Achromatization of nonimaging Fresnel lenses for photovoltaic solar concentration using refractive and diffractive patterns

In the field of concentrated photovoltaics, the main disadvantage of lenses compared to
mirrors lies in their chromaticity: Snell's law is related to the refractive index which is
wavelength dependent. Consequently, even for purely collimated beams under normal
incidence, the maximum concentration achievable with typical lenses made of PMMA is
limited to ~1000×. This maximum value becomes even lower when considering Sun's
angular aperture. Since the law of reflection is not wavelength dependent, mirrors can
theoretically achieve the thermodynamic limit of concentration which is about 46'000×.
This thesis aims at the design and the manufacturing of an achromatic Fresnel lens
suitable for photovoltaic solar concentration, i.e. combining high concentration, low
production cost and tolerance to manufacturing errors.
Firstly, we investigated a hybrid lens made of a refractive lens and a diffractive lens.
The investigations showed that the concentration ratio could be multiplied by 4. A full
chapter is dedicated to the optimisation of blazed diffraction gratings to finally achieve
the design of the diffractive lens. Nevertheless, a bilayer diffractive lens is needed to
obtain a high diffraction efficiency which makes the diffractive lens highly sensitive to
manufacturing errors and consequently not suitable for photovoltaic solar
concentration.
Purely refractive achromatic Fresnel doublets were then investigated and several
designs were compared. They allow for very high concentration ratios in the case of
collimated beams under normal incidence, higher than 100'000×. Therefore, contrary to
singlets, Fresnel doublets are much more affected by the angular size of the source than
by the chromatic aberration. Moreover, it was shown that they are tolerant to
manufacturing error, change of temperature and uncertainty on the refractive index.
It emerges from this thesis that the concept of achromatic doublets is a tolerant and
low-cost production solution to achieve a highly concentrated white flux. Although
bilayer diffractive lenses are not suitable for concentrated photovoltaics, the
combination of refractive with diffractive structures seems to be promised to a bright
future for spectrum splitting applications, including spectrum splitting for concentrated
photovoltaics.