How to Measure Solar Radiation?

Solar energy results from the nuclear fusion process that takes place in the sun. This energy is the engine that drives our environment, being the solar energy that reaches the earth's surface 10,000 times greater than the energy consumed currently for all humanity.

Radiation is the transfer of energy by electromagnetic waves produced directly from the source to the outside in all directions. These waves do not need a material medium to propagate, and they can travel through space interplanetary and reach Earth from the Sun.

Solar radiation is measured by its energy power transferred per unit area (watts / m 2). In general, the Earth receives less than 0.5 × 10 −9 of the energy of its radiation from the Sun.

The functional unit that describes solar radiation reaching Earth is solar irradiance, or unit of power per square meter (w / m²).

Why Is It Important to Measure Solar Radiation?

Getting reliable solar radiation data is vital for:

• Study the transformations of energy in the Earth-Atmosphere system.

• Analyze the properties and distribution of the atmosphere, the elements that constitute it, such as aerosols, steam, ozone, etc.

• Study the distribution and variations of the incident, reflected, and total radiation.

• Satisfy the needs derived from biology, medicine, agriculture, architecture, engineering, and industry related to solar resources.

• To make predictions before installing a PV system with a solar array of solar panels.

• Flux density measurement is precious for operation as well as for performance testing of solar power towers. 

How to Measure Solar Radiation?

Measurement systems are outlined for determining the spectral response, the external quantum efficiency, and solar cells' internal quantum efficiency.

An essential point to bear in mind is the sunshine duration. A simple way of recording sunshine hours is using a sunshine recorder, a card system that focuses sunlight. A mark is burned onto the recording chart if the sunshine is more significant than 200W/m2. Thus, the number of sunshine hours is determined as the number of hours the sun is shining.

To measure solar radiation, we distinguish three methods depending on whether it is:

• Direct.

• Indirect.

• Infrared.

Measurement of Direct Solar Radiation

We can measure Direct Normal Irradiance (DNI) or beam radiation at the surface of the Earth at a given location with a surface element perpendicular to the Sun. It excludes diffuse solar radiation. With shutters, only the sun's radiation and an annular sky's region very close to the sun are measured.

All pyrheliometers are required to be mounted on a mechanism that allows accurate monitoring of the Sun.

How Is Global and Diffuse Radiation Measured?

Diffuse radiation is solar radiation that reaches the earth's surface after being scattered by molecules or particulate matter in the atmosphere.

If the sky is cloud-covered, most of the direct sunlight does not reach the ground. Instead, what comes down is refracted by water droplets suspended in the air. There are many drops, and each has its shape and, therefore, refracts in its way. The clouds scatter light from the sky, and as a result, white light reaches the earth. Clouds absorb part of the light, and it is the reason because the sky looks greyish.

Radiation during scattering does not change much in spectral composition: water droplets in clouds are more significant than the wavelength, so the entire visible spectrum (from red to violet) is scattered in about the same way. In intensity, the radiation varies (estimated) from 1/6 the power of direct sunlight for relatively thin clouds to 1/1000 for the thickest thunderstorm clouds.

Global radiation is defined as the solar radiation received from a solid angle of 2π steradians on a horizontal surface. Thus, global radiation includes that obtained directly from the solar disk and also diffuse radiation from the sky scattered through the atmosphere.

Global radiation is measured by the pyranometer. To measure only the diffuse component of solar radiation, the direct component is covered using a screen or shading system. 

How Is Infrared Radiation Measured?

Infrared radiation - or IR radiation - is a type of electromagnetic radiation with a greater wavelength than visible light. We use pyrgeometers to measure it.

IR radiation is often associated with the concepts of "heat" and "thermal radiation" since every object with temperature emits radiation in this band. According to Wien's law, by increasing the temperature, the emission peak moves more and more towards the visible until the object becomes incandescent ).

Most of these remove short wavelengths by filters that exhibit constant transparency at long wavelengths while being nearly opaque at shorter wavelengths.

Short-wave infrared energy comes directly from the sun but is not felt as heat. Instead, it converts into heat when it strikes an object. Long-wave infrared energy is the heat radiated from an object, which has received short-wave infrared radiation.

What Is the Total Solar Radiation in the Northern Hemisphere?

In this table, we present the data collected in different latitudes in the northern hemisphere.

(Unit: calories / cm²)

What Is the Total Solar Radiation in the Southern Hemisphere?

(Unit: calories / cm²)

Published: July 8, 2021
Last review: August 30, 2022