Experimental Investigation of Solar Heat Pipes with Ethanol Solution as Working Fluid

As working fluid of heat pipes, water is widely used in solar heat pipe water heater. But water causes poor performance of heat transfer under low temperature and freezing in cold winter. In this paper, ethanol-water solution is used as working substance in heat pipe. The static heat transfer and start-up performance of heat pipe are experimentally studied in this paper. The experimental results showed that when the heat flux is low, the heat transfer of heat pipe with 40% voluminal concentration ethanol solution is better than that of heat pipe with pure water. Start-up speed and temperature of ethanol solution heat pipe are higher than that of water heat pipe. In the low temperature condition, ethanol solution is more competent than water for working medium of solar heat pipe. In addition, heat transfer performance is also influenced by incline angle of heat pipes, charge quantity and heat flux. The effect of incline angle on heat transfer performance of heat pipes becomes distinct with the increasing of heat flux. The reason is that the heat flux becomes high and reflux condensate becomes much with the increasing of heating power. The optimal incline angle of heat pipe with 40 vol. % ethanol solution is 45 and the optimal charge quantities is 11.8 vol. %. The experimental results are helpful for widely use of solar heat pipe water heater in north China.