Page 32 - RSE - Results of the Apollon Project

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Optics

(ENEA, RSE, SOLARTEC, CPOWER)

Optics in CPV technology have the fundamental function to concentrate the solar fux onto the solar cell, allowing
the reduction of the semiconductor material needed to produce a given electrical power. With a constant aperture
cross-section of the optical system, the higher the geometrical concentration factor the lower the surface of the
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semiconductor materials needed to produce the same power. For this reason, the development of optics having
high concentration factors is of great interest. An ideal CPV optical system should have the following features: high
concentration factor, long term durability, low cost, adequate irradiation uniformity in full wavelength spectrum
and high acceptance angle . Depending on their operating mode two optical systems can be used for concentrating
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the radiation: Refective and Refractive.

State of the art

the Problem of illumination uniformity and Low Acceptance Angle at high
Concentration Factors

Among the refractive optical systems, Fresnel based technology is certainly the most widely used. Fresnel
lenses are composed of several trapezoidal torus sections stacked on each other (see Figure 25). However at high
concentration factors (≥500x), Fresnel lenses have shown a non-uniform spatial distribution of the light in the
focal plane and a reduction of the acceptance angle (±<0.5°). The former effect penalizes the solar cell fll factor,
while the latter makes tracking requirements more severe, thus producing an increase in the CPV system cost. In
order to maintain high system performance and competitive CPV costs when using high concentration factors, the
manufacturing process of a Fresnel lens must be optimized. Furthermore, a Secondary Optical Elements (SOEs)
can be used as a second stage, located after the primary lens, for homogenizing the incident spectrum over the PV
cell surface and for increasing the acceptance angle of the optical system. The development of the SOE has to be
addressed by considering the low cost requirement for the whole system.

FiguRE 25. Grooves-in (left) and grooves-out (right) Fresnel lenses

27 The geometrical concentration factor is given by the ratio of the aperture area (collecting surface) of the optical system and the designated illumination
area of the cell.
28 The acceptance angle is the maximum tolerated misalignment between the sun and the input optical axis of the concentrator.

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