SOLAR PANEL

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• General Information
• Solar Cell Circuitry
• Solar Cell Layout
• Structural Design
• Solar Panel Electrical Properties
• Solar Array Fabrications
• Testing
• Assembly

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SOLAR CELL

The solar cells used on SAPPHIRE are 1.6 inch x 0.8 inch Gallium Arsenide and Germanium substrate cells with cover glass. Both are rated at 15% efficiency, although the Germaniums are supposed to have better efficiencies as well as more resistance to cracks and performance degradation resulting from voltage reverse biasing due to shadowing effects. They are obtained from Crowder College as left overs from another project (very inexpensively). Each cell provides 0.8 Volts open circuits.

They are first inspected for cracks. The cracks are classified into three :

• Tranverse cracks : these are bad since the cells are electrically-connected internally in the longitudinal directions. Any severe tranverse cracks can eventually opens and will interrupt this connections thus causing the cell to open.
• Longitudinal cracks : although cracks are by-definition bad, cells with this type of cracks are still usable - at least for our purposes since we cannot afford to buy them.
• A Combination of the Two Types
• Corner cracks : these are somewhat acceptable since the crack is localized and unlikely to propagate.

These cells are then inspected for their ratings that are given by the manufacturers/supplier. The ratings corresponds to how well each cell produces power. The cells that are of the same ratings are best grouped together for optimal performace.

SOLAR CELL CIRCUITRY

The positive terminal of this type of solar cell islocated at one of the short end, also known as the P-Joint. The negative terminal is located on the back side of the cell, at 2 solder spots already pre-tinned with solder.

The cells are connected in series into strings of 20 cells, to produce 16 Volts. While the 16 Volts may seemed to be enough for providing charge voltages required for the battery packs, the cell voltages output actually very sensitive to the temperature at which the cell is operating at. In general, the solar cells and series strings will lose about 0.24% in voltage for each 1 °C increase in operating temperature.

Each of the strings is connected to a diode to prevent power loss in case one of the solar cell string is shorted. The diode will also prevent reverse charging, for example when the solar panel being charged by the battery as the satellite goes into an eclipse, or in case one string has a lower voltage output than the other string(s) on the same solar panel.

SOLAR CELL LAYOUT

The cell layouts on the panels are optimized for the available surface areas. The top and bottom panel each have 4 strings, totalling to 80 cells each, while the six side panels each have 40 cells made up into 2 strings, bringing the total number of cells to 480.

The solar cells are spaced 0.016 inch apart on the long side, and 0.030 inch apart on the short side, conforming with Lockheed-Martin specifications on these type of cells.

STRUCTURE

The panel structures are made of Aluminum-6061. Side panels #2,3,5 and 6, are made of 0.25 inch thick solid Aluminum, while side panels #1 and 4 are made of 0.5 inch thick Aluminum honeycomb. The top and bottom panels consists of 2 parts: the solar panel containing the cells which is made out of 0.063 inch Aluminum sheet, and a structural support panel which is made out of 0.5 inch Aluminum honeycomb.

The outer Aluminum surfaces which will hold the solar cells are overlaid with dielectric material, for electrical insulating purpose. This process is done at Lockheed-Martin facility.

PANEL ELECTRICAL PROPERTIES

Because the operating voltage of SAPPHIRE solar array is low (16 Volts open circuit), Lockheed-Martin engineers felt that there is no need for electrical insulation tests. Normally, this test is done using some sort of liquid which is spread over the dielectric-covered Aluminum panels and applying potential difference, similar to what will be generated by the solar array, between the liquid and the panel. Should there be an exposed area of the panel, a spark will result which will leave a black mark on the light green/blue-colored FM73. The exposed areas revealed on this procedure would then undergo further treatment for electrical insulation.

Extra attention is paid on electrical grounding of honeycomb face sheets. The side solar panels have "ground plugs"to ensure electrical ground connections between each of the side solar panel face Aluminum sheet. The top and bottom panels also employs "ground plugs", connection the inner honeycomb face sheet to the the 0.063 inch thick Aluminum sheet that contains the solar cells. The outer face sheet is connected to the 0.063 inch Aluminum sheet through copper grounding strips.

SOLAR ARRAY FABRICATION

The solar cells are connected one to another using interconnects. The interconnect are custom made by one of Lockheed-Martin’s supplier. The base material of the interconnect is Molybdenum, and it is plated with Silver. The Molybdenum is choosen for its thermal characteristics which matches well with the thermal characteristics of the Ga-As and Ge cells. The Silver plating will ensure the best electrical conductivity.

There are four type of interconnects :

• Positive : connects to the positive terminal of the solar cell (which is also known as the P-Joint).
• Negative : connects to the negative terminal of the solar cell
• Common : which connects the negative terminal of one solar cell, to the positive terminal of the next cell in the string, with both cells being in the same solar cell row.
• Turn-around : which connects the negative terminals of one solar cell, to the positive terminal of the next cell in the string, with one cell being in the next row of solar cell.

The interconnects have stress-relief areas to accomodate the expansion and contraction of the cells due to temperature changes.

The interconnects are first soldered to the solar cells. A number of this
sub-assembly is then assembled to make a string. conforming with the cell layout of their respective panels. The assembling processes use a (number of) jig that was custom machined for laying out this specific type of solar cell.

The cell are bonded to the FM73 covered solar panel surface using a controlled volatility RTV supplied by Lockheed-Martin. The RTV would adhere to the area between the 2 negative terminal solder spots on the back of the cell. Once the layout of the cells on a panel is determined. these areas are located on two 10-mils Mylar sheet. These areas are then cut out, forming masks. One of the mask is used to held the prearranged cells, while the other one is used for masking the areas on the FM73 covered solar panel that were not to be glued.

A side note : The Mylar used for this masking process should have been 20-mils thick. This will ensure that enough RTV are present for optimum bonding between the cell and the F-77 covered solar panel surface. However, we have no access to 20-mils Mylar, and this forces us to use 10-mils Mylar. Despite of this, however, SAPPHIRE test solar panel have successfully underwent shake test at Lockheed-Martin facility.

TESTING

Completed solar panels are tested to see how well they perform their power generation. Follow the folliwing link for more info on how we test them :

• Solar Panel Testing

If defect(s) is found during these test, those areas will be re-worked. This usually involves removal of the cell identified as "bad" (of course our cells are bad ie. rejected cells already) and putting on "better" ones and hand-soldering its interconnects to the rest of the solar cell string.

ASSEMBLY

This is how SAPPHIRE's solar panel are put on.

• Basically all solar panels except Panels 2/3can be put in any order.
  
• The thru-bolts that holds the satellite together is the tighten at the top panel. There is no space on the bottom panel that will allow the feets or the cup-cone assembly to turn. Also the tools used for tightening will come in contact with the bottom solar cells.
  
• Panel 2/3 is put on last, or taken off first. There is very little slack in the cabling of the solar panels, due to launch considerations. Doing so to panel 2/3 will allow access to the other solar panel connectors.

Lastly, take extreme care with solar panels.