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IEC 62108: Concentrator Photovoltaic (CPV) Modules — Design Qualification and Type Approval
A comprehensive technical examination of IEC 62108, the international standard for design qualification and type approval of concentrator photovoltaic (CPV) modules and assemblies. This article covers the complete test sequence including thermal cycling, damp heat, UV preconditioning, hail impact, water spray, and tensile strength tests, along with practical engineering guidance for achieving CPV module reliability in long-term outdoor operation.
1. Introduction to CPV Technology and Standardization
IEC 62108:2007, developed by IEC Technical Committee 82 (Solar photovoltaic energy systems), establishes the design qualification and type approval requirements for concentrator photovoltaic (CPV) modules and assemblies intended for long-term operation in general open-air climates. Unlike flat-plate PV modules covered by IEC 61215, CPV modules use optical elements (Fresnel lenses, parabolic mirrors, or refractive concentrators) to focus sunlight onto small, highly efficient multi-junction solar cells. This concentration of light—typically 100 to 1000 suns for high-concentration CPV (HCPV) systems—introduces unique reliability challenges that IEC 62108 specifically addresses.
The standard recognizes the fundamental difference between CPV and flat-plate PV: CPV modules require precise optical alignment between the concentrator optics and the receiver cells; they must track the sun; and they experience highly non-uniform illumination profiles on the cell surface. These factors make CPV module qualification a fundamentally different engineering problem from flat-plate module qualification, requiring dedicated test sequences that stress the optical, mechanical, and thermal interfaces unique to concentrator systems.
Design Insight: A critical parameter that distinguishes CPV qualification from flat-plate PV testing is the use of direct normal irradiance (DNI) rather than global horizontal irradiance (GHI). CPV modules only utilize direct-beam sunlight—diffuse light cannot be focused by the concentrator optics. This means CPV modules are tested under DNI conditions, and the standard requires that spectral mismatch corrections account for the specific spectral response of the multi-junction cells, which varies significantly with air mass and atmospheric conditions.
2. Test Sequence and Qualification Requirements
IEC 62108 defines a comprehensive test sequence divided into three major groups. Group A tests are performed on a single module and establish baseline performance, visual inspection criteria, and electrical safety. Group B tests are performed on two or more modules and constitute the accelerated stress sequence that qualifies the design. Group C tests cover specialized requirements for assembly components, including receiver assemblies and optical elements tested independently. The standard requires that all ten specimens in the test sample pass each applicable test with specified pass/fail criteria.
2.1 Thermal Cycling and Temperature Extremes
Thermal cycling is arguably the most demanding test for CPV modules. The standard specifies 200 cycles from −40 °C to +85 °C with a dwell time of at least 10 minutes at each extreme and a transition rate of ≥ 100 °C/hour (for large modules, ≥ 60 °C/hour). The rate of change is critical: CPV modules experience the most severe thermal stress during rapid cloud transients, where direct sunlight can be replaced by shade in seconds, causing the receiver cell temperature to drop from 80–100 °C to near-ambient temperature within minutes. The 100 °C/hour rate in the test chamber simulates approximately 100 cloud-transient events per year.
| Test Group | Test Name | Conditions | Specimens |
|---|---|---|---|
| A | Visual Inspection | IEC 61215 criteria + CPV-specific | All |
| A | Performance Measurement | DNI ≥ 850 W/m², spectrum corrected | All |
| A | Ground Continuity / Wet Leakage | 500 V DC, Ileak < 50 µA | All |
| B | UV Preconditioning | 15 kWh/m² UV (280–385 nm) | 2 |
| B | Thermal Cycling | 200 cycles, −40 °C to +85 °C | 4 |
| B | Damp Heat | 1000 h at 85 °C / 85% RH | 2 |
| B | Hail Impact | 1-inch ice ball at 23 m/s | 2 |
| B | Water Spray | 10 mm/min for 1 hour | 2 |
| B | Robustness of Terminations | Pull/torque test | 2 |
2.2 Damp Heat and Humidity Freeze
The damp heat test (1000 hours at 85 °C / 85% relative humidity) evaluates the moisture ingress resistance of the CPV module encapsulation and sealing. This is particularly critical for CPV modules because the receiver assembly—containing the multi-junction solar cell, bypass diode, and interconnect—is typically encapsulated in silicone or epoxy within a cavity below the primary optical element. Moisture ingress into this cavity can cause corrosion of the cell metallization, delamination of anti-reflection coatings, and degradation of the silicone encapsulant’s optical transmission properties.
Engineering Note: CPV modules are generally more susceptible to damp heat degradation than flat-plate modules because of the larger number of interfaces and material transitions in the optical path. Each interface—air-to-lens, lens-to-secondary-optic, secondary-optic-to-silicone, silicone-to-cell—presents a potential moisture ingress pathway. The standard’s 1000-hour damp heat test at 85/85 is considered a minimum; many CPV manufacturers now qualify their modules to 2000 hours or more based on field experience in high-humidity locations like Florida, Singapore, and southern China.
3. CPV-Specific Performance Measurement
IEC 62108 specifies that performance measurements be conducted under natural or simulated sunlight at direct normal irradiance (DNI) of at least 850 W/m², with spectral mismatch corrections applied using the reference spectrum for concentrator cells. The measurement procedure requires that the CPV module be mounted on a two-axis solar tracker aligned to within ±0.1° of normal incidence. For flash testing (the preferred method for production testing), the flash duration must be sufficient to allow the multi-junction cells to reach thermal equilibrium while avoiding excessive heating—typically 10–100 ms for a single flash.
The standard defines power rating at concentrator standard test conditions (CSTC): 25 °C cell temperature, 1000 W/m² DNI, and ASTM G173-03 direct reference spectrum. Additionally, concentrator standard operating conditions (CSOC) are specified: 20 °C ambient temperature, 1000 W/m² DNI, and 4 m/s wind speed. The ratio of CSTC to CSOC power is a key design parameter, typically 0.75–0.85 for HCPV modules, reflecting the significant cell temperature rise under concentration.
| Parameter | CSTC (Rating) | CSOC (Operating) |
|---|---|---|
| Irradiance (DNI) | 1000 W/m² | 1000 W/m² |
| Cell Temperature | 25 °C | Not specified (derived) |
| Ambient Temperature | — | 20 °C |
| Wind Speed | — | 4 m/s |
| Reference Spectrum | ASTM G173-03 Direct | |
| Tracking Accuracy | ±0.1° normal incidence | |
Key Insight: The spectral mismatch correction for CPV measurements is substantially more complex than for flat-plate silicon modules because multi-junction cells have multiple bandgaps. A typical triple-junction CPV cell has bandgaps of approximately 1.9 eV (top cell), 1.4 eV (middle cell), and 0.7 eV (bottom cell). The current output of each subcell must be balanced—the cell current is limited by the subcell generating the least photocurrent. This means the spectral content of the test light source must match the natural spectrum across three separate wavelength bands simultaneously. A mismatch of 10% in the blue/red ratio can change the measured power by 5% or more.
4. Engineering Design for CPV Reliability
Achieving IEC 62108 qualification requires careful design of several critical subsystems. The optical assembly must maintain focus over the temperature range −40 °C to +85 °C without significant beam displacement on the receiver cell. This typically requires matching the coefficient of thermal expansion (CTE) of the lens plate material (typically PMMA or silicone-on-glass) with the receiver substrate (typically aluminum nitride or direct-bonded copper on aluminum). A CTE mismatch of more than 3 ppm/°C between the lens plate and receiver substrate will cause measurable focus shift during thermal cycling, potentially reducing power output by 5–15% at temperature extremes.
The receiver assembly—the most thermally stressed component—must manage heat fluxes of 50–150 W/cm² at the cell level (compared to 0.1–0.3 W/cm² for flat-plate modules). This requires the cell to be soldered or sintered to a high-thermal-conductivity substrate using materials like silver-sintered die-attach or gold-tin eutectic solder, with thermal conductivities exceeding 200 W/m·K. The standard’s thermal cycling test is particularly demanding on these interconnects because the CTE mismatch between the germanium or gallium arsenide cell substrate (approximately 6 ppm/°C) and the copper heat spreader (approximately 17 ppm/°C) generates cyclic mechanical stress that can cause fatigue failure of the solder joint.
Critical Consideration: One of the most common CPV qualification failures is bypass diode failure during the thermal cycling test. The bypass diode in a CPM receiver must handle the full module current (typically 5–10 A at 600–1000× concentration) when a cell is shaded or fails. Under concentration, the bypass diode can experience reverse voltage transients exceeding 50 V during cloud edge events. The standard’s 200 thermal cycles often reveal inadequate diode thermal management or insufficient reverse voltage ratings. Design the bypass diode heatsink for at least 150 °C junction temperature margin, and select Schottky diodes with a reverse voltage rating of at least 2× the maximum expected reverse voltage.
5. Frequently Asked Questions
Q: What is the difference between IEC 62108 and IEC 61215?
A: IEC 62108 is specific to concentrator photovoltaic modules, while IEC 61215 covers flat-plate (non-concentrating) PV modules. The key differences include: (1) CPV testing uses direct normal irradiance (DNI) while flat-plate uses global irradiance; (2) CPV modules are tested on a two-axis tracker; (3) CPV has additional tests for optical alignment stability and receiver assembly integrity; (4) the thermal cycling profile for CPV is more stringent (200 cycles from −40 °C to +85 °C vs. 200 cycles from −40 °C to +85 °C for flat-plate — though the profiles differ in ramps and dwells). The wet leakage and insulation test requirements are similar between the two standards.
Q: Can low-concentration PV (LCPV) modules be qualified under IEC 62108?
A: Yes. The standard covers all CPV modules regardless of concentration ratio. However, the test requirements may be adjusted for low-concentration systems (< 10 suns). For LCPV systems that do not use active tracking, some tests (particularly those requiring two-axis tracker mounting for performance measurement) may be modified with the agreement of the certification body. The standard's applicability to LCPV has been a subject of discussion, and some LCPV manufacturers have qualified under both IEC 62108 and IEC 61215 to cover all aspects of their design.
Q: How does the hail impact test differ for CPV modules?
A: The hail impact test in IEC 62108 uses the same 1-inch (25.4 mm) diameter ice ball at 23 m/s as IEC 61215. However, the impact locations are different: CPV modules must be tested at the center of the primary optical element (the most vulnerable point for lens cracking) and at the junction between optical elements. A cracked Fresnel lens can reduce the module power output by 20–50% due to defocused light on the receiver cell. For glass-based CPV designs, the impact may also be applied to the glass surface over the receiver assembly.
Q: What is the typical failure rate for CPV modules in the field?
A: First-generation HCPV modules experienced field failure rates of 1–5% per year, primarily due to receiver interconnect fatigue, optical delamination, and tracking system failures. Modern CPV modules qualified to IEC 62108 with > 1000 hours of damp heat survival and > 500 thermal cycles have demonstrated field failure rates below 0.5% per year. The most common field failures observed in contemporary CPV installations are not module failures but rather tracker control system faults (position sensors, motor drives) and soiling of the primary optics in arid environments where water for cleaning is scarce.
