NATIONAL RENEWABLE ENERGY LABORATORY
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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
4
Deg.Rate (%/year)
3 2 1 0
Pre Post Pre Post Pre Post Pre 1 2 1 2 1 2 1
a-Si CdTe CIGS Date of Ins tallation within Technology c-Si
Pre Post 1 2
poly-Si
NATIONAL RENEWABLE ENERGY LABORATORY
• 40 module types from 10 different manufacturers • Earliest installation dates from 1993 • Technologies represented
• • • • • Mono-Crystalline Silicon Multi-Crystalline Silicon Amorphous Silicon Cadmium Telluride Copper Indium Gallium Selenide
• A long lifetime > 25 years (most have 25 year warranty) • A low infant mortality rate • A low degradation rate < 0.5%/year
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• Reviewed the literature on PV field performance. • Plotted data in accompanying figure. • It is likely that modules with large power degradations are considered to have failed and are not listed. • Most frequently reported annual degradation rate is 0.5%.
• 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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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)
OUTLINE
• PV Module Overview • Evaluating Long Term Performance • Developing Accelerated Stress tests • Qualification Tests (IEC 61215) • Full Product Lifecycle • Lifetime Testing • SuLong Term Performance
• To evaluate long term performance outdoors
we really need outdoor performance data. • On the other hand we can not wait 25 years to determine if a module is going to have a 25 year lifetime. • Therefore, we have to utilize outdoor test data to develop accelerated stress tests. • Since outdoor data has to come first I will start there.
•All mounted at latitude tilt.
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PERT Data Analysis
• NREL has normalized data to PVUSA Conditions
DC Power (W)
• Irradiance = 1000 • Ambient Temperature=20 °C • Wind speed = 1 ms-2 Wm-2
100 80 Frequency 60 40 20 0
Median: 0.5 %/year Average: 0.7 %/year N = 780
Degradation Rate (%/year)
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NREL Performance & Energy Rating Test Bed
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PURPOSE OF PV 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.
PV Module Reliability
John Wohlgemuth
May, 2011
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.
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Degradation Measurements from PERT
• Figure shows annual degradation rates for PERT modules. • Partitioned by technology and date of installation (before or after 2000) • Degradation rates of CdTe, CIGS & pc-Si have improved significantly since 2000. • Degradation rates of a-Si is still high. • Degradation rates of c-Si has always been quite low.
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Economics of Photovoltaics