Electro-optical study of a ×1024 concentrator photovoltaic system

APPLICATION

Electro-optical study of a �1024 concentratorphotovoltaic systemAlexandre Pereira1*, Loïc Dargent1, Géraldine Lorin1, Wilfrid Schwartz1, Thierry Baffie2,Christophe Mangeant3, Mathieu Mariotto3, Jean-Edouard de Salins4 and Guillaume Vives4

1 CEA-Grenoble/DRT/LITEN/DTNM/LCCE, 17 rue des Martyrs, 38054 Grenoble, France2 CEA-Grenoble/DRT/LITEN/DTBH/LCTA, 17 rue des Martyrs, 38054 Grenoble, France3 CEA-INES, 73377 Le Bourget du Lac, France4 HELIOTROP SAS, 24 rue de l’Est, 75020 Paris, France

ABSTRACT

Concentrator photovoltaic (CPV) systems are one of the most promising technologies for future energy supply. Severalstudies reported the interest of using a Fresnel lens coupled with a secondary optical element in such a system. For highconcentration factor, the optimization of the optical configuration plays a key role regarding electrical performances. Onthe other hand, the thermal management of the solar cell is also critical to ensure a better module efficiency. This paperpresents a study of a �1024 CPV system performances and a methodology for estimating the optical chain efficiency,the cell temperature impact and the alignment requirements. Module efficiencies were then measured as a function ofthe cell temperature and correlated to optical performances through current-tension characterizations under real solar illu-mination conditions and the estimation of the power density received by the solar cell. The system yield was up to 27% fora cell temperature around 30 �C, confirming that high concentration ratio should be of great interest in the near future. A 1Dmodel was also developed in order to quantify the possible improvements of this CPV system. Using a solar cell with an effi-ciency of 36.7% at�600, we then demonstrated that the�1024 CPV system could reach up to 30% in standard test conditions.Copyright © 2012 John Wiley & Sons, Ltd.

KEYWORDS

concentrator photovoltaic system; optic

*Correspondence

Alexandre Pereira, CEA-Grenoble/DRT/LITEN/DTNM/LCCE, 17 rue des Martyrs, 38054 Grenoble, France.E-mail: [email protected]

Received 28 March 2012; Revised 10 May 2012; Accepted 26 June 2012

1. INTRODUCTION

Since the 1990s, concentrator photovoltaic (CPV) systemsusing Fresnel lenses have been considered as an attractivetechnique for the reduction of the photovoltaic electricityproduction cost. Recently, significant advances have beenperformed on �300 up to �600 modules [1–7]. The useof secondary optical element (SOE) was largely studiedbecause it enables to improve the alignment and assemblyrequirements, minimizing this way the energy cost produc-tion (€/kWh). The acceptance angle of CPV systems is alsoincreased by using SOEs. Reflective truncated pyramidwith an on-axis optical efficiency around 89% was the firstoptical component studied [8]. In this case, for an off-axisangle around 1�, 80% of the on-axis power is guided to thecell. Refractive SOEs (truncated pyramidal waveguide andspherical dome) bring the homogenization of the flux den-sity received by the cell, which reduces serial resistances

usually generated by a nonuniform irradiation (Gaussian-shaped beam profile). Dielectric-filled truncated pyramidexhibited on-axis optical efficiency of 81.8% [9]. TheLight Prescriptions Innovators developed Fresnel–Köhleroptical components that gave on-axis efficiency around85% for a�625 concentrator module. For an off-axis anglearound 1.4�, this concentrator guides 90% of the on-axispower to the cell [10,11]. They showed that this concentratortechnology enhances both acceptance angle and homoge-nization of the flux received by the cell compared withconventional reflexive and refractive SOEs.

The size reduction of expensive epitaxial semiconduc-tors is one of the most promising ways to reduce theelectricity cost production in CPV power plant. This ispossible by increasing the concentration ratio and byusing a standard cell area of 0.3 to 1 cm². For high con-centration ratio (typically up to �1000 on a 1 cm² cellarea), the choice of the SOE is critical to ensure the best

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONSProg. Photovolt: Res. Appl. (2012)

Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2271

Copyright © 2012 John Wiley & Sons, Ltd.

module efficiency. Indeed, the focal length of such a sys-tem is higher than for conventional concentration ratio,needing an accurate optical alignment. Furthermore, thepower density to be dissipated by the cell is also greaterfor high concentration ratio. The energy production cost isdirectly impacted by the choice of the heat sink technology(active or passive).

In this way, this paper presents optical, electrical andthermal characterization of a �1024 concentrator systembased on a reflective truncated pyramid SOE and an activeheat sink. A real solar illumination set-up was developedthat comprises a solar tracker, a �1024 CPV module, aCCD camera and a cooling and micrometric translatingsample holder. The first objective was to understand the

Figure 1. (a) Scheme of the �1024 concentrator photovoltaic (CPV) module and (b) real solar illumination set-up.

Electro-optical study of a CPV system A. Pereira et al.

Prog. Photovolt: Res. Appl. (2012) © 2012 John Wiley & Sons, Ltd.DOI: 10.1002/pip

on-axis and off-axis optical chain performances, thethermal solicitation of the cell and its impact on moduleefficiency. The second one was to identify and quantifythe efficiency improvement of the system on the basisof an electro-optical 1D modelling.

2. EXPERIMENTAL SET-UP ANDSOLAR CELL CHARACTERIZATION

The�1024 CPV module studied in this work is presented inFigure 1(a). It was composed of a 320� 320mm² silicone onglass (SOG) Fresnel lens and a 1 cm² Concentrating Triple-Junction (CTJ) EMCORE (Somerset, New Jersey, US) solarcell mounted on a copper/alumina receiver and encapsulatedwith a silicone-based material (Dow Corning Sylgard 184,Midland, Michigan, US). The SOE was an inverted pyra-midal reflector fixed on the receiver with the encapsulatingmaterial. A cooled sample holder was used for the manage-ment of thermal dissipation, and micrometric moving plateswere mounted for better optical alignment accuracy. The

module was fixed on a specific real solar illuminationset-up (Figure 1(b)), which is a two-axes rotation trackingsystem coupled with a pyrheliometer for direct normalirradiance (DNI) measurements.

Current-tension and current-power behaviours of thesolar cell were first characterized with a flash tester undervarious incident power densities at 25 �C by the Institutoof Energia Solar (IES) (Figure 2(a)). From these measure-ments, the solar cell efficiency of 36.7% was found for a�600 concentration ratio. In order to understand the thermalsolicitation of the cell and its impact on the module effi-ciency, we also determined the open-circuit voltage (Voc)variations as a function of the power density (dP) receivedby the cell. Figure 2(b) presents these results comparedwith EMCORE datasheets (Table I) [12]. As alreadyestablished in the state of the art [13], the open-circuitvoltage (Voc) follows a logarithmic behaviour with theincident power density (dP≥ 1). In our case, we foundEquation (1).

Voc ¼ Ln dPð Þ � 8:96� 10�2 þ 2:81 (1)

Figure 2. (a) Current-tension/current-power curves and (b) power density dependence on the Voc variation for the EMCORE concentratortriple-junction solar cell studied.

Electro-optical study of a CPV systemA. Pereira et al.


3. OPTICAL CHARACTERIZATIONRESULTS

The use of SOG Fresnel lens induces optical losses becauseof glass and silicone absorption, reflexion at interfaces andnonideal Fresnel grating. Spectrophotometric analysis, con-centration efficiency measurement and imaging analysis ofthe spectral and geometrical energy distribution at the focalpoint were performed. The imaging analysis is based on a

Lambertian diffusive screen placed at different distancesfrom the Fresnel lens. An energy mapping of the light spotimaged on the screen was then acquired with a CCD camera.A spectral cartography of the light spot was recorded byplacing an optical filter centred at 532nm between the lensand the Lambertian screen. A picture was then taken for sev-eral screen positions allowing this way a whole descriptionof the spectral and spatial distribution of the light aroundthe focal point of the lens. Figure 3(a) presents the spotlight sizes measured at 532 nm for different lens to screendistances. The spot light was considered as Gaussian-shapedbeam, and its size was measured at 90% of the total energy(Figure 3(b)). For AM1.5D solar spectrum, the limitingjunction of GaInP/GaAS/Ge solar cells in terms of currentdensity is the top one (i.e. GaInP) [14]. Considering chro-matic aberrations of the Fresnel lens, the energy spatialdistribution in the 400–600nm range has to match the solarcell size. In other words, the optical system (Fresnel lens

Table I. Temperature dependence of the current density, the opencircuit voltage and the efficiency of the concentrator triple-junction

EMCORE solar cell [12].

ΔJsc +7.2mA/m²/�C

ΔVoc �4mV/�CΔ� 6� 10�2%/�C

Figure 3. (a) Spatial energy distribution measured on the screen at 532 nm for different lens to screen distances and (b) energy profileat 532 nm for a lens to Lambertian screen of 505mm.



coupled with SOE) has to be designed in such a way that thespectrum of light transmitted to the cell closely matches thespectrum for which this cell exhibits its highest efficiency.We found that for a distance of around 505mm, 90% ofthe incident energy density at 532 nm is intercepted by thesolar cell (Figure 3(a and b)). This position is consideredas the best one to transmit the maximum light to the limitingjunction with an illumination area close to the solar cell size(i.e. 1 cm²).

Spectrophotometric measurements were performed on aglass sample with a 200 mm-thick silicone layer in order toestimate the spectral response of the Fresnel lens. Thetransmission of this test sample balanced by the AM1.5 so-lar spectrum gives a 300–1700 nm integrated value of89.5%. In order to determine the concentration efficiencyof the lens, the grating geometrical defects such as tiprounding and a slightly different slope of the pitch haveto be taken into account [15]. For a DNI of 650W/m²,

we estimated the power transmitted through the Fresnellens around 54W. Considering a lens area of 0.1024m²,the lens efficiency is then 82%.

The SOE was a reflector with dimensions optimizedthrough ray-tracing simulations. Its reflexion propertiesare plotted in Figure 4(a) as a function of the irradiation in-cidence angle. The SOE transfer function was calculatedthrough its reflexion coefficient (average values of thecurves in Figure 4(a)) and the energy profile at 532 nmfor a lens to cell distance of 505mm (Figure 3(b)). At thesewavelength and position, only 10% of the incident flux isintercepted by the SOE, and 90% of the light coming fromthe Fresnel lens is directly collected by the solar cell. Fur-thermore, this element was bonded to the receiver with a300mm-thick silicone encapsulate that induced opticallosses because of reflexion at the encapsulate/air interface(Figure 4(a)). The power density recovered by the SOEand the total power density received by the cell are plotted

Figure 4. (a) Reflectivity of the secondary optical element (SOE) as a function of the incident angle and of the silicone encapsulate/airinterface and (b) decomposition of the power density in the optical chain.



in Figure 4(b). We finally found a theoretical efficiencyof the optical chain around �Opt = 77% for a DNI of650W/m². The power density recovered by the SOEwas around �SOE = 7% of the incident power density,and losses due to the encapsulating material were esti-mated around 3%.

The optical chain efficiency also depends on the operatingtemperature. It is well known that the thermo-mechanicaldeformation of the grating and the refraction index variationof the silicone with temperature have a significant impact onthe concentration performances of the Fresnel lens. Theytend indeed to shift the lens focus and increase the spot size[16–19]. As shown in Figure 5(a and b), the efficiency andthe temperature of the lens were simultaneously measuredas a function of DNI. One can notice that DNI stronglyimpacts the lens temperature. The efficiency of the opticalchain was then modelled through the dependence of theFresnel lens performances on the DNI and on the basis

of the optical chain efficiency previously calculated for650W/m² (�Opt = 77% for lens efficiency of 82%). As forthe Fresnel lens, we assume that the yield of the optical chainfollows a linear behaviour with DNI (Figure 5(b)).

4. SYSTEM EFFICIENCYMEASUREMENTS

Because the cell temperature assessment strongly dependson knowing Voc at 25 �C and at a given power density,we used Equation (2) to determine the temperature duringirradiation, where T0 is the room temperature, V0

oc is theopen-circuit voltage measured at T0 for a given powerdensity, VM

oc is the measured open-circuit voltage and sthe temperature dependence coefficient of the open-circuitvoltage (Table I) [12].

Figure 5. (a) Temperature variations of the Fresnel lens as a function of the direct normal irradiance (DNI) and (b) Fresnel lens and opticalchain efficiencies as a function of the DNI.



TCell ¼ T0 þ V0oc�VM

ocð Þ.s

h i(2)

With the cell receiver clamped on the cooled sampleholder, we performed efficiency measurements of the �1024CPV system. A pyrheliometer is used to record the DNI,allowing this way an accurate measurement of the efficiency.The system efficiency was calculated by using Equation (3),where SLens is the lens area and PMax the maximum outputpower of the system:

� ¼ DNI�SLensð Þ.

PMax

(3)

The lens–cell distance was optimized using micrometricmoving plates through the maximum short-circuit current(Icc) output. A distance of 500� 5mm was found, which isin a good agreement with the results obtainedwith the imaginganalysis (Figure 3(a)). The fill factor (FF) was quasi-constant

for a lens to cell distance from 495 to 510mm (around81%). As reported in Figure 6(a), the efficiency varied from25% up to 27% in the 30–70 �C range. The temperaturedependence coefficient was Δ� =�5� 10�2%/�C, which isin good agreement with the EMCORE datasheet (Table I:6� 10�2%/�C for a CTJ cell at 80W/cm²) [12].

For a cell temperature (Tcell) around 75 �C, we foundthat the system efficiency varied from 23.4% to 24.5%depending on the DNI. The best efficiency was obtainedfor DNI = 800W/m². These measurements underline thatthe performances of a CPV system obviously depend onthe DNI, which impacts not only simultaneously the cellefficiency and the cell temperature but also the lens effi-ciency. Indeed, the performances of SOG Fresnel lensesare known to be sensitive to temperature, so indirectly toDNI. The lens used was optimized by the manufacturerfor a working room temperature. This failure was charac-terized by means of short-circuit current measurements as

Figure 6. (a) System efficiency measurements as a function of the cell temperature and (b) short-circuit current variations as a functionof the direct normal irradiance (DNI).



a function of the DNI (Figure 6(b)). One can notice a breakslope in the Icc variations for DNI> 800W/m².

In the same graphic, we plotted IES and EMCOREmeasurements that take into account the optical efficiencyof the system (�Opt = 77%). The discrepancy betweenthis curve and experimental dots obtained for DNI950W/m² can be explained by the impact of the lenstemperature on its optical performances and by thedecrease of the cell efficiency in the case of high powerdensity. This will be discussed in more details in themodelling section.

Furthermore, measurements of the optical alignmenttolerances of the system were carried out using the micro-metric moving plates for x, y and z displacements of thesolar cell. The data are plotted in Figures 7(a and b) com-pared with ray-tracing simulations results. Consideringthat 95% of the optimal efficiency (on-axis system) shouldbe preserved in case of misalignments, the combination

of the SOE with the Fresnel lens leads to �3mm forx displacement, �13mm for z displacement and �0.3�

for the off-axis angle. These results were in good agree-ment with the ray-tracing simulations, even if only thephotons flux was taken into account in the modelling.This method is then well adapted and might be suffi-cient for the characterization of on-axis and off-axisCPV system performances.

5. MODELLING

Using the previous cell characterization and the estimation ofthe optical chain performances, we developed a 1Dmodel onthe basis of the external quantum efficiency (EQE) of theCTJ EMCORE solar cell measured with a Spequest instru-ment (LOT ORIEL, Massy, France) (Figure 8). The currentdensities of the top, middle and bottom junctions were

Figure 7. Measurements and ray-tracing simulations of (a) the optical alignment tolerances (Δx and Δz) and (b) the off-axis tolerancesof the system.



respectively calculated from AM1.5D solar spectrum usingEquation (4), where SI(l) is the solar irradiance, q is theelectronic charge, h is the Planck constant and c the lightvelocity.

Jsc ¼Z1700 nm

300 nm

EQE lð Þ � SI lð Þ � qlhc

� dl (4)

The output power is the product of the lower currentdensity of the three junctions by the open-circuit tension(Voc) and the FF. The Voc depends both on the cell temper-ature and on the power density received by the solar cell.We used the temperature dependence coefficient fromthe EMCORE datasheet (Table I) and the logarithm lawmeasured on the flash tester for the power density depen-dence (1). The temperature dependence of the currentdensity was also taken into account from the EMCOREdatasheet (Table I) [12].

The optical chain efficiency estimated in the optical char-acterization chapter was used to calculate the power densityreceived by the solar cell: �Opt =�0.0214�DNI+90.702.From these results, the system efficiency can be expressedas Equation (4). In this equation, the FF is equal to 81%.The variation of this parameter with the DNI is estimatedto be �1%.

�Syst ¼VocJsc�OptFF

DNI(4)

In Figure 9(a), measured and calculated short-circuitcurrent (Isc) variations as a function of the DNI areplotted. One can observe a good agreement betweenmodelling and experimental results, which indicates thatthe DNI dependence of the optical chain efficiency waswell modelled. In the same graphic, the results of themodelling without this DNI dependence are also plottedgiving this way the theoretical short-circuit current thatcould be achieved in the case of a constant opticalchain efficiency (�Opt = 77%= constant). In Figure 9(b),the measured and modelled system efficiencies arerepresented as a function of the estimated cell tem-perature. As for short-circuit current modelling, wecalculated the theoretical system efficiency achievedwithout the DNI dependence of the optical chain yield.The efficiency of the CPV module could then rise up to28.5% for Tcell = 30 �C. We believe that the opticaldesign (e.g. Fresnel lens grating design) and the manu-facturing process have to be optimized to guaranteestable performances and then an optimal yield in the25 to 50 �C range.

An antireflective coating on the entrance side of theFresnel lens can also significantly enhance the short-circuit current and then the �1024 system efficiency.First measurements were performed and showed anincrease of the short-circuit current and consequentlyof the system efficiency (Figures 10(a and b)). For anestimated cell around 30 �C, the system efficiency wasincreased up to 27.7%. The optimal lens to cell dis-tance was found to be around 497� 5mm instead of

Figure 8. External quantum efficiency (EQE) of the concentrator triple-junction EMCORE solar cell—current densities of the top, middleand bottom junctions calculated from the AM1.5 solar spectrum.



505� 5mm without antireflective coating. Alignmenttolerances measurements will be performed and presentedin a future study.

6. CONCLUSION

In this paper, an electro-optical study of a�1024 CPV sys-tem was presented. The cell temperature impact and thealignment requirements were analysed in particular. Themodule efficiency was measured as a function of celltemperature and correlated to optical performances throughcurrent-tension characterizations under real solar illumina-tion conditions. The efficiency was found to vary from27% to 25% for Tcell ranging from 30 �C to 70 �C withan optical yield around �Opt = 77% at 650W/m². Almost95% of this on-axis efficiency is preserved within align-ment tolerances of �3mm in x, �13mm in z and off-axisangle of �0.3�.

Finally, a 1D model that takes into account the EQE ofthe solar cell and the dependence of the lens efficiency onlens temperature was developed. We showed that by usinga thermally stable lens, an improvement of the short-circuitcurrent (+1A) could be achieved for high DNI. The effi-ciency of the CPV module could then rise up to 28.5%for Tcell = 30 �C.

The future work concerns the development of a low-cost cooling system (active or passive) to ensure a celltemperature around 40 �C in operational conditions andthen system efficiency around 27% for high direct solarirradiance. The use of the next generation of III–V solarcell could significantly increase the electrical performances.Indeed, we can expect cell efficiency around 39% for�1024concentration ratio inducing an STC yield around 30%.Regarding the dependence of the optical efficiency on lenstemperature, future works have to be carried out to ensurean optimal yield in the temperature range of interest (typicallyfrom 25 to 50 �C).

Figure 9. Comparison between experimental data and modelling: (a) short-circuit current as a function of direct normal irradiance (DNI)and (b) system efficiency as a function of estimated cell temperature.



ACKNOWLEDGEMENTS

This work has been financially supported by HELIOTROPSAS. The authors would like to acknowledge Mr Jean-LucMartin, Mr Frédéric Mezzasalma, Mr Colasson Stéphaneand the IES team for their support and for their significantcontribution.

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Figure 10. Enhancement of the performances using a Fresnel lens with antireflective coating: (a) short-circuit measurements and (b)system efficiency measurements.



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http://emcore.com/assets/photovoltaics/CTJ_Cell_10mm.pdf

http://emcore.com/assets/photovoltaics/CTJ_Cell_10mm.pdf

Application

Electro-optical study of a �1024 concentrator photovoltaic system

Alexandre Pereira, Loïc Dargent, Géraldine Lorin, Wilfrid Schwartz, Thierry Baffie, Christophe Mangeant,Mathieu Mariotto, Jean-Edouard de Salins and Guillaume Vives

This paper presents a study of �1024 concentrator photovoltaic system performances and a methodology forestimating the optical chain efficiency, the cell temperature impact and the alignment requirements. Moduleefficiencies were measured as a function of the cell temperature and correlated to optical performances throughcurrent-tension characterizations under real solar illumination conditions. The system yield was up to 27% fora cell temperature around 30�C, confirming that high concentration ratio should be of great interest in the nearfuture.

Electro-optical study of a ×1024 concentrator photovoltaic system

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