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

Shanguo Zhao,Guoying Xu,Ning Wang,Xiaosong Zhang

doi:10.3390/nano8020072

Abstract

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 nanofluid as the working fluid instead of deionized water. The stability of the prepared graphene/water nanofluid 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 nanofluid 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 nanofluid-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 nanofluid 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.

Highlights

Heat pipe solar collectors, which use the heat pipe as a core component of heat transfer, have been widely used in solar thermal water heating applications
Menlik T. et al [11] experimentally demonstrated that the performance of a heat pipe was improved by 26% under 200 W heating power when MgO/water nanofluid was charged into the system instead of water
As it is operated in fluctuant solar radiation conditions, the solar gravity heat pipe may frequently start up and this severely affects its solar thermal collection performance

Summary

Introduction

Heat pipe solar collectors, which use the heat pipe as a core component of heat transfer, have been widely used in solar thermal water heating applications. The structure types of heat pipe solar collectors were mainly divided into (1) evacuated-tube heat pipe and (2) flat-plate heat pipe solar collectors. In both types of solar collectors, the evaporation sections of heat pipe arrays are placed inside the vacuum tubes or on the flat-plate solar absorber, transfers the absorbed solar heat to the condensing section. Commonly used as the working fluid, has a liquid-steam phase transition inside the sealed heat pipe. A gravity heat pipe solar collector could achieve higher thermal efficiency and allow more pressure-bearing operations [1,2] compared with a conventional solar collector

Methods

Results

Conclusion