# Thermal Performance Analysis of a Double-Pass Solar Air Collector: A CFD Approach
###### Abstract
performed three-dimensional modeling of a flat-plate receiver CPC to determine the thermal losses in the cavity as a function of its aspect ratio and tilt, the optical properties of the materials, and the absorber and ambient temperatures [18]. To model the internal radiative heat transfer, they used a discrete ordinary radiation model, and for the external one, they established the thermal boundary conditions and emissivity. As mentioned above, another strategy to improve the efficiency of solar air heating collectors is to increase the number of passes. Thus, [PERSON] et al. analyzed the effect of double-pass configuration in a solar air heater when operating in concurrent parallel flow, parallel in counterflow, and double U-pass [19]; the latter presented the best thermal performance. [PERSON] et al. analyzed, through CFD simulation, two flat-plate solar collectors for air heating with three and four passes and determined which one had the best performance to evaluate it experimentally [20]. In both solar collectors analyzed, air enters through the lower pass and exits through the upper pass, which has the radiant heat gain. They found that the four-pass collector has a heat gain 3 \({}^{\circ}\)C higher than that obtained with the three-pass collector and that the maximum deviation between the CFD model and the experimental results was 10%. In addition, [PERSON] and [PERSON] performed a CFD analysis of a two-covers, double-pass flat-plate solar collector for air heating, which was validated with experimental results [21]. The air enters through the space between the two covers and returns between the second cover and the absorber plate. The temperatures on the absorber plate obtained with the model had a standard deviation from experimental results between 1.05 K and 4.65 K, while for the cover, it was between 0.1 K and 0.45 K.  
Likewise, improved surfaces or novel geometries have been incorporated in multi-passes solar collectors, such as the work of [PERSON] and [PERSON] [22]; they analyzed the impact of using smooth, rough, and corrugated surfaces in a double-pass flat-plate air solar collector and obtained average thermal efficiencies of 78%, 62%, and 90%, respectively. On the other hand, [PERSON] determined the performance of double-pass flat-plate air solar collectors with different fin configurations [23]. They varied in size, angle, arrangements (in-line, staggered, and hybrid), and hydraulic diameter. Finally, [PERSON] et al. proposed a curved air heater with asymmetric double-pass counterflow turbulators, whose design was determined from CFD analysis by comparing various flow configurations and geometric parameters [24]. Two or more pass technologies have been applied in flat-plate solar collectors to improve their efficiency; however, this strategy has not been applied in CPCs for air heating. This study proposes the CFD analysis of a CPC-type solar air heater with U-shape double-pass airflow. The air first circulates through the trapezoidal cavity contained in the volume formed by the cover, the reflecting walls of the CPC, and the flat-plate receiver and then circulates in counterflow through the receiver's duct interior. The objective of the numerical analysis presented is to test different inlet and outlet configurations in the CPC array to determine how these configurations influence the velocity distribution, outlet temperature, and instantaneous efficiency of the U-shape double-pass CPC solar heater. Section 2 of this manuscript describes the main characteristics of the U-shape double-pass CPC and the four air inlet/outlet configurations considered in its design. It also defines the mesh design to perform the CFD simulation, the mathematical model for such simulation, the boundary conditions applied in the study, and the methodology followedto estimate the thermal efficiency of the U-shape double-pass CPC. Section 3 includes a summary of the simulation results obtained and their discussion and concludes with a summary of the efficiencies calculated for each of the four configurations analyzed.