Heat pipe augment flat plate collector for water heating

A well-designed and fabricated heat pipe has effective thermal conductivity about 500 times greater than copper. The aim of this research was to test the principle of using heat pipes attached to a flat plate solar collector so that water can be heated at higher temperature than that can be attained with simple flat plate solar water heaters. Six heat pipes were designed and fabricated for experimental verification. The maximum theoretical power output of each heat pipe, governed by the entrainment limit, was 480W. The heat pipes were so designed that they start functioning when the evaporator temperature reaches 76 deg C. A black painted aluminum fin with a surface are of 0.12 m by 1.85 m was attached along the evaporator part of the heat pipe as a solar collector plate. The condenser of the heat pipe was cooled by water flowing at a rate of 0.013 kg/s through a manifold attached on the condenser part of the heat pipe. Performance of the heat pipe reduces as the temperature of water in the manifold increases. When the evaporator of the heat pipe was set at 76.5 deg C, the heat transfer ability of the heat pipe was 63 W for a water temperature of 70 deg C in the manifold. However, the net available heat energy delivered by the absorber at 76.5 deg C and 745 W/square m irradiance is only 43 W rendering a great deal of incoming solar energy to losses. Experimental measurements were taken with a water flow rate of 0.013 kg/s at 30 deg C and solar irradiance of 745 W/square m. The power output obtained was 40.8 W which is very much close to the theoretical estimation. As it stands now, though it is possible to raise the temperature of the water at a higher temperature than that is possible with simple flat plate collectors, the overall performance efficiency was too low owing to the huge amount of heat loss at the absorber plate at such high temperature. Double glazing or use of low conductive materials such as perspex instead of glass and low emissivity paints on the absorber surface will greatly improve the overall performance that make use of the system possible and affordable for high water temperature applications.