PV Module Degradation — PID/LID Mechanisms, EL Imaging Measurement, and Warranty Claims in Thailand

Factory solar systems cost millions and are expected to last 25-30 years — but panels don't degrade in the neat straight line shown in brochures. There are specific mechanisms that accelerate degradation past spec, especially in Thailand's hot, humid conditions.

This article builds on How to choose Solar panels meeting IEC 61215 / IEC 61730, which covers "acceptable degradation rates" — here we go deeper: why panels degrade (the mechanisms) and how to measure/prove/claim it.

1. Panels Degrade via 5 Mechanisms — Not Just "Age"

flowchart TD
    M["PV Degradation"] --> A["LID
Light-Induced"]
    M --> B["PID
Potential-Induced"]
    M --> C["LeTID
Light + elevated Temp"]
    M --> D["Thermal/UV
cycling + EVA browning"]
    M --> E["Microcrack/Hotspot
mechanical/install"]
    B --> note["Biggest problem in Thailand
(heat+humidity+high voltage)"]

2. PID — Why Thailand Is Hit Especially Hard

PID (Potential-Induced Degradation) is charge leakage from a high potential difference between cells and the frame/ground. Thailand accelerates PID by combining three factors at once:

1. High heat — speeds Na-ion migration in the glass/EVA
2. High humidity — creates a charge-conducting path on the panel surface
3. High DC system voltage — factory installs often run strings at 1000-1500V, giving panels at the string ends a large potential difference to ground

Preventing PID:

• Choose panels rated PID-resistant (passing IEC TS 62804)
• An inverter with an anti-PID / PID-recovery function (reverse voltage at night to recover)
• Functional grounding per the manufacturer's instructions
• Design string voltage within rating + avoid trapped humidity at panel edges

The upside of PID: unlike permanent degradation, PID is partially recoverable if the root cause is fixed in time — so early detection matters.

3. Measuring/Proving Real Degradation — 4 Tools

The classic problem: you know output dropped, but you can't prove it degraded "beyond spec" → you can't claim the warranty. You need quantitative evidence:

flowchart LR
    A["IV Curve
flash test"] --> P["compare to baseline
commissioning"]
    B["EL Imaging
(electroluminescence)"] --> P
    C["IR Thermography
find hotspots"] --> P
    D["Performance Ratio
(PR) monitoring"] --> P
    P --> CLAIM["evidence to claim
power warranty"]

Key: you must have a commissioning baseline (first IV + EL at handover) — otherwise you can't prove "degraded from what."

4. Claiming the Power Warranty — What You Need

Panels carry two warranties: product warranty (materials/workmanship ~10-15 years) and power output warranty (guaranteeing output ≥80% at year 25). Claiming the power warranty requires proving degradation beyond the guarantee:

1. Baseline flash test report at installation (kept from day one)
2. Current IV curve at standard conditions → measured degradation %
3. EL image showing internal damage (microcrack/PID)
4. PR data from the monitoring system confirming the trend

No baseline = very hard to claim. The manufacturer will argue low output came from installation/environment, not the panel itself.

5. Reduce Degradation With Correct O&M

flowchart TD
    Q1{What to inspect} --> A["Commissioning:
EL + IV baseline"]
    Q1 --> B["Annual: PR trend
+ IR hotspot scan"]
    Q1 --> C["Before warranty expiry:
EL + IV to claim degraded panels"]
    A --> R["Protect the 25-year investment"]
    B --> R
    C --> R

• Commissioning: capture EL + IV baseline + catch transport/install cracks from the start
• Clean soiling on schedule (dust/bird droppings cut output + create hotspots)
• Monitor PR to catch anomalies early (especially recoverable PID, if caught in time)
• EL/IV before warranty expiry to claim panels degraded beyond spec before the right lapses

Conclusion

Solar panels don't degrade in a straight line — mechanisms like LID, PID, LeTID, thermal/UV and microcracks accelerate degradation past spec, with PID the biggest problem in Thailand from combined heat + humidity + high voltage.

Protecting the 25-year investment requires: choosing PID-resistant panels, preventing with an anti-PID inverter + grounding, capturing a commissioning IV/EL baseline, continuously monitoring PR, and keeping complete evidence to actually claim the power warranty.

Sahawatthanakit (1988) handles survey, design, installation and O&M planning of Solar systems for factories and warehouses — our engineering team builds in PID prevention and monitoring from the design stage, so the system pays off across its full 25 years.