Acceptance angle (solar concentrator)

Maximizing the acceptance angle of a concentrator is desirable in practical systems and it may be achieved by using nonimaging optics.

In the center, this figure shows another set of parallel rays, now incident on the concentrator at an angle α = θ to the optical axis.

However, on the right, this figure shows yet another set of parallel rays, now incident on the concentrator at an angle α > θ to the optical axis.

The acceptance angle θ of a concentrator may be seen as a measure of how precisely it must track the sun in the sky.

The smaller the θ, the more precise the tracking needs to be or the concentrator will not capture the incoming sunlight.

However, real optics are never perfect and the right part of the figure shows the effect of a badly made bottom surface s2.

At the end, the concentrator must still have enough acceptance to capture sunlight which also has some angular dispersion θS when seen from earth.

Perfectly parallel rays (shown in blue) would be captured, but sunlight, due to its angular aperture, is partially lost.

This can be summarized in the expression which defines a quantity CAP (concentration acceptance product), which must be smaller than the refractive index of the medium in which the receiver is immersed.