Sunlight Story (3)

Editor : Optomon

 On my previous article, Sunlight Story (2), I explained about a Solar Simulator and its standards. In this article, Sunlight Story (3), I am going to talk about how sun’s radiant energy can be measured.

How sunlight, sun’s radiant energy can be measured?

 There are four methods for measuring sun’s radiant energy. The first method is measuring the radiant directly by Pyranometer and the second method is by photovoltaic sensor, the third method is by analyzing meteorological data from a satellite and the last method is by using Spectroradiometer. In this article, I am going to talk about Pyranometer and Photovoltaic Sensor which are commonly used.

Solar Radiation Spectrum Pyranometer


 You can consider a Pyranometer as a precise sensor measuring sun’s radiant energy which radiates on a horizontal surface. As you can see from the image below, it is consisted of two hemispherical domes and it is a commonly used radiometer to measure Global Irradiance (or diffuse irradiance using shield band or disk) on horizontal surface, in other words, it is a radiometer used to measure Global Irradiance or Total Irradiance which even includes reflected light from a surface with angle. Pyranometer can be used to measure Total Irradiance on a surface with a specific angle. In this case, a factor which is resulted by reflected irradiance is commonly included. In short, Pyranometer measures temperature increase by absorbing solar energy using black metal plate.


Thermopile Pyranometer


 Photovoltaic Sensor is much cheaper than Pyranometer. It is normally crystalline silicon sensor and generates current in proportion to radiant energy. However, it cannot detect all data due to wide spectrum of solar radian energy. PV sensor is also used as a component of a monitoring system for large PV power generation system’s operation management. The measured data (logs) is saved on advanced inverter or industrial computer and can be used for analysis.


Si-Pyranometer (Photodiode)


Reference Cell (K801, McScience)

Directional Error

What are quantitative expression ways of Solar Radiant Energy?

 As I explained on my previous “Sunlight Story(1)”, sun’s surface is about 5,800K. Since sun is spherical, sun radiates light and heat in all directions. When sun’s irradiance E0 is measured, 1,325 W/m2 and 1,412 W/m2 can be obtained and Solar Constant E0 (1367 W/m2) is used which is average of the two values. 1 Sun is 1,000 W/m2 and it is a value excluding light which are scattering, reflecting and being absorbed in atmosphere.
 The graph below shows range of visible, ultraviolet and infrared light, and AM is short for Air Mass. AM0 is an irradiation outside atmosphere and AM1.5 is one at the surface of the earth where we live. Also, you can notice that blue light has more energy than red light.

Tilt of incidence plane

The spectrum of the solar energy

To be continued on February’s issue…