Altermatt Lecture:   The Solar Spectrum


4.6:  The reflectance of the ground

On the previous page, the albedo was defined as the broadband average of the reflectance. The albedo serves as an approximation to calculate backscattering.

Because solar cells are usually mounted at a tilt angle s from the ground, they face not only the sky, but partly also the ground, expressed by the ground view factor:

Eq Ground view factor

Hence, a part of the sunlight that enters the cell does not come directly from the sky, but is first reflected at the ground in the near or far surroundings.

The reflectance in the surroundings may be approximated again by a broadband average, a sort of albedo that is not measured from perpendicular above the ground, but from a tilt angle. It is called the tilt albedo or the local albedo. However, it has turned out that this albedo is often too approximate: it is necessary to take the spectral reflectance into account, called the reflectance of the ground.

Typical reflectance spectra are shown below [18][19][20]. The reflectance of some surfaces depend on the angle of the observer and the sun, as for example the sea and some metallic roofs [21].

Why does live vegetation cause a strong increase in reflectance around 700 nm?

→ Hover here for a hint.

→ Hover here for the answer.

Reflectance vs wavelength of natural surfaces Reflectance vs wavelength of roofing materials

Figure: Reflectance vs wavelength for natural surfaces (left) and roofing materials (right).


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