Daniel Philipp
Head of Group Module Testing, Fraunhofer Germany

Biography

Daniel Philipp studied renewable energy engineering in Berlin. In 2005, he finished his studies with his diploma thesis which he wrote at Fraunhofer ISE, titled: Influence of humidity on the degradation of PV modules and permeability of components“. After his diploma, he stayed at Fraunhofer ISE and supported the building-up and founding of the Test Lab PV Modules, where he started to work as testing and certification engineer. In parallel, he also worked in several R&D projects in the field of quality and reliability of PV modules. In this function, he collected substantial experience in quality and reliability related aspects of PV modules. He conducted factory inspections and training programs for PV module testing engineers. He contributed to the founding of further PV testing laboratories in Albuquerque (USA) and Singapore. Since 2012, he is head of the accredited TestLab PV Modules.
 
Abstract
A combined test cycle, which was developed in the project “PV-Zuverlassigkeit II” (PVZ-II, BMWi FKz 0329978) at Fraunhofer ISE, is applied as a realistic and highly challenging multi test cycle in order to qualify two different encapsulation materials for full-size PV modules. On the one hand a commercial, high performance EVA material and on the other hand a new polyolefin material from Borealis called “Quentys BPO” is used.

The Multi-Cycle Test approach is based on experimental outdoor and indoor testing as well as on theoretical investigations to simulate the moisture ingress [1]. Realistic outdoor conditions are simulated by combining realistic and constant humidity level in the PV module with combined stresses, such as temperature cycling (TC / constant humidity) and UV (UV / DH). The stresses are applied in special sequences and create a harsh, accelerated, but, compared to standardized and common tests, a more realistic stress to the PV modules. The degradation characterization is evaluated by performance, electroluminescence, color, thermography and peel force measurements. In addition, (wet) leakage testing is used to monitor the insulation behavior of the modules.

In the presentation this Multi-Cycle Test will be introduced and explained. A comparison of two high quality encapsulants, tested accordingly, will be presented.
 
Results
The Multi-Cycle Test causes accelerated aging close to real outdoor stress by combining the most relevant degradation factors:
  • UV- irradiation
  • Humidity ingress
  • Temperature and thermo-mechanical stress
 
    
 
Fig. 1: Multi-Cycle sequence and combined UV-Damp-Heat Chamber at Fraunhofer ISE.
    
 
Fig. 2: Test Conditions and UV Spectrum.
In total four cycles have been applied on two full-size 60-cell modules. The samples have been produced by project partner BOREALIS using identical materials for all samples (e. g. commercial c-Si cells) except for the encapsulant: Polyolefin (Quentys BPO) vs. high-performance commercial EVA.
    
 
Fig. 3: Samples and Materials.
Degradation of electrical characteristicse and electroluminescence imaging showed good stability of both samples during up to three cycles, while there is a trend to slightly stronger degradation of the EVA sample. After four cycles the EVA sample has degraded significantly.
 
Fig. 4: Degradation of power and EL appearance
The EL images indicate a degradation of the metallization of the EVA sample after four cycles which surely has the most significant impact on the electrical degradation However, the Yellowing on the front side of the cell of the EVA sample (Fig. 5b) indicates also an increase of optical losses.
 
Fig. 5: Yellowness Index (YI) against test cycle and cross sections of the modules after four cycles.
Although both modules show an increase in YI only the EVA sample discolored in front of the cell. This could be at least partly responsible for the degradation of the Isc, as displayed in the following figure.
 
Fig. 5: Degradation of Isc, Voc and fill factor after four cycles.
A qualitative comparison of the results of a 23 years field aged module in Delhi shows strong similarities of the degradation indicators. This supports the practical relevance of the applied test method, but should of course not be understood as service life correlation. Further analytical work is ongoing to verify the Multi-Cycle Test.
 
Fig. 6: Degradation of appearance in electroluminescence. The EVA based module, exposed for 23 years in New Delhi shows similar degradation pattern.
 
Conclusions
Combined testing allows more realistic accelerated aging. The Multi-Cycle test is a comprehensive, model-based testing sequence proposal. The results of the herein presented work as well as experience in typical failure modes in the field support the practical relevance.
 
The benchmarking of two encapsulants unveils that EVA sample degraded more strongly compared to Quentys BPO, especially in the final stress cycle. Electroluminescence images indicate degradation of cell metallization of the EVA sample, possibly due to acetic acid formation. Cross-sections show that discoloration above the cells is stronger in EVA than in BPO, which could also explain the degradation in Isc of the EVA sample.