It is the amount of energy absorbed per square meter by a surface (free from any shade or shadow) that does not travel directly from the sun but is instead evenly distributed from all directions due to atmospheric molecules and particles.
The amount of solar radiation received by a surface that is constantly held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky is known as direct normal irradiance (DNI).
A surface can often get the most irradiance per year by maintaining a normal angle to incoming radiation. Both concentrated solar thermal plants and installations that track the location of the sun are particularly interested in this amount.
How can Direct Normal Irradiance be measured?
A device called a pyrheliometer is the main tool used to measure DNI. Pyrheliometers frequently use thermopile sensors at the base of a light-collimating tube and a glass window face, though they can alternatively be made with another photosensitive component in place of the thermopile.
The pyrheliometer can only measure the direct beam and circumsolar radiation because of the light-collimating tube’s 5° field-of-view restriction. Moreover, this restricted range of view necessitates tracking pyrheliometers parallel to the sun.
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In the absence of direct observations of DNI, co-planar measurements of the diffuse and total radiation by tools with a 180° field of view can be used to determine DNI (the incident angle between the collection plane and sun must also be known).
Models have been devised to estimate DNI from the global horizontal irradiance (GHI) and other environmental parameters if co-planar diffuse and total radiation measurements are not available. The DISC and DIRINT models, created by E. Maxwell and R. Perez, respectively, are two well-known models.
