Solar Panel Performance - The Good, the Bad and the Ugly!
Peter Green Silcar Energy Solutions
1/19 Tamara Drive, Cockburn Central, Perth
Western Australia. [email protected]
In Australia telecommunications networks have utilised solar power to support critical loads since the early 1970s. These systems have formed the back bone for the powering of major and minor networks ever since. As major long haul, inter capital radio systems were replaced with optical fibre systems, these solar systems continued to support the load requirements without issue.
The renewable energy industry will normally showcase the impressive vista of large scale solar installation and compete for attention, but rarely do they show how the panels look after twenty to thirty years once they have been subjected to the elements.
This paper is not intended to be a scientific address on manufacturing techniques but a pragmatic review of panel performance.
This paper will examine some typical examples of the degradation of two brands of solar panels over their lifetime. The paper provides an insight into when asset owners and operators should consider replacing the panels and if in fact the degradation in output warrants the replacement. Manufacturers will often provide warranties to 80% of rated output at 20 years; so how do operators determine end of life and rate efficiency of solar panels getting to 20 years operation are they able to safely support critical infrastructure?
Consideration is given to utilising the legacy solar array systems with on grid or off grid applications.
Data and samples are taken from recovered solar panels that have been used in the harsh Australian outback with temperatures ranging from -2 degrees to + 50 degrees Celsius with relative humidity up to 98%. The seasonal transitions in the regions where solar systems are typically deployed are extreme and violent, with areas of the Australian coastline
subjected to severe tropical cyclones to Category 5 with wind speeds to 300 kph .
Figure 1. Cyclone Damaged Solar Array.
The other extremes include snow capped alpines of the south east of Australia. Samples used have been subjected to the high extreme temperatures with humidity levels up to 98% from the north west of Western Australia with two samples recovered from the Nullarbor Plain in the south with low winter temperatures (- 2 degree Celsius) and high summer temperatures to 45 degree Celsius.
The particular samples used in this assessment were manufactured in France by Phillips  and in Australia by Solarex, the latter having since ceased production in Australia. The panel year of manufacture ranged from 1979 through to 1986 with six of each brand used for the initial assessment, Refer Table 1.
Table 1 DETAILS OF SAMPLE PANELS USED IN THIS REVIEW.
Brand DOM Rating Service life to date.
Aug 1984 to 1986 33 & 37W < 26 years.
Phillips RTC BPX 47C
Circa 1979 to 1982. 33 W < 30 years.
978-1-4673-1000-0/12/$31.00 2012 IEEE
The fundamental differences between the two panels relate to the construction methods:
The Phillips BPX 47C panels were constructed with the single crystal silicon wafers sandwiched between two layers of clear glass. These are held in place with a polymer resin to seal the cells from moisture ingress and provide strength to resist thermal and mechanical shock.
Figure 2. Phillips BPX47C panels.
Solarex LX81 BGT series panels utilised a silicon rubber backing sheet that was heat vacuum sealed and white in colour. This method is still often utilised by the majority of panel manufacturers but with the use of cheaper ethylene vinyl acetate (EVA) products and Tedlar.
Figure 3. Solarex X81BGT panel. (The Good)
These panels were deployed in their thousands across Australia  and whilst end of life (EOL) replacement
programs have recovered a large number, many are still in use today.
Many of the EOL replacement criteria are subjective and can lead to either replacing the panels too early or simply too late.
The criteria included:
II. Cracks in the glass. III. Shattered glass. IV. Corroded inter-cell tracks. V. Dirt build up in the substrate between the glass
surface and backing. VI. Junction terminal failures.
VII. Discolouration of the wafers. VIII. High resistance interconnect joints from corroded
solder joints: and IX. Perceived reduction in output.
The point, at which degradation should be acted upon, is highlighted in the following sections.
The output characteristics of each panel were measured and compared against the original manufacturers data to determine the level of degradation and assess if the degradation is directly attributed to any one of the above causes.
2 CRACKS IN THE PANEL GLASS - BPX47C SERIES.
This is most likely caused by the prevailing weather conditions such as severe cyclonic storm events with objects or material being lifted from the ground and/or hail which has struck the front or rear of the glass laminated panels. This has allowed moisture to enter the laminated glass as it heats and cools and tracks along the interconnecting wiring and onto the actual wafers. Given the high costs of solar panels (in the 1980s and 1900s), attempts were made to seal the cracks in the glass. This proved futile as the cracks extended to the inaccessible parts of the support frame allowing moisture to continue entering at those points or by the time the cracks were found the moisture had already entered the laminate. With surface temperatures on the wafers as high as 60 degrees Celsius the cracked laminated glass allowed atmosphere to enter the panel. The process of delaminating continued to a point where the resin was allowed to heat up and run to lower parts of the void consequently holding the glass apart by as much as 6mm. This void simply became a well for moisture and contaminants to promote decay and corrosion. This is clearly evident in Figure 4.
Figure 4. Delaminated BPX47 C panel.
3 SHATTERED GLASS LX81BGT.
These panels suffered the same fate as the glass laminated BPX 47C panels when subjected to severe weather. In this case with the tempered glass, EVA and tedlar backing sheets the seals were rarely broken, with little or no signs of moisture ingress from the rear. Moisture had entered the panel through crack lines in the glass and has commenced discolouring (brown) the silicon wafer and interconnection tracks. This is highlighted in Figure 5 where the discolouration follows the cracks.
Figure 5. Moisture ingress on X81BGT shattered panel, (The Bad)
4 CORRODED INTER-CELL TRACKS.
Corrosion is highly visible on the damaged BPX 47C laminated glass modules. Once the moisture had entered the panel it crept along the tracks causing corrosion.
5 DIRT BUILD UP IN THE SUBSTRATE BETWEEN THE GLASS SURFACE AND BACKING.
This issue was not as evident as the moisture damage.
6 JUNCTION BOX FAILURES.
The junction boxes fitted to the Phillips BPX 47CF clear glass panels were directly exposed to the sun and have become very brittle and disintegrated exposing the connections terminals and diodes. The earlier model BPX47C panels were fitted with diecast aluminium junction boxes that were in very good condition internally and externally. It should be noted that the cable entry conduits had to be sealed with silicon compounds to prevent moisture from travelling up the conduits and collecting in the junction boxes causing terminal corrosion and high resistance joints.
The junction boxes fitted to the Solarex LX81BGT panels are protected from direct sunlight by the silicon rubber backing to the panels. As a result the junction box plastics were in good condition and were not brittle. These particular junction boxes do not provide a positive seal and as such allowed any collected moisture to evaporate or drain off without adversely affecting the connection terminals.
All junction boxes are mechanically fixed to the panel support frame. It was found on later model panels that the junction box is secured to the tedlar with common silicon which falls off after a few years compromising the connection cables and the seal where they enter the panel face. Strong winds then blow the junction box around causing further damage and stress to the module cabling.
7 DISCOLOURATION OF WAFERS.
Both module types experienced discolouration.
Panels operating in the Australian tropics (to approximately16 degrees south of the equator) with high levels of humidity suffer from severe mould growth and discolouration whilst the same panel operating in the drier southern regions of Australia appear to be in near perfect condition.
This mould growth condition is exaggerated if the glass has been damaged. Field inspections have found this to be an ongoing issue even with more modern panels manufactured to this day.
On a number of the BPX47C panels the resin between the layers of glass surrounding each wafer has been badly disturbed and would appear to be caused by hot spots as a result of shading. As noted in Intelec papers from 1984 Solar Power For Telecommunications The Last Decade . The reverse currents can excessively heat the wafer melting and bubbling the surrounding resin. The damage and disfigurement has allowed moisture to enter these crevices and voids.
Figure 6. BPX 47 C Deteriorated Laminated Panel (The Ugly)
On at least one sample panel (LX81BGT) some wafers displayed a symmetrical brown/orange discolouration in the centre of the wafers covering about 70% of the surface of