Harnessing solar energy through solar air heaters (SAHs) offers a sustainable solution for heating and drying applications. This study presents a computational investigation of SAHs with novel inclined offset ribs aimed at enhancing thermal performance. Unlike most studies which focused on first law efficiency, the present work emphasizes second law optimization through evaluation of the energy devaluation number ( \(N_{{{\text{dev}}}}^\text{en}\) ) and exergy destruction number ( \(N_{{{\text{des}}}}^{\text{ex}}\) ). The Reynolds-averaged Navier–Stokes equations, coupled with the \(RNG k - \varepsilon\) turbulence model, are solved for rib inclination angle \(\emptyset\) (= 45° to 150°), with corresponding relative angle ( \(\alpha = \frac{\varphi }{{90^{0} }}\) = 0.5 to 1.66). Additionally, the vertical length of the ribs ( \(l_{{\text{r}}}\) ) is varied from 0.25 to 2.00 mm, with corresponding blockage ratio \(\beta\) ranging from \(0.0125\) to \(0.1\) . Reynolds numbers \(\left( {Re} \right)\) ranges from 6000 to 21,000. The entropy generation due to viscous dissipation \(\dot{S}_{{{\text{gen}},\text{D}}}^{{\prime \prime \prime }}\) and heat transfer (conduction) with finite temperature difference \(\dot{S}_{{{\text{gen}},{\text{C}}}}^{\prime \prime \prime }\) are studied along with Nusselt number \(\left( {Nu} \right),\) friction factor \(\left( f \right)\) , and thermal enhancement factor \(\left( {TEF} \right)\) . The results show that \(N_{{{\text{des}}}}^\text{ex}\) increases, while \(N_{{{\text{dev}}}}^\text{en}\) decreases with rising \(Re\) , due to turbulence and convective heat transfer, which intensify temperature gradients and viscous dissipation, leading to greater entropy generation and more effective energy utilization. However, both \(N_{{{\text{dev}}}}^\text{en}\) and \(N_{{{\text{des}}}}^\text{ex}\) gradually decline reaching a minimum at \(\alpha = 1.33\) and 1.50. The near-wall regions are dominated by direct (mean) dissipation \(\dot{S}_{{{\text{gen}},{\overline{\text{D}}}}}^{\prime \prime \prime }\) due to molecular viscosity, whereas indirect (turbulent) dissipation \(\dot{S}_{{{\text{gen}},{\text{D}}^{\prime } }}^{\prime \prime \prime }\) dominates the regions around the ribs, particularly in the wake. Total entropy generation \(\dot{S}_{{{\text{gen}},{\text{T}}}}^{\prime \prime \prime }\) follows a pattern similar to \(\dot{S}_{{{\text{gen}},{\text{C}}}}^{\prime \prime \prime }\) , confirming that thermal irreversibilities predominantly govern overall entropy generation. An optimal blockage ratio of \(\beta\) = 0.05 yields minimum values of both \(N_{{{\text{dev}}}}^\text{en}\) and \(N_{{{\text{des}}}}^\text{ex}\) . Therefore, the configuration with \(\alpha = 1.5, \beta = 0.05,\) and \(Re = 15000\) yields the optimal overall thermo-hydraulic performance based on both first and second law assessments.
