Thermal Efficiency (\( TE = \alpha \tau \cos(\beta) - \mu \frac{T_a - T_u}{Q_g} \)):

Where:

• \( TE \) is the thermal efficiency, in percent (%).
• \( \alpha \) is the heat transfer coefficient of the absorber material.
• \( \tau \) is the transmissivity of the covering surface.
• \( \beta \) is the angle between a line perpendicular to the surface and the direction of radiation, in degrees.
• \( \mu \) is the coefficient for losses through convection, conduction, and insulation.
• \( T_a \) is the average temperature of the absorber, in degrees Celsius (°C).
• \( T_u \) is the ambient air temperature, in degrees Celsius (°C).
• \( Q_g \) is the global radiation intensity, in kilowatts per square meter (kW/m²).