This experimental study investigated the thermal performance of three distinct external collector geometries of a single-pass solar air heater under both natural and forced air circulation processes to demonstrate the best design between them. Rectangular, trapezoidal, and novel hybrid collectors were selected. Experiments were performed using real-time climatic data from Iraq's winter season. Key performance indicators, including the temperatures of the inlet air, outlet air, absorber plate, and glass cover, were measured to evaluate thermal efficiency and energy yield. The results revealed that the hybrid model consistently outperformed the other designs in terms of thermal efficiency and loss reduction. Its peak instantaneous efficiency reached 95% at a mass flow rate of 0.018 kg·s −1 near sunset, which reflects a brief optimal condition rather than sustained daytime performance. During daylight hours, the efficiency ranged from 31% under natural convection to 52% under forced convection, after which it gradually increased as solar radiation decreased toward the evening. This trend highlights the hybrid design’s adaptability to varying operating conditions. Under natural convection, the maximum outlet air temperature reached 66.6 °C at peak solar radiation, whereas under forced convection (0.018 kg·s −1 ), it reached 37.5 °C. The hybrid model also demonstrated the lowest thermal losses, decreasing from 27.2 W under natural convection at sunset to just 4.2 W at the highest airflow rate. These findings demonstrate that optimizing the external geometry of air heaters can significantly enhance performance without the complexity of extended surfaces, offering a simpler and more cost-effective design strategy. This study provides a valuable foundation for future developments, including refined internal geometries, integration of thermal storage media, and coupling with industrial waste heat sources, paving the way for more efficient and adaptable solar heating systems.
Salih et al. (2026) studied this question.