The University of Kansas

School of Engineering Design Project

A Sustainable Approach to Automobiles and Energy Infrastructure

EcoHawks Solar Team (2009-2010)

Bryan Strecker, Alfonso Bortone, Mike Rollins, Joseph McCracken, David McNally and Rob Low

For our senior year, students were tasked with building a small, portable and light solar charging station.  The goal of our setup was to charge our various Remote Control car battery packs.  The goal of this page is to give you, the reader, an overview of how the solar team went about constructing our solar panel.  Hopefully, you will find this site useful if you plan on constructing your own personal panel. 

For our solar panel, we chose Everbright Polycrystalline 3” ´ 6” cells.  Each cell was rated at 3.6 amps and 0.5 volts.  This rating can also be thought of as a power output (1.85 Watts per cell).  Commonly, you wire the cells in series, adding the voltage of each cell while keeping the current constant.  If you so choose to wire the cells in parallel, the opposite is true.  This is known as Kirchhoff’s law.  Generally speaking, you will need about 1.5 times the voltage of whatever you are charging.  So, for a 12 volt battery this is about 16 volts.  If you are going to have some deviation from that value, make sure to error on the high side.

**There are many different ways solar cells are packaged and sent.  Regardless of how your shipper does this, make sure you order some extra cells.  Most companies will include extra cells at no charge.  Expect some cells to brake during shipping and manufacturing of your panel.** 

So, you are ready to build a solar panel…..let’s get some supplies!!

Once you know how many cells you need for your panel, a rough sketch of what you are doing should be constructed.  Pictured on the right is our layout.  We used OSB board for the backing of the panel ( and used 3/4” ´ 3/4” trim to section the OSB board into two compartments.  Each one of the compartments had four rows of six cells, giving the panel a total count of 48 cells.  All the dimensions of the OSB backing were based off of the number of cells used with the goal to leave a half of inch between each cell.  Home Depot cut all of our material to the proper dimensions. (OSB was convenient, but heavy as we found out later).

This is our total parts list.  Most of these items where purchased from either Home Depot or E-bay.  We were fortunate enough to have our solar cells donated ($213.00 retail price).  Other different lightweight backing materials exist that could be used for a panel.  One of the more interesting possibilities that our team considered was coroplast.  It is available in many different thicknesses and some of it even comes aluminum coated.  Materials available to cover solar panels include glass, Lexan, and Plexiglass.  Our group choose Plexiglass for its light weight and relative strength.   

Pictured above are soldering iron, sponge, flux, Q-tips and tabbing wire. 

Soldering Cells Together

Figure 1 - 32 of the 96 packaged cells from Everbright Solar.  Often, tabbed cells will come with a tabbing wire bundle.  Our team opted to use pre-tabbed cells.  We strongly recommend this because you will be spending A LOT of time soldering. 

 

Figure 2 - the back of one pre-tabbed cell.  Flux will be applied to the six small boxes on the back of the cell.  The purpose of flux is to give you a clean surface to solder on, providing the best conductive interface.  Our team tried soldering a couple of cells without it and we can assure you it does make a difference in the amount of current output produced. 

 

Figure 3 - the beginnings of one of the six cell strands needed for our panel.  One end cell on each strand will need additional leads soldered to it.  This can be seen in Figures 3 and 4. 

 

Figure 4 - the attachment of the second cell in the strand of six and two red circles indicated where to place the silicon caulking.  Remember that you will be soldering from the top of one cell to the bottom of the other. 

 

Figure 5 - Figure 4 in better detail, with the appropriate solder bead. 

 

Figure 6 - another good example of solder technique.  Notice the Q-tips for applying the flux.

Figure 1.  Packaged cells

Figure 2.  Back of one pre-tabbed cell

Figure 3.  Beginning of cell strand

Figure 4.  Attachment of second cell

Figure 5.  Better detail of Figure 4

Figure 6.  Example of solder technique

Testing Cells and Panel

Figure 7 - shows us testing five cells in series.  If you take this voltage output and divide by five you get 0.240 volts.

 

Figure 8 - shows us testing six cells in series.  If you take this voltage output and divide by six you get 0.238 volts which is roughly the same as when we tested our strand of five.  Through testing various strands of six, we determined that a reading of 1.42 volts was optimum for each strand under unnatural lighting conditions (testing indoors is not ideal). After completing all the strands, we attached them to peg board by applying a small dab of silicone to the bottom of each cell and gently pressed down. 

 

Figure 9 - shows us placing the peg board into one of the two sections.  Small squares of foam were glued to the OSB board providing a gap between the OSB and the peg board.  This provides needed air flow to keep the cells cool. 

 

Figure 10 - Individual strands were connected in series by soldering two additional tabbing wire strips on the inner and outer tabbing wire leads coming from the cells.

 

Figure 11 - shows us placing electrical tape over one of the connections between strands.

 

Figure 12 - shows the total panel output at 16.88 volts.  Taking this number and dividing by 48 gives us the average value of 0.35 volts which under indoor lighting is not bad.  Our panel outputted 27 volts and 3.5 amps on a bright sunny day.  This is obviously much higher then the 16 volts required for a 12 VDC battery (second panel iteration should use less cells).  Doing some quick math, each cell is producing about 0.56 volts and 3.5 amps.  This is only a 0.1 amp decrease with respect to manufacturer’s specifications!!

Figure 7.  Testing five cells in series

Figure 8.  Testing six cells in series

Figure 9.  Placing peg board into one of the two sections

Figure 10. Diagram of how strands were connected in series

Figure 11.  Use of electrical tape for safety

Figure 12.  Total panel output

Final Product

Figure 13 - The final panel weight came out to be around 25 lbs.  For our application, this was quite a bit more then we were hoping.  Other backing options should be used in future iterations in order to keep the final weight down.

 

Figure 14 - Drilling screw holes through the Plexiglass proved to be a bit challenging as some cracking occurred.  The Plexiglass is very brittle and has the tendency to climb the bit as you drill.  Finally, we ended up having Brian hold down the Plexiglass as Dave drilled through the material into a hole that was already in the work bench.  This proved to be a very effective method preventing any further cracking.  Some thought should go into this process in the future.

 

Figure 15 - shows all the members of the solar team minus Mike, who was taking the picture. Nick Surface is shown wearing a white and blue tee-shirt.  A special thanks goes out to him for helping out the team on multiple occasions. 

 

***Another great link on building your own panel***  http://www.mdpub.com/SolarPanel/index.html

Figure 13.  Final panel

Figure 14.  Drilling through the Plexiglass

Figure 15.  The solar team!