Christian Vodermayer* • Marcel Mayer* • Maurice Mayer* • Tobias Müller* • Monika Niess* • Gerald Wotruba* Gerd Becker** • Mike Zehner*** • Jürgen Schumacher****

*BEC-Engineering GmbH · Mitterfeldring 41 · D-85586 Poing · Tel.: +49 (0)8121 884567-0, Fax: +49 (0)8121 884567-8, vodermayer@bec-engineering.de**Bavarian Association for the Promotion of Solar Energy (Solarenergieförderverein Bayern e. V.) · Elisabethstraße 34 · D-80796 Munich • www.sev-bayern.de

***University of Applied Sciences Munich · Department of Electrical Engineeringand Information Technology · Working Group SE-Laboratory · D-80323 Munich****University of Applied Sciences Stuttgart · Schellingstraße 24 · D-70174 Stuttgart

First Results – Correlation between IR Images and Electrical Behavior and Energy Yield of PV Modules

TaskIn the past field IR imaging of PV modules showed different thermal effects on PV modules (figure 2 - 6). But IR images itself are difficult to interpret without more background information regarding the influence of thermal anomalies to the electri-cal operation behavior. By analyzing the IR images, the following topics are of interest:

– Impact to electrical behavior of the PV module(s)/ strings– Impact to energy yield– Long term behavior of thermal effects– Impact to the lifetime of PV modules

Figure 1: Outdoor Test Stand BEC-Engineering GmbH

Figure 2: PV system with different thermal phenomena

Figure 3: PV module with hot spot (∆T = 40 °C) with an open loop cell array.

Figure 4: PV module with defect bypass diode (∆T = 8° C) and resulting cell array heating (∆T = 2° C)

Figure 5: PV module with heated cell (∆T ~ 20 K)

Figure 6: PV module with overheated bypass diode (∆T ~ 40 K)

Figure 7: Overview about electrical failures which can cause noticeable thermal effects

Figure 8: Relation between thermal abnormality and possible electrical reasons

Figure 9: PV module with one hot cell measured at an irradiation in tilted plane of 622 W/m2 and an ambient temperature of 24.3 °C

Figure 10: I-V curve of one PV module and its four separate cell arrays (shown in figure 9, G

mod = 622 W/m2, T

amb = 24.3 °C)

Figure 11: PV module with multiple heated cells measured at an irradiation of 817 W/m2 in tilted plane and an ambient temperature of 19.3 °C

Figure 12: I-V curve of one PV module and its four separate cell arrays (shown in figure 11, G

mod = 817 W/m2, T

amb = 24.3 °C)

Figure 13: Exemplary time variation curve of temperature and power difference (of the PV module out of figure 9) comparing normal cell strings to those with thermal anomalies

Figure 14: Mpp power deviation between normal cell strings to those with thermal anomalies in relation to the irradiance (figure 9)

Test environment:All measurements were realized on an Outdoor Test Stand (figure 1), located in Bavaria, Germany. One representative PV module taken from the ana-lysed PV plant without thermal or electrical pheno-mena with average output power is used as refe-rence for every I-V curve measurement. Technical equipment:

The interpretation of thermal effects can be diffi-cult. There are several different electrical failures, which can cause a thermal phenomenon. For exam-ple hot cells in a cell array can cause an overhea-ting of associated bypass diodes. Figure 8 gives an overview on the thermal effects, in relation to the IR images in figure 3 - 6 and some of their possible reasons. In figure 7 these reasons are shown exem-plarily on a schematic PV module. The thermal ef-fects of broken cells are not yet verified, also effects without impact to the electrical behavior of the PV module are not considered in figure 8 (for examp-le delamination). Some effects are interconnected to each other, for example a hot cell array can be caused by interconnection failures or bypass diode defects.

A PV module with one single hot solar cell (figure 9) is analyzed more in detail. This PV module con-sists of 4 cell arrays, each with 18 cells and one by-pass diode per cell array parallel (figure 7). The I-V curves of all 4 cell arrays, the entire module and one reference module were measured. Figure 10 shows the results for one measurement cycle. All cell array I-V cures, mathematically added, give the measured I-V curve of the complete module.

A PV module with multiple heated cells has been examined. Like the module shown in figure 9, this module consists of 4 cell arrays. Figure 11 shows se-veral different temperature levels (∆T

max = 25.4 K) of

the solar cells within the module under operating conditions. The resulting I-V curve measurements are shown in table 2.

In figure 13, the module shown in figure 9 is mea-sured over a longer period with a measurement cy-cle of one minute. The selected profile of 1.5 hours shows a significant correlation between the tem-perature difference of the hot cell and the average temperature of the surrounding cells and the po-wer difference between the concerning cell arrays. The irradiation and ambient temperature give no indication of an influence to the rapid changes of the characteristic at 09:14 and 09:56 shown in fi-gure 13. But the correlation between temperature and power difference can still be recognized at ra-pid changes. The reason for this effect is not clear and needs to be further researched.

This Poster gives an overview of most of the ther-mal effects which are detectable by IR imaging. They can indicate malfunction regarding the elec-trical behavior and the resulting power loss. Some exemplary measurement results of analysed PV plants with single and multiple hot cells are shown.

One main result is a direct relation between the power- and the thermal differences which is ad-ditionally shown in a daily profile in figure 15. An

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Conclusion and Outlook

– Weather station with mono- and polycrystalline calibrated solar cells, pyranometer, air tempera- ture sensor, wind speed- and direction sensor with a sampling rate of 5 seconds– Free programmable high accuracy Electronic Load samples every 1 minute one complete I-V curve of the complete module and all cell arrays– High resolution IR camera takes every minute one image

Cell array 4 shows a decreased power of -14.8 % compared to the average of cell arrays 2, 3 and 4 (table 1: ∆_1). This is caused by a decreased mpp current, which shows a decrease of -19.6 %. The hea-ted solar cell in array 4 has an obvious reducing ef-fect on the maximum power output of the entire module. Compared to the reference module, this module has a decreased power of -12.0 % under these environment conditions (table 1: ∆_2).

Figure 14 shows the power difference values sor-ted by irradiation classes on a sunny day with only few clouds. The behavior seems to be approxima-tely linear. The data points are based on the same series of measurements as in figure 13. The power loss increases with rising irradiation values.

impact to the energy yield seems obvious but fur-ther evaluations are necessary to quantify the ef-fective loss in kWh/kW

p per year. The topic will be

observed in the future with more detailed long

Due to thermal effects in every cell array, the com-plete module shows -24.3 % decreased maximum power compared to the reference module (table 2: ∆_2). The difference of maximum power between cell array 1 and 4 amounts -18.3 %. Cell array 1 has the most thermal abnormalities. The average cell array temperature seems to correspond with the resulting power loss.

term analyzes and measurements of single cells (disassembling of PV modules). With these results simulations will be done with the block orientated simulation environment INSEL to verify the thesis

and get more information regarding the impact to the energy yield.

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Figure 15: Diurnal variation curve of temperature and power difference (of the PV module out of figure 9) comparing normal cell strings to those with thermal anomalies

Overview