Category Solar Cells: Materials, Manufacture and Operation

Spacecraft Solar Array Anomalies in Orbit

Another source of data for improving solar array design comes regrettably from anomalies faced by spacecraft SA in orbit. Investigations of the failure mechanism in-orbit are much more complicated due to the small quantities of data often available. However, some anomalies in orbit could be acceptably explained; a few of them are depicted here below:

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Monitoring of Solar Array Performance in Space

Monitoring of solar arrays in space is mainly needed to verify that their performance meets the spacecraft power requirements for planned operations

Table 2 Mechanical/physical characterisation tests on space solar cells

Test name or phys./charact.

Purpose

Test method

Requirements

Visual inspection

Find solar cell or component obvious defects

Unaided eye or low magnification

(5x-10x)

Several defects are not allowed at component level, relaxation criteria for higher levels of integration exist.

Interconnector

adherence

Interconnector weld and cell contact adhesion

Pull test

Maximum pull force value and breakage mode.

Coating adherence

Contact and coatings adhesion

Tape peel test

Percentage of delaminated area below certain value.

Solar cell dime...

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Monitoring of Space Solar Cells and Arrays

3.1 Flight Experiments

Several flight experiments have been conducted with solar cells/coupons in order to verify their endurance to the space environment and ground radiation testing assessment. Most of the flight experiments measure main electrical parameters of the cells and coupons (Isc, Voc, and power at certain voltage, full I­V curve), sun aspect angle and the operational temperature. In Table 3 some of the most relevant recent flight experiments are listed together with the publication reference, dates of data acquisition, orbit (apogee/perigee), cell/ coupon types and main conclusions achieved.

Some flight experiments are, at the time of writing, in preparation:

• Mars array technology experiment (MATE): Several solar cell technologies shall be sent to Mars surface for checking t...

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Physical Characteristics and Mechanical Tests

Several tests are presented in this section, not only tests to check mechanical characteristics as adhesion of coatings, contacts or interconnectors, but also measurements of some physical characteristics needed for solar array sizing or essential inputs to other solar array analysis (Mass Budget, Thermal Analysis, etc). A summary is depicted in Table 2.

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Humidity

Accelerated humidity/temperature testing of solar cells is conducted to check the stability of solar cell contacts and anti-reflection coatings for long storage periods

[63] . GaAs solar cells with AlGaAs window layers are submitted to this test in order to assure the effective protection of the anti-reflection coating to the corrosion of this window layer [64, 65].

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Electrostatic Discharge

Dielectric solar array surfaces, mainly solar cell coverglass and kapton layers, are subject to electrostatic charging due to geomagnetic substorm activity or by the spacecraft surrounding plasma. Subsequent sudden electrostatic discharge (ESD) effects may permanently damage solar array components [60]. Coverglasses are coated with conductive coatings (i. e. ITO) and grounded [61] to lighten charging and to give an equi-potential surface for scientific field measurements. Tests are conducted at coupon level to check adequacy of components and interfaces [62] and at component level for the survival of the conductive coating to the mission environment [17].

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Micrometeoroids

More frequent impacts from micrometeoroids and space debris (between 10~6 and 10”3 g) mainly erode coverglass and solar array exposed coatings, with small solar array performance degradation due to optical losses. Predictions are in agreement with in-orbit degradation [55] and permanent loss of solar array sections by impacts on harnesses, though these are rare [56]. Hypervelocity impacts of particles are simulated with plasma drag accelerators [57, 58] and light gas guns [59].

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Vacuum

A space vacuum might vaporise metals (Mg, Cd and Zn) and also volatile materials like adhesives. Thermal vacuum is a standard test performed at component and up to solar array level, for endurance testing of components and interfaces. Chambers as described in [52] are commonly available in the space photovoltaic industry and test houses. Failures coming from wrong manufacturing process or contamination of materials are also quickly revealed with these tests [54].

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