Solar Cells Selected
Determining cell quantity and array design are next steps
One of the most important questions that the Array Team has faced to date is that of which solar cell to use. There are a number of manufacturers in the United States that make solar cells, and we ordered samples from each one. We tested the samples and found that cells made by Siemens Solar Industries and Photon Technologies would work the best for us. Siemens cells were used on Aurora-I, but the Photon Technologies cells are one percent more efficient, cheaper than Siemens, and available presorted. By selecting the Photon Technologies cells, we have eliminated the need to test more than 1,500 cells.
Once the solar cell manufacturer was known, we could make final decisions about the design and layout of the solar cells on the array. Now the trick is to find the maximum number of cells to fit into the allowable array area. So as the design stage for Aurora-II comes to an end, the Array Team is faced with the imposing task of constructing by hand a surface of eight square meters in area--over 86 square feet--filled with solar cells.
The array designer's rule of thumb is that a solar panel is only as good as its worst cell, so the rule is to gather like cells into panels. By gathering like cells together, it will be possible to prevent bad cells from limiting output of good cells and will increase the overall solar array power.
Cells will be interconnected using a flat copper ribbon and special solder paste compatible with the silver pickup traces on the cell. Cells are connected in series, just like batteries would be.
After the strings, or modules, of cells are connected together, they must be encapsulated to keep them clean and insulated from the environment. The team will be using a thin plastic film which will be placed over the top of the cells once the modules have been mounted to the car.
Because the majority of the structures of the array will be made of carbon fiber, which is conductive, great care must be taken to make sure that there is no contact between the module interconnections and the shell. Wires connecting the array
panels and the peak power trackers, which go though the shell, must be properly insulated.
Because of varying light conditions and varying temperature, voltage from the array can vary significantly, while batteries like to have a constant voltage for charging. Peak Power Trackers (PPTs) take electricity from the array and make sure that is in the form that the batteries can use most efficiently.
The Array team is hoping to be able to purchase PPTs from Australian Energy Research Labs(AERL), which makes them specifically for solar racing cars. The AERL product was used by a majority of top teams in past races.
With the planned completion date for the array coming up in the last week in November, there is a lot of work to do. The shortened schedule is necessary in order to be able to test the car over winter break. The tasks associated with array construction are large, and often complex, but necessary as well in order to gain the needed edge in Sunrayce 95.