Solar Radiation

Solar radiation is often called the solar resource or sun radiation or just sunlight, is a general term for the electromagnetic radiation emitted by the sun which is the only source for natural light in our solar system. 

There are different types of solar radiation. Solar radiation or solar rays can be captured and turned into forms of energy that are beneficial to humankind. Solar rays can be converted into heat and electricity, using a variety of technologies.  Depending on the availability of the solar resource the technical viability as well as economical operation of these technologies at a specific location is considered.

Pyranometer

A pyranometer is a solar irradiance sensor. Pyranometers can measure global irradiance. Solar Irradiation is defined as the amount of solar energy per unit area per unit time that has been incidented on a surface of specific orientation emanating from a hemispherical field of view (2π sr) and is denoted as Eg↓. The global irradiance encompasses direct sunlight as well as diffuse sunlight. 

The contribution from direct sunlight is given by E⋅cos(θ); w

here, θ - the angle between the surface normal and the position of the sun in the sky and E- the maximum amount of direct sunlight. The global irradiance is then presented as:

Eg↓ = E⋅cos(θ) + Ed; where Ed accounts for the diffuse sunlight.

The extraterrestrial radiation is also usually expressed in Irradiance Unit (Watts/m²). Due to Earth's elliptical orbit around the sun, extraterrestrial radiation varies throughout the year. It depends on the distance between the Sun and the Earth.

Incoming Solar Radiation 

The incoming solar radiation which is also known as insolation is the amount of solar energy reaching the Earth of about 70 percent.

The three relevant bands, or the ranges, along with the solar radiation spectrum are ultraviolet, visible (PAR), and infrared. Of the light reaching the surface of the earth, infrared radiation makes up 49.4%, while visible light provides about 42.3% 9.

Approximately 23 percent of the incoming solar energy is absorbed in the atmosphere by water vapor, dust, as well as by ozone. About 48 percent passes through the atmosphere and is absorbed by the surface and 23% of incoming energy is absorbed in the atmosphere by atmospheric gases, dust, and also by other particles.

Insolation Means

Sun is the only primary source of energy on this planet. Planets rotation, as well as the revolution, can all be considered based on the sun. So the sun plays a very significant role in the entire solar system.

The sun radiates its powerful energy in and every direction into space in short wavelengths, which are referred to as solar rays. The energy received by the surface of the earth in the form of short waves is termed Incoming Solar Radiation or Insolation. The amount of insolation that has been sustained on the surface of the earth is far less than what has been radiated from the sun because of the small size of the earth and its distance. The temperature of a place is also composed by the insolation received. The insolation differs according to the latitude, which leads to the difference in the temperature.

Solar Ray: Definition 

Solar rays or solar radiations are described as the imaginary lines between the sun and an observation point on the earth.

Diffuse Sky Radiation

Diffuse sky radiation is solar radiation that reaches the surface of the earth after having been scattered from the direct solar beam by molecules or particulates in the atmosphere.  It is also called sky radiation, or diffuse skylight, or just skylight, which is the determinative process to change the colors of the sky. Approximately 23% of the direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere. And of this amount only about two-thirds finally reaches the earth as photon diffused skylight radiation.

Diffuse Radiation and Direct Solar Radiation

As sunlight passes through the atmosphere, some of the sun rays are absorbed, scattered, and reflected by the following:

• Air molecules

• Water vapor

• Clouds

• Dust

• Pollutants

• Forest fires

• Volcanoes

This is known as diffuse solar radiation. The solar radiation that reaches the surface of the earth without being diffused is known as the direct beam solar radiation. The sum of the diffuse as well as the direct solar radiation is called to be the global solar radiation. Atmospheric conditions may lead to the reduction of direct beam radiation by 10 percent on clear and dry days whereas it can be reduced by 100 percent during thick and cloudy days.

Basic Principles of Solar Radiation 

Every place on the Earth receives sunlight for at least part of the year. The amount of solar radiation or solar rays that reach any spot on the Earth's surface varies according to the following:

• Geographic location

• Time of day

• Season

• Local landscape

• Local weather