Read Microsoft PowerPoint - PV Tutorial_4 (Modules).ppt text version

Module Component of PV Tutorial

"Integration of Renewable & Distributed Energy Resources Conference

John Wohlgemuth December 6, 2010

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

PURPOSE OF MODULE PACKAGE

· Mechanical support ­ hold the cells in place pointing toward the sun. · Dielectric protection ­ keep high voltage away from people and keep current from flowing out of the array circuit (to ground or in a loop). · Protect the cells, diodes and interconnects from the weather (UV, rain, humidity, hail etc.) · To couple as much light energy as possible into the solar cells (at all angles at the wavelengths that the cell can utilize). · To minimize the temperature increase of the cells.

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Module Package Should

·Be qualified to IEC 61215 or 61646. ·Be safety certified to UL 1703 if being sold in US. ·Be safety certified to IEC 61730 parts 1 and 2. ·Have at least a Class C fire rating if it is going to be used on a building. ·Carry an extended warranty. (25 years is typical for all technologies today)

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TYPICAL CONFIGURATIONS

Glass Superstrate o Glass/encapsulant/cells/encapsulant/backsheet o Glass/thin film cells/encapsulant/backsheet o Glass/encapsulant/thin film cells/glass or other substrate Flexible o Transparent frontsheet/encapsulant/thin film cells/flexible substrate

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Glass/encapsulant/cells/encapsulant/backsheet

GLASS CELLS EVA

BACKSHEET

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Glass/thin film cells/encapsulant/backsheet

Thin Film PV

Glass Encapsulant

Backsheet

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Glass/encapsulant/thin film cells/substrate

Encapsulant Glass Thin Film PV Substrate

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Transparent frontsheet/encapsulant/thin film cells/flexible substrate

Transparent Frontsheet Encapsulant Thin Film PV Flexible Substrate

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Typical component selection for Cr-Si

·Superstrate ­ Glass ·Substrate or Backsheet ­ Multi-layers of Tedlar and /or PET ·Encapsulant ­ EVA

· (also PVB, TPU and Ionomers)

·Edge Seal ­ usually none

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Typical components for Thin Films

·Superstrate ­ Glass ·Substrate - Glass ·Encapsulant ­ EVA

· (also PVB, TPU and Ionomers)

·Edge Seal ­ PIB (polyisobutylene)with additives

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Development of Reliable Modules

· Development of reliable PV modules required an understanding of potential failure mechanisms. · The most straightforward way to determine these failure mechanisms is to observe them in the field. · We can't wait 20 or 25 years to see what failure mechanisms a module type might suffer from nor to get an estimate of lifetime or degradation rate. · Therefore we try to develop stress tests that accelerate the same failure mechanisms. · So I am going to take you on a short review of history of PV module failure mechanisms and how this information was utilized to develop accelerated stress tests and ultimately the module qualification tests.

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History of Field Failures

· Broken interconnects · Broken cells · Corrosion · Delamination and/or loss of elastomeric properties · Encapsulant discoloration · Solder bond failures · Broken glass · Hot Spots · Ground faults · Junction box and module connection failures · Structural failures

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Broken Interconnects

· Interconnects break due to stress caused by thermal expansion and contraction or due to repeated mechanical stress. · Early modules suffered open circuits due to broken interconnects. · What makes it worse

­ Substrates with high thermal expansion coefficients ­ Larger cells ­ Thicker ribbon ­ Kinks in ribbon

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Broken Cells

· Crystalline Si cells can (and will) break due to mechanical stresses. · Early modules suffered open circuits due to broken cells since there was only one attachment point for each polarity. · What makes it worse ­ thinner cells ­ Single crystal especially if cleave plane is oriented along bus bar ­ Pre-stressed or chipped cells ­ Larger cells in large modules ­ Poor packaging of modules during shipment

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Corrosion

· Moisture induced corrosion of cell metallization. · Key to survival is to minimize the ionic conductivity in the package, especially the encapsulant. · Field failures of PVB encapsulated modules in 1980's was due to high ionic conductivity in moist PVB. · What makes it worse

­ Metallization sensitivity to moisture ­ Encapsulant with humidity dependent conductivity ­ Encapsulant that absorbs a lot of moisture.

Figure 2.

Solar-Cell Electrochemical Corrosion

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Power Loss Due to Corrosion in PVB (JPL Picture and Data)

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Delamination

· Delamination observed to varying degrees in a small percentage of PV module types. · Delamination can be between superstrate (ie. glass), substrate (ie. Backsheet) and encapsulant or between encapsulant and cells. · Usually the result of an adhesive bond that is sensitive to UV, humidity, or contamination from the material (Excess Na in glass or diffusion dopant glass left on cell as was case in picture)

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Encapsulant Discoloration

· Will result in some loss of

transmission and therefore reduced power. · Worst reported case was in slow cure EVA caused by low concentration system at Carissa Plains. · Standard cure EVA formulation A9918 does discolor.

­ ­ ­ Caused by heat and UV. Bleached by oxygen So with breathable backsheet center of cells discolor while outside ring remains clear. Without concentration it takes 5 to 10 years to see discoloration and longer to start appreciably reducing output power.

­

· It was not EVA itself that discolored,

but additives in the formulation. Changing additives dramatically reduced the problem.

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Hot Spots

· Hot-spot heating occurs in a module when its operating current exceeds the reduced short-circuit current (Isc) of a shadowed or faulty cell or group of cells. · When such a condition occurs, the affected cell or group of cells is forced into reverse bias and must dissipate power. · If the power dissipation is high enough or localized enough, the reverse biased cell can overheat resulting in melting of solder and/or silicon and deterioration of the encapsulant and backsheet.

Maximum shunt/cell temperature in degC

2 3

1 4

5 6

Candidate low shunt cells: 3, 5, 6 Worst case low shunt cell: 3

Reverse current in Amps measured at ­10 Volts

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Hot Spots in Cells (Pictures from TUV)

1 2 3

4

5

6

· Most cells can be adequately protected by use of bypass diodes (say 1 diode every 20 cells). · Also need to screen out cells with low shunt resistance.

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Structural Failures

· Sometimes the module itself is not strong enough to survive mechanical stresses (particularly wind and snow load). · These failures can involve the superstrate, substrate, frame or frame adhesive. · Snow load can deform the frame and break the glass. (See picture) · Often it is the way the module is mounted that determines whether it can survive a particular load.

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Difference in Mounting Method

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Accelerated Stress Tests

· So

now that we have a list of failures, we can develop tests that duplicate the failures in a fairly short time frame (at least compared to outdoor exposure). · Our goals should be:

­ To use the results of the tests to improve the module's ability to withstand this specific stress. ­ To use the results of the accelerated tests to predict module lifetime

· In using accelerated stress tests we must cause degradation in order to verify that our accelerated test is duplicating the failure mechanism we saw outdoors.

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Accelerated tests for PV

Thermal cycling ­ Broken interconnects ­ Broken Cells ­ Solder bond failures ­ Junction box and module connection failures Damp Heat Exposure & Humidity Freeze ­ Corrosion ­ Delamination ­ Junction box and module connection failures UV Test ­ Delamination ­ Encapsulant discoloration

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Accelerated tests for PV

Mechanical Load

­ ­ ­ ­ Broken interconnects Broken cells Broken glass Structural failures

Dry and Wet Insulation Resistance

­ Delamination ­ Ground faults ­ Electro-Corrosion

Hot Spots

­ Localized shunts that lead to hot spots ­ Inadequate bypass diode protection

Hail test

­ Broken cells ­ Broken glass

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Qualification testing or Certification

· Qualification tests are a set of well defined accelerated stress tests developed out of a reliability program. · They incorporate strict pass/fail criteria. · The stress levels and durations are limited so the tests can be completed within a reasonable amount of time and cost. · The goal for Qualification testing is that a significant number of commercial modules will pass. Then if module manufacturer uses an adequate QA system, all subsequent production modules will be built the same way as the test modules were built. · So passing the Qualification test says the product meets the specific set of tests, but doesn't predict product lifetime nor indicate which product will last longer or degrade in operation. · Qualifies the design and helps to eliminate infant mortality

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IEC 61215 (61646 very similar)

· 200 Thermal Cycles from -40 to + 85 C with Imax current flow during heat up · 1000 hours of damp heat exposure at 85 C and 85% RH · UV Preconditioning/50 TC/ 10 Humidity Freeze Cycles · Bypass diode thermal test

· One hour at Isc and 75 °C with diode below temperature rating · 1 hour at 1.25 times Isc and 75 °C with diode surviving

· Hot Spot test

· 3 lowest shunt resistance cells subjected to 1 hour exposure to 1000 Wm­2 irradiance in worst-case hot-spot condition and · 1 highest shunt cell subjected to 5 hours exposure to 1000 Wm-2 irradiance in worst-case hot-spot condition

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IEC 61215 (Continued)

· Mechanical Load Test

· 3 cycles of 2,400 Pa uniform load, applied for 1 h to front and back surfaces in turn. · Optional snow load of 5,400 Pa during last front cycle

· Hail Test ­

· 25 mm diameter ice ball at 23,0 ms-1, directed at 11 impact locations

· Outdoor Exposure Test - 60 kWh.m­2 total solar irradiation · Pass/Fail

o < 5% power degradation o no major visual defects o pass dielectric withstand at 1,000 V d.c. + twice the maximum systems voltage for 1 min the measured insulation resistance times the area of the module shall be not less than 40 Mm 2 measured at 500 V or maximum systems voltage, whichever is greater o Pass wet leakage current test with the measured insulation resistance times the area of the module shall be not less than 40 Mm 2 to be measured at 500 V or maximum systems voltage, whichever is greater

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Safety Qualification IEC 61730

· Part 1 provides safe construction requirements including material tests especially for polymers. · Part 2 provides test requirements

· Uses the accelerated stress tests from IEC 61215. · Adds specific safety tests

· · · · · · · Ground Continuity Accessibility Impulse Voltage Reverse Current Overload (to set fuse rating) Partial Discharge on backsheet or package Module Breakage Cut Susceptibility

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Updating

· Qualification and Safety Tests are continually being updated. · WG2 of IEC TC-82 now working on edition 2 of IEC 61730

· Make IEC 61730 consistent with requirements of IEC 61140 · Completely changed polymer section. · Trying to replace RTI with realistic UV test and a high temperature creep test · Added retest guidelines · Added recommendations for factory testing

· WG2 of IEC TC-82 now working on edition 3 of IEC 61215

Made maximum power at STC a pass/fail criteria Added retest guidelines Completely changed the hot spot test Combined the NOCT test with the Outdoor exposure test and renamed the resultant temperature NMOT · Modified to cover low concentration modules that utilize diffuse light · · · ·

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Status of Packaging Technologies ­ Cr Si

· More than 20 years of field experience with the present package. · Modules easily pass the IEC 61215 Qualification Test · EVA discoloration from 1990's appears to have been solved, but must be revisited every time a new EVA formulation or supplier appears. · Adhesion of encapsulant to glass and backsheet appears adequate for 25 year life as long as processing is under control. · Annual module degradation rates of < 0.5% per year are being reported.

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Status of Packaging Technologies ­ Thin Films

· Edge seals appear to do a good job of keeping moisture away from sensitive thin film layers. · Most modules pass the IEC 61646 Qualification Test · Recent thin film modules appear to suffer much less degradation than earlier types. · Many thin film modules are now sold with a 25 year power warranty, matching today's crystalline silicon modules.

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SUMMARY

· PV modules can be extremely reliable. · Need to maintain that reliability through comprehensive testing program that should include accelerated stress tests and long term outdoor exposure. · Use accelerated stress tests to:

­ Assure that commercial processes and materials are under control. ­ Verify that proposed changes will not degrade module lifetime, reliability or safety. ­ Assist in identifying those failures observed in the field helping to eliminate them.

· Continue to monitor outdoor performance via.

­ ­ ­ ­ Analysis of warranty returns Deploying and monitoring individual modules over long time periods Monitoring the performance of PV systems over time. Utilizing added accelerated stresses on fielded modules (such as high voltage).

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