Solar Irradiance Measurement - Part 1 - Basic Definitions

Solar Irradiance

Irradiance is defined as the amount of solar power falling onto a unit area. In other words it is the solar power flux. The SI unit of solar irradiance is Watts per square meter. 

Types of Solar Irradiance

Sun light does not reach the earth directly. It travels through the space and strikes the upper atmosphere of earth. Then it crosses the atmosphere and reaches the earth's surface. While passing through the atmosphere, a considerable amount of light gets scattered because of aerosol particles, gas molecules and dust particles. The sun light reaching the earth's surface is either the direct sunlight which makes its way through the atmosphere without getting scattered and the scattered sunlight. These two parts of sunlight are measured as Direct Normal Irradiance (DNI) and Diffused Irradiance. 

Direct Normal Irradiance (DNI)

Direct Normal Irradiance or DNI is the sunlight that comes directly from the solar disk without getting scattered in the atmosphere. 

Diffused Irradiance (DI)

Diffused Irradiance is the measure of scattered sunlight. The standard way to measure this irradiance is to use a shaded pyranometer mounted horizontally. This arrangement measures what is known as Diffused Horizontal Irradiance (DHI). 

Global Irradiance (GHI)

In general the Global irradiance measured on a horizontal surface is a function of direct normal irradiance and diffused horizontal irradiance. The three types are related as follows

GHI =  DHI + DNI * cos(zenith angle) ----------- (1)

Where the zenith angle is measured between 0 & 90 degrees and is the representative of height of sun in the sky. It is zero when the sun is directly overhead (see sub-solar point) and is 90 degrees when the sun is at the horizon.

Ground Reflected Irradiance

Equation 1 is a general formula for calculating the global horizontal irradiance but it lacks one component known as the ground reflected irradiance which as the name implies is the sunlight reflected from the ground and reaches the measuring surface. 

The amount of ground reflected irradiance depends upon the reflectivity of the ground (also known as Albedo). The albedo is very high after snowfall. 

What is  Sub-Solar Point?

A sub-solar point is a location on any planet which, at some time of a year, comes directly under the sun or the the sun is at the zenith.

The sun-light at the sub-solar point hits the surface, perpendicularly.

The sub-solar point can also be perceived as the the location closest to the sun.

Significance of Sub-Solar Point:

The sub-solar pint defines the locations of Tropic of Cancer and Tropic of Capricorn on Earth. The tropics are the two latitudes which mark the extreme North and South movement of sub-solar point throughout the year.

On Summer Solstice (21 June) the sub-solar point is on the tropic of cancer whereas on winter solstice (22 December) the sub-solar point is on the tropic of Capricorn. Moreover the sub-solar point always remains between the two tropics and reaches the equator, two times a year known as equinoxes.


The Tropics:

The tropics are two latitudes on the surface of earth that correspond to the maximum and minimum solar declination angle throughout the year.
The maximum declination angle occurs on June 21 and is equal to +23.4 degrees whereas the minimum declination angle occurs on December 21 and is equal to -23.4 degrees.