Solar PV inspection: finding the one hot module among tens of thousands

Faults in a solar plant fall into three families: module degradation (cell cracks, hot spots, failed bypass diodes, potential-induced degradation (PID), discoloured or delaminated encapsulant, broken glass), electrical faults (open strings, overheating connectors and junction boxes, tripped inverters), and structure, ground and surroundings (corrosion, loose bolts, vegetation, erosion).

In Japan, the 2017 revision of the feed-in tariff (FIT) scheme required generators to carry out maintenance and inspection in line with their approved business plan. Neglecting maintenance can put the approval at risk. Industry associations publish checklists and guides, and inspection typically combines visual checks, electrical measurements and infrared imaging.

The problem is area. Utility-scale plants cover several to dozens of hectares, many on hillsides. Walking the rows in midsummer with a handheld thermal camera, module by module, is slow and carries a real risk of heat stroke.

The other pain is who does the work, and at what scale. Large plants have dedicated O&M contractors, but small plants are far more numerous and often get little attention. Japan has been extending notification and safety-check requirements to smaller commercial PV systems as well.

Solar is one of the first fields where drone inspection became routine. The sky is open, GNSS is reliable, and the targets lie flat in neat rows. A drone carrying a thermal and a visible camera together flies an automated route and covers the plant in one go.

The basic pattern is a back-and-forth automated route along the rows. Thermal capture needs strong in-plane irradiance: an IEA PVPS review recommends imaging at more than 600 W/m²; without it, faulty and healthy cells do not separate in temperature. Altitude is set backwards from the resolution needed to resolve a single faulty cell in the thermal image.

How to plan a capture around sun, reflection and infrared conditions is laid out in planning a thermal and visual capture around sun and reflection (Airffic Flight Institute). The same reasoning applies to solar arrays.

Solar sites are open and easy to fly. But the best time to inspect is midday under strong sun — the hottest part of the day. Aircraft, batteries and people all bake. And module glass is also a mirror for the sky and sun, which can fool thermal images.

• Heat: at midsummer noon battery temperatures climb and flight time can shrink. Store packs in shade, let them cool after charging and rotate spares. Plan for heat stroke prevention for the crew too. Heat
• Reflections: the sun or the drone itself mirrored in the glass can look like a hot spot. Capture close to perpendicular to the module plane and plan routes that avoid solar reflections.
• Clouds and wind: passing clouds change irradiance and scramble temperature differences; strong wind cools the surface and hides contrast. Log conditions with an irradiance meter.
• Floating and rooftop PV: floating plants risk a ditched aircraft; rooftops often have people below. See water, spray & humidity and urban areas & people below.

A single flight produces thousands of thermal images. From them you find thermal anomalies, classify them, identify the exact module and decide repair priority. Thermal patterns have recognisable shapes: one hot cell, a hot stripe (suspect a bypass diode), a whole hot module, a whole hot string. The pattern points to the likely cause.

In Japan, PV plants fall under both the Electricity Business Act (conformity to technical standards and a safety organisation scaled to plant size) and the Renewable Energy Act (maintenance in line with the business plan). Whether to use drones is the operator’s choice, but recording the results and fixing faults is the operator’s responsibility.

Internationally, IEC TS 62446-3 sets out how infrared inspection of PV plants should be carried out and has become the common language for capture conditions and fault classes. Warranty claims sometimes require reports in this format.

Flight rules are separate. Most plants are outside densely inhabited districts, but distance to roads and houses beyond the fence and whether you fly beyond visual line of sight (BVLOS) automatically change what permissions you need — all the more so for unattended patrols from a drone dock. Compare countries in world regulations.

• Confirming the cause: you can see that a module is hot but often not why. Back it up with I-V tracing or a close-up check.
• Electrical safety: insulation resistance tests and work inside combiner boxes are for qualified electricians.
• Fine cell cracks: some cracks never show in daytime thermal images. They need EL imaging or another method.
• Under the modules: racking and bolts under the panels are invisible from above; check them from the ground.
• Weak-sun days: under cloud or weak winter sun, thermal contrast simply does not appear. Inspection windows are limited.

In the corner of a hillside plant sits a drone box. When the irradiance sensor reads strong enough sun, the drone takes off on its own, maps the rows thermally and returns. By evening, a list of modules worth fixing — thermal images matched against production records — reaches the O&M contractor. After heavy rain it checks the slopes for washouts too. That is the picture our editors expect.

Nobody walks the rows under a burning sun. Instead people fix faults in order of lost energy, handle warranty claims and plan for recycling when modules reach end of life. Inspection shifts from searching module by module to caring for the whole life of the plant.