a new model to assess the reliability of cpv modules in real time outdoor tests

8/3/2019 A New Model to Assess the Reliability of Cpv Modules in Real Time Outdoor Tests

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A NEW MODEL TO ASSESS THE RELIABILITY OFCPV MODULES IN REAL TIME OUTDOOR TESTSJ. R. Gonzalez", M. Vazquez", c. Alqora' and N. Nluiez1

"lnstituto de Energia Solar & EUIT de Telecomunicaci6n, Universidad Politecnica de MadridCiudad Universitaria, E-28040 Madrid (Spain)21nstituto Nacional de Tecnica Aeroespacial (INTA) - SPASOLABCarretera de Ajalvir, km 4, E-28850 Torrej6n de Ardoz, Madrid (Spain)

ABSTRACTThe reliability of CPV systems is a hotquestion considering that they mustcompete with silicon flat modules, which inturn have been demonstrated to becapable of withstand 25 years in fieldoperation. In this paper, a summary of theIES-UPM achievements in this field ispresented. A new method for assessingthe reliability of CPV systems in realoperation is needed and a first approachis presented. INTA-SPASOLAB is alsogetting involved in this subject and a newtesting field has been installed.

INTRODUCTIONPhotovoltaic concentration has beenconsidered as an interesting way forreducing the cost of photovoltaic electricitysince early 70's of the last century [1].lnrecent years, there has been a significantadvance in the field of III-V highconcentrator solar cells, achieving peakefficiencies of 32.6% at 1000 suns for adouble junction solar cell [2], and 40.8% at326 suns for a triple junction solar cell [3].It is well known that silicon flat modulesare very reliable systems capable of

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withstanding 25 years in field operation[4]. Nowadays, warranties offered by themanufacturers are precisely of about 25years, but this value is expected toincrease until 30 years in the short future[5]. If Concentrator Photovoltaic(hereinafter CPV) systems are expectedto be competitive with silicon flat modules,the former ones must be capable ofreaching similar warranties to the later.But there are still many open questionsregarding the reliability of these devicesthat should be answered in order to get awarranty.The need of accelerated ageing tests todetermine the reliability of CPV systemshas been pointed out in a previous work[6]. Accelerated ageing tests allow thereduction of the test-time, considering thatCPV systems are expected to last no lessthan 25 years. The main drawback of thiskind of tests is that, if the stress applied toaccelerate the test, is too high, they canintroduce failure mechanisms that wouldnever appear in real operation. Therefore,one question arises from this idea, whatshould be considered a too high stress?

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Reliability assessment of the solar cell and the module. Failure mechanismsdetermination. Optimization of Reliability in the design of th e device

TASK 4: an outdoor real-timedegradation test has already beenfinished. The setup will be brieflydescribed in the next section andresults will be soon available.

Figure 1: diagram containing the working plan to assess the reliability of CPV cells and systems .To answer the previous question is the life-time of the solar cells.necessary to carry out real time outdoor Results are available in [7].tests. With them is possible to evaluate TASK 2: a measurement system forthe failure mechanism of the systems and data acquisition has beenfind a correlation between real time and specifically design to carry outaccelerated tests. Therefore, the aim of accelerated ageing tests inthis paper is to emphasize the importance simulated operation conditions.of carrying out real-time outdoor testingfor assessing the reliability of CPV TASK 3: a step-stress temperaturesystems. ageing test was carried out.Reliability analysis was carried out,

as well as a thorough assessment ofthe cause of failure. Results areavailable in [6], showing a MTTF(Mean Time To Failure) of 69.2years.

WORKING PLAN.The working plan that has been followedin this research to determine the reliabilityof III-V solar cells and CPV systems isoutlined in the diagram of Figure 1. Beloware listed the tasks that have beenprogrammed, most of them are alreadyfinished and the rest are ongoing. TASK 1: reliability prediction basedon comparison with LEOs. This taskwas used to make estimations about

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TASK 5: a new temperature ageingtest has been started with cellscovered with silicone to protect theperimeter. This test is ongoing andwill provide a more accurate resultthan the one described in TASK 3 aswell as information about theactivation energy.EXPERIMENTAL SET-UP.

For the test introduced in TASK 4 of theprevious section, a module with 32 GaAssingle junction solar cells and TIR opticshas been used. This CPV modulepresents a particular assembling thatallows independent electrical access toevery receiver contained in it. This way,the evolution of each one of thesereceivers can be followed independently,avoiding the hiding-effect in theperformance degradation that representsthe series/parallel connection in aconventional assembling. The modulewas mounted on a two-axis sun trackerinstalled on the flat roof of the IES-UPM(see Figure 1).

Figure 2: Two-axis sun tracker installed in the flatroof of IES-UPM.

The data acquisition system recordsperiodically the following information: Illumination I-V curves of eachindividual solar cell each 15minutes since 10:00 up to 17:00.

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Dark I-V curves of each individualsolar cell once a day at 02:00. Irradiance and temperature at thesame time that I-V curves.

Dark and illumination I-V curves havebeen recorded to follow the evolution ofthe solar cells. With every measurement,the temperature and irradiation data havebeen also collected, in order to getnormalized results. The illumination I-Vcurves have been normalized using theequations proposed by ISFOC [8]. Basedon this method the points( ~ I i) measured at a cell temperatureTeet! and a direct normal irradiance Bmedare converted to a new operatingcondition characterized by a new celltemperature Toper and a newirradianceBoper. The new points will be+ b. .I, BoperJ being b. defined byBmedthe following equation:

b.Vi = 0,02 57(Toper - Teen ) [In I Lmed - I i ]297 I med

( { Toper )+ Eg - Vocmed 1-- -i .:being the temperatures measured inKelvin, Eg is the gap band energy in eV,ILmed is the short circuit current inamperes and Voc is the open circuitmeasured in volts. The temperature of themodules has been measured in thebackside.RELIABILITY EVALUATION MODEL: AFIRST APPROACH.It is essential to understand the definitionof reliability to realize the importance thatits knowledge has in the goodperformance of any device. Reliability isthe ability of a system or component toperform its required functions under stated

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conditions for a specified period of time.Therefore, in order to evaluate thereliability is necessary to define veryprecisely when the system performs itsrequired functions.In a CPV module, the required function isrelated with the output power it candeliver. This output power required for themodule to work will be considered withrespect to its initial value. If, at a givenpoint, the power generated by a module ishigher than a predefined value (Plimit) ,then the module will be considered towork properly. On the contrary, if thepower produced by a module is lower thanPlimit then the module will be considered tohave suffered failure. Therefore, in orderto evaluate the reliability of CPV modules,it is necessary to take into account notonly catastrophic failures but also failuresrelated with power degradation or"pseudofailures".The main failure mechanism ofconventional PV modules andoptoelectronic devices, as Light EmittingDiodes, is usually performancedegradation instead of catastrophicfailure. Taking this into account, areliability evaluation model based ondegradation is an important issue forassessing the reliability of CPV modules.This statistical method to assess thereliability will give the followinginformation: A prediction of reliability for highlyreliable devices when theobservation of failures is rare. Analytical expressions for thereliability function, the failureprobability density function, theinstantaneous failure rate function,the MTTF.

A method for establishingreasonable power warranties to thecustomers.

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IES-UPM and INTA-SPASOLAB areworking in this subject to develop this newstatistical model to assess the reliability ofa CPV module and making and importanteffort to install a full equipped testing fieldas can be seen in Figure 3.

Figure 3: INTA-SPASOLAB's testing field.SUMMARY AND CONCLUSIONS.

A big effort has been made in the lastyears to try to solve the problem ofreliability for CPV solar cells and systemsbased on them. Nevertheless, there is stillmuch work to be done.In this paper, a review of the lastachievements in this regard can be found.IES-UPM and INTA are working hard togive an answer to the nowadays openquestions regarding the reliability of thesesystems. These institutions are making abig investment in time and facilities tocarry out a research essential for theimplementation of this technology in thephotovoltaic sector.

BIBLIOGRAPHY.[1] J. Luther, A. Luque, A. W. Bett, F. Dimroth, H.Lerchenm uller, G. Sala and C. Algora, "Concentrationphotovo/taics for highest effciencies and cost reduction", 20thEuropean Photovoltaic Solar Energy Conference, Barcelona,Spain 2005.[2] C. Algora, I. Rey-Stolle, I. Garcia, B. Galiana, P. Espinet, M.Baudrit and E. Barrigon, "A dual junction solar cell with anefficiency of 32.6% at 1000suns and 31.0% at 3000 suns", 5thInternational Conference for the Generation of Electricity, PalmDesert, Nov. 2008.

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[3] J. F. Geisz, D. J. Friedman, J. S. Ward , A . Duda, W. J.Olavarria, T. E. Moriarty, J. T. Kiehl, M. J. Romero, A. G.Norman and K. M. Jones, "40.8% efficient inverted triplejunction solar cell with two independently metamorphicjunctions"[4] A. Skoczek, T. Sample and E. D. Dunlop, "The results ofperformance measurements of field-aged crystalline siliconphotovoltaic modules". Progress in Photovoltaics: Researchand applications, April 2009, vol. 17, pp. 227-240.[5] Wohlgemuth JH, Cunningham OW, NguyenAM, Miller, J."Long term reliability of PVmodule." 20th EuropeanPhotovoltaic Solar EnergyConference, Barcelona, Spain, 2005;1942-1946.[6] J. R. Gonzalez, M. Vazquez, N. Nunez, C. Algora, I. ReyStolle and B. Galiana, "Reliability analysis of temperature stepstress tests on III-V high concentrator solar cells".Microelectronics Reliability (2009),doi:10.1016fj.microreI.2009.04.001[7] M. Vazquez, C. Algora, I. Rey-Stolle and J. R. Gonzalez."III-V concentrator solar cell reliability prediction based onquantitative LED reliability data". In Progress in Photovoltaics:Research and Applications,vol. 15, issue 3, May 2007.[8]http://www.icmf.es/icmf/ISFOC/PLIEGOCONDICIONES Esp.pQf

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a new model to assess the reliability of cpv modules in real time outdoor tests

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