Theoretical investigation of the thermal performance of evacuated heat pipe solar collector with optimum tilt angle under various operating conditions

The thermal performance of an individual pipe in an evacuated tube solar collector with a heat pipe is investigated by an analytical method based on the energy balance of the collector. A simulation method was developed and compared to experimental results under different operating conditions, such as different ambient temperature and changes in weather. After comparing the simulation and experimental results, a discrepancy of about 0–6% is observed due to the simple assumptions in the simulation process and unavoidable human factors. The results show that under conditions of poor radiation and low ambient temperature, the efficiency changes rapidly, while it is relatively steady in high-radiation situations, and is influenced by differences in the ambient temperature and the collector inlet temperature. In addition to the change in the tilt angle of the solar collectors, the solar radiation collected by the absorber unit varies, which leads to differences in the outlet temperature and the efficiency.