Solar energy is the result of the nuclear fusion process that takes place in the sun. This energy is the engine that drives our environment, with the solar energy that reaches the Earth's surface being 10,000 times greater than the energy currently consumed by all of humanity.
Radiation is the transfer of energy in the form of electromagnetic radiation. The Sun produces these electromagnetic waves and emits them outwards in all directions.
Solar radiation is measured by its energy power transferred per unit area (w/m²). 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 the solar radiation arriving on a flat surface of the Earth is solar irradiance.
How is solar radiation measured?
Radiation sensors are measurement systems for determining the global solar radiation, spectral response, external quantum efficiency and internal quantum efficiency of solar cells.
A fundamental point to take into account is the duration of sunshine. A simple way to record sunlight hours is to use a sunlight logger, a card system that focuses sunlight. A mark is burned on the recording table if the sunlight is more significant than 200W/m2. Therefore, the number of hours of sunshine is determined as the number of hours the sun shines.
To measure the amount of solar radiation received, we distinguish between three methods:
1. Measurement of direct solar radiation
We can measure Direct Normal Irradiance (DNI) or beam radiation on the Earth's surface at a given location with a surface element perpendicular to the Sun. It excludes diffuse solar radiation. With shutters, only the radiation from the sun and the region of an annular sky very close to the sun are measured.
All pyrheliometers must be mounted on a mechanism that allows precise control of the sun.
2. Measurement of global and diffuse radiation
If the sky is covered in clouds, most of the direct sunlight does not reach the ground. Instead, what descends is refracted by water droplets suspended in the air. Clouds scatter light from the sky and as a result, white light reaches the earth.
Global radiation is defined as 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 the diffuse radiation from the sky spread through the atmosphere.
Global radiation is measured with the pyranometer. To measure only the diffuse component of solar radiation, the direct component is covered by a screen or shading system.
3. Measurement of infrared radiation
Infrared radiation, or IR radiation, is a type of electromagnetic radiation with a wavelength longer than visible light. We use pyrgeometers to measure it.
IR radiation is often associated with the concepts of "heat" and "thermal radiation", since any object with a temperature emits radiation in this band.
Most of these eliminate short wavelengths using filters that exhibit constant transparency at long wavelengths while being nearly opaque at shorter wavelengths.
Shortwave infrared energy comes directly from the sun, but it does not feel like heat. Instead, it turns into heat when it hits an object. Longwave infrared energy is the heat radiated by an object that has received shortwave infrared radiation.
Measurement tools
To measure solar radiation, various instruments specifically designed to capture and quantify the solar energy that reaches a particular location are used.
- Pyrheliometer : This device measures direct solar radiation and consists of a sensor that collects the radiation of incident sunlight at an angle of 90 degrees. Provides an accurate reading of direct solar radiation.
- Pyrgeometer : Unlike the pyrheliometer, the pyrgeometer is used to measure total solar radiation, including direct radiation and diffuse radiation. It is especially useful for calculating the net radiation at a location.
- Pyrranometer : This instrument measures global solar radiation, which includes direct radiation and diffuse radiation. It is widely used in the solar energy industry to evaluate the power generation potential at a location.
- Ultraviolet (UV) Solar Radiation Sensor : These sensors measure ultraviolet radiation from the sun, which is important for assessing UV radiation levels that can affect human health and sun protection systems.
- Global solar radiation sensor : Similar to the pyranometer, these sensors measure the total solar radiation that reaches a surface, including direct and diffuse radiation. They are used in applications such as meteorology and solar energy.
- Solar radiometer : These portable devices measure solar radiation in real time and are useful for evaluating the intensity of solar radiation at different times and places.