Solar Irradiation of a Horizontal Surface
A horizontal surface is irradiated from two sources:
Direct beam radiation from the sun, which for many purposes can be treated as an intense point source of energy.
Diffuse irradiance from the sky which can be treated as a hemispherical source of energy. The distribution of irradiance from this hemisphere is not always even. Under a clear sky it will be higher in the direction of the sun. Under an overcast sky it will be more or less uniform.
This is illustrated in the diagram below:
The total irradiance (often referred to as the global horizontal irradiance) is the sum of the horizontal beam and diffuse components:
From Lambert's Cosine Law, the horizontal beam component is a function of beam irradiance and the zenith angle:
The equation for global irradiance can then be written as:
Typically, horizontal irradiance is measured by instruments known as pyranometers or solar radiom meters. These consist of a sensor mounted in the horizontal plane. A variation of the basic design is to shield the sensor from the direct beam of the sun, either with a narrow band or disk, thus the sensor only receives diffuse irradiance:
After the output of the sensor has been corrected for the presence of the band, an estimate of the beam irradiance can be made by using a re-arrangement of the above forma:
The mechanical arrangements for the shadow band vary considerably, the two common ones are:
Two separate instruments, one with a shadow band and one without The upside of this arrangement is that measurements of global and diffuse irradiance are for the same instant in time.
A single instrument with some form or mechanism to position the shadow band. Measurements are alternatively taken when the shadow band is in place, then a short period later when it has been moved below the sensor field of view.
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