Category Solar Cells: Materials, Manufacture and Operation

MIR Solar Array

In January 1998, a segment of the MIR solar array was retrieved by the space shuttle. The segment, composed of eight panels, spent 10.5 years in a 380 km orbit. The panel design is exclusive; a laminated sandwich of coverglass, glass cloth, silicon solar cells (11% efficiency), glass cloth and optical solar reflectors

(OSRs) [97]. Two post-flight investigation programmes have been conducted in the USA and Russia. The main conclusions are the following:

• Hot spots are the main reason for the 50% power degradation of the solar array. By-pass diodes were not installed on the panel, relying especially on solar cell screening for handling full reverse currents...

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EURECA

The European Retrievable Carrier (EURECA) was launched in July 1992(500 km orbit) and completely retrieved in July 1993 by the Space Shuttle. The SA consists of two interchangeable wings of five rigid panels (~100 m2) each providing initially 5 kW. Silicon BSFR 10 £2 cm solar cells of two sizes were used to manufacture the charge and load array networks. The solar array post flight investigation programme had the objectives of studying LEO environment effects and mainly the anomalies faced during the mission [96]. Main conclusions are depicted here:

• Failures by fatigue (inadequate bend radii in the stress relief loop) in the Wiring Collecting Panels (WCPs) were responsible for open circuits on solar cell strings...

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Hubble Space Telescope Solar Array 1

One wing of the Hubble Space Telescope SA was retrieved from space in December 1993, after more than 3.5 years operating in a low earth orbit. The SA of the Hubble Space Telescope consisted of two wings of a double roll-out concept using two flexible solar cell blankets on each wing. The 48760 Silicon BSFR solar cells should provide the required 4.4 kW after two years in operation.

The post-flight investigation programme carried out between 1994 and 1995

[28] had the following main objectives:

• Assess the effect of different LEO interaction and environments as: thermal fatigue, ATOX, meteoroid and space debris damage, contaminations, UV, etc.

• Explain the anomalies experienced in orbit.

During the investigation programme the SA was submitted to several tests like detailed visual insp...

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Post-flight Investigations on Returned Solar Arrays

Returned SAs from space are valuable opportunities to assess their predicted behaviour in the space environment. Few SAs have been returned to Earth and a brief summary of their investigation programmes and the major conclusions are outlined in subsequent paragraphs.

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Pioneer Venus Orbiter SA

Pioneer Venus orbiter was a spin-stabilised (5 rpm) cylindrical spacecraft that operated in a high eccentric near polar orbit around Venus for more than eight years. After two years orbiting, power drops correlated with string losses were observed depending on the vehicle rotating angle. This suggested failures on strings due to reverse bias of cells (no shunt diodes protected the strings) produced by cyclic shadows made by the magnetometer boom cast, not predicted and unavoidable for the mission success. Ground tests were not conclusive that the cyclic reverse bias operation ended in cell breakdowns. Therefore, other interactions, as the ATOX environment in the Venus upper atmosphere, could favour the S A degradation [94].

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GPS Navstars 7-6

Six GPS Navstars satellites were placed in 20,000 km circular orbits from 1980. Mission lifetime for each spacecraft was five years and silicon solar cells K4 or K6 were in the SAs. After two years in orbit all spacecraft suffered an unexpected additional degradation of 2.5%. Investigations carried out in optical reflectors surfaces of one of the spacecraft revealed traces of contamination covering all spacecraft external surfaces. These contaminants mainly come from the outgassing of materials from the spacecraft, leading to reflectivity degradation of the coverglasses [93].

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X-ray Timing Explorer (XTE)

The XTE spacecraft was launched in December 1995. SA is composed of two wings of three rigid panels each, with silicon solar cells. Shortly after launch, the array showed discontinuous current drops, consistent with the loss of a part of a cell, when coming out from eclipse. The failure mechanism seems to be cell cracks not detected in ground inspections that became open in orbit due to the temperature gradients. These cracks were probably produced during the extensive tap tests, performed to detect SCA to substrate delaminations. During testing on the ground, following the same activities as for the flight SA, the qualification panel showed these effects, giving high confidence to this theory [92].

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European Communication Satellite (ECS) and Maritime European Communication Satellite (MAR. ECS)

After 1.5 years in GEO both SA (virtually identical, two wings of three rigid panels each with silicon solar cells) started to suffer partial loss of power [60]. The failures seemed to be short-circuits between the cell network and panel structure. These failures continued intermittently until the end both missions, however, for ECS the power losses were recovered. Several potential failure modes were identified; imperfections of the Kapton insulation layer or embedded particles in between layers, insulation breakdown by electrostatic discharge, thermal cycling, corona effects, micrometeoroids or a combination of all of them...

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