Thermal Cycle of Solar Cells for Space Applications According to ECSS and ST 08C

with Solar panels photovoltaic technology are among the cleanest and safest energy sources available to us.

Not everyone knows, however, that the aerospace makes extensive use of solar cells to meet the energy needs of orbiting objects.

These solar cells must be thoroughly tested before being used in orbit, and thermal cycling in an environmental chamber is among the mandatory tests required by international standards. 

But let's proceed in order.

What are space solar cells used for?.

The marriage between photovoltaic technology and the aerospace sector is as old as the space race itself. In fact, photovoltaic technology was developed in the middle of the last century, and Vanguard 1, the first US satellite, launched into orbit in 1958, was already equipped with six microcells that produced 1 watt of electrical power.

Since then, technology has made great strides and each satellite has employed increasingly efficient and long-lasting photovoltaic models.

Today, satellites must remain in orbit for 10-15 years. To ensure the investment is not wasted, all satellite components must be rigorously tested before launch.

The solar panels that guarantee energy self-sufficiency are no exception, and their production is regulated by a series of international organizations.

In Europe we have the ECSS.

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The ECSS E ST 08C Standard for Thermal Cycling of Solar Cells

The European Cooperation for Space Standardization, abbreviated to ECSS, was founded in 1993 and is responsible for developing international standards that serve as a reference for both European space missions and those of the space agencies of individual member countries.

ECSSE ST 08C is one of these standards and deals with the tests to which photovoltaic technologies for space use must be subjected.

Satellite solar panels are then divided into the following components: photovoltaic assemblies, solar cell assemblies, bare solar cells, solar cell protective glasses, and protective diodes.

Thermal cycling testing is recommended for bare solar cells and solar cell protective glass.

What is the purpose of thermal cycling on solar cells?

Orbiting satellites must survive for over ten years in a very particular climate, subject to extreme and sudden temperature changes.

The climatic test known as thermal cycling simulates this type of condition in an environmental chamber . We've covered it in detail here.

As for its application on solar cells for space uses, the thermal cycles will have to be repeated for the number of times prescribed by the aforementioned standard in order to simulate the effect that a whole year in orbit will have on the satellite.

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The FDM Environmental Chamber for Thermal Cycling Testing on Solar Cells

FDM environmental chambers are the ideal tool for performing extreme temperature tests such as thermal cycling on space-grade solar cells.

They have a temperature range from -70°C to +180°C and a relative humidity control from 10% to 98%.

Furthermore, they have a heating rate of 5°/min and a cooling rate of -4°/min as standard, but which can be further customized according to customer requests.

Thanks to the practical revoFACE, it is also possible to program the process.

For any further questions or concerns, please do not hesitate to contact us.

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Reference Standards

• ECSS-E-ST-20-08C Rev.1 — Photovoltaic Assemblies and Components, ESA main standard for space solar cell qualification tests.
• ECSS-E-ST-10-03C — Testing, ESA general standard for environmental testing of space equipment.
• ECSS-Q-ST-70-02C — Thermal vacuum outgassing test for the screening of space materials.
• AIAA S-111A-2014 — Qualification and Quality Requirements for Space Solar Cells, complementary American standard.
• IEC 60904 series — Photovoltaic devices – Part 1 (measurement IV), Part 3 (reference solar spectral irradiance AM1.5G).
• ASTM E595-15 — Total Mass Loss and Collected Volatile Condensable Materials from outgassing in vacuum.
• NASA-STD-6016 — Standard Materials and Processes Requirements for Spacecraft, reference NASA.

Authoritative Sources

• ECSS – European Cooperation for Space Standardization. A cooperative standards system between ESA and European industry, it manages the ECSS-E series for engineering.
• ESA – European Space Agency. European Space Agency, primary user of the ECSS standards for missions and hardware qualification.
• NASA. US space agency, publishes NASA-STD-xxxx complementary to ECSS for space hardware.
• IEC TC 82 – Solar photovoltaic energy systems. IEC Technical Committee for Standardization of Photovoltaic Devices.

Frequently Asked Questions about Thermal Cycling in Space-Based Solar Cells

How many thermal cycles does ECSS-E-ST-20-08C prescribe?

The number of cycles depends on the mission and the satellite's orbit. For LEO (Low Earth Orbit) missions , typically at an altitude of 400-800 km, the satellite completes approximately 15 orbits/day with shadow/sun passages each lasting 35-45 minutes, equivalent to 5,500 cycles/year. For a 10-15 year life, qualification tests involving 30,000-80,000 thermal cycles between -150°C and +100°C (extremes dependent on the specific orbit) are required. For GEO (Geostationary Orbit) missions, the rate is 1 cycle/day (during the equinoxes with shadow passages), for a total of 1,300-2,500 cycles over 15 years, with less severe ΔT (-100°C to +80°C). Deep space missions with flyby or free attitude may require cycles over extreme ranges of -180/+150 °C with dedicated numbering. ECSS-E-ST-20-08C prescribes a safety factor of 1.25x on the nominal number of cycles for type qualification.

What electrical parameters are measured pre/post heat cycle?

Space solar cells are characterized using an IV curve under AM0 conditions (Air Mass Zero, extraterrestrial spectrum 1367 W/m²) before and after thermal cycling. The monitored parameters are: short-circuit current I_sc (sensitive to optical degradation of the anti-reflection windows), open-circuit voltage V_oc (a function of junction T and bulk quality), power at the maximum power point P_max , and fill factor FF (junction quality and series resistance). ECSS requires P _max degradation < 2% after qualification. Shunt resistance R_sh and series resistance R_s are also measured to detect micro-cracks or contact delamination. The test is performed with an AM0 class A+A+A+ solar simulator calibrated with a NASA/ESA calibrated reference cell, sample temperature controlled at 28 ± 1 °C, and acquisition times < 100 ms to avoid heating. Current GaInP/GaAs/Ge triple-junction cells have 30-32% BOL efficiency and must maintain > 80% at EOL (15 years).

How is the vacuum of space simulated in an environmental chamber?

ECSS tests integrate thermal cycling with thermal vacuum (TVAC) . ECSS-E-ST-10-03C requires pressure < 10 ^-5 mbar to simulate an orbital environment. TVAC chambers combine rotary pumps + turbomolecular + ion pumps with cryogenic trap to reach 10 ^-7 mbar. Thermal control is via copper shrouds cooled by LN ₂ or GHe and radiant lamps or resistances for heating, allowing a range of -180/+150 °C. The TVAC test combines TRM (Thermal Reference Mission) with a reduced number of cycles (8-20) for system qualification, followed by thermal cycling tests in air (ECSS-E-ST-20-08C) with many more cycles but less outgassing control. The integration of the two approaches fully qualifies the system. FDM environmental range chambers (-70/+180 °C) support standalone thermal cycling tests according to ECSS; For a complete TVAC, dedicated vacuum-thermal chambers with specific space technology are required.

Which auxiliary materials (adhesives, coverglass) must be qualified?

The assembly of a space solar cell includes a bare cell, coverglass (typically CMG – Cerium-doped Micro-sheet Glass, 100-200 µm thick) with UV filter, optical adhesive (silicone DC93-500, RTV-S691) and bypass diodes. All materials must pass ASTM E595 for outgassing: TML (Total Mass Loss) < 1% and CVCM (Collected Volatile Condensable Material) < 0.1%. ECSS-Q-ST-70-02C tests are performed at 125 °C for 24 hours in a 10^-6 mbar vacuum with a cold finger at 25 °C that collects volatiles. Each material must also be qualified for: resistance to proton/electron radiation (MeV fluence equivalent to mission dose), UV resistance in vacuum (VUV test), atomic oxygen (AO) compatibility for LEO. Approval requires inclusion in the ESCIES database (ESA) or MAPTIS database (NASA). The thermal cycle assesses the integrity of the laminate (cell + coverglass) by checking for cracks in the coverglass, adhesive separation, and fractures in the bonding contacts.