Performance improvement of a double-pass solar air heater with fins and baffles under recycling operation

The performance of a solar air heater featured with double-pass as well as fins and baffles design was investigated for the effect of recycling operation via both experimental and theoretical approaches. The experimental results deviate by 1.5–23% from the theoretical predictions. The performance of solar air heaters with different designs are compared, including the single-pass, double-pass with recycle, fined double-pass with recycle, and fined plus baffled double-pass with recycle. The double-pass device introduced in this study was proposed for aiming to strengthen the convective heat transfer coefficient and enlarge the heat transfer area. Based on both theoretical and experimental results, the collector efficiency of the fined plus baffled double-pass with recycle design is much higher than the other designs under different reflux ratios and mass flow rates. The optimal reflux ratio for the fined plus baffled double-pass design is about 0.5 while considering both the collector efficiency and the pumping power requirement. An economic consideration in terms of the heat transfer efficiency and power consumption increment for double-pass operation is delineated.