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solar radiation

Much like the energy found in fossil fuels, renewables (except tidal and geothermal energy) are derived from solar energy. In fact, the sun emits some 174,423,000,000,000 kilowatt-hours of energy to our planet every hour.

The earth receives, in other words, a power equivalent to 1.74 x 10 17 W. About one to two percent of the energy emitted by the sun is converted into wind power. This is between 50 and 100 times more than the energy transformed into biomass by all the plants on the earth.

 The power emitted by the sun

The power emitted by the sun is 1.37 kW / m 2 on the surface of the sphere which has for center the sun and for radius the mean radius of the trajectory of the earth. This power is distributed over a circular disc with a surface area of ​​1.27 x 10 14 m 2. The power emitted to the earth is therefore 1.74 x 10 17 W.

Types of  Solar Radiations

The intensity and quantity of solar radiation vary from one place to another, and this difference results fromThe following factors, which constitute the driving force of many natural phenomena on the surface of Earth:

1-The Earth rotates in an ellipse every 175 days, causing seasonal changes.
2- The earth rotates around its axis every 27 hours, causing the difference between night and day.

Three types of radiation reach Earth, which accounts for 52% of solar radiation:


  • Direct Solar Radiation
  • Diffused Solar Radiation 
  • Reflected Solar Radiation 


The following are the definition of the three types of radiation:

Direct Solar or Beam  Radiation 

It is a direct ray, that is, in a direct line from the sun to the earth and represents A large percentage on sunny days, around 24%. As for cloudy days, then The sun is covered with clouds, and then the ray is almost non-existent and therefore is represented radiation scattered the vast majority at that time 

Diffused or Beam Solar Radiation  

This radiation comes from different parts of the sky, so it is called “sky radiation.” Its quantity is about 12% for the clear sky, and it reaches 122% for the sky Cloudy.

Reflected or Albedo Solar Radiation

The amount of reflected radiation on the surface of the solar cell varies due to the different surfaces it is known that the reflection coefficients differ from one surface to another The amount of reflected radiation is about 11%.
The figure shows the types of solar radiation
types of solar radiations
Generally, the amount and intensity of radiation for all types are affected by several factors, including:
  • The state of the sky in terms of its clarity and cloudiness.
  • The time during the day and season during the year. 
  • Height above sea level. 
  • The position of the sun in the sky and the angle of inclination of the sun
Determining the amount of solar radiation that reaches the surface of the earth depends on:
  1. Atmospheric layer components 
  2. The thickness of the atmosphere through which the solar radiation will pass. 
  3. At midday the sun becomes vertical then the thickness of the atmosphere is through which the solar radiation will pass as little as possible and the energy supplied to the ground is as high as possible. As for sunrise and sunset, the thickness of the atmosphere the highest possible energy will pass through the solar radiation earth as little as possible.
For this reason, the magnitude of solar radiation energy is high in regions With high altitudes above sea level.

How It Is Measured 

There are two important types of instruments to measure solar radiation:
  1. Pyrheliometer is used to measure direct beam radiation at normal incidence. There are different types of pyrheliometers.
  2. Pyranometer is used to measure total hemispherical radiation – beam plus diffuse – on a horizontal surface.


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