2014 NREL Photovoltaic Reliability Workshops

The 2014 Photovoltaic Module Reliability Workshop featured more than 100 presentations and posters from workshop participants. The following topics were covered:

The presentations listed are available as Adobe Acrobat PDFs. A compilation of the 2014 PVMRW proceedings is also available for download.

This annual workshop is sponsored by the U.S. Department of Energy Solar Energy Technologies Office.

Introduction and Plenary

  • Welcome—Bill Tumas, National Renewable Energy Laboratory (NREL)
  • Welcome—Shubhra Bansal, U.S. Department of Energy (DOE)
  • Accessing Low Cost Financing Through Standardization, Best Practices, and Risk Scoring—Michael Mendelsohn, NREL

Field Experience

Session Chair: John Wohlgemuth, NREL



Potential Induced Degradation

Session Chair: Max Koentopp, Hanwha Q CELLS GmbH


Physics of Failure

Session Chair: David Miller, NREL


Field Imaging

Session Chair: Peter Hacke, NREL


Inverter Reliability

Session Chair: Rob Sorensen, Sandia National Laboratories


Thin Film Technologies

Session Chair: Tim Silverman, NREL


Packaging Materials and Accelerated Testing


Quality Management

Session Chair: Sarah Kurtz, NREL


Standards Work and Thin-Film Technologies


IEC Task Force Update

Session Chair: Nick Bosco, NREL


Session Notes

Session: Field Experience

  • Broken glass appeared to be the results of thermal expansion issues associated with the racking system.
  • The solder bond failures reported in "Reliability Evaluation of PV Power Plants: Input data for Warranty, Bankability and Energy Estimation Models" were judged to be more related to workmanship than design, but the detailed cause has not been determined.
  • The bypass diode failures reported in "Reliability Evaluation of PV Power Plants: Input data for Warranty, Bankability and Energy Estimation Models" resulted in open-circuited bypass diodes. Almost all of these were found in a system that had shading.

Session: Potential Induced Degradation

  • The comparison of damp heat and foil testing presented in "Testing Modules for Potential-Induced Degradation — A Status Update of IEC 62804" would have given a better comparison if weight had been applied to the foil.
  • Some attendees raised the question of using a foil test that applies the stress across the entire module instead of simulating the situation in the field. Others noted that the foil test does a good job of simulating the leakage when it is raining and that they like the foil test because it helps to easily identify whether there is a problem. The current draft of IEC 62804 includes both the damp heat and the foil method.
  • Volker Naumann reported that after a regenerative bias has been applied (after degradation induced by application of a bias voltage), they have observed evidence that the sodium migrated back out of the stacking faults. Annealing can also lead to regeneration, but takes a longer time.

Session: Physics of Failure

  • In "Guidelines for Comparing indoor Accelerated Stress Tests to Outdoor Use," it was assumed that there was a single reaction occurring with a single activation energy, but in some situations there are multiple processes occurring that have different activation energies. In some cases, the rate-limiting step may be the diffusion of the moisture, which gives a different result than modeling the activation energy of the chemical reaction.
  • We need to be cognizant of the difference between the application of damp heat and the photolysis process that occurs in the field. To be quantitative, we need to model the specific reactions.
  • Measuring the activation energy may give a clue to the mechanism.
  • When looking at the effects of moisture ingress, one should include thermal cycling. Inclusion of thermal variation may open paths for moisture ingress that one would not see in an isothermal damp-heat test.

Session: Inverter Reliability

  • A separate study showed that 1000 cycles corresponded to 10 y in the field, but the details of the temperature fluctuations during those 10 y are unknown.
  • When comparing the effect of temperature fluctuations in the field with thermal cycling in a chamber, the order of the fluctuations may change the damage done by a specific change in temperature. There's a hysteresis effect.
  • The temperatures experienced for microinverters are higher than ambient. Typically, SolarBridge keeps the inverters 3-4 inches off of the roof, which keeps the temperature in a manageable range.
  • During testing, Advanced Energy installs as many as 200 thermocouples to monitor every IGBT, every PC board, and many other places.
  • To enable the writing of good standards, we need the manufacturers to communicate about the failures modes that are being seen.
  • There is talk of people disconnecting from the grid, but battery storage is still expensive and unreliable, so its time has not yet come.
  • There is a trend toward undersizing the inverter, then clipping the power (by operating the modules in forward bias) when the inverter rating is exceeded. Advanced Energy reports that some of their 1 MW inverters may clip as much as 8 hours in a day. The design intent is to have a steep power-up curve.
  • The operating voltage and power point (clipping) need to be considered when testing for degradation and failure mechanisms.
  • Testing the firm ware is essential. Advanced Energy uses a hardware emulator to test the firmware in the laboratory and also tests firmware on site in beta testing. Much of this testing is directed by UL standards.
  • Customers want longer reliability from inverters. However, this message isn't driving the standards development — customers need to participate in standards writing to demand that testing is done to give the confidence that customers look for.

Session: Quality Management

  • Hand-soldering can yield superb results, but movement toward automation may help to provide better consistency when there is a diverse work force.