One of the largest sub-teams we have at Illini Solar Car is our Solar Team. Our Solar Team is responsible for the fabrication, testing, and mounting of the the vehicle solar array.

The solar array team is made-up of students from several engineering majors with Materials Science and Engineering having the largest representation.  These students are currently hard at work fabricating each of the 20 modules that will be put together to form the array.

The array is primarily put together in the ECE Building Clean Room with encapsulation taking place in the ECE Open Lab. Read below for a more detailed look into this process!

The Process of Creating a Solar Module:

There are four main components that go into creating a solar array:
  • Top Film 
  • EVA Film
  • Solar Cells
  • Back Sheet
Thank you to Isovoltaic for donating EVA and back-sheet and to Sunpower for giving us a discount on Solar Cells!

The process starts in the Clean Room in the ECE Building.

Each solar module is put together on a large stainless steel sheet. This sheet is first polished and then cleaned with Acetone and IPA (Isopropyl Alcohol). 

After the metal sheet is cleaned, top-sheet and EVA are cut, cleaned, and placed on the stainless steel with top sheet on the bottom (the array is assembled upside down).

The top film is the clear outer polymer layer which acts as a shield from the outside world. Most commercial solar arrays utilize thick glass or plastic sheets as this outer layer. Solar vehicles, on the other hand, utilize top sheet to reduce weight and allow the needed flexibility for mounting the cells on the slightly curved vehicle bodies. 

Underneath the top sheet is the EVA film. EVA is ethylene vinyl acetate - a copolymer material. Upon encapsulation the EVA melts and essentially laminates the individual solar cells. This protects the solar cells from dirt, humidity, and other contaminants. Additionally, EVA helps to absorb shocks from objects that may hit the solar array (such as a rock kicked up by a leading vehicle).

Cutting the Top Film

Once the top film and EVA are cut, cleaned, and placed onto the stainless steel, each solar cells must be cleaned. Each cell is rinsed with acetone, IPA, and deionzied water and then dried with pressurized nitrogen gas. 

Washing Solar Cells in the Clean Room


Drying with nitrogen gas
Rinsing with deionized water
After the cells washed they are laid out correctly for that specific module. There are 20 modules on the car - many of which are unique in some way. Modules are different shapes and have connections at different places due to constraints at different locations on the car. This module is a 4 x 6 module with a simple internal routing (zig-zag from one end to the other). Other modules, however, may not be rectangular or may have complex or inconsistent routing to fit their connections to the car.

Placing the last cell of a 4 x 6 module

The module is next transferred to a different room within the clean room for soldering. Soldering is the most involved process of each module. Each cell has multiple connections to solder together. Additionally, the solderer must verify that cells are lined up correctly to ensure the module works as expected. Once the module is encapsulated nothing can be changed. One bad connection will render a module unusable on the vehicle. On a 4 x 6 module, like the one above, one bad connection will result in no output from 8 of the 24 cells.

Soldering in the Clean Room

The soldered cells are then arranged back onto the top film and EVA as before. The last piece is the back sheet. The back sheet protects the wiring and the backs of the cells when the module is mounted onto the car. It also acts as an insulator since the body is made of carbon fiber - a conductive material.

Now the module is fully assembled and it is transferred back to the ECE Open Lab where the module is encapsulated. The encapsulation process involves placing the module under heat and pressure. First, breather material is placed on top of the module and the module is sealed inside vacuum bagging. Then a pump is attached to increase the pressure to as close to 29.92 Hg (inches of Mercury) as possible. We are able to reach levels above 28 Hg. Lastly, plywood is clamped down on top of the vacuum bagging. Together these provide the pressure and ensure the module is encapsulated flatly.

Thank you to Airtech for donating the vacuum bagging materials!

The module is placed underneath the plywood while encapsulated
To heat the module, silicon heating pads are used. The heating pads are the red/orange pads underneath the stainless steel. The pads are incrementally increased to 135° C over the course of 30 minutes. The temperature is then left at 135° C for 60 minutes. The module is then left to cool (which can take a while). Once it reaches a temperature below 40° C, the vacuum and plywood are removed. Once cooled to room temperature, the vacuum bag and breather are removed and the module is complete!

Removing Breather Material

Flipping over the module

A completed 4x6 module!

First, an Engineering Open House Update:

The 2017 Engineering Open House (EOH) at the University of Illinois is next weekend - Friday March 10th and Sunday March 11th. Illini Solar Car will be there showing off our bottom shell (which you can read about below), our nearly completed electrical system (including our custom built Lithium-Ion battery box), and a completed solar module. We will have some hands on demos as well! If you are coming to EOH be sure to stop by! Watch on social media for an announcement about our location later this week.

Bottom Shell Layup:

by Jenny Chu; Mechanical Team

At the beginning of February, Illini Solar Car gathered up its forces to start the layup of the bottom shell with carbon fiber prepreg. Members across all sub teams pitched in to spend their evenings massaging the car into being. 


Step 1: Cut the many Carbon Fiber sheets needed to make the shell. Thanks to Gurit for supplying the prepreg!

Mapping out the staggered ends of each ply of carbon fiber in each overlapping region.



Right: 
Test fitting the honeycomb core into the bottom of the driver's compartment. Thank you to Plascore who donated the honeycomb core!



Left:
Laying the film adhesive (donated by Patz Materials and Technologies) onto the honeycomb core. Balsa triangles were placed along the edge of the honeycomb core to provide a transition from areas with the core to areas without core. The film adhesive was used to connect all the pieces.



As the carbon fiber was being laid it was important to be meticulous in removing any air bubbles or dust. This made for many long nights just to keep on schedule.

Pressing down out the air bubbles and using the heat gun to make the prepreg stick and conform better to the mold.
It was 18F that night :O So cold.

Checking the mold for debris, air bubbles and other imperfections to fix prior to vacuum bagging and curing the molds.

In order to cure our carbon fiber body parts they must be vacuum bagged and heated for 80°C for many hours. In order to do so we build our own large oven a few months ago. We’ve already tested the oven several times for heat distribution. The data showed the driver compartment was cold spot. To solve this problem, a high temperature blower was added above the driver compartment to draw air from hotter areas in the oven to blow air down into the driver compartment.

One of the temperature sensors taped on the outer surface of the vacuum bag
The graph of temperature data points at eleven locations within the oven over our 12 hour cure time. You can see two dips near the end when we had to replace the propane tank on each of the two heaters. One tank wasn't quite enough to get the oven to over 80° C and keep it there for 12 hours. The average temperature difference between the warmest and coolest location was 6° C.

Our oven with the bottom shell curing inside. Thanks to all our members who took shifts to monitor the oven. The curing went from 11pm to 11am!

Removing the vacuum bag after curing. Excess resin from the carbon fiber was drawn through the perforated peel ply (like a big piece of paper) and was soaked up by the breather (the white fabric).

Removing the peel ply and breather from the shell. Thanks to Airtech for donating all of the vacuum bagging materials!

The cured bottom shell!

The bottom shell will be the most difficult carbon fiber part of the car body to complete. It was a big milestone to successfully get it done! It may look rough around the edges (since they will be cut to shape around the edges once the top shell is fit to it) but it is the first tangible piece of our first car of the 21st century!

Thanks to all of our sponsors and the University for helping to make this possible - especially our work space sponsor!