Evaluation of Heat Dissipation Performance of Thermally Conductive Gels in Photovoltaic Modules

Thermally conductive gels are increasingly used in photovoltaic modules to lower cell operating temperatures and recover power losses caused by heat buildup. A professional evaluation of thermal gel heat dissipation performance combines material-level thermal characterization with module-level testing of temperature reduction, power gain, and long-term reliability. This article explains the sample requirements, testing items, test methods, and typical acceptance criteria applied by accredited third-party laboratories for such assessments.

Technical Overview

Photovoltaic modules typically convert only 15% to 23% of incident solar irradiance into electricity, while the remainder of the absorbed energy is dissipated as heat. Under real operating conditions, cell temperatures commonly reach 50 to 75 degrees Celsius depending on irradiance, mounting configuration, and ambient airflow. Because crystalline silicon cells lose roughly 0.3% to 0.45% of peak power for every degree of temperature rise, elevated operating temperatures translate directly into measurable energy-yield losses.

Thermally conductive gels are low-modulus, dispensable thermal interface materials designed to fill microscopic air gaps between the cell strings, encapsulant, backsheet, and heat-spreading structures such as metallic frames or cooling plates. Since stagnant air has a thermal conductivity of only about 0.026 W/(m·K), replacing interfacial air with a gel of 1 to 7 W/(m·K) significantly reduces the total thermal resistance of the heat rejection path.

Sustained high temperature also accelerates degradation mechanisms such as encapsulant discoloration, hydrolysis, metallization corrosion, and solder bond fatigue, following approximately Arrhenius-type kinetics in which every 10 Kelvin rise can double the chemical degradation rate. A comprehensive evaluation must therefore quantify not only the immediate cooling benefit, but also the durability of the thermal path after accelerated environmental stress.

Testing Samples

Cured Thermal Gel Material Specimens

Neat gel is dispensed and cured under the supplier-specified conditions into discs or sheets, typically 25 to 50 mm in diameter and 0.5 to 3 mm in thickness. These specimens are used for intrinsic thermal conductivity, thermal impedance, hardness, density, and electrical insulation measurements.

Laminated Coupons and Mini-Modules

Representative glass, encapsulant, cell, gel, and backsheet stacks of approximately 200 by 200 mm are laminated to reproduce the actual interface conditions inside a module. These coupons support interface resistance measurements, adhesion testing, and accelerated aging studies at a manageable scale before full-module validation.

Full-Scale PV Modules

Identical production-grade modules are prepared in a paired control design, with one group incorporating the thermal gel and a reference group built without it. Thermocouples or resistance temperature detectors are embedded at defined positions on cell interconnects, the backsheet, and the frame to enable accurate temperature mapping during illumination testing.

Testing Items

• Thermal conductivity, thermal diffusivity, and specific heat capacity of the cured gel
• Thermal impedance and total thermal resistance as a function of bond line thickness and applied pressure
• Contact (interface) resistance at gel-to-backsheet and gel-to-cell assembly interfaces
• Density, Shore 00 or Shore A hardness, and compression force-deflection characteristics
• Adhesion and peel strength to glass, EVA or POE encapsulant, backsheet films, and aluminum
• Dielectric breakdown strength and volume resistivity for electrical insulation safety
• Weight loss and outgassing potential after prolonged thermal exposure
• Coefficient of thermal expansion of the cured gel
• Module I-V performance parameters, including Pmax, Voc, Isc, and fill factor, before and after gel integration
• Cell and module operating temperature distribution measured by infrared thermography and embedded sensors
• Power temperature coefficient and nominal module operating temperature
• Hot spot endurance and electroluminescence imaging to detect hidden defects
• Thermal cycling, damp heat, and humidity-freeze endurance with post-aging thermal performance retention

Testing Methods

Material-Level Thermal Characterization

Intrinsic thermal performance is measured using the ASTM D5470 steady-state heat flow method with instrumented meter bars. Thermal impedance is recorded at several bond line thicknesses under controlled clamping pressure, and the slope of the impedance-versus-thickness curve yields intrinsic thermal conductivity while the intercept yields total contact resistance. Results are cross-validated with the ASTM E1461 laser flash method or ISO 22007 transient techniques for thermal diffusivity and specific heat.

Module-Level Heat Dissipation Evaluation

Paired modules with and without gel are mounted in identical fixtures and illuminated at 1000 W/m² using a class AAA solar simulator in accordance with IEC 60904-9, or under stable natural sunlight. Steady-state temperatures are recorded through calibrated infrared thermography together with attached thermocouples on cells, backsheet, and frame. Current-voltage characteristics are captured per IEC 60904-1, and power temperature coefficients are determined following IEC 60904-10, with nominal module operating temperature optionally assessed under IEC 61853-2. Heat dissipation effectiveness is expressed as the cell temperature reduction, the module thermal resistance calculated from the temperature difference divided by dissipated heat, and the resulting peak power gain of the gel-equipped module relative to the control.

Accelerated Aging and Reliability Verification

The durability of the thermal path is verified using the IEC 61215-2 qualification sequence, including 200 thermal cycles between -40 and +85 degrees Celsius, 1000 hours of damp heat at 85 degrees Celsius and 85% relative humidity, and humidity-freeze cycling. After each stress sequence, thermal impedance measurements, infrared thermography, I-V testing, and electroluminescence imaging are repeated to confirm that the gel has not pumped out, dried out, delaminated, or lost adhesion. Electrical safety aspects such as insulation coordination are verified under IEC 61730, and flammability of the cured gel may be assessed under UL 94.

Data Analysis and Acceptance Criteria

Typical acceptance benchmarks include intrinsic thermal conductivity within 80% or more of the datasheet value, a measurable reduction in total interface resistance, a steady-state cell temperature reduction of 2 to 8 degrees Celsius for backside gel designs, and peak power improvement in the range of 0.5% to 2.5% depending on the heat rejection configuration. After accelerated aging, retention of at least 95% of the initial thermal performance, compliance with the 5% maximum power degradation criterion of IEC 61215, and the absence of gel migration or delamination are generally required.

Applications

Evaluation of thermal gel heat dissipation performance supports product development and market entry across a wide range of photovoltaic and adjacent applications.

• High-power large-format modules using 182 mm or 210 mm cells, half-cut designs, and multi-busbar interconnection where heat flux density is elevated
• Building-integrated photovoltaics such as roof tiles, curtain walls, and facades where rear-side convection is restricted
• Vehicle-integrated and portable photovoltaic products operating in confined spaces with limited airflow
• Modules incorporating heat-spreading backsheets, graphite layers, or aluminum cooling plates that require a low-resistance interface
• Concentrated photovoltaic and photovoltaic-thermal hybrid collectors with high thermal loads
• Performance upgrades for modules deployed in hot climates where operating temperature losses dominate annual yield
• Interface management around junction boxes and bypass diodes where localized heat dissipation is critical

FAQ

How much temperature reduction can a thermal gel realistically deliver in a PV module?

In typical backside configurations with a heat spreader or conductive backsheet, cell temperature reductions of 2 to 8 degrees Celsius are commonly achieved, with larger gains possible when an external heat sink or active cooling path is present. The gel itself does not absorb heat; it lowers interface resistance, so a downstream conduction path is essential for the benefit to materialize.

What thermal conductivity rating is appropriate for a PV thermal gel?

Commercially available gels for photovoltaic use typically range from 1 to 7 W/(m·K). Beyond approximately 5 W/(m·K), total thermal resistance is usually dominated by contact resistance and wetting coverage rather than bulk conductivity, so interface quality and bond line thickness control matter more than chasing higher datasheet values.

How long does a complete evaluation program take?

Material-level characterization of conductivity, impedance, and electrical properties generally requires one to two weeks. A full module-level program including baseline comparisons and the complete accelerated aging sequence typically requires eight to fourteen weeks, depending on the scope of stress testing selected.

Will adding a thermal gel affect module certification?

Introducing a new material inside the laminate or on the backsheet constitutes a design change, so key IEC 61215 and IEC 61730 test sequences should be re-run or supplemented. The gel must demonstrate adequate dielectric strength, chemical compatibility with encapsulants and backsheets, low outgassing, and stable adhesion to ensure that safety and reliability certification is maintained.

How is heat dissipation performance confirmed after simulated field aging?

After thermal cycling, damp heat, and humidity-freeze exposure, the laboratory repeats infrared thermography, I-V measurement, electroluminescence imaging, and peel or adhesion testing on the aged specimens. Retention of thermal performance and power output at or above 95% of initial values, together with the absence of pump-out, dry-out, or delamination, confirms that the cooling benefit remains stable throughout the module service life.