Hotspots impact site performance and safety

Hotspots in PV modules are a commonly encountered defect that significantly impact performance on sites large and small. Caused by a number of underlying defects, hotspots present with a wide variety of thermal signatures. While remediation is essential, the severity of the underlying defect is the primary determinant of the appropriate remediation action.

Hotspots can be broadly categorized into four types based on their cause:

Soiling/Shading: Hotspots caused by shading from vegetation, adjacent structures, module racks, debris on the surface, bird droppings and more

1. Soiling/Shading: Hotspots caused by shading from vegetation, adjacent structures, module racks, debris on the surface, bird droppings and more

2. Mechanical damage: Hotspots caused by cracked and broken glass, improper handling and installation, physical impact damage, 

3. Module/Cell defects: Hotspots caused by cell defects, cell cracks, de-lamination, bad solder joints, stuck bypass diodes, diode failures and more

4. Installation defects: Hotspots caused by unconnected strings and modules, reverse polarity

The focus of investigation and remediation action is always in the reverse order of defect types listed above, with the most bang-for-the-buck delivered by correcting installation defects and module/cell defects.

Beyond operational performance, hotspots also impact safety on sites as they can lead to runaway thermal events and fires. Hotspot detection and correction is, therefore, an important O&M activity on sites of all sizes. While annual hotspot checks are recommended, high costs often meant that sites were not checked completely to identify defects. This has changed with the development of drone-based thermal scanning procedures. What was once a manual job that took weeks is now completed in a matter of hours with drones, enabling single operators to thermally image over 50 MWp of installed solar PV in a single day.

Although data collection is simpler, the emphasis is now on accurately processing collected information to identify and classify hotspots. As operators start to rely solely on drone-based scans, accuracy of the results is the primary determinant of site performance and safety.

SenseHawk’s findings from first 14 GW of thermography analytics

While processing 14 GW of thermal scans across sites of all sizes and technologies in 17 countries, SenseHawk continued refined its detection and classification engines to ensure that nothing is ever missed. Constantly examining the data collected helps infer trends and spot areas for improvement.

1. Asset owners lose 2% of generation on average due to defects that can be detected easily with thermal scans.

   • The median loss observed was 1.4%, while the range was wide with the best performing site losing as little as 0.1% while the worst performing site was losing as much as 20%. Without considering outliers, the loss range was between .5 and 3%

2. Thermal scans at commissioning pay back instantly and should be a part of the handover/takeover process.

   • On newly constructed sites, an average loss of 2% was observed just from strings that were offline

   • Additionally, an average of 2200 modules qualified for warranty replacements per 100MWp of installed capacity

   • An average of 4 potentially hazardous defects (reversed strings/ short circuits) were detected per 100MWp

   • Minor defects with low delta T at commissioning often resulted in failures in the first year of operation, with the most common failure being burnt out interconnect solder

   • Broken/damaged modules were easily found at commissioning and could be flagged for replacement by the EPC

3. Single cell hotspots are a mixed bag.

   • Single cell hotspots displaying high delta T were often flagged for inspection. However, causes varied and a majority were caused by bird droppings, dust or vegetation shading. About 20-35% of single cell hotspots were caused by cell defects

4. Stuck open diodes are a common occurrence.

   • Between 400-3500 modules with stuck open diodes were observed per 100MWp on sites scanned. Of these, 80% were warranty replaceable with diode/junction box failures

5. Thermal defects occur as often on large sites as on small sites.

   • Defect density was not correlated to size of installation and small sites between 1 and 20MWp, with limited maintenance fared as well as sites that were 100MWp or larger

   • However, rooftops did not fare as well

6. Small rooftops fare poorly compared to larger sites.

   • The average rooftop site lost 4% of production due to thermal defects. Defect density correlated with the age of the site

7. Thermal scans pay back in as little as 3 months.

   • On utility scale sites, the pay back from thermal scans was within 3-5 months and was a function of the PPA rate and installation size

8. A map-based app with a defect location and management system improves outcomes.

   • In a number of sites, thermal scan results did not get the attention they deserved as field teams found it difficult to accurately locate and fix problems

   • With the SDP desk and app companion products, resolution rates increased from 46% to 91% while resolution time per 100MWp reduced by 60%

Best practices that are evident from the analysis

Based on the findings, SenseHawk recommends the following best practices:

1. Thermal drones should become an integral part of the solar PV commissioning and O&M toolkit. 

2. An annual thermal scan should be a part of the Preventive Maintenance plan.

3. Annual scans can be coupled with on-demand scans based on specific production problems. These will need in-house drone capability.

4. Surveys should be scheduled immediately after module cleaning to eliminate dust and bird dropping related hotspots.

5. A platform approach to managing scan data with cloud-based software and analytics tools is critical.

6. Issue tracking software coupled with a map-based field app closely integrated with the analytics platform is critical to tracking, remediation, warranty claims, and long-term monitoring.

Standard Test Conditions: normalized to 1000 W/m2 irradiance

To know how the SenseHawk SDP can help you design and build your project better, book a demo by dropping an email at sales@sensehawk.com