Performance of natural and forced convection heat pipe solar water heaters.

The thermal performances of the evacuated heat pipe solar water heaters operated under natural and force convection were tested. Three evacuated heat pipe solar collectors were connected in series and coupled with an insulated storage tank. Force convection system utilized a circulation pump while the natural convection system operated under thermal buoyancy. The results showed that the natural convection system performed better than the forced convection system due to the difference in the performance of the heat pipe. A natural convection system is able to provide the heat pipe condenser with sufficient cooling water drained from the lower section of the storage tank. Temperature stratification within the storage tank prevents the fast mixing of the heated water returning from the solar collectors with the cold water. Hence, constant low temperature cooling water can be supplied to the condenser to absorb heat energy transferred by heat pipe. On the other hand, force convection system will have a well mixed water temperature within the storage tank. As the circulation pump circulates the heated water back to the solar collectors, the heat transfer within the heat pipe will be decreased due to the insufficient cooling of the condenser.