/ Findings
Top Performance Killers
Which findings arise more often and which have the highest impact on the performance?
The following chart shows the top 10 findings detected on site having a negative impact on the performance of the analysed PV plants. The number attached to each bar shows in how many PV plants each finding was present.
It can be seen how heavy soling and near shading, two findings linked to the design constraints of the site, appear in a significant amount of PV plants, while inactive cell strings and mechanical damages in cells, both related to the electromechanical integrity of the modules, are present in around one third and two thirds of the inspected plants respectively. The findings (i) strong deviation from true South, (ii) self- shading and (iii) the inconsistent tilt angle, belong to the third level and all of them are related to an inadequate design of the PV plants. Finally, Potential Induced Degradation (PID), disconnected circuits and inverter clipping represent a mix of findings related to (i) an inadequate design of the electrical architecture of the DC part, (ii) failures in the installation and (iii) unexpected module degradation mechanisms.
Another aspect that also contributes to the loss of energy production is an operation and maintenance plan below market standards. Specifically, the lack of spare parts on site coupled with high reaction times, are two aspects that directly result in loss of availability and therefore in a drop of the yield. Only one of the PV plants had spare parts on site and in most of them there was no written agreement setting the contractual reaction times. In regards to the extremely low specific yields recorded in some of the PV plants, it is important to consider that although in some of them less nominal power was installed than initially planned, the calculation of the specific yield is carried out with the latter. This undoubtedly leads to unfairly low performance indicators. The following graph shows which of the findings shown previously have the highest impact on the performance of the inspected PV plants. The graph also indicates the maximum energy loss values associated to each of these findings.
The graph below shows how an advanced stage of Potential Induced Degradation (PID) can lead to a loss of global production at the PV plant level of more than 30%.
It is important to bear in mind that the aforementioned PID degradation has been reached in only three years. This phenomenon can be explained by the high sensitivity of the modules and the typical hot and humid conditions in the region. On the other hand, the presence of bird droppings, debris or pollution, result in soiling losses of up to 13% in some of the PV plants. In this regard, it is important to bear in mind that the PV plants visited by PI Berlin underwent a natural cleaning a few days before by means of a heavy rainstorm and hail that likely removed much of the accumulated dirt on the modules. The values measured on site by PI Berlin can therefore be greatly exceeded during the dry season. The losses caused by near shading and strong deviation from true South are estimated in some of the inspected PV plants at around 8% and 7% respectively.
These losses are difficult to mitigate since they are due to design constraints which should have been properly addressed during the design phase. Finally, the losses associated to (i) mechanical damage of cells and (ii) inactive cell strings induced by soiling, shading or cracks, can reduce the production of some PV plants by 6% respectively. These losses are often caused by bad transport to the roof, mishandling of the modules during the installation and O&M phase, and in others due to product failures. Only in the case of the latter can cooperation of the manufacturer in relation to the supply of replacement modules be expected. Since in most of the analyzed cases the warranties offered by the installation companies are limited to the product and do not include workmanship, the damages resulting from mishandling during installation remain uncovered.

Description of Findings

Deviation from South
15%
prevalence
3.8%
average loss
7%
max loss
In some of the inspected plants, the PV modules were installed parallel to the facades despite the building’s strong deviation towards South-East or South-West. Thus, when aligned with the building, the PV modules’ orientation is significantly deviated from the optimum.
Inverter clipping
2.5%
prevalence
3%
average loss
3%
max loss
Inverters are designed to operate at certain ranges of DC and AC voltages. In cases when the amount of modules in series or the DC/AC overbuilt are not properly selected, disruptions may occur.
Disconnected circuits
5%
prevalence
7.3%
average loss
7.5%
max loss
One of the characteristics of the series connection of modules is that if one element of the circuit is in open circuit, then the whole circuit is off. As a consequence, no current is produced and production losses occur.
Inhomogeneous tilt angle
7.5%
prevalence
2.1%
average loss
3%
max loss
The energy production of a PV panel is proportional to the solar irradiation it receives, and the latter obviously changes depending on the tilt angle of the PV panel.
Self Shading
0%
prevalence
0%
average loss
0%
max loss
Shadow cast by PV modules onto other PV modules should be minimized in order to prevent energy production losses and hotspots development.
Near Shading
67.5%
prevalence
4.6%
average loss
15%
max loss
This type of shading is caused both by objects inside the PV plant and along the perimeter of the building area. For the former category, examples are walls on the roof of the building, antennas and water tanks.
Soiling
95%
prevalence
6%
average loss
19%
max loss
Dirt and particles can accumulate on the PV module’s front glass over time. Depending on the environmental conditions, PV plants need different cleaning frequencies.
Cell Cracks
47.5%
prevalence
5.4%
average loss
10%
max loss
Photovoltaic cells are made of silicon. This makes the cells very fragile. Cell cracks are cracks in the silicon substrate of the photovoltaic cells that often cannot be seen by the naked eye.
Inactive Cell Strings
12.5%
prevalence
4.6%
average loss
6%
max loss
In parallel to a certain number of solar cells, bypass diodes are integrated into the PV module. These bypass diodes reduce the power loss caused by partial shading on the PV module.
Potential Induced Degradation
32.5%
prevalence
17.3%
average loss
56%
max loss
The phenomenon of Potential-Induced Degradation (PID) is based on a power loss degradation caused by a negative potential of the solar cells towards earth, which leads to an accumulation of Na+ located in the glass and migrating into the solar cells damaging the p-n junction responsible for the electron flow.