density near the top of the SOE (see schematic ray-trace of Fig. 9). In the early 90’s Alpha Solarco reported problems of degradation of the SILO glass due to UV absorption by undesired particle traces (solarization) [13]. By now glass technology has improved, and this risk may be already solved, but even so the 4-fold FK concentrator divides that risk by four (since the high radiation is split in the four units of the SOE).

(a) (b)

-5

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Fig. 15. Irradiance distributions, in suns (1 sun = 1kW/m2), on the cell for the (a) spherical dome with f/1.5 and the (b) f/1 FK concentrator (this is the average of the graphs in Fig. 7). Both have geometrical Cg = 625x; and α* = ± 0.61° for the spherical dome and α* = ± 1.30° for the LPI’s FK concentrator.

5. Practical considerations for mass-production

There are several features of the SOE of the FK concentrator that make it advantageous with respect to other dielectric SOE’s that use flow-line type [9] total internal reflectors, as for instance the RTP.

First, all the surfaces of the RTP are optically active (see Fig. 16). This makes it complex to hold it without introducing optical losses and compromising the mechanical stability (some CPV manufacturers hold the RTP from the joint to the cell, which compromises the requirements for the RTP-cell coupling material). In contrast, a large portion of the SOE surface has no optical function, and consequently can be used to make that mechanical fixing and to introduce alignment features.

Another problematic issue is that the optical coupling of the cell and the SOE is very critical for the RTP, because lateral spillage of the silicone rubber causes significant optical losses from leakage through it. If to avoid spillage the joint is under-filled, the resulting air gap produces optical losses too. These losses cannot be quantified until full production is achieved. This particularly discourages the use of RTP for small cells (<5mm side). In the FK concentrator, however, the overflow does not affect the optical performance, which greatly simplifies the joint coupling. Particularly, the optical coupling and mechanical fixturing functions can be totally separated in the FK concentrator.

#122958 - $15.00 USD Received 19 Jan 2010; revised 26 Feb 2010; accepted 26 Feb 2010; published 26 Apr 2010(C) 2010 OSA 26 April 2010 / Vol. 18, No. S1 / OPTICS EXPRESS A39

refraction

Lost ray

Every surface is

optically active

refraction

Total

Internal

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(TIR)

Only these

surfaces are

optically active

Silicone optical coupler

Fig. 16. Practical aspects of the R-TP secondary versus the LPI’s FK secondary.

6. Conclusions

The FK concentrator is a reliable and robust high-performance device that allows mass production thanks to its high tolerance and practical manufacturability aspects. The FK optical surfaces, from the manufacturing point of view, are very similar to a conventional flat Fresnel lens and a conventional dome. This means that they can be manufactured with the same techniques (continuous roll embossing, hot embossing, compression molding, etc. for the POE; glass molding for the SOE) and that their production cost is essentially the same, but their optical performance (CAP*) is much better.

The FK concentration acceptance angle products, CAP and CAP*, are the highest among the concentrators based on flat Fresnel lenses considered here, and are also superior to other designs reported in [19]. However, two designs using mirrors as POE have been reported with a higher CAP than the FK [18]. The irradiance uniformity obtained by the FK concentrator is excellent, without the chromatic aberration so typical of other Fresnel concentrators. This minimizes the losses associated with such inhomogeneities, while diminishing the reliability risks associated with elevated local concentrations. It is also remarkable that the FK concentrator maintains its high CAP* for rather small f-numbers, which is very interesting if compactness of the CPV module is desired.

The FK concentrator is in our opinion an excellent candidate to make low-cost high-concentration CPV modules that combine very high electrical efficiency at the array level (and not only at the single cell level) with cost-effective assembly and installation.

The free-form Köhler array solutions are not limited to the present FK concentrator. Even for the flat Fresnel lens as a primary, multiple options for the secondary are envisaged for future improvements. The true innovation in these new designs is that they are free-form Köhler integrating arrays. This degree of freedom enables the design of optical surfaces that can perform different functions at the same time (improving the device performance without affecting its cost). This allows good irradiance uniformity and high tolerance angle at high concentration values.

Acknowledgments

The LPI’s FK device presented in this paper is protected under US and International patents pending by LPI, LLC, 2400 Lincoln Avenue, Altadena, CA 91001 USA http://www.lpi-llc.com/. The authors thank the Spanish Ministry MCI (project SIGMASOLES PSE-440000-2009-8) and the Madrid Regional Agency IMADE (project PIE/469/2009) for their partial support. The authors also thank Bill Parkyn for his help in editing the paper.

#122958 - $15.00 USD Received 19 Jan 2010; revised 26 Feb 2010; accepted 26 Feb 2010; published 26 Apr 2010(C) 2010 OSA 26 April 2010 / Vol. 18, No. S1 / OPTICS EXPRESS A40