Solar Irradiance

Solar irradiance is the output of energy from the sun received at an area on the Earth. It is measured in W/m² (Watts per meter squared) in the form of electromagnetic radiation.

The Sun’s output of energy is the primary source of energy to Earth, it affects everything from plant metabolism to climate change. Small changes in output can have dramatic effects here on earth.

direct normal radiation sensor
Pyrheliometer, used to measure direct normal irradiance.

Using historic and current measurements the agricultural sector can better plan when to grow crops. Climate modellers use the data to obtain precise outputs from their models, helping them make more accurate predictions.

Solar irradiance can be measured here on Earth or in space. On Earth, measurements are affected by atmospheric conditions, the position of the sun in the sky, and the angle of the measuring surface. Whereas in space, where there is no atmosphere, solar irradiance is largely a function of the distance from the sun, though the solar cycle also plays a part.

Large solar farms are particularly interested in solar irradiance. Measurements are used to determine the optimal sites to build new solar farms and to optimise panel performance. When operational, solar irradiance is used to monitor PV panels’ performance.

As solar irradiance on Earth is subject to numerous factors several sensors are used to take measurements.  We will explore the primary ones below.

Diffuse Horizontal Radiation (DIF)

As the Sun’s radiation enters the Earth’s atmosphere it is scattered by particles and molecules.

Rayleigh Scatter is one form of this, and occurs when radiation interacts with small particles in the atmosphere. Shorter wavelengths are scattered more readily, hence why the sky is blue (Blue and violet light are in the short wavelength end of the spectrum).

Solar irradiance
Pyranometer and pyrheliometer measuring solar irradiance.

Mie scatter and Non-Selective scatter have a larger effect on diffuse horizontal irradiance. Mie scatter occurs when the radiation wavelength is of a similar size to atmospheric particles e.g. pollution, dust and pollen.
Non-Selective scatter occurs when the particles are larger than the wavelength of the radiation, scattering visible and infrared spectrums of radiation; clouds being a primary contributor to non-selective scatter.  

Pyranometers measure diffuse horizontal radiation. When tilted at an angle a pyranometer can measure tilted global irradiance, an important measurement for solar farms.

Direct Normal Irradiance (DNI)

The amount of radiation received per unit area on a surface perpendicular (normal) to the sun is called direct normal irradiance, it is measured using a pyrheliometer which needs to track the sun’s location. The pyrheliometer does not measure diffuse radiation. Direct normal irradiance is important to concentrated solar power stations, which rely on the measurements to actively position mirrors or lenses to optimise reflection of the sun’s energy towards a receiver.

Global Horizontal Irradiance (GHI)

The total radiation received on a horizontal surface on the Earth. Global Horizontal Irradiance factors in both Direct Normal Irradiance and Diffuse Horizontal Irradiance, while also accounting for the angle of the sun.

If you would like to know more about our range of solar radiometers then please contact ESS Earth Sciences.