I: Solar Cells:
The factory standard cell specifications are the
following (under perfect testing conditions), though the
individual cells can be above or below these
standards.
Cell Specifications:
Average Power (Watts): 1.75 Wp
Average Current
(Amps): 3.5 Imax
Average Voltage (Volts): 0.5
Vmax
Thickness 200 µm = 0.2 mm
Exact
dimension: 3 1/4 inches x 6 inches, or 80 mm by 150
mm
Weight: Just above 6 grams, or 0.2 oz.
II: Tabbing wires: $2 value - additional
5 feet of tabbing wires that Evergreen Solar uses to
connect the solar cells. Cut them into 6 inch sections to
solder onto the bus bars or back side of the cells. You
will get slightly more than 110 feet, but 108 feet is what you
need to tab 108 cells.
III: Bus wires: $5 value - 11 feet of bus wires. Use them to connect them between the series and connect the tabbing wires ends to the junction box at the back of the panel.
IV: Flux: $7-$14.95 value - an easy-to-use flux pen that dispenses flux onto the bus bar or your tabbing wires without leaving a mess. It's used to aid soldering and bonding of the wires to the cells. A must have item. The flux liquid in the pen has a seal. At first use of the brand new flux, remove cap, hold the flux and point the tip upward, press the tip inside a few times so that the liquid can flow to the tip later on.
V: Solder: $7-$9 value - a tube of solder to help you
solder the tabbing wires and the bus wires together, or
whenever you need extra solder.
Some useful information in making
solar power:
How to make sense of the above
numbers:
The basic and very important
formula you need to know to make sense of solar cells is
this:
Power (P) = Current (I) Multiply by Voltage (V), or
P=I*V
Power's unit of measure is watt,
Current's unit of
measure is amp,
Voltage's unit of measure is
volt.
So in our
solar cell's specific example, if each solar cell is rated at
1.75 watts on average, and the voltage is 0.5, you do a little
algebraic math, then to get the amps (I), since P=I*V, so I =
P / V ,therefore I (amps) = 1.75 watts divided by 0.5 volts =
3.5 amps.
It's
important to understand the concepts of series connection and
parallel connection when stringing these solar cells together
to make a solar panel. Series connection of the cells
increases voltage but not amperage; parallel connection of the
cells increases amperage but not voltage. Series connection is
when you connect the positive terminal of a cell with the
negative terminal of the next cell. Parallel connection is
when you connect the positive terminals of all cells in the
set of cells with a tabbing wire and all the negative
terminals of all cell in the same set. You can use a
combination of series and parallel connections to get the
right voltage and amperage for your solar
panel.
The cell specifications above were
given to us by the manufacturer as averages. Variations
are possible. To make a 18 volts panel, for example, you
connect 36 cells in series (36 cells times 0.5 volt each = 18
volts). And 36 * 1.75 (each cell in theory averages 1.75 watt)
watt = 63 watts. The amps you will be getting is 63 watts
divided by 18 volts = 3.5 amps.
Solar cells basics:
The front of the cells (blue side, aka Sunny side) has two thick white lines, called bus bars. They are the negative terminals of the cell. The back side, where the 6 square dots are, are positive terminals. Connect the tabbing wires from the bus bar of the first cell on one side to the three dots of on the back side of the next cell. That way you have a series connection. Repeat the process on the other side of the cells. In panel making you should connect all the tabbing wires on the front for all the cells first, then flip the cells over to solder the back side in a second step. Some solar cells such as SunPower cells have both positive and negative terminals at the back of the cells, but we don't have to worry about them. Most cells are negative on the front and positive on the back.
The white color bus bars on front and the contact points on the back are made out of silver, and you should keep them intact. Apply solder on your them and the wires should bond.
Testing:
Make sure that you test the cells under strong sun light with the front of the cell facing the sun, at the same time connect your meters to the right terminals. If you are inside the building, make sure you shine your sun simulating lamps onto the front of the cell while testing. Place the solar cells on a metal plate, ideally made from copper. With a warm light (ie halogen lights) shining on the cell, put your positive lead from your multimeter on the plate, and the negative lead of your multimeter on the bus bar. Your multimeter will be set to typically check amps or voltage at one time.
Panel configuration:
For those who are new to solar panel making or if you don't have an engineering background, here is our recommendation:
1) Make small panels first so you gain experience.
2) In most situations, make either 36 or 72 cell panels, and connect them in series. 36 cells give you 17.5 or around 18 volts or so. And 72 cells doubles that. These panels are very useful, and you will find a lot of matching products that will want to buy! For example, you almost always want to use a charge controller if you make your panels to charge batteries. Charge controllers usually come in 12 or 24 volt settings, which match your panels' 18 and 36 volts very nicely. Yes, your panel voltage should be 1.5 times the voltage of the battery you intend to charge. Connect more panels together if you want to tie to the grid, in which case, always use an inverter.
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