Experimental Investigation of the Thermal Performance of a Flat Plate Solar Collector with a Spherical Protrusions Enhanced Absorber

In this study, the thermal performance of a flat-plate solar collector equipped with a protruded absorber surface was experimentally investigated. The absorber consisted of 92 spherical protrusions with a diameter of 20 mm, resulting in an increased developed surface area compared with a conventional flat absorber. Experiments were conducted under quasi-steady-state conditions during the morning-to-noon period, and key parameters including solar irradiance, ambient temperature, inlet and outlet fluid temperatures, flow rate, and collector thermal efficiency were measured and analyzed. The results showed that increasing solar irradiance from approximately 480 to 900 W m⁻² led to a noticeable rise in the outlet fluid temperature and the temperature difference across the collector, while the thermal efficiency increased from about 56.5% to more than 62%. In addition, increasing the flow rate from 0.5 to 2 L min⁻¹ improved the collector efficiency, which can be attributed to the enhanced rate of energy transfer to the working fluid and improved heat-transfer conditions. Furthermore, the reduction in wind speed during the experimental period contributed to lower thermal losses and more favorable operating conditions for the collector. The analysis revealed that the observed improvement in collector performance was primarily associated with the modified absorber geometry and the increased heat-transfer surface area between the absorber and the working fluid. The presence of spherical protrusions facilitated more effective transfer of the absorbed thermal energy to the fluid, resulting in higher outlet temperatures and improved thermal efficiency.