Friday, July 11, 2014

solar bbq

This home built solar cooker is a parabolic solar reflector with a spit rod at the focus. A simple aligner post at the bottom front shows clearly when it is perfectly aimed at the sun. Aiming seems not that critical and the post makes it easy. We checked and moved very slightly only once during 1/2 hour of cooking.

Today was the first time I was able to try it. For my test, I cooked one hot dog to perfection. The test was done again the next day with four hot dogs. I would like to do a non-meat test.

Two things weren't perfect: I'd started cooking in the late afternoon, at about 4pm as the sun was already part way down the sky. Also, the reflective surface is not a mirror finish but shiny aluminum roofing flashing as it comes off the roll which might give I think about 1/2 performance, yet a useful test in itself. In spite of these two factors, I had a steaming hot dog (160F) in about a half an hour. I was hoping to be able to grill somewhat (brown the surface) but that didn't happen, it did not get hot enough. I will use a mirror film shortly and start cooking earlier in the day.

I can heat small objects which skewer onto the thin stainless rod which slides into a slot along the focal line. I used a round stainless rod but a square, flat, oval or triangular cross section might work better since the payload (what's on the spit/skewer) has a tendency to slide around on the round road as it cooks and shrinks.

The characteristics of the surface of what I put on the spit will determine how it heats. I thought that this might be useful for marshmallows but their bright white color makes it almost certain that they will reflect away most of the heat focused perhaps up to some point at least?

The aiming is manual. The small disk with a perpendicular dowel makes a sensitive indicator of the sun's position. If the shadow is small and falls almost directly near the dowel, the aim is good and the sun's concentration achieved is about 18 times.

click any picture to enlarge

Aiming disk. The shadow of the small dowel on the disk shows less than perfect alignment and give an indication of which way the bbq should be turned in order to point more directly at the sun. In practice, the aiming is not super critical. If the shadow is within about 5cm of the dowel, this is good enough. We moved the trough only once in the half hour and probably didn't need to do that.

View of the rear. I am using a video camera tripod which makes an excellent mount. Not a polar mount but it gets the job done. I have attached a small plate with a captured nut the same as on the bottom of cameras (1/4-20) so I attach the bbq to the stand with one screw, the regular one used to attach a camera to the stand.

Trying again today with four dogs and starting mid-day. My assistant Justin is showing how bright it is standing close to the reflector. We have put a strip of aluminum foil over the bottom third (it is not very smooth) to see if it was more reflective than the aluminum roof flashing (if dogs heat quicker over the foil) and possibly to catch any drips.

One of the dogs after about 25 minutes. The glisten makes it difficult to measure the temperature with an optical temperature meter, there is scatter depending on how it was pointed. Done as before by removing the spit from the bbq am moving it to a shady area to measure.

It is hottest closest to the spit. It would not be effective, for example to thread the hot dogs onto the spit through their radial center as opposed to lengthwise, through their axial center even though it might seem to increase the capacity of the bbq (more hot dogs cooked at once?).
With the aluminum roll flashing as the reflector, I estimate about 200 watts of cooking power available. This could possibly reach 400 watts with a mirror reflector.

The finished product. Could be used also for kebabs or other long, narrow items.Temperatures as high as 160F to a low of about 70F so I don't have faith in the measurement. The dogs were warm to the touch and mouth though and already cooked (according to the package). I would still like to broil them so need to get mirror material for the reflector.

Still, I am encouraged to see that this works. If you only had access to roof flashing and a bit of plywood, some straight angle metal (aluminum, steel, plastic, fiberglass etc), a workable spit bbq can be achieved at very low cost. The cost of the materials as I did it (not including the tripod) is about Cdn$10-$25. Using flexible mirror in place of the roof flashing might double cost but improve performance, probably near double (will cook faster and hotter).

This is an example of what can be done with the gen2 design in a simplified version. The gen2 book is available here in either downloadable or paper version.