Let the sun, not profits, burn

DuPont’s Stephan Padlewski (sixth from left) and Fiona Paul Pinter (far right) pose with a selection of presenters invited to talk at the DuPont Photovoltaic Solutions event held last month in Rome, Italy.

Experts from the solar industry and the financial sector gathered in Rome in October at a conference organized by U.S. chemical company DuPont to discuss how to improve mitigating risk strategies for solar investments. The conference, which identified that the task of protecting investments in solar parks worldwide is becoming a more and more complex one, examined how the number of factors that may negatively impact projects has increased considerably over the past few years, as more surveys and analyses on solar project portfolios are being conducted.

The results of one of these surveys, the Global DuPont Field Surveys (2016), were presented at the conference by Stephan Padlewski, the Regional Marketing Manager EMEA, at DuPont Photovoltaic Solutions.

The survey comprised data from 190 solar facilities totaling 450 MW spread across North America, Europe, and Asia Pacific, and data on modules provided by 45 different manufacturers.

The survey revealed that 22% of the modules installed at these facilities were affected by performance issues, and that backsheets are the most affected components. According to Padlewski, around 11.3% of problems came from chemical and mechanical backsheet degradation, while defective solar cells and EVA degradation had contributions of 7.4% and 2.8% respectively, with other minor defects having a minimal share of 0.5%. The number of defects, however, was considerably higher in hot and arid climates, accounting for around 41.6% of affected modules. Backsheet-related issues represented 25.9% of defects, followed by EVA degradation with 10.4% and solar cells with a 5.2% share.

In temperate climates, however, the number one problem remains defective solar cells, which had a share of around 16.4%, with defective backsheets representing only 8.7% of the facilities’ issues.

Furthermore, the survey has revealed that backsheets are also the main critical factor for rooftop projects, independent of climate conditions, due to the fact that rooftop systems are usually exposed to higher temperatures than ground-mounted arrays. According to the survey, 37% of panels in rooftop projects were affected and backsheet issues reached a 25.3% share. In comparison, ground-mounted solar parks had 21.6% of modules affected and only 7% of problems with backsheets.

Backsheets: a closer look

The types of degradation affecting backsheets are varied, although the most serious threats, which are cracking and delamination, had percentages of 6.9% and 6.8% respectively. Yellowing, which is an indicator that the polymer has started to degrade, was the most common issue, with front side yellowing reaching a share of 69.3% and air side yellowing representing 17% of degradation factors. “We still don’t know, however, how yellowing may really impact PV plant performance to date,” stressed Sara Di Mario, the Operations Director of Italy’s largest PV plant operator EF Solare Italia, during the conference. Padlewski also commented that IEC standards may not solve all of the problems, because these do not take into account specific stresses endured by the solar panels.

“The standards for the type testing of PV Modules (IEC 61215, IEC 61730) define requirements for construction and testing of modules used in a moderate climate,” Willi Vaassen from Germany’s TÜV Rheinland told pv magazine. “The higher stress of the modules used in hot and tropical regions are not well taken into consideration yet.”

Vaassen presented a second survey at the conference which showed that 30% of inspected solar plants have serious or particularly serious defects, and that around 55% of defects are caused by installation errors.

The survey was conducted on data collected in 2014 and in the first quarter of 2015 from PV projects located in Europe, Asia and North America that ranged in size from 100 kW to 30 MW.

Serious defects were detected by TÜV Rheinland experts in modules in 48% of cases, while cabling represented 28% of identified issues. Defects in connection and distribution boxes reached a share of 16%, while inverter and mounting systems had both a minimal share of around 4%.

The impact of degradation

Potential-induced degradation (PID) was identified as performance killer number one, while early degradation of backsheets was pointed out as the second most serious threat for a project’s yield.

“We have the situation,” Vaassen told pv magazine, “where the assessment of different power plants in different countries shows whitening effects of the backsheet and partly cracks between the cells. This happens in some cases after only a few years of operation. In general, the cause of both failures is the unsuitable characteristics of the backsheet.

“With this whitening and these cracks, we face problems of personal safety – now or sometimes in the future. In other words: Protection against electric shock might not be a given. Hence, the encapsulation itself seems in some cases to still be functional, but with respect to PV module safety standards it cannot be considered a functional insulation.

“Beside this, in the long run we face performance problems when moisture gets into the laminate and corrosion effects occur. If there is an indication that these failures are present in a plant, a risk assessment shall be performed and risk mitigation measures have to be defined.”

To avoid or strongly mitigate risks early, measures taken during the project development and procurement phase are most effective, Vaassen recommends. Such measures include vendor qualification, component testing, qualification of EPC, advanced inspection, and advanced monitoring.

“Technical risks can be systematically organized in a risk matrix,” Vaassen stressed. This means that a professional risk management strategy should become an integral part of each PV investment. That backsheet faults are becoming a serious threat was also confirmed by Sara Di Mario, who revealed that evidence of big problems with backsheets has emerged only over the past two years, and that EF Solare Italia had restructured its business also on these kinds of issues. PID, hot spots, and oxidation were also pointed out as major critical issues impacting PV project performance. “These are all factors,” Di Mario said, “that must be carefully considered for capital expenditure decisions.”

The head of engineering of Italian renewable energy company Enel Green Power, Fabrizio Bonemazzi, also highlighted the seriousness of the backsheet degradation threat, “especially when it occurs in an accelerated and homogeneous way within a single project.” Backsheet issues, he added, were detected across several PV plants located in Italy, and especially in modules of the older generation that were manufactured with low-quality materials.

Potential-induced degradation was also indicated by Bonemazzi as a factor that may even lead to a 70% performance loss at the module level.

Backsheets, after all, are responsible for a multitude of tasks in ensuring module reliability. But generally, it is not possible to find the ‘right’ material combination that can win in all categories. In another article published in this issue, pv magazine’s backsheet expert Sraisth discusses how polyolefin-based backsheets may impact LCOE, particularly concerning coextrusion technology (p. 60).

However, as DuPont’s conference in Rome has shown, backsheets are a major issue – but not the only one. The world’s solar portfolio is relatively new and the number of challenges for PV plant operators willing to maintain acceptable margin levels is probably doomed to increase. As a consequence, risk mitigation strategies, which also have their cost, are becoming unavoidable for both new projects and operational solar plants. An investment in these strategies, however, may save investors a lot of trouble.

On the other hand, as DuPont’s speaker Padlewski pointed out in his presentation, “a five year reduction of a project’s lifetime could lead to a 30% LCOE increase and result in a rise in project costs by €0.25 per watt.”