Additive Manufacturing a Solar Trough for Cooking

NOTE: This project is a submission for the Make it Real Challenge, so if you like the instructable, your vote would be appreciated.

This project details work done in the UNC Charlotte Department of Engineering Technology using MATLAB, ProE, and a VFlash 3D printer from 3D Systems to build a parabolic trough for a solar cooker.  The goal was to see how well the VFlash did at producing a curved surface on a part that encompassed most of the available build volume, and maybe even cook a hotdog as a secondary benefit.

Step 1: Equipment and Materials

You will need the following tools:
A 3D printer (3D Systems VFlash in this case)
A CAD package capable of creating parabolic curves (proE in this case)
meat thermometer
infrared thermometer (optional for the curious)
flush cut pliers
pipe cutter
sun

and materials:
A copper tube (1" ID 1/16" wall) 12" long
A sheet of mirror board
high temperature black paint
hot pipe foam insulation
sandpaper
1 hot dog (ideally organic w/o too many nitrites etc.)

Step 2: A Little Math

The nice thing about a parabola is that it will focus the light coming from a distant source down to a single focal point.  Following the nice derivation of the focal point provided on wikipedia (http://en.wikipedia.org/wiki/Parabola) I used MATLAB to play with the available build dimensions to find a focal point that would work for my design.

Step 3: The CAD Work

Given the equation for the parabola derived from my MATLAB experiments, I used ProE Wildfire to create a trough with parabolic inner surface.  Slots matching the parabolic curve were also design into the ends and central support in order to form the mirror board to the appropriate shape.  Note that these screen shots do not include some cosmetic features that were added later (pocketing for weight reduction and aesthetics).  Once the geometry was set, the file was exported as an STL with the highest resolution options (small chord lengths and high angle control).

Step 4: Setting Up the Build

The STL file exported from ProE was then uploaded to the software controlling builds on the VFlash where position and orientation on the build platform were set.  The trough was centered on the build platform with a slight rotation (3 degrees) about the long axis as recommended by the manufacturer to prevent sticking to the film during building.  The orientation of cavity up was chosen so that support material would be adhered to the outside of the trough and not interfere with the parabolic inner surface as well as being easier to access for removal.

Step 5: Extracting the Trough and Removing Supports

Once the build finished, the part was cleaned, rinsed, and given a final cure in a UV curing chamber.  Interestingly, the support structure that was generated on one side of the trough was a interconnected scaffolding, while the other side was simple support columns.  The support structures on both sides were clipped off with flush cut pliers.

Step 6: Finishing

After removing the support structure, the apertures of the models were then sanded to open them up a bit to allow for easier access of the mirror board and heating tube.  These were "post production engineering enhancements" and most definitely not me correcting for sizing the holes/slots too tightly initially.

Step 7: Assembling the Cooker

The mirror board is then cut to size to fit into the slots on the trough.  You get an indication that the parabola is correct when looking at the trough straight on from a distance and all you see in the mirror is the color of the cooking tube.  The copper tube is then painted black with the high temperature paint to increase absorption.  At this point the copper tube was cut down with a pipe cutter to 9" to minimize the tubing hanging outside the trough which would just provide additional area for heat loss.

Step 8: Prepping the Cooking Tube

The first step in preparing the cooking tube is to plug one end with the foam pipe insulation that has been tightly rolled into a cylinder.  This should help to keep the heat inside the tube from escaping, allowing for higher temperatures.  Next, skewer the hot dog with the thermometer, leaving some room at the end to wrap foam to seal the second end.  Insert the skewered hot dog into the tube and replace the tube into the parabolic trough.

Step 9: Results, Future Work, and Updates

Results:
The solar cooker was tested in the morning in Charlotte, North Carolina.  The outside temperature was 79 F and there was 50% cloud cover. 
The initial temperature of the hotdog was measured as 50 F (having come out of the fridge then waiting to pose for photos).
After 15 minutes, the temperature had risen to 72 F.
After 45 minutes, the temperature had risen to 90 F.
After 75 minutes, the temperature had risen to 98 F.
During this time, the solar cooker was oriented towards the sun, with adjustments made every 15 minutes.  If anything cloud cover increased during the testing up to ~75%.

Using an infrared thermometer, the temperature on the surface of the cooking tube was measured as high as 148 F.

Future Work:
Repeat the test on a sunnier day.
Test the setup after covering the top to the trough with a taut layer of plastic wrap to minimize convective cooling of the tube.

Update (2-Jun-2012):
I made the following modifications:
-Drilled a small hole in the central web and inserted a screw to assist in aligning with the sun, when the shadow of the head was centered at the base of the screw, I knew it was aligned
-covered the top with saran wrap to minimize convection

Condtions: Sunny with intermittent clouds, temperature 74 F, intermittent slight winds
The hotdog inner temperature measured 58F initially
-84 F after 15 min
-122 F after 30 min
-160 F after 60 min  (I was on a phone call an missed the 45 min measurement)
-197 F 75 min
-209 F 90 min
-213 F 105 min
at that points some clouds rolled in and the temperature dropped back down to 209 F
at each temperature measurement, I adjusted the position to keep alignment and pulled the saran wrap taut to minimize wrinkles

I brought the device inside, removed the saran wrap and let the tube cool for 10 minutes, then careful of the hot dripping hot dog juices extracted the hot dog.