Thermal Performance of Thermosyphons Intended for Evacuated Tube Solar Collector Using Graphene Oxide Nanofluids

Abstract

Vacuum tube solar collectors are composed by two concentric glass tubes with the annular space evacuated. At the inner tube a thermosyphon is placed inside a metallic fin in order to absorb sun’s irradiation and heat running water placed at a manifold. Thermosyphons are passive heat transfer devices that absorb heat at the evaporator region, evaporating the working fluid that reaches the condenser in the form of steam. At the condenser, heat is dissipated to the environment, condensing the working fluid that returns to the evaporator, closing the thermodynamic cycle. In this study, thermosyphons with three different working fluids (5 and 10% graphene oxide nanofluids and distilled water) were built and experimentally tested. The evaporator and the adiabatic section have an outer diameter of 8.33mm and lengths of 1,600mm and 40mm, respectively. The condenser has an outer diameter of 13.40mm and a length of 35mm. The filling ratio used was 50% of the evaporator’s volume. A resistive tape wrapped at the evaporator and connected to a power supply was responsible for heating the working fluid by Joule effect, and water flow rates of 0.50, 0.75, and 1.00L/min were responsible for condensing the working fluid at the condenser. Heat loads of 35, 55, and 75W were applied to the devices and K-type thermocouples were responsible for acquiring temperature data from the thermosyphons, allowing the thermal analysis based in the temperature distribution and thermal resistance for each working fluid. The best working fluid for the conditions proposed, out of the three investigated, was 5% graphene oxide.