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Energy Express

  • Editor: Christian Seassal
  • Vol. 21, Iss. S6 — Nov. 4, 2013
  • pp: A1028–A1034
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Luminescence inverse method For CPV optical characterization

R. Herrero, C. Domínguez, S. Askins, I. Antón, and G. Sala  »View Author Affiliations


Optics Express, Vol. 21, Issue S6, pp. A1028-A1034 (2013)
http://dx.doi.org/10.1364/OE.21.0A1028


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Abstract

The luminescence inverse method may be used to optically characterize a concentrator photovoltaic module. With this method, the module angular transmission is obtained by evaluating the light emission of a forward biased module. The influence of the emission of the cell when measuring the angular transmission is evaluated, and the process of building a global angular transmission from the set of individual optics-cell unit functions is explained. A case study of a module composed by several optics-cell units is presented. In order to validate the proposed measurement, results for five different CPV technologies are compared for both direct methods (i.e., solar simulator) and indirect methods (i.e., Luminescence inverse method).

© 2013 Optical Society of America

1. Introduction

The angular transmittance function T(θ,φ) of a CPV module is defined as the ratio between the output power P(θ,φ) for a given illumination direction (θ,φ) relative to the maximum output power P(0,0) at the best module alignment with respect to the light source. The angular transmittance function depends not only on the module properties but also on the characteristics of the light source impinging the CPV module, i.e., angular distribution and spectrum. Regarding the angular distribution, the angular transmittance T(θ,φ) can be theoretically expressed as the convolution of the light source angular distribution S(θ,φ) and the impulse-response angular transmittance H(θ,φ) of the module [1

1. A. Rabl and P. Bendt, “Effect of circumsolar radiation on performance of focusing collectors,” J. Sol. Energy Eng. 104(3), 237 (1982). [CrossRef]

]. The impulse-response angular transmittance H(θ,φ) corresponds to the angular transmittance of a module when it is illuminated by a beam of parallel rays with zero angular spread (as given by a Dirac delta (δ) function).

The angular transmittance T(θ,φ) can be obtained by direct methods, (i.e., direct illumination of the module aperture) by measuring the module under the Sun or in a flash solar simulator [2

2. C. Domínguez, I. Antón, and G. Sala, “Solar simulator for concentrator photovoltaic systems,” Opt. Express 16(19), 14894–14901 (2008). [CrossRef] [PubMed]

], and recording the maximum power at different angular alignments of the module to the light source. As an alternative to direct methods, one can study the transferred flux when the concentrator is illuminated from the output by indirect methods [3

3. A. Parretta, A. Antonini, E. Milan, M. Stefancich, G. Martinelli, and M. Armani, “Optical efficiency of solar concentrators by a reverse optical path method,” Opt. Lett. 33(18), 2044–2046 (2008). [CrossRef] [PubMed]

5

5. R. Winston, “Thermodynamically efficient solar concentrators,” J. Photon. Energy,2 025501 (2012).

]. Specifically, the luminescence inverse (LI) method was proposed [6

6. R. Herrero,  C. Domínguez, S. Askins, I. Antón, G. Sala, and J. Berrios, “Angular Transmission Characterization of CPV Modules Based On CCD Measurements,” 6th International Conference on Concentrating Photovoltaic Systems, A. W. Betts, F. Dimroth, R. D. McConnell, y G. Sala, Eds. Melville: Amer Inst Physics, 131–134 (2010). [CrossRef]

] to provide the 2-dimensional (2D) impulse-response angular transmittance H(θ,φ) of a CPV module at high resolution, without the need for mechanical module rotation.

2. Robustness of the luminescence inverse method against non-idealities of the cell emission

The excess carriers injected into a solar cell by forward biasing are preferentially recombined at the lowest energy gap between the valence band and the conduction band. If this minimum gap is direct, then the photon energies of the emitted radiation are narrowly distributed around the energy of that bandgap. The materials for top and middle junctions of the state of the art three junction (3J) lattice matched solar cells (GaInP/GaInAs/Ge) have direct gaps, and their peak emissions are at 680 nm and 890 nm respectively. The angular transmission measured by the LI method might be incorrect if the cell emission is not Lambertian and uniform in the whole area, thus how the cell emission affects the measurement must be studied.

To evaluate all possible optical paths that rays can follow when coming from the Sun to the module, the cell emission must be Lambertian at least in the angular range needed to cover all those paths. This angular range is given by the rim angle of the light beam impinging on the cell when the optical system is illuminated by the Sun. The radiant intensity curves of common 3J solar cells at different incident angles have been analyzed by using a CCD camera, confirming that the emission is close to Lambertian [Fig. 2].
Fig. 2 Lambertian emission vs. GaInP/GaInAs/Ge cell emission.

To cover all possible optical paths with the same intensity level, the emission of the solar cell must be free of defects (i.e., spatially uniform). Different mechanisms such as impurity density variation, alloy clustering, or interface roughness in heterostructures can vary the electronic properties across the cell, causing non-uniform electroluminescence emission [10

10. P. Espinet, et al., “Electroluminescence characterization for III-V multi-junction solar cells,” Photovolt. Spec. Conf. 33rd IEEE, 147 (2008).

], [11

11. C. Honsberg, M. Z. Bernett, “Shunt effects in polycrystalline GaAs solar cells,” Photovolt. Spec. Conf. 21st IEEE, 1, 772-776 (1990).

].To evaluate how the cell emission is, electroluminescence maps of a number of GaInP/GaInAs/Ge cells (10mmX10mm) have been measured using a CCD camera with a Si sensor, and the addition of high/low pass filters to analyze separately the emission from the top and middle subcells. Two cells were selected from those measured as representing very uniform emission (Cell 1) and very non-uniform emission (Cell 2), and the results measured emission maps of these two cells are presented in Fig. 3(a).
Fig. 3 (a) Emission maps for top and middle subcells of Cell 1 and Cell 2 (b) Angular transmittance for the same optical system measured by LI method with Cell 1 vs. Cell 2.

3. Optical characterization of CPV modules using the LI method

To obtain the angular transmission function of a concentrator, the monochromatic flux transferred by the optics or the concentrator electrical performance (i.e., Pmp or Isc) are studied in the the LI and direct method respectively. In both methods, the angular transmission of the CPV module depends on the performance of every unit in the module according to their electrical connections. A case study based on a module composed by 6 optical system-3J cell units connected in series is analyzed in this section. Each of the 6 optical units is composed by a Fresnel lens as primary optical system (POE) and a glass homogenizer [12

12. K. Araki et al.,”Development of a metal homogenizer for concentrator monolithic multi-junction-cells,” Photovolt. Spec. Conf. 29th IEEE, 1572–1575 (2002).

] as secondary optical system (SOE), and each unit has a bypass diode for reverse current protection.

v(i)=ILk>iVc(i)ILk<iVd(iILk)
(1)

The 1D angular transmittance curves with Isc and Pmp as output signal (i.e., TPmp(θ) and TIsc(θ)) are presented in Fig. 4(b). The most significant function for a CPV module is that for the Pmp, which provides data of module power as a function of misalignment in real operation.

In the LI method, the 2D function T(θ,φ) of the module can be reconstructed as a composition of the 2D angular transmission function of every single unit. Electroluminescent emission varies from one solar cell to the next; Although a perfect solar cell would be a perfect emitter via the principle of detailed balance, it is not true that all good absorbers are good emitters (i.e., it would depend on non-radiative and radiative recombination rates) [14

14. E. Yablonovitch, O. D. Miller, and S. R. Kurtz, “The opto-electronic physics that broke the efficiency limit in solar cells,” Photovolt. Spec. Conf. 38th IEEE, 001556-001559 (2012).

]. Thus, the curve measured for each unit is normalized to its integrated or maximum value so that only relative differences on the emissions as function of the angles are considered. In performing this operation, we must also assume that the following parameters are constant for all units across the module:

  • Optical system maximum (on-axis) efficiency: (e.g., Absorption in the lens). Since the main drivers for optical efficiency are material parameters, this is a reasonable assumption, although should be evaluated for each module technology, especially if anti-reflective coatings are used which may have significant process variation to be tested.
  • Photocurrent generated by the cell in on-axis condition: This is an optical test and therefore it must be assumed that each cell has identical electrical performance. If cells are binned before being mounted in the modules this is a reasonable assumption. If there are differences in the performance of the cells used in the module, the calculation of the full module transmission curve will be less accurate.

To calculate the angular transmission of the CPV module, the electrical connection scheme between optics-cell units has to be considered. For the case study module with 6 units series connected, the unit that emits with lower intensity at a given angular position can be assumed to be the one with lower photo-generated current at that angular position and therefore the one defining the Imp. Therefore, the equivalent power transmission of the module TPmp(θ,φ) calculated by the inverse method is the lower envelope of the transmission curves T(θ,φ) of every unit in the module. At the same time, the equivalent short circuit current transmission of the module is calculated as the upper envelope of all curves T(θ,φ), which corresponds to the best illuminated unit an each particular angular position.
Fig. 5 (a). Angular transmittance curves for every single unit in the module from LI method (b). Angular transmittance curves from LI and solar simulator.
In Fig. 5(a) the 1D T(θ) curves corresponding to all single units in the module, and the equivalent Pmp (i.e., minimum curves envelope) and Isc (i.e., maximum curves envelope) transmission curves measured by the LI method are presented. The curves obtained by the LI method are compared with the ones from solar simulator with a good agreement as shows Fig. 5(b).

Table 1. Acceptance angles measured by direct method (AAD) and indirect (AAI) methods for five different CPV technologies, and the relative error (AAD-AAI)/AAD

table-icon
View This Table
Table 1 summarizes the acceptance angles measured for 5 different CPV technologies (included the module case study of Fig. 5). The maximum relative error between results from direct (i.e., solar simulator) and indirect measurements (i.e., SLI) is 2.44% (related to the module case study).

It must be pointed out that the angular transmission curves calculated by the lower and upper envelopes in the LI method do not corresponds always to the transmission curves calculated by direct method when recording Pmp or Isc values. For example, the power output could not be given by the worst illuminated cell if its photocurrent is much lower than the others so the transmission function given by the LI method would be slightly narrower compared to the module performance; also the short circuit current of the module does not correspond exactly to the photo-generated current of the best single unit when many units are connected in series because one single unit cannot compensate the rest of the forward biased bypass diodes which leads to a transmission function slightly wider.

3. Conclusions

We have evaluated the requirements of the cell emission (i.e., Lambertian and spatially uniform) to validate the viability of the LI method. The solar cell emission has been measured to be Lambertian, and it has been shown that the angular transmission measured using the LI method is almost un-affected even when solar cells have non-uniform light emission.

The LI method provides useful information not only for single optics-cell units but also for CPV modules. We have presented a method for calculating CPV module performance from the individual optics-cell unit measurements, and discussed the assumptions implicit in this method. CPV module-level results using this method have been compared with results from conventional methods (e.g., solar simulator) for 5 different CPV technologies, and they have shown good agreement. Therefor the LI method is a good candidate for production line testing of CPV module acceptance angle characteristics.

Acknowledgments

This work has been supported by the European Commission and NEDO through the project NGCPV (EU Ref. N: 283798), by the Community of Madrid through Numancia-2 (S2009/ENE-1477), by the Spanish Ministry of Economy and Competitiveness under Project SIGMA-E (IPT-2011-1218-92000). Rebeca Herrero is thankful to the Spanish Ministry of Education for her FPU grant.

References and links

1.

A. Rabl and P. Bendt, “Effect of circumsolar radiation on performance of focusing collectors,” J. Sol. Energy Eng. 104(3), 237 (1982). [CrossRef]

2.

C. Domínguez, I. Antón, and G. Sala, “Solar simulator for concentrator photovoltaic systems,” Opt. Express 16(19), 14894–14901 (2008). [CrossRef] [PubMed]

3.

A. Parretta, A. Antonini, E. Milan, M. Stefancich, G. Martinelli, and M. Armani, “Optical efficiency of solar concentrators by a reverse optical path method,” Opt. Lett. 33(18), 2044–2046 (2008). [CrossRef] [PubMed]

4.

J. H. Atwater, P. Spinelli, E. Kosten, J. Parsons, C. Van Lare, J. Van de Groep, J. Garcia de Abajo, A. Polman, and H. A. Atwater, “Microphotonic parabolic light directors fabricated by two-photon lithography,” Appl. Phys. Lett. 99(15), 151113 (2011). [CrossRef]

5.

R. Winston, “Thermodynamically efficient solar concentrators,” J. Photon. Energy,2 025501 (2012).

6.

R. Herrero,  C. Domínguez, S. Askins, I. Antón, G. Sala, and J. Berrios, “Angular Transmission Characterization of CPV Modules Based On CCD Measurements,” 6th International Conference on Concentrating Photovoltaic Systems, A. W. Betts, F. Dimroth, R. D. McConnell, y G. Sala, Eds. Melville: Amer Inst Physics, 131–134 (2010). [CrossRef]

7.

V. M. Andreev, V. A. Grilikhes, and V. D. Rumyantsev, Photovoltaic Conversion of Concentrated Sunlight (John Wiley & Sons, 1997) Chap. 4.

8.

V. D. Rumyantsev and M. Z. Shvarts, “A luminescence method for testing normal operation of solar modules and batteries based on AlGaAs solar cells with radiation concentrators,” Geliotekhnika 28, 5 (1992).

9.

R. Herrero, C. Domínguez, S. Askins, I. Antón, and G. Sala, “Two-dimensional angular transmission characterization of CPV modules,” Opt. Express 18(4S4), A499–A505 (2010). [CrossRef] [PubMed]

10.

P. Espinet, et al., “Electroluminescence characterization for III-V multi-junction solar cells,” Photovolt. Spec. Conf. 33rd IEEE, 147 (2008).

11.

C. Honsberg, M. Z. Bernett, “Shunt effects in polycrystalline GaAs solar cells,” Photovolt. Spec. Conf. 21st IEEE, 1, 772-776 (1990).

12.

K. Araki et al.,”Development of a metal homogenizer for concentrator monolithic multi-junction-cells,” Photovolt. Spec. Conf. 29th IEEE, 1572–1575 (2002).

13.

I. Antón and G. Sala, “Losses caused by dispersion of optical parameters and misalignments in PV concentrators,” Prog. Photovolt. Res. Appl. 13(4), 341–352 (2005). [CrossRef]

14.

E. Yablonovitch, O. D. Miller, and S. R. Kurtz, “The opto-electronic physics that broke the efficiency limit in solar cells,” Photovolt. Spec. Conf. 38th IEEE, 001556-001559 (2012).

OCIS Codes
(040.1520) Detectors : CCD, charge-coupled device
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(350.6050) Other areas of optics : Solar energy

ToC Category:
Solar Concentrators

History
Original Manuscript: July 17, 2013
Revised Manuscript: September 27, 2013
Manuscript Accepted: October 3, 2013
Published: October 15, 2013

Citation
R. Herrero, C. Domínguez, S. Askins, I. Antón, and G. Sala, "Luminescence inverse method For CPV optical characterization," Opt. Express 21, A1028-A1034 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S6-A1028


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References

  1. A. Rabl and P. Bendt, “Effect of circumsolar radiation on performance of focusing collectors,” J. Sol. Energy Eng.104(3), 237 (1982). [CrossRef]
  2. C. Domínguez, I. Antón, and G. Sala, “Solar simulator for concentrator photovoltaic systems,” Opt. Express16(19), 14894–14901 (2008). [CrossRef] [PubMed]
  3. A. Parretta, A. Antonini, E. Milan, M. Stefancich, G. Martinelli, and M. Armani, “Optical efficiency of solar concentrators by a reverse optical path method,” Opt. Lett.33(18), 2044–2046 (2008). [CrossRef] [PubMed]
  4. J. H. Atwater, P. Spinelli, E. Kosten, J. Parsons, C. Van Lare, J. Van de Groep, J. Garcia de Abajo, A. Polman, and H. A. Atwater, “Microphotonic parabolic light directors fabricated by two-photon lithography,” Appl. Phys. Lett.99(15), 151113 (2011). [CrossRef]
  5. R. Winston, “Thermodynamically efficient solar concentrators,” J. Photon. Energy,2 025501 (2012).
  6. R. Herrero,  C. Domínguez, S. Askins, I. Antón, G. Sala, and J. Berrios, “Angular Transmission Characterization of CPV Modules Based On CCD Measurements,” 6th International Conference on Concentrating Photovoltaic Systems, A. W. Betts, F. Dimroth, R. D. McConnell, y G. Sala, Eds. Melville: Amer Inst Physics, 131–134 (2010). [CrossRef]
  7. V. M. Andreev, V. A. Grilikhes, and V. D. Rumyantsev, Photovoltaic Conversion of Concentrated Sunlight (John Wiley & Sons, 1997) Chap. 4.
  8. V. D. Rumyantsev and M. Z. Shvarts, “A luminescence method for testing normal operation of solar modules and batteries based on AlGaAs solar cells with radiation concentrators,” Geliotekhnika28, 5 (1992).
  9. R. Herrero, C. Domínguez, S. Askins, I. Antón, and G. Sala, “Two-dimensional angular transmission characterization of CPV modules,” Opt. Express18(4S4), A499–A505 (2010). [CrossRef] [PubMed]
  10. P. Espinet, et al., “Electroluminescence characterization for III-V multi-junction solar cells,” Photovolt. Spec. Conf. 33rd IEEE, 147 (2008).
  11. C. Honsberg, M. Z. Bernett, “Shunt effects in polycrystalline GaAs solar cells,” Photovolt. Spec. Conf. 21st IEEE, 1, 772-776 (1990).
  12. K. Araki et al.,”Development of a metal homogenizer for concentrator monolithic multi-junction-cells,” Photovolt. Spec. Conf. 29th IEEE, 1572–1575 (2002).
  13. I. Antón and G. Sala, “Losses caused by dispersion of optical parameters and misalignments in PV concentrators,” Prog. Photovolt. Res. Appl.13(4), 341–352 (2005). [CrossRef]
  14. E. Yablonovitch, O. D. Miller, and S. R. Kurtz, “The opto-electronic physics that broke the efficiency limit in solar cells,” Photovolt. Spec. Conf. 38th IEEE, 001556-001559 (2012).

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