# Thermal Performance Analysis of a Double-Pass Solar Air Collector: A CFD Approach
###### Abstract  
Solar air heaters can reduce climate change by replacing conventional fossil fuel-burning technologies in drying and space heating applications. Concentrating solar technologies, such as compound parabolic concentrators, allow air temperatures up to 120 \({}^{\circ}\)C; however, it is desirable to improve their heat transfer to reduce the space requirements for their installation. In this work, a parabolic concentrator composed of a flat receiver designed to recover heat from the cover-receiver-reflectors cavity is analyzed, operating it as a U-shape double pass solar heater. With this operation, first, the air flows through the cavity, and then it is incorporated into the duct, where the dominant heat gain occurs due to the capture of solar radiation. Thus, four input-output configurations in the cavity were modeled through dynamic simulations to determine the influence of the inlet and outlet air flow positions on the solar concentrator outlet temperature. Therefore, the incorporation of the first pass has a contribution of between 36% and 45% in useful energy gain, showing that this appropriate and relatively simple strategy can be implemented to improve the thermal performance of solar air collectors, resulting in instantaneous efficiencies higher than 75%. However, the simulation results demonstrate that the position of the inlets and outlets does not significantly impact the efficiency and outlet temperature. solar energy; CPC; solar heating; solar drying; industrial process; solar air heater; space heating +
Footnote †: journal: Applied Sciences  
technologies [2], so it is essential to develop reliable and economically efficient solar air heating technologies. Flat-plate solar collectors are recommended for temperatures below 70 \({}^{\circ}\)C because of their ease of manufacture and operation. For higher temperatures, it is necessary to use some solar concentrating technology, such as compound parabolic concentrators (CPCs), which allow fluid heating temperatures up to 120 \({}^{\circ}\)C, depending on their design, and are easy to operate and maintain. However, to ensure good efficiency of CPCs, it is necessary to perform an optimal optical design and minimize thermal losses or improve heat transfer. Strategies to reduce convection losses in the receiver of a solar collector include using evacuated tubes or filling the CPC cavity with gases such as Argon and Krypton [3,4], which are denser gases and have lower thermal conductivity than air, or even applying a vacuum throughout the cavity [5]. In contrast, double absorbers have been proposed to reduce conduction losses [6]. In other technologies, such as flat-plate solar air heaters, it has been proposed to increase the heat transfer rate by incorporating multiple passages, including extended surfaces, artificial roughness, and packed mesh [7]. This multi-pass strategy is used in hybrid CPCs (PV/T) to cool the photovoltaic cells on the flat-plate receiver with fins on the back side [8]. In the general design of solar collectors, computational fluid dynamics (CFD) tools can be used to reliably estimate their thermo-hydraulic performance before building them, saving time and resources. Several analyses of solar collectors by computational fluid dynamics (CFD) can be found in the literature, both for liquid and air heating. Table 1 provides an overview of the different solar collector models and assumptions found in the literature review.