The aforementioned solutions and methods to concentrate the light are not the only developed for photovoltaic applications. One important limitation of these designs is the necessity to use tracking structure to follow the sun. This constrain must not be considered always a limitation; indeed, especially for utility scale installations, tracking structure are used for standard flat plate modules too, in order to improve the energy harvesting, being always on the plane perpendicular to the sunrays. However, the possibility to use static photovoltaic concentrator able to capture also the diffuse radiation has been developed, using a different optical approach, not just the geometrical optics, but involving also some physical properties of particular material like the luminescence; these concentrators, named luminescent concentrators, are usually made of a flat plate of transparent material, with solar cells connected to the sides of the plate; inside the transparent material, luminescent particles like organic dyes or quantum dots are dispersed, absorbing part of the light spectrum and re-emitting light with shifted wavelengths, matching the spectral response of the cells. The re-emitted light is than guided toward the solar cells through the transparent mean, using the total internal reflection at the surface. The limiting point of this technology is the low efficiency achieved due to the losses in the different physical processes involved; it is currently in the order of 6-7% for record prototypes; moreover, the usual concentration for this kind of modules is in the order of 10-40 and the overall size of each luminescent concentrator, to avoid significant losses for light absorption from the transparent material, must be limited.
In fig. (7) a sketch describing the basic concept of these concentrator is reported.
Fig. 7. Simplified drawn of a luminescent solar concentrator