Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
1998-02-21
2001-06-19
Pyon, Harold (Department: 1772)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C136S244000, C136S259000
Reexamination Certificate
active
06248950
ABSTRACT:
BACKGROUND
The present invention relates generally to solar arrays used on spacecraft, and more particularly, to improved solar cell circuit layouts and solar cell structures for protecting solar arrays located on spacecraft disposed in geosynchronous earth orbit from electrostatic discharge.
During 1997, the assignee of the present invention launched five high-powered spacecraft which generate over 10 kW of electrical power at the beginning of their life. On two of those spacecraft there has been damage to the solar arrays during the first year of operation. Extensive analysis and ground testing has demonstrated that a damage mechanism exists in which electrostatic discharges occurring between pieces of cover glass and the solar cells on the solar arrays can be sustained by current from the solar array itself. Depending on the physical construction of the array, local heating can cause pyrolization of the insulation which separates the solar cells from the conductive substrate, thus resulting in short circuits of individual strings of solar cells. Susceptibility to this phenomenon is likely to increase throughout the industry as spacecraft power increases lead to larger solar arrays operating at higher voltages. However, analytical modeling and laboratory experimentation have verified the phenomenon and validated the preventative actions undertaken by the assignee of the present invention so that this phenomenon can be controlled on future spacecraft.
It would therefore be desireable to provide for technical approaches that protects solar cells on geosynchronously orbiting spacecraft from damage caused by electrostatic discharge. Accordingly, it is an objective of the present invention to provide for improved solar cell circuit layouts and cell structures for protecting solar arrays located on spacecraft disposed in geosynchronous earth orbit from electrostatic discharge.
SUMMARY OF THE INVENTION
To accomplish the above and other objectives, the present invention comprises improved solar cell circuit layouts and cell structures that protect solar arrays located on spacecraft disposed in geosynchronous earth orbit from electrostatic discharge. The present invention provides for the use of an insulating material as a barrier, such as using room temperature vulcanizable (RTV) adhesive or insulating material, for example, disposed in intercell gaps between solar cells. The use of such insulating material modifies sparking in the gaps caused by electrostatic discharge so that, while the spark still occurs, it has different non-destructive characteristics. The use of the insulating material causes no damage to other solar cell materials, such as Kapton insulating material disposed between the solar cells and the substrate to support the cells. Furthermore, unique solar cell wiring schemes are provided that limit the voltage between adjacent cells to 50 volts or less.
In developing the present invention, a model was developed for spacecraft charging that shows that the solar panels of large geosynchronous earth orbit communications satellites can exhibit an “inverse potential gradient” in which the solar cell cover glass charges less negatively than the spacecraft body. The amount of inverse potential gradient is strongly dependent on the bulk resistivity of the cover glass.
A model of the arc discharge that can result from this potential gradient was also developed that shows that a plasma created by the discharge can trigger a sustaining arc, with current fed from the array itself. It has been found that there is a threshold cell-to-cell differential voltage below which the sustaining arc cannot be created, which may be why it has not been a problem with spacecraft in the past.
The arc discharge model has been verified by testing at NASA Lewis Research Center. This verification has shown that arrays that have been flown with high reliability for years (such as Intelsat VII, for example) can fail if they are operated at sufficiently high cell-to-cell voltage. Although heritage construction processes have been used, for both high power 100 V GaAs and Si arrays developed by the assignee of the present invention, the threshold for damage has been shown to be just at the limit of the normal operating range. The fact that the high power Si arrays currently in use have not been damaged can be attributed to detailed construction differences and it may be strongly influenced by the use of cover glass having a relatively low bulk resistivity.
In implementing the present invention, a number of construction techniques have been developed to provide a margin against failure from the secondary arc. Once the failure mechanism was understood, combining these techniques provides a very large margin to prevent the arc discharge phenomenon from occurring in the future. The corrective action implemented on future spacecraft to be launched by the assignee of the present invention will provide a safety margin significantly higher than that of previous solar arrays that have operated successfully for years.
Regarding the specifics in producing a solar array in accordance with a preferred embodiment of the present invention, the intercell voltage differential is lowered by 62.5% by rewiring the solar panels, the voltage threshold at which damage can occur is increased by a factor of 3 to 4 by adding the insulating material barrier between cells, and the current available to the arc is decreased by a factor of 2 to 3 by adding solar cell string isolation diodes.
The susceptibility of solar arrays to electrostatic damage is a function of their construction details, which determines how susceptible they are to damage. However, using the principles of the present invention, solar arrays can safely be produced that operate at today's 10 kW power levels, and that will also operate at significantly higher power levels that will be used in the near future.
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Hoeber Christopher F.
McVey Michael J.
Neff Robert E.
Pollard Howard E.
Float Kenneth W.
Miggins Michael C.
Pyon Harold
Space Systems Loral, Inc.
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