Improving stability and thermal properties of TiO2-based nanofluids for concentrating solar energy using two methods of preparation

Nanofluids are considered a promising alternative to the classic fluids used in heat transfer processes. One interesting application of nanofluids is their use as a heat transfer fluid in thermosolar plants, such as those using concentrating solar power (CSP) technology. Therefore, this study presents the preparation of nanofluids based on TiO2 nanoparticles and the most common thermal oil used in CSP plants. The nanofluids were prepared using a one-step method by means of the solvothermal synthesis of TiO2 and also using a two-step method by means of ultrasonic waves and stabilizing the nanoparticles using 1-octadecanethiol as a surfactant. The stability of the nanofluids was analysed using UV–Vis spectroscopy, particle size and ζ potential measurements. The formation of TiO2 nanoparticles in the one-step method was detected using X-ray diffraction, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Thermophysical properties were also measured obtaining an atypical improvement of isobaric specific heat of up to 25.4% for the nanofluids prepared by the two-step method. For the nanofluids prepared by the two-step and one-step methods, thermal conductivity increased by up to 52.7% and 31.4% at higher temperatures, respectively. Finally, enhancements of up to 35.4% of the heat transfer coefficient were estimated, which means these nanofluids are suitable for being used as heat transfer fluids in CSP plants.