**6. Solar radiation measurement and analysis**

It is inevitable to know the potential of solar energy available on daily and monthly bases at the site for solar energy application, not only in amount but in quality, particularly its spectral composition. For this, the measurement of solar radiation energy and its spectral distribution under all atmospheric conditions is undertaken also at many radiation networks around the world.

Solar radiation energy arriving at the edge of the earth's atmosphere is carried or conveyed in electromagnetic spectrum, of wavelengths ranging from about 0.2µm to 4µm, as said above. These groups of wavelengths of the solar radiation are of principal importance to the earth and its atmosphere, especially for the calculation of absorption by gases, clouds and aerosols in the atmosphere and to calculate the spectral variation of the earth – atmosphere albedo, and also essential for photosynthesis, photobiology and photochemistry in the atmosphere.

#### **6.1 Basic radiation measurements**

The basic radiation fluxes being actively measured and studied in many radiation network stations globally include the sw-total (global) solar irradiance, sw-direct solar irradiance, swdiffuse or sky irradiance. Other radiation fluxes measured are global and diffuse photosynthetic active radiation( PAR), ultraviolet total optical depth and the sun photometric measurement, and commonly measured radiation parameter is the sun shine hours. However the brief analysis here on radiation measurements is on the global (total) solar irradiance, H, direct solar irradiance, Hb, and diffuse sky irradiance ,Hd.

#### **6.1.1 Global (total) solar irradiance**

Global solar irradiance, H, which is the total sw-radiation flux, measured on a horizontal surface on the ground surface of the earth, comprising the direct sw- solar irradiance, Hb and diffuse sw- sky irradiance, Hd . In simple mathematics, the three fluxes are connected as in the following

$$\mathbf{H} \equiv \mathbf{H}\_b + \mathbf{H}\_d \tag{9}$$

If all measurements were accurate, wherever two of these fluxes are measured, the third can easily be obtained, but this is not always so.

Global (total) solar radiation flux is the most easily and commonly measured of all the radiation fluxes in almost all the radiation network throughout the world. Measurement is done in the shortwave regions, 0.2 to 4.0µm wavelengths, which includes the photo synthetically Active Radiation (PAR).

The measurement is done to date, for example, at BSRN station, Physics Department University of Ilorin using Eppley Precision Spectral Pyranometer (PSP), serial number, SN17675F3 and 28866F3 with calibration constant of 8.2 x 10-6 V/ Wm-2 and well documented calibration history. Data quality is ensured by eliminating spurious errors that could arise from incidental and shading or partial un-shading of sensor by discarding all observations for which the insolation is less than 20Wm-2. The data assembled on minute – by – minute basis was used to generate the hourly, daily and monthly averages.
