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IEC 62670: Photovoltaic Concentrators (CPV) — Performance Testing and Standard Conditions
Concentrated photovoltaic (CPV) technology uses optical elements such as Fresnel lenses or parabolic mirrors to focus sunlight onto high-efficiency multi-junction solar cells, achieving conversion efficiencies that exceed 40% under laboratory conditions. Unlike flat-plate photovoltaic modules, CPV systems require direct normal irradiance (DNI) and precise tracking to function effectively, making their performance characterization fundamentally different from conventional PV. IEC 62670 establishes the standard conditions and procedures for testing CPV modules and systems, providing a unified framework for manufacturers, testing laboratories, and project developers to compare performance on a consistent basis. This article explores the technical requirements of the standard and their practical implications for CPV system design and deployment.
1. Standard Test Conditions for CPV Performance Rating
IEC 62670 defines specific standard test conditions (STC) for CPV that differ significantly from those used for flat-plate PV modules. The key distinction is the use of direct normal irradiance (DNI) rather than global irradiance, reflecting the optical concentration principle that underpins CPV technology:
| Parameter | CPV Standard (IEC 62670) | Flat-Plate PV (IEC 61215) | Rationale |
|---|---|---|---|
| Irradiance type | Direct Normal (DNI) | Global (GHI) | CPV optics only collect direct beam radiation |
| Irradiance level | 1000 W/m2 DNI | 1000 W/m2 GHI | Reference condition for power rating |
| Cell temperature | 25 degC | 25 degC | Standard reference temperature |
| Air Mass | AM1.5D (direct spectrum) | AM1.5G (global spectrum) | Direct spectrum matches CPV optical path |
| Spectral range | 280–4000 nm | 280–4000 nm | Full solar spectrum consideration |
| Wind speed | Not specified at STC | Not specified at STC | Cell temperature is controlled parameter |
The standard also defines extended rating conditions that account for variations in DNI, spectrum, and cell temperature. This is critical because CPV performance is highly sensitive to spectral composition — multi-junction cells with three or more junctions can experience current mismatch when the spectrum deviates from AM1.5D, leading to power losses of 5–15% under blue-rich or red-rich atmospheric conditions.
Engineering Insight: When designing CPV test facilities, the placement of the DNI pyrheliometer is critical. IEC 62670 requires that the DNI sensor be mounted on a tracking platform with the same pointing accuracy as the CPV module under test (typically within 0.5 degrees). A misalignment of just 1 degree between the DNI sensor and the CPV aperture can introduce measurement errors of 2–3% due to the circumsolar radiation component that the sensor captures but the concentrator optics reject. This is one of the most common sources of inter-laboratory measurement discrepancy in CPV testing.
2. Measurement Procedures and Power Rating Protocol
IEC 62670 specifies a detailed measurement protocol for determining the rated power of CPV modules and systems. The protocol accounts for the unique characteristics of concentrator technology, including angular sensitivity, spectral dependence, and thermal behavior:
- Electrical I-V Curve Measurement: Current-voltage curves are measured under stabilized conditions using a capacitive load or electronic load sweep. The standard requires that the DNI remain within 900–1100 W/m2 during measurement, with spectral corrections applied if the actual air mass deviates from AM1.5D by more than 0.5 air mass units.
- Angular Response Characterization: The standard requires measurement of the angular acceptance function, which quantifies how CPV power output decreases as the incidence angle increases. This is typically characterized by a Gaussian or super-Gaussian function with a half-width at half-maximum (HWHM) of 0.5–1.5 degrees depending on the concentrator optics.
- Spectral Mismatch Correction: For multi-junction CPV modules, the spectral mismatch factor (MM) is calculated using the actual spectral irradiance, the reference spectrum, and the spectral response of each sub-cell. The correction can exceed 10% when testing under significantly non-standard atmospheric conditions.
- Temperature Coefficient Determination: The standard specifies methods for measuring the temperature coefficient of power, which is typically 0.1 to 0.3 %/degC for multi-junction CPV modules — lower than the 0.3–0.5 %/degC typical of crystalline silicon flat-plate modules.
- Power Rating at Standard Conditions: The final rated power is determined by correcting measured values to STC (1000 W/m2 DNI, 25 degC cell temperature, AM1.5D spectrum) using the measured correction factors.
Critical Consideration: The cell temperature measurement methodology in CPV systems is significantly more challenging than in flat-plate modules. Because CPV cells are small (typically 3–7 mm2) and operate under concentration ratios of 300–1000x, they cannot be measured with conventional back-of-module thermocouples. IEC 62670 recommends two methods: (1) the IR thermometer method, which requires knowledge of the cell emissivity (typically 0.85–0.95 for III-V cells with anti-reflection coatings), and (2) the forward voltage method, which uses the temperature-dependent open-circuit voltage of a reference cell. The forward voltage method is generally more accurate (within 1 degC) but requires prior calibration.
Engineering Design Insights for CPV Systems
Beyond the testing framework, IEC 62670 provides valuable insights for CPV system designers and project developers:
Design Guidance: For CPV power plant design, the annual energy yield prediction must account for the DNI resource rather than global irradiance. Sites with high DNI fractions (DNI/GHI > 0.7) such as desert regions in the Middle East, Chile’s Atacama Desert, and the US Southwest are ideal for CPV deployment. The standard’s spectral correction methodology should be applied to hourly simulation models to accurately predict multi-junction cell performance across varying atmospheric conditions. Ignoring spectral effects can lead to energy yield overestimation of 3–8% annually.
Common Pitfall: Applying flat-plate PV testing protocols to CPV modules is a fundamental error. The use of global irradiance instead of DNI, the absence of spectral mismatch correction, and the use of incorrect angular acceptance functions can lead to power rating errors of 20–40%. CPV modules tested under diffuse or global illumination will appear to have extremely low efficiency because the concentrator optics cannot focus diffuse light onto the cell. Always ensure that CPV testing is performed under the IEC 62670 framework with appropriate DNI measurement and spectral correction.
FAQ
Q1: Why does CPV require direct normal irradiance instead of global irradiance?
CPV systems use optical concentrators (Fresnel lenses or mirrors) that can only focus direct beam radiation onto the solar cell. Diffuse radiation, which is scattered by the atmosphere, arrives from many angles and cannot be concentrated. In clear-sky desert environments, DNI constitutes 75–85% of GHI, making CPV highly effective. However, in cloudy or hazy locations where diffuse radiation dominates, CPV performance drops dramatically — unlike flat-plate PV which can still generate power from diffuse light.
Q2: How does multi-junction cell technology affect CPV testing requirements?
Multi-junction cells contain 3–5 sub-cells connected in series, each absorbing a different portion of the solar spectrum. When the incoming spectrum deviates from the AM1.5D reference, the current matching between sub-cells changes, affecting overall power output. IEC 62670 requires spectral mismatch correction to account for this effect. This is unnecessary for single-junction flat-plate modules, making CPV testing inherently more complex and requiring spectral irradiance measurement equipment.
Q3: What is the typical concentration ratio range covered by IEC 62670?
IEC 62670 applies to CPV systems across a wide range of concentration ratios, from low-concentration (2–20x) systems using refractive optics to high-concentration (300–1000x) systems using point-focus Fresnel lenses or dish reflectors. The testing principles are the same regardless of concentration ratio, but the practical measurement challenges — particularly cell temperature measurement and angular alignment sensitivity — become more severe at higher concentration levels.
Q4: How do soiling and optical degradation affect CPV performance testing over time?
CPV systems are particularly sensitive to optical degradation because any reduction in lens transmittance or mirror reflectivity directly reduces the concentrated irradiance reaching the cell. Soiling on the primary optic can reduce DNI collection by 0.1–0.5% per day in arid environments. IEC 62670 testing provides a baseline power rating, but long-term performance monitoring should include periodic re-testing to track optical degradation. The standard’s angular acceptance measurements can also detect optical misalignment caused by structural settling or tracker drift.
