DNI (Direct Normal Irradiance)
Solar radiation arriving in a straight line from the sun disc, measured on a surface held perpendicular to the sun rays.
DNI counts only the beam component of sunlight that has not been scattered by the atmosphere, measured on a plane that always faces the sun. It is measured with a pyrheliometer mounted on a sun-tracking device, whose narrow field of view (about 5°) excludes most sky light.
DNI is the decisive resource for technologies that concentrate light, because mirrors and lenses can focus only the direct beam:
- Concentrated solar power is generally considered viable above about 1,900–2,000 kWh/m²/yr of DNI (IRENA, NREL); leading sites in Morocco, Chile, South Africa and the UAE exceed 2,500.
- Concentrator PV and dual-axis trackers also depend on high DNI.
Clear-sky DNI at noon can reach 900–1,050 W/m² in dry, high-altitude locations, but it collapses to near zero under clouds, and haze, dust or wildfire smoke can cut it sharply even when GHI changes little. DNI is therefore more variable than GHI, and annual DNI totals in humid tropics can be lower than in temperate deserts despite stronger sun.
where θi is the angle of incidence between the sun rays and the surface normal; trackers try to keep θi near zero.
Related terms
All terms →- GHI (Global Horizontal Irradiance)
Total shortwave solar radiation received on a horizontal surface, the sum of direct beam and diffuse sky components.
- DHI (Diffuse Horizontal Irradiance)
Solar radiation reaching a horizontal surface after being scattered by molecules, aerosols and clouds, excluding the direct beam from the sun disc.
- Concentrated solar power (CSP)
Power plants that use mirrors to concentrate direct sunlight onto a receiver, heating a fluid that drives a steam turbine, usually with built-in thermal storage.
- Solar tracker
A motorised mounting structure that rotates PV modules or mirrors to follow the sun, raising the irradiance they receive over the day.
- Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).