Experimental investigation of extra-long pulsating heat pipe application in solar water heaters

In this study, the aim is to investigate the application of pulsating heat pipes (PHPs) as a heat transfer tool in a solar water heater (SWH). For this purpose, an extra-long pulsating heat pipe (ELPHP) is designed, constructed and installed in a thermosyphon solar water heater. In this work the ELPHPs are made of copper tubes of internal diameter 2.0 mm. The number of meandering turns is 6 and the working fluid employed is distilled water. The lengths of condenser and evaporator sections are 0.8 and 0.96 m, respectively. The length of adiabatic section varies between 0.7 and 1.8 m. Inclination of the ELPHPs varies between 15° and 90° but is 45° for evaporator section. Four different tests are carried out; SWH with thermosyphon cycle, SWH with PHP filling ratio (FR) = 30%, FR = 50% and FR = 70%. The results show that PHP with FR = 70% has the most stable and the longest functioning duration. While mean temperature of container water is 35.3 °C and 34.5 °C for thermosyphon cycle and PHP FR = 70%, respectively, the surface area under the container water temperature curve vs. time for thermosyphon is 87.69% of that of PHP FR = 70% which indicates the higher heat transfer rate for the ELPHPs. In addition, calculations show efficiency of PHP FR = 70% is equal to 53.79% while efficiency of thermosyphon mode is in range of 31% and 36%.

► Extra-long pulsating heat pipes can enhance the performance of solar water heaters.
► PHP mode with FR=70% has better performance compared with other filling ratios.
► In the best case, performance is increased by 22% using pulsating heat pipes.