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Project
Construct a solar concentrator
Primary Objective
Through various experiments, demonstrate the viability of a personal solar concentrator to produce energy through various techniques
Secondary Objective
Have fun catching stuff on fire using a backyard death ray.
Background
Originally I saw an episode of MythBusters called “Archimedes Death Ray” where they were testing the ancient myth of a large array of mirrors used to concentrate sunlight to set fire to the tar on invading ships. They busted the myth but it made me want to try it too.
I stumbled across some blogs and videos about constructing a personal solar concentrator. The difference between a solar concentrator and a solar collector is the collector is just an array of solar cells laid flat to collect sunlight, while the solar concentrator is a concave reflective surface that reflects light across a large surface into a small focal point. It looked like a fun project that required minimal supplies. Also, all the pictures and video I saw were weak. Most were either using aluminum foil (sometimes the dull side of the foil?!), small dishes, large mirrors, inefficient mirror patterns, prime focus, or they didn’t take it a step beyond catching stuff on fire to actually producing some usable energy. I thought I could better.
It also brought back childhood memories of frying ants with a magnifying glass on a hot summer day. It also brought back good memories of high school physics projects with my friend Nathan. We built boats out of cardboard big enough to hold us and paddle around the school’s swimming pool to demonstrate buoyancy and built an 8 foot tall hot air balloon out of nothing but tissue paper and rubber cement.
Safety Precautions:
None
Step 1 - Gather Materials
I sent my partner-in-crime, Nathan, the links to the blogs and videos and he like the idea of the project. Nathan used to install satellite dishes a long time ago and had a lot of left over dishes in storage.
We knew we wanted the smallest mirrors we could find so we could maximize the surface area of whatever dish we found. Nathan went online and found a place to order 1” square mirrors. They were $10 for 80 so he bought 240 thinking that would be about right to fill up a 15” dish.
Nathan then visited his storage area and gathered up several dishes ranging in size from 15”, like you see on most people’s home today, all the way up to a 53”x49” old PrimeStar dish. We decided that there was no reason to start out small and went with the PrimeStar dish. Apart from its size, this dish was the best because it was lightweight fiberglass and had an offset focus. This means that it’s shape is not a semi-sphere where the focal point would be directly over the center of the dish and it has to be pointed directly at the sun. Rather, this dish is a specially designed semi-ellipsoid whose focal point is off to one side and it stands upright and only slightly tilted. This is good for two reasons. First, whatever we place at the focal point wouldn’t block any of the mirrors and secondly if we were going to be burning stuff then we didn’t want to make a mess over the top of the mirrors. This particular dish also had a nice bracket where the focal point was to hold the LNB that could be adapted to hold whatever we want.
We didn’t have enough mirrors to cover the whole dish but at least we could start out with a parabola across the center. The surface area of a flat ellipse is (Pi * L * W ) / 4. That come out to 2040 mirrors! It would be even more for the interior of an elliptical dish. With 240 1” square mirrors we had only 12% of the mirrors we needed if we wanted to cover the entire dish.
We also did some research on collimator lenses, magnifying lens, Peltier coolers, small steam engines, etc. We also picked up a Peltier cooler and a solar cell from a local electronic parts outlet to play around with.
Step 2 – Attach the Mirrors to the Dish
We toyed around with several options for attaching the mirrors to the dish. One concern we had was if we weren’t happy with the results we wanted to be able to salvage the mirrors. We decided that concern wasn’t that important and went ahead and started attaching the mirrors with superglue. We couldn’t figure out why some of the people in the videos we’d seen wanted to arrange the mirrors in concentric circles. We thought this was a poor design because you lose a lot of surface area between the mirrors, and it’s not about making a pretty pattern but covering the maximum surface area of the dish with mirrors.
We washed all the gunk that had collected on the dish from it being stored outside and attached a row of 40 mirrors across the middle. It wasn’t perfectly aligned with the edges of the dish, but again, it doesn’t matter if they are straight across or diagonal, as long as you cover the entire thing. It was about 5:00 in the evening and the sun was still out so we rolled the dish into the driveway and pointed it at the sun. The focal point was pretty good with only a little variance around the edges. You could clearly make out a 1” square focal point. We didn’t take any measurements, but the early results looked promising so we pressed on.
We put the other 200 mirrors we had on the dish. It came out to 6 rows of 40 with a couple of mirrors offset for the mount bolt holes. By then the sun was down so we couldn’t test it, but there happened to be a full moon that night and clear skies so we were actually able to get some heat from of the sunlight reflected off the moon and focused on our hands!
The next day was sunny, so we did this test:
More to come when I get around to ordering more mirrors.
