Overheating limitation of thermosiphon solar collectors by controlling heat pipe fluid in all glass evacuated tubes

Overheating is a major problem in all glass evacuated tube with heat pipe when hot water in a pressurized thermosiphon solar water heater is not consumed. Stagnation temperature of a system can be limited in a controllable manner without significantly degrading operating temperature performance. In this approach the dry out limit is used, which can be reached when the filled amount of heat pipe fluid is quite small. In this study, it is aimed to prevent the overheating problem by controlling the heat pipe fluid while almost keeping the efficiency at the same level up to 80 degrees C with the existing one. In order to stabilize the temperature at 95 degrees C, an efficiency reduction above 80 degrees C is utilized on the summer days which are the hottest and have the highest radiation. As the water temperature in the tank increases, the heat transfer of all glass evacuated tube to the water decreases. Another important factor to achieve this balance is nocturnal cooling of the water in the tank during the night. Three different liquids which are ethanol, methanol, and acetone are used as the heat pipe fluid in the experiments in addition to the existing one. Daily conversion efficiency for new designed all glass evacuated tube containing ethanol is equal to 77.5%, based on collector aperture area, up to water temperature of 80 degrees C in field test. It is observed that the experimental results are consistent. The all glass evacuated tube designed in this study has been started to be used commercially in pressurized thermosiphon solar water heaters, taking into account the results obtained from field experiments.

Automated summary

The study aims to prevent overheating issues in glass evacuated tubes with heat pipes in solar water heaters by controlling the heat pipe fluid, while maintaining efficiency up to 80 degrees Celsius. By using ethanol as the heat pipe fluid, new designed glass evacuated tubes achieve a daily conversion efficiency of 77.5% up to a water temperature of 80 degrees Celsius in field tests, showing consistent experimental results.