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IEC qualification testing of bifacial PV modules –Test conditions and test requirements

bifiPV Workshop26/27 May 2014, INES, Chambery

Dr. Werner Herrmann, René DüpontTÜV Rheinland Energie und Umwelt GmbH51101 Cologne, GermanyPhone: +49-221/806-2272Email: [email protected]

27.05.2014

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Product certification of c-Si PV modules

IEC EN 61215 Ed. 2Crystalline silicon terrestrial photovoltaic (PV) modules -Design qualification and type approval

IEC EN 61730 Ed. 1Photovoltaic (PV) module safety qualification - Part 1: Requirements for construction- Part 2: Requirements for testing

Note: UL certification issues are not considered in this presentation

Modifications required for bifacial PV modules?

27.05.2014 bifiPV Workshop26/27 May 2014, INES, Chambery

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Specific test requirements for bifacial modules

Specific characteristic:

PV modules with bifacial solar cells deliver a higher module current compared to the rated values of the front side

Qualification tests that include test conditions for module current need to be performed at higher test severity to consider the rear side current.

Reference rear side irradiance needs to be defined to derive test levels for ISC,MAX total and IMP,MAX.

Ideally these shall reflect worst case operating conditions

Impact on IEC 61215 qualification testing:

No impact on IEC 61730-2 safety qualification testing


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Real operating conditions of bifacial PV modulesMaximum generated photocurrentDirect sunlight to front side

Reflected sunlight to rear side

Albedo factor A of ground surface

Height H above ground

Inclinationangle

IFRONT = ISTC x (1 + x (T – 25°C)) x Gt/1000 W/m²IREAR = ISTC x ( … ) x GREAR/1000 W/m² x BFF

GREAR = f (H,,A,GHI,DHI) BFF = Bifacial Factor


http://horoskop-online.com/html/frameset_wolke.htm

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Real operating conditions of bifacial PV modulesImpact of albedo

Albedo is the fraction of the Global Horizontal Irradiance (GHI) that is reflected.

The PVsyst modeling software provides the following guidance for estimating an appropriate value for albedo.

High variability Impact of albedo on GREAR?

Albedo

Urban environment 0.14 – 0.22

Grass 0.15 – 0.25

Fresh grass 0.26

Fresh snow 0.82

Wet snow 0.55 – 0.75

Dry asphalt 0.09-0.15

Wet asphalt 0.18

Concrete 0.25-0.35

Red tiles 0.33

Aluminum 0.85

Copper 0.74

New galvanized steel 0.35


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Real operating conditions of bifacial PV modulesSimulation of maximum irradiance on rear side

Source: Y. Yusufoglu (RWTH Aachen)

Input data Solar irradiance data: Cologne, 12.09.2011 (GHI, DHI) PV module dimensions: 6 x 10 x (6” solar cell) Inclination angle: 35° Height above ground: 1 m Albedo: 0.2, 0.5, 0.8, 0.9

Results: Rear side irradiance at noon in W/m²GHI = 1080 W/m², DHI = 478 W/m²

albedo Maximum Minimum Average

0.2 157 150 153

0.5 358 340 348

0.8 559 530 543

0.9 626 593 608

<3% non-uniform irradiance on rear side


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IEC qualification testing of bifacial PV modulesTest programme for crystalline silicon PV modules

Initial measurements: Visual inspection, electrical performance, insulation, wet leakage current

Preconditioning (5 kWh/m²)

Temperature cycling (200)

Electrical parameters

Outdoor exposure

Hot-Spot

UV preconditioning

Temperature cycling (50)

Humidity freeze

Electrical terminations

Damp heat

HailMech.load

Final measurements: Visual inspection, electrical performance, insulation, wet leakage current

Bypass diode


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Thermal Cycling testThis test is primarily a mechanical fatigue test where differential thermal expansion may cause cells or interconnects to crack. This test can also address any thermal mismatch between components. (Test conditions: 200 temperature

cycles between -40°C and +85°C)

Hot Spot testHot-spot heating occurs in a module when its operating current exceeds the reduced short circuit current of a shadowed or faulty cell or group of cells within it. When such a condition occurs, the affected cell or group of cells is forced into reverse bias and must dissipate. power, which can cause overheating.

Bypass diode testTo assess the adequacy of the thermal design and relative long-term reliability of the bypass diodes used to limit the detrimental effects of module hot-spot susceptibility. During the test the module is heated to 75 °C and the module’s short circuit current at STC is applied to the diodes.

IEC qualification testing of bifacial PV modulesStress tests affected by bifaciality


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Accelerated stress test 2

IEC qualification testing of bifacial PV modulesPass Criteria

InitialDiagnostic

Measurements

Accelerated stress test 1

FinalDiagnostic

Measurements

IntermittentDiagnostic

Measurements

Pass criteria:

Maximum power: Degradation of output power of front side at STC (rear side covered) for each test < 5% and <8% for each test sequence

Electrical safety: Fulfilling the minimum requirements for insulation (high voltage test, wet insulation test)

Visual defects: No delamination forming a continuous path to modules edges, no severe cracks in solar cells check by electroluninescenceanalysis)

Other: Determination of BFF after each sequence Information about front side and backside behavior, but no pass criterion


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IEC qualification testing of bifacial PV modulesOutput power determination of bifacial PV modules

Rating of nominal output power (Global agreement within TÜV Rheinland Group)

Power rating/labelling of module type shall comply to nominal power of the front side only.

Front side output power at STC is measured by covering rear side with an opaque sheet.

Power rating of rear side is performed at STC or 400 W/m². This value is to be regarded as additional information on module label or datasheet.


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IEC qualification testing of bifacial PV modulesOutput power determination of bifacial PV modules

Measurement of front side: 1000 W/m² 25°C Backside covered

Measurement of rear side: 1000 W/m² 25°C Front side covered

400 W/m² 25°C Front side covered

Determination of maximum current

BFF

Max. modulecurrent

Bifacial Factor (BFF): 𝐵𝐹𝐹 =𝐼𝑠𝑐,𝑏𝑎𝑐𝑘@1000

𝐼𝑠𝑐,𝑓𝑟𝑜𝑛𝑡@1000

𝐼𝑆𝐶,𝑀𝐴𝑋 = 𝐼𝑠𝑐,𝑓𝑟𝑜𝑛𝑡@1000 + 𝐼𝑠𝑐,𝑏𝑎𝑐𝑘@400

𝐼𝑀𝑃,𝑀𝐴𝑋 =𝐼𝑆𝐶,𝑡𝑜𝑡

𝐼𝑠𝑐,𝑓𝑟𝑜𝑛𝑡@1000𝑊/𝑚²𝐼𝑀𝑃,𝑓𝑟𝑜𝑛𝑡@1000𝑊/𝑚²


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IEC qualification testing of bifacial PV modulesThermal cycling test (200 cycles)Test procedure

Test conditions: 200 cycles -40°C to +85°C Current injection with Imp

𝐼𝑇𝐸𝑆𝑇 = 𝐼𝑀𝑃,𝑀𝐴𝑋 =𝐼𝑆𝐶,𝑡𝑜𝑡

𝐼𝑠𝑐,𝑓𝑟𝑜𝑛𝑡@1000𝑊/𝑚²𝐼𝑚𝑝,𝑓𝑟𝑜𝑛𝑡@1000𝑊/𝑚²


Heat the module to 75 °C. Apply a current to the module equal to the short circuit current of the module as measured at STC ± 2 %. After 1 h measure the temperature of each bypass diode

𝐼𝑆𝐶,𝑀𝐴𝑋 = 𝐼𝑠𝑐,𝑓𝑟𝑜𝑛𝑡@1000 + 𝐼𝑠𝑐,𝑏𝑎𝑐𝑘@400

Increase the applied current to 1.25 times the short circuit current of the module as measured at STC while maintaining the module temperature at 75 °C. Maintain the current flow for 1 h.

𝐼𝑇𝐸𝑆𝑇2 =1.25 𝑥 𝐼𝑆𝐶,𝑀𝐴𝑋

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IEC qualification testing of bifacial PV modulesBypass diode thermal test

Test procedure


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IEC qualification testing of bifacial PV modulesHot-spot test

Test is performed with a steady state solar simulator

Irradiation from both sides with solar simulator is not possible

Back side current is compensated by higher irradiance setting on front side

No change in the further test procedure

Test procedure

𝐼𝑆𝐶,𝑓𝑟𝑜𝑛𝑡 = 𝐼𝑆𝐶,𝑀𝐴𝑋 = 𝐼𝑠𝑐,𝑓𝑟𝑜𝑛𝑡@1000𝑊/𝑚² + 𝐼𝑠𝑐,𝑟𝑒𝑎𝑟@400𝑊/𝑚²


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Conclusions

Test standard IEC 61215 is applicable for bifacial PV modules. Just test levels of current driven tests need to be modified: a) Thermal cycling 200, b) Hot-Spot test, c) Bypass diode thermal test

Test current is related to 1000 W/m² front side irradiation and 400 W/m² rear side irradiation.

Assumption of 400 W/m² irradiation on rear side has been confirmed by simulation study with real weather data.

Power rating of a bifacial PV module type shall be referenced to the front side only. Power contribution of rear side shall be informative.


27.05.201416

Thank you for your attention.

bifiPV Workshop26/27 May 2014, INES, Chambery

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For product qualification of PV modules commonly IEC test standard are referred. The stress tests defined in the test programmes are short-duration accelerated tests that are performed at stress levels higher than use stress in order to facilitate failures in a timely manner. The qualification tests can be considered as a minimum requirement to undertake reliability testing.

The primary goal of IEC qualification testing is to identify the initial short-term reliability issues in the field. This means that mainly early product failures are detected.

The actual lifetime expectancy of qualified modules will depend on their design, their environment and the conditions under which they are operated. Time of Operation

Open the box failure

Early failures

Random failures Degradation

Wear-out failures

Failu

re R

ate

IEC Qualification Testing of PV modules


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Real operating conditions of bifacial PV modulesIrradiance data Tempe, Arizona, 7–18 April 2014

Clear sky dayCloudy day


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Real operating conditions of bifacial PV modulesMaximum irradiance on rear side (Tempe data)

albedo 11.04.2014GREAR in W/m²

14.4.2014GREAR in W/m²

0.2 166 125

0.5 376 297

0.8 587 468

0.9 657 525

Source: Y. Yusufoglu (RWTH Aachen)

Cloudy day Clear sky day


Presentations & Public Speaking

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