#### LESSONS OF EASTER ISLAND

March 17th, 2016

As a result of the presentations during the workshop and the subsequent

discussion, the participants have agreed the following conclusions:

• Present solar cells are not likely to reach a cost that will allow penetration of the PV electricity market because, in their present form—with poor utilization of the solar spectrum—they make ineffective use of the solar resource that—although immense—comes with moderate densities.

• A number of options were presented that, in principle, may ensure better use of the solar resource.

• Of such options, multi-junction solar cells seem to be the one closer to practical exploitation. It was generally agreed that high concentration was needed to render them cost effective...

Read MoreWe will formally prove with the model presented in section 13.1.5 that for cells whose grid-line series resistance is negligible, when the local concentration distribution is assumed to vary slowly between the grid-lines, the distribution providing maximum cell efficiency is the uniform one.

Let us consider the cell model presented in section 13.1.5. In the trivial case in which the parameter rs is negligible (i. e. J(C, V)rs ^ VT for any C < CMAX and any V < Voc), any local concentration distribution f (C) produces the same efficiency (equation (13.11) does not depend on f (C)).

Assuming now that rs is not negligible, consider two concentration distributions with the same average concentration (C): one uniform distribution, i. e...

Read MoreThe recent progresses in non-imaging optics research are focused on to designs in 3D geometry. The interest in 3D designs for PVs lies in the potential capability of 3D methods to obtain practical solutions close to the optimum PV concentrator performance, defined in section 13.1.6.

Not only do 2D designs not control most of the 3D rays, but the rotational or linear symmetric devices are theoretically unable to solve some design problems due to its symmetry. For instance, linear symmetric concentrators cannot achieve isotropic illumination of cell surrounded by an optically dense medium (n > 1) [46], and rotational concentrators cannot achieve maximum concentration on a spherical receiver [47].

Designing in 3D is more difficult than in 2D, due to the greater number of rays to be controlled...

Read MoreUsing the SMS method to design concentrators has produced new families during the last decade [35]. Among them, several families (RR [30], RR-RRIF [30,36], XR [30], XXf [37]], SMTS [38], DSMTS [6], RXI [39], RXIf [40], RXI-RX [40], TIR-R [41]) have been suggested for PV applications.

The nomenclature used for referring to the different designs (excluding SMTS and DSMTS) is as follows: each concentrator is named with a succession of letters indicating the order and type of incidence of the optical surfaces that the sun-ray encounters on its way to the cell. The following symbols are used: R=refraction, X=reflection, I=total internal reflection. The sub-index F is added to X and I of these mirrors coincidence with the flow line).

In the case of high concentration systems (i. e...

Read More13.2.1 Classical concentrators

The progress in solar cells development since its invention has been huge. The investment needed has also been important. Present cells have become much more efficient and the technology much more advanced. The next-generation approaches discussed in this book try to further cell development, increasing cell sophistication if necessary.

However, PV concentrator development has not been accompanied by such progress. This can be seen in the fact that most PV concentrator systems have been based either in the parabola or in the Fresnel lens (see figure 13.11): the parabola has been known since Menaechmus (380-320 BC) and its optical properties since Apolonius (262-190 BC), and Galileo (1564-1642) used lenses for making the first refractive telescope...

Read MoreNon-imaging optics (also called anidolic optics) is the branch of optics that deals with maximum efficiency power transfer from a light source to a receiver [], and, thus, it is the best framework for PV concentrator design. The term non-imaging comes from the fact that for achieving high efficiency the image formation condition is not required (but neither is it excluded, as the RX showed!), and then, in contrast to imaging optical systems, the ray-to-ray correspondence will not be restricted (see figure 13.9)...

Read MoreEquation (13.1) showed that if a high concentration is needed to reduce the receiver cost and a sufficiently high acceptance angle a is needed to make the system practical, the illumination angles в on the receiver must, of necessity, be high. A first consequence of illuminating the cell with wide angles в is that such an illumination will make the cell design (especially antireflection coatings, thickness of tandem cells, etc) more difficult and, in general, for the same cost, the final cell performance will be worse than for the case of a low illumination angle в.

The higher the angles a and в are the more difficult the design problem for achieving good illumination uniformity will be, which can be critical as we have just seen in section 13.1.5...

Read MoreIt is well known that non-uniform illumination of solar cells under high concentration may decrease the power output significantly. For the concentrator designer, the key question is what local concentration distribution can be considered good enough for a given cell.

Usually, the only information available for concentrator designers is the cell efficiency under uniform illumination for different concentration factors Cu. With this information, the common rule of thumb is that the cell efficiency under a certain local concentration distribution C(x, y) of average concentration (C) and peak concentration Cmax will be between the cell efficiencies under uniform illuminations of Cu = (C) and Cu = Cmax...

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