# Thermal Performance Analysis of a Double-Pass Solar Air Collector: A CFD Approach
###### Abstract
\begin{table}
\begin{tabular}{c c c c c} \hline
**Reference** & **Solar Technology** & **TES Fluid** & **CFD Model** & **Radiation Model** \\ \hline
[9] & Flat-plate & Air in gap of upper part &
\begin{tabular}{c} 2D model \\ Newtonian and incompressible gas \\ Boussinesq approximation \\ \end{tabular} & ER: Constant radiative flux \\ \hline
[10] & Heat-pipe evacuated & \begin{tabular}{c} Water in heat pipe \\ Air or OCMs in inner \\ evacuated tube \\ \end{tabular} & \
\begin{tabular}{c} 3D model \\ Boussinesq approximation \\ \end{tabular} &
\begin{tabular}{c} ER: Time-dependent polynomial \\ function for the solar irradiance data \\ as a boundary condition \\ \end{tabular} \\ \hline
[11] & Evacuated tubes with & \
\begin{tabular}{c} Air \\ \end{tabular} &
\begin{tabular}{c} 3D steady-state model \\ RNG k-\(\epsilon\) turbulence model with \\ enhanced wall treatment model \\ under periodic flow conditions \\ \end{tabular} & ER: Uniform and constant heat input \\ \hline
[12] & Flat plates with &
\begin{tabular}{c} 3D steady-state model \\ Incompressible air \\ Boussinesq approximation \\ Standard k-\(\epsilon\) turbulence model \\ \end{tabular} & IR: Discrete ordinates model \\ \hline
[13] & \begin{tabular}{c} CPC with a \\ single-pass evacuated \\ tube receiver \\ \end{tabular} & Air & \
\begin{tabular}{c} 3D steady-state model \\ k-\(\epsilon\) turbulence model \\ \end{tabular} &
\begin{tabular}{c} ER: Ray-tracing and finite volume \\ solver to determine the non-uniform \\ solar flux distribution on the \\ absorber surface \\ \end{tabular} \\ \hline
[14] & CPCs with three types & \begin{tabular}{c} Water \\ \end{tabular} & \
\begin{tabular}{c} 3D transient-state model \\ Incompressible fluid \\ Spalart-Allmaras turbulence model \\ \end{tabular} &
\begin{tabular}{c} ER: Ray-tracing and finite element \\ solver to determine the incident solar \\ flux on the absorber surface \\ \end{tabular} \\ \hline
[15] & CPCs with tubular & \
\begin{tabular}{c} Water in absorber \\ Air in cavity \\ \end{tabular} &
\begin{tabular}{c} 3D steady-state model \\ Boussinesq approximation \\ Standard k-\(\epsilon\) turbulence model \\ \end{tabular} & ER: Ray-tracing used to determine a \\  & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Solar collectors CFD and radiation models in the literature review. Thus, [PERSON] et al. determined the influence of the thickness and number of transparent covers to reduce the heat losses of a flat-plate solar collector [9], and [PERSON] and [PERSON] modeled an evacuated heat-pipe solar collector with and without integrated phase change materials as a thermal storage medium [10]. In the particular case of solar air collectors, [PERSON] et al. analyzed an evacuated tube collector with ribs of different roughness [11]. At the same time, [PERSON] et al. performed a three-dimensional CFD model to optimize the design of a solar air collector with an extended surface area by a different number of rectangular fins [12]. In addition, they analyzed the effect of adding a selective surface on the absorber. Regarding the analysis of CPC collectors, [PERSON] et al. analyzed by CFD the thermal behavior of an evacuated tube collector as a receiver of a compound parabolic concentrator, and the simulation was validated with experimental data [13]. [PERSON] et al. performed the numerical simulation of CPCs with tubular receivers, with and without fins, for residential water heating [14]. This study focused on the ray-tracing analysis of radiation and heat transfer by coupled finite element and CFD methods. In addition, [PERSON] et al. developed two simplified computational fluid dynamics models to determine the temperature and velocity distribution in two almost identical parabolic tube-receiver CPCs [15]. One had a transparent ETFE sheet around the receiver to reduce convective heat losses. The models included the reflector, receiver, cover, and back insulating material and allowed the analysis of both air movement in the cavity and water movement in the absorber tube.