Page 33 - RSE - Results of the Apollon Project

Results of the APolloN PRoject ANd coNceNtRAtiNg PhotovoltAic PeRsPective

Apart from Fresnel lenses, other interesting optical solutions can be included for a comparative test. Namely,
an original solution was proposed by ENEA in the Photovoltaic Concentrators to Utility Scale Project (PhoCUS).
That was the prismatic/hybrid lens optical system. In addition to their high optical effciency, exceeding 80%, these
lenses offered a further beneft: they did not need to be covered by any glass sheet, therefore their excellent optical
performance could be attained also under the actual working condition, allowing the manufacturing of lightweight
modules. Since under the PhoCUS project such lenses were manufactured for medium concentrations, APOLLON’s
challenge in this respect was to push this technology toward higher concentration factors on a reduced area.

the Problem of Low Optical Effciency

The constant improvement in solar cell effciency provided by the multi-junction technology has to be followed
by a constant increase in the effciency of the optical system. As a matter of fact, huge differences can be found
between the CPV cell effciency value and the module effciency value. At the beginning of the APOLLON project
the effciency of a module was around 22%, obtained by using MJ solar cells with effciency values around 35%. This
remarkable difference was actually due to the low optical effciency of the concentrator, together with the mismatch
losses and tracking inaccuracy losses.

Refective Optics: a Better understanding of Spectrum-Splitting-based Solutions in
Comparison with Simple Mirror-based Ones

Dense array High CPV systems use large mirrors to concentrate the light onto a group of solar cells. In order to
simplify the thermal management, mirror-based spectrum-splitting systems have been proposed.
The following question needs answering: does the performance of mirror-based spectral splitting systems justify
their increased complexity (and cost)? What are the most economically competitive solutions?

FiguRE 26. Linear (left) and Cassegrain (right) refective concentrator

Optical Development Within the Apollon Project

Under the APOLLON project, different solutions for optical concentrators have been investigated with the aim
of reaching high concentration factors, wider acceptance angles, high optical effciency, long-term stability and a
low manufacturing cost. For this purpose, both refractive and refective optics have been designed, and tested by
the different partners in the project. Among the mirror-based solutions, a comparison has been drawn between the
spectrum splitting based solutions and the simple mirror based ones. In particular SolarTec, SE SRTIIE, CRP and
ENEA worked on concentrator systems based on refractive primary lenses, while C-Power, UNIFE, RSE and ASSE
developed mirror-based concentrators. All these systems feature a secondary optical element to homogenise power
density distribution on the cell plane and to increase the system’s angular acceptance.

Optimization of Refractive Optics

The Fresnel geometry has been studied by SolarTec taking into account several aspects like the size of the cell,
the infuence of the working temperature, the misalignment of the impinging light beam and the spectral behaviour
related to the different spectral response of each sub-cell in a triple junction device. At the same time, SE SRTIIE

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