The photovoltaic glass hail impact test is a critical mechanical robustness evaluation that determines whether the front glass of a solar module can survive hailstone strikes without cracking, delaminating, or suffering performance degradation. Performed in accordance with international standards such as IEC 61215 and ASTM E1038, the test launches calibrated ice balls at controlled velocities against module surfaces to simulate real-world hail exposure. This article provides a comprehensive technical overview of the test standards, sample requirements, procedures, equipment, and acceptance criteria used by accredited third-party laboratories and module manufacturers worldwide.
Technical Overview
Photovoltaic (PV) modules are designed for outdoor service lifetimes of 25 years or more, during which they may be exposed to severe weather events including hailstorms. Hailstones impacting the front glass of a module transfer substantial kinetic energy in a fraction of a millisecond, which can fracture the glass, damage underlying solar cells, rupture encapsulant layers, and compromise electrical insulation. Glass breakage is particularly dangerous because it creates a direct pathway for moisture ingress, leading to corrosion, ground faults, and accelerated power loss.
The photovoltaic glass hail impact test addresses this risk by firing artificially frozen ice balls of specified diameter, mass, and velocity at predefined locations on a full-size module or glass laminate. The test is primarily governed by the following standards:
- IEC 61215-2 (Test MQT 10) – Design qualification and type approval of terrestrial PV modules, hail impact resistance.
- IEC 61730-2 – PV module safety qualification, which references mechanical impact robustness requirements.
- ASTM E1038 – Standard test method for determining the resistance of photovoltaic modules to hail impact using launched ice balls.
- GB/T 9535 and related national standards – Chinese equivalents aligned with the IEC framework.
- UL 61730 – North American safety standard incorporating comparable impact evaluation.
By standardizing projectile size, velocity, impact location, and evaluation methodology, these standards enable consistent comparison of glass strength, lamination quality, and overall module mechanical durability across manufacturers and product generations.
Testing Samples
Standard Front-Glass Modules
The most common test article is a complete crystalline silicon module with a 3.2 mm thermally tempered soda-lime glass front cover, laminated with EVA or POE encapsulant. Full modules are preferred because the test evaluates the entire mechanical system: glass, encapsulant, cells, backsheet, and frame interaction.
Glass-Glass Bifacial Modules
Double-glass (glass-glass) constructions, typically using 2.0 mm or 2.5 mm heat-strengthened front and rear glass, are increasingly tested for hail resistance because the thinner glass presents a different fracture behavior than conventional 3.2 mm tempered cover glass.
Thin-Film Module Glass
Thin-film technologies such as CdTe, CIGS, and amorphous silicon use TCO-coated front glass. Hail testing verifies both the mechanical integrity of the glass and the continuity of the transparent conductive oxide and absorber layers after impact.
Specialty and Functional Glass
Anti-reflective (AR) coated glass, patterned light-trapping glass, and laminated safety glass used in building-integrated photovoltaics (BIPV) may be evaluated separately or as complete laminates, since coatings and interlayers influence both impact energy absorption and post-impact fragmentation behavior.
Testing Items
A complete hail impact evaluation typically includes the following test and measurement items:
- Pre-test baseline characterization: visual inspection, maximum power (Pmax) measurement at STC, electroluminescence (EL) imaging, and insulation resistance.
- Ice ball impact at specified diameter, mass, and velocity on designated target points.
- Post-impact visual inspection for glass cracks, chips, shattered areas, delamination, bubbles, and interconnect damage.
- Post-impact maximum power measurement to quantify power degradation.
- Electroluminescence imaging to reveal hidden cell micro-cracks invisible to the naked eye.
- Insulation resistance and wet leakage current testing to confirm electrical safety after mechanical damage.
Testing Methods
Ice Ball Preparation
Ice balls are molded from deionized water or water of equivalent purity and frozen in controlled molds to produce smooth, spherical projectiles free of cracks and air inclusions. The frozen balls are stored at a temperature of approximately -10 °C or colder until immediately before launch. Before firing, each ice ball is checked for dimensional conformity and mass, and only projectiles within tolerance are used.
Launch Equipment and Velocity Measurement
A pneumatic hail cannon or compressed-gas launcher propels the ice ball through a calibrated barrel toward the mounted specimen. Projectile velocity is measured immediately before impact using optical light gates or a calibrated chronograph positioned close to the target plane. Velocity accuracy of a few percent is required to ensure the delivered kinetic energy matches the standard specification.
Standard Severity Levels
The reference condition in IEC 61215-2 uses a 25 mm diameter ice ball at 23.0 m/s. Higher severity levels may be agreed between manufacturer and purchaser for hail-prone installations. Commonly applied combinations include:
- 25 mm diameter, approximately 7.53 g, at 23.0 m/s – standard qualification level.
- 35 mm diameter, approximately 20.7 g, at 27.2 m/s – elevated severity.
- 45 mm diameter, approximately 35.9 g, at 30.7 m/s – severe hail regions.
Impact Locations and Procedure
The specimen is mounted and supported according to the manufacturer's installation instructions, typically at ambient laboratory temperature. Ice balls are directed at predefined impact points specified by the standard, which generally include:
- Four corners of the module, inside the frame area.
- Midpoints of the edges and one interior corner region of a cell.
- The center of the module and central cell areas.
- Locations near the junction box and areas above soldered interconnect strips.
Each specified point receives one impact at the calibrated velocity. Any impact that strikes outside the designated target zone is repeated on a valid location.
Evaluation and Acceptance Criteria
After the impact sequence, the module is inspected and retested, and it is considered to have passed if the following criteria are met:
- No major visual defects: the glass shows no through-cracks, shattered areas, or broken edges that could compromise safety or function.
- Maximum power degradation does not exceed 5% compared with the initial measurement.
- Insulation resistance and wet leakage current requirements continue to be satisfied.
- EL imaging shows no significant new cell cracking beyond defined acceptance limits, where specified.
Applications
Hail impact testing of PV glass serves multiple purposes across the solar industry value chain:
Certification and market access: passing the hail test is a mandatory requirement within IEC 61215 and IEC 61730 type-approval sequences, which are prerequisites for sale in most global markets.
Product development: manufacturers use hail testing to optimize glass thickness, tempering quality, coating processes, and encapsulant selection while balancing cost, weight, and optical transmittance.
Incoming quality control: module assemblers verify the mechanical consistency of glass supplied from different production batches or new suppliers.
Site-specific risk mitigation: project developers in hail-prone regions specify elevated severity levels (larger ice balls and higher velocities) to match local meteorological records and insurer requirements.
BIPV and special installations: laminated safety glass used in facades, skylights, and vehicle-integrated photovoltaics must demonstrate both hail resistance and safe post-breakage behavior.
Failure analysis: comparative hail testing helps identify root causes of field breakage, such as edge damage, nickel sulfide inclusions, or inadequate tempering.
FAQ
What is the standard hail test condition for PV modules?
The reference condition defined in IEC 61215-2 is a 25 mm diameter ice ball, approximately 7.53 g in mass, launched at 23.0 m/s against designated points on the mounted module.
Can the test severity be increased beyond the standard level?
Yes. Standards such as IEC 61215 and ASTM E1038 allow the involved parties to agree on larger ice balls and higher velocities, for example 35 mm or 45 mm projectiles, to simulate severe hail climates or to satisfy insurance specifications.
What constitutes a failure in the hail impact test?
A module fails if it exhibits major visual defects such as through-cracks or shattered glass, if maximum power degrades by more than 5% relative to the pre-test value, or if it no longer meets insulation and wet leakage requirements.
Is the test performed on full modules or on glass samples alone?
Certification testing is performed on complete modules because the backing structure, encapsulant, and frame significantly influence impact energy absorption. Standalone glass or laminated coupons may be tested during research and development or supplier qualification.
Why is distilled or deionized water used for the ice balls?
Pure water produces homogeneous, transparent ice that freezes without trapped air bubbles or mineral inclusions, ensuring consistent density, mass, and fracture behavior of the projectile and therefore repeatable impact energy delivery.
How long does a hail impact test take?
The impact sequence itself is typically completed within a few hours, including ice ball preparation, velocity calibration, and the series of impacts. Total turnaround depends on the time required for pre-test and post-test electrical characterization and safety measurements, and on the overall certification test sequence of which hail testing is one part.