Infrared picture of a MWT-testpanel taken after 1,000 hours exposure to moisture and heat in the climate chamber. The white dots (about 40 °C) mark local heat spots due to high amps in certain contacts. The temperature of the coolest parts (green) is about 25 °C (Illustration ECN).

In many ways, the solar panels developed by ECN that have contacts on the back are superior to ‘classic’ solar panels. There is no disagreement about this. However, will these panels still supply energy after 20 years of use in the harsh conditions of tropical humidity and extreme desert heat? Wilma Eerenstein of ECN Solar Energy put eight panels into a climate chamber. “They were in there for much longer than prescribed by the IEC standard. We wanted to test how rugged the panels really are and be sure that any mechanisms of failure would be exposed.”

The solar panels currently on the market are based on technology developed 30 years ago using soldered connections to link the solar cells. The high temperatures involved often damage these cells. Although the damage can be repaired manually, this will ultimately lead to higher sales prices. Eerenstein: “Panels built based on our new concept (MWT cells) do not have this problem. The MWT technology uses a special new type of foil plus conductive glue. How reliable are the panels if they are assembled using a particular manufacturing method? This is something every potential manufacturer of these solar panels would like to know.”
The solderless panels using ECN technology were developed at the right moment in time. A European agreement (ROHS) is now in place to banish lead, a toxic metal, from products. Current solar panels contain lead, because it is used in solder. However, the manufacturers were granted exemption based on the fact that no reliable alternative was available. “It is expected that this exemption from the ROHS will be terminated in around two years’ time. All solar panels will have to be lead-free by then. MWT panels are lead-free, but, of course, they also need to be reliable,” explains Eerenstein.

Twenty years in the desert
The climate chamber looks like a bread oven at a bakery. It is as tall as an adult and is double-walled to enable quick climate changes inside the chamber. The electrical power supplied by a test panel is measured beforehand. The panel is then tested in the climate chamber in accordance with the IEC standard. After the test, the power capacity of the panel is measured once more. If the decrease in power capacity is less than 5% compared to the capacity measured before the test, the panel scores a ‘passed’ label.  
The IEC standard prescribes that the temperature should cycle 200 times between -40 °C and +85 °C during the heat test. A complete test takes around six weeks because only complete cycles are carried out each day. This test simulates the most extreme conditions in a desert climate. It is virtually impossible to draw direct conclusions about the expected life span of the panels, but a successful test says a lot about the quality of the materials used to manufacture the panel. In the moisture and heat test, the panel is continuously exposed to a relative humidity of 85% and a temperature of 85 °C for 1,000 hours. These conditions are typical of a tropical climate. Eerenstein: “Naturally, we were interested to find out when the panels would actually fail. This is why we subject the panels to conditions well beyond the requirements of the standards. The temperature cycles have been increased to 300 times. We doubled the duration of the moisture level and heat test to 2,000 hours. After the tests, we inspected the panels thoroughly.”

Other testing and analysis methods
In the MWT panel tests, we test more than just the durability of the materials used. ECN also uses this test to investigate quite different testing and analysis methods. It is not intended to replace the IEC standard, but rather to speed up ECN’s investigations. “The design concept of MWT cells has clearly been validated. We strongly suspect that we can also be successful if we start looking for the best materials. Obviously, we want to test reliability too, but time is short and we don’t intend to wait 1,000 hours. Often panels also need to be opened to find out what’s going on inside. In this case, you have to break the panels so you can't do any more tests on them. This is why we intend to develop two alternative methods of non-destructive testing aimed at exposing weaknesses in the panels.
ECN is not alone in this quest for a much quicker testing method. Within the framework of the ‘Pieken in de Delta’ project ‘geZONd’, ECN, Solland, Philips, Holst-centrum, OM&T and Holland Innovative are jointly striving to develop such testing methods. If this effort proves successful, the method will be performed at ECN’s site in Eindhoven. The plan is to concentrate the research of materials and testing methods at ECN Eindhoven.

Infrared and luminescence
Research into the mechanisms of failure is still a big puzzle, says Eerenstein. “We used current knowledge of weaknesses as a starting point. Examples of weak points are the conductive glue connections between the cells and the foil at the back of the panel. For each cell there are 31 connections, and a standard panel contains 6 x 10 = 60 cells. If a single connection fails electrically, the surrounding contacts can take over from the failed connection. These connections then need to conduct a higher current, and will therefore have a higher temperature than other connections. Within the scope of the geZONd project, ECN has developed a method to visualise these minor temperature differences with an infrared camera. Using this method, bad connections are spotted immediately because they do not generate any heat.”

Cross section of a MWT panel. Locations where several different materials come together are risk areas for contact failure (illustration ECN).

An additional method of analysis that can be used to identify poor connections is electroluminescence (EL). This method reveals which parts of a solar cell are active and which parts have failed, for example, as a result of bad contacts. “A panel that we preanalysed using electroluminescence shows a number of weak points. Following exposure to the trying conditions in the climate chamber, we used electroluminescence to reanalyse the panel. This showed that the weak points had not weakened any further, proving that MWT panels are particularly rugged. But I am especially pleased that the EL analysis was able to expose the weak points without the need to cut the panels open.”

Electrocution
In addition to the two IEC-related tests described above there is yet another test: the leakage current test. The panel is kept immersed in water for two minutes while a minimum of 500 V is applied to the electrodes. This analysis needs to be the final test conducted. After the test, the specific electrical resistance of the panel should not be less than 40 MΩ/m². All panels that passed the higher-level climate chamber tests, also easily passed the higher-level leakage current test (1,000 V put on the electrodes rather than 500 V (180 and 320 MΩ/m², respectively). This removes any doubts regarding the ruggedness of MWT panels.

Contact
Wilma Eerenstein
ECN Solar Energy / PV Module technology
Tel.: + 32 (0)22 456 44 35
E-mail: Wilma Eerenstein  

Information
Climate chamber test results of MWT back contact modules 

This ECN Newsletter article may be published without permission provided reference is made to the source: www.ecn.nl/nl/nieuws/newsletter-en/