*2.3. Efficiency Calculation*

The thermal efficiency of the collector is calculated as the ratio between the useful energy gained by the fluid on the collector's cavity and absorber and the net solar energy on the collector's aperture, using Equation (16):

$$\eta = \frac{Q\_u}{A\_c I} \tag{16}$$

The heat transfer of the analyzed solar collector resembles a counterflow heat exchanger, where the cold fluid flows through the cavity, and the hot fluid flows through the duct. Therefore, the useful energy gain of the collector is calculated using Equation (17).

$$Q\_u = \dot{m} \mathbb{C}\_{p, \text{avg}} \left( T\_{p \approx 2, \text{out}} - T\_{p \approx 1, \text{in}} \right) \tag{17}$$

#### **3. Results and Discussion**

Once the simulation model of the four CPC configurations with different inlet and outlet positions was implemented (see Figure 2), simulations were carried out considering the boundary conditions for two different values of mass flow rates to analyze: 0.01 kg/s and 0.02 kg/s. The materials, air properties, and boundary conditions are presented in Tables 2–4, respectively. In addition, the results of the hydraulic and thermal behavior of the air in the cavity of the CPC for each of these configurations are presented below.
