Fabrication and thermal performance of a solar-driven heat pipe filled with reduced graphene oxide nanofluids

Nowadays, the solar-thermal technology, converting solar energy into useful thermal energy, was regarded as a direct and facile method to harvest solar irradiation for alleviating the energy crisis and environmental issues. In this work, we reported a transparent solar-driven heat pipe filled with rGO nanofluids, which combined volumetric solar-thermal harvesting and heat pipe technology to realize efficient solar capture while fast transporting of the collected thermal energy to the application ends simultaneously. The rGO nanofluids not only were used as solar volumetric absorbers to absorb solar radiation directly, but also completed the whole thermal cycle as the working fluid. We investigated the impact of filling ratio, inclination angle, working fluids, and solar power density on the thermal property of the heat pipe experimentally. It was found that 20% was the optimal filling ratio for the heat pipe with a lowest thermal resistance of 0.25 K/W at the solar power density of 17.5 W/cm2. Under the vertical operation mode, the heat transfer performance of the solar-driven heat pipe was better than that under the horizontal operation mode. Finally, this developed solar-driven heat pipe had been successfully applied for solar water heating systems, showing excellent heat transfer performance.