Experimental Study on the Thermal Start-Up Performance of the Graphene/Water Nanofluid-Enhanced Solar Gravity Heat Pipe

The solar gravity heat pipe has been widely used for solar thermal water heating because of its high efficient heat transfer and thermal diode characteristics. Operated on fluctuant and low intensity solar radiation conditions, a solar gravity heat pipe may frequently start up. This severely affects its solar collection performance. To enhance the thermal performance of the solar gravity heat pipe, this study proposes using graphene/water nano fluid as the working fluid instead of deionized water. The stability of the prepared graphene/water nano fluid added with PVP was firstly investigated to obtain the optimum mass ratios of the added dispersant. Thermophysical properties-including the thermal conductivity and viscosity-of nano fluid with various graphene nanoplatelets (GNPs) concentrations were measured at different temperatures for further analysis. Furthermore, based on the operational evaluation on a single heat pipe's start-up process, the performance of nano fluid-enhanced solar gravity heat pipes using different concentrations of GNPs were compared by using water heating experiments. Results indicated that the use of 0.05 wt % graphene/water nano fluid instead of water could achieve a 15.1% and 10.7% reduction in start-up time under 30 and 60 W input heating conditions, respectively. Consequently, a higher thermal efficiency for solar collection could be expected.