Thermal performance improvement of a solar air compound parabolic collector through double-pass operation: An experimental approach

The current study evaluates improvements in the thermal efficiency of solar air compound parabolic collectors through double-pass operation. Since previous studies have analyzed this operation mode alongside other heat transfer enhancement techniques, no research has focused solely on the advantages of modifying the airflow operation. Therefore, an experimental comparison between two compound parabolic concentrator solar air heaters was made: a single-pass and a double-pass. Both collectors were evaluated simultaneously under identical conditions of solar irradiance, ambient temperature, and mass flow rate, using the ISO 9806 methodology. The results show that the double-pass CPC significantly enhances performance, with higher outlet temperatures and thermal efficiencies than the single-pass arrangement, since additional heat is recovered from the receiver plate. In particular, operating in steady-state with ambient inlet temperature and mass flow rates of 0.0074 kg/s and 0.0081 kg/s, the double pass achieved an additional temperature increase of up to 4 °C, with a collector thermal efficiency up to 30 % higher. The double-pass configuration also yields superior useful energy gains, outperforming the single-pass CPC by 25-30 %. These findings highlight the potential of the double-pass operation as a practical, impactful improvement for solar air-heating applications such as material drying, heat treatment, and space heating.