Direct Solar Radiation (\( Q_i = Q_o \times D \times A \times \cos(\alpha) \)):

Where:

• \( Q_i \) is the direct solar radiation, in kilowatts (kW).
• \( Q_o \) is the solar constant, in kilowatts per square meter (kW/m²).
• \( A \) is the absorber surface area, in square meters (m²).
• \( D \) is the transmission factor (0.06 – 0.82).
• \( \alpha \) is the angle between a line perpendicular to the surface and the direction of radiation, in degrees.