Thermal performance of an evacuated tube heat pipe solar water heating system in cold season

Highlights: • The cold-season performance of a heat pipe solar water heating system was studied. • Water consumption pattern plays a crucial role in optimum design of the system. • Working fluid mass flow rate impacts on the outlet temperature and thermal efficiency. • The highest thermal and exergy efficiencies were 92.7% and 15.58%, respectively. -- Abstract: This study evaluates the performance of a heat pipe solar water heating system to meet a real residential hot water consumption pattern theoretically and experimentally under non-ideal climatic conditions during a cold day in Perth, Western Australia. A mathematical model was developed and used to calculate the optimum number of glass tubes of the heat pipe solar collector. Based on the obtained data, an experimental rig with 25 glass tubes was designed, built, and tested as the temperature changes after 25 tubes reached the insignificant value of 0.6%. The results showed that hot water extraction had significant impact on the thermal performance of solar water heating system by increasing the amount of the absorbed energy and overall efficiency and decreasing exergy destruction. This indicates the importance of considering hot water consumption pattern in design and analysis of these systems. Auxiliary heating element was a necessary component of the system and played an important role mainly at the beginning of the operation in early morning (operation time of 19 min) and partly during the cloudy and overcast periods (operation time of 8 min). Two empirical equations relating the thermal and exergetic efficiencies of the heat pipe solar collector to the operational and environmental parameters were proposed. Comparison of the theoretical and experimental outlet temperature of the collector showed very good agreement with the maximum absolute and standard errors being 5.6% and 1.77%, respectively.