Beam, diffuse and global radiation

When sunlight passes through the earth's atmosphere, part is absorbed and scattered by air, gases, dust and clouds. The radiation reaching the surface therefore has two components.

• Beam (direct) radiation, Gb: solar radiation that reaches the earth's surface without scattering, travelling in a straight line from the sun. It casts a shadow and can be concentrated by mirrors and lenses.
• Diffuse radiation, Gd: solar radiation that reaches the surface after being scattered by molecules, clouds, dust and aerosols, coming from all directions of the sky. It casts no shadow. On cloudy days nearly all the radiation is diffuse.
• Global (total) radiation, G: the sum of beam and diffuse radiation received on a surface (usually horizontal). It is what a pyranometer measures.

Relation

G = Gb + Gd (on a horizontal surface, Gb = Gbn cos θz, where Gbn is the beam normal radiation and θz is the zenith angle)

For a tilted surface, the total also includes the radiation reflected from the ground (albedo): Gt = Gb·Rb + Gd·(1+cos β)/2 + G·ρ·(1−cos β)/2, where β is the tilt angle and ρ the ground reflectance.

The diffuse fraction Gd/G is high on cloudy days (up to 100%) and low (10–20%) on clear days. Flat-plate collectors and PV use both beam and diffuse radiation, but concentrating collectors can use only the beam part.