from 5 to 45, with heating times between 5 to 60 min. The results showed that heating temperature, heating cycles, and heating time all exacerbated agglomeration of samples. Following these experiments, orthogonal experiments were designed to improve the preparation process and the resultant thermal-impulse stability. Thermal properties of these samples were characterized, and thermal performance in an “on-sun” linear Fresnel solar collector was measured. All tests revealed that thermal performance of a solar collecting system could be enhanced with nanofluids, but thermal stability still needs to be further improved for industrial applications.