Introduction: Multifacet Parabolic Solar Concentrator
I live in Phoenix, Arizona. Phoenix is among the areas in the US that gets the most insolation (incident sunlight) each year. This is partly due to the latitude and party due to the dry climate and therefore low humidity and low rainfall. These led me to think about building a solar water boiler. I've always been interested in old style steam piston engines and thought it would be fun to build a solar concentrator to boil water and run an engine. This project gets as far as building the solar concentrator. The engine and other aspects of steam piston engines operation are yet to come.
For now, let's content ourselves with just a parabolic solar concentrator.
Before we begin, it is important to know that concentrated solar energy is very dangerous. Reasonable precautions should be taken. I cannot take responsibility for individual choices you make in the design, construction, modification, or use of this project. Have fun and be safe.
Step 1: Obtain Parts
You will need several parts for this project. The cost ranges from $300-$500, so this is not a poor man's hobby. What you will need is:
1) About 17 aluminum angle stock rods 5/8 inches (or thereabouts). An aluminum siding store will probably have this much cheaper than your corner hardware store. They're friendly folks, usually.
3) Some chrome-plated aluminum sheeting. Again, an aluminum siding store will usually provide this. You may have to special-order it. I ordered it under the name "light sheet". I am not sure if it was a brand name or a description. You will need approximately 40 square feet of it and ask them to cut it into 6 inch square mirrors. They will try to talk you out of it and think you're crazy for asking them to. They will also charge you for the cutting service. Let them. They deserve to make an honest living, but ask them to be careful in cutting it into _exactly_ 6 inch squares.
4) About 300 machine screws, 500 machine nuts and about 500 large washers. I used 3 inch screws in a 10-32 pitch. 10-24 is sometimes cheaper. It doesn't really matter, but be consistent. 10-32 and 10-24 look just alike and with so many of them, you won't want to waste time sorting them.
5) A screwdriver, drill, wrench, and ruler that will measure inches and millimeters.
6) Various other common household tools.
7) Some creativity and basic high-school level geometry.
For the heat-exchanger (if you choose to build it)
8) A length of 1/2 inch copper pipe and about 20 "T" junctions, some end caps, solder, propane torch, gloves, a little soldering know-how.
Step 2: Measure, Cut, Measure Cut
Cut all but 4 of the aluminum angle stock to a uniform 6 feet 6 inches in length. Measure fairliy carefully, but an exact length is not too important.
Next, measure accurately an interval of 6 and 3/8 inches. This is VERY important. I took a small piece of medium density fiber board and drilled two holes VERY CAREFULLY to be exactly that distance apart. Then, I took a hole punch and a hammer and pounded small dimples into the soft aluminum at this interval all across the length of the angle stock. Do this for each of 13 of the angle stock pieces. This means more than 160 dimples exactly the same distance apart.
The reason they must be so carefully measured is that the mirrors are all 6 inches square. The machine screws are 3/8 inches wide. This means you will have space for one mirror, one bolt, one mirror, one bolt, etc.
Step 3: Drill, Drill, Drill
Now that all of the measurements are made, measure some more and then drill some holes.
Measure halfway in between the 6 3/8 inch intervals on the first, last, middle, and 1/4, 3/4 up each bar.
These will be the holes for the cross-beams.
Now, each of the dimples that was made will be drilled out with a 3/8 inch drill bit. This is a lot of drilling and I highly recommend an electric drill and some understanding neighbors. It will be loud because the aluminum will ring as you drill. That's ok. Also, because you're using power tools, don't forget to keep your fingers clear and your eyes protected. I got a piece of aluminum in my eye once. I do not consider eye injury to be an important step for completing the project, so wear eye protection.
Once drilled, you will want to file the holes flush because aluminum is soft and as such creates lots of sharp edges when you drill it out. An ordinary file will help you remove those edges so you don't slice your fingers.
Step 4: Assemble the Frame
Now, you will want to assemble the frame. Do this by taking some short 10-32 machine screws and nuts and simply attaching the 13 angle stock pieces to the 5 cross-beams. You should wind up with 13 parallel angle-stock pieces in the vertical and 5 in the horizontal. You may notice that it's difficult to keep them at a 90 degree angle from one another. There is a simple solution.
Take another couple of scraps of aluminum and do a pythagorean theorem problem (you know, x2+y2=d2) to figure out how long the triangle brace should be. A 3-4-5 right triangle is a pretty good one to apply. In the picture, you will see 2 of these triangle pieces.
Now that the assembly is fairly stable, start inserting the 3 inch machine screws. You may want to use 4 inch screws around the outer edges. You'll see why later. Hold them in place using a machine nut. This is perhaps the most time consuming part of the project.
Step 5: Mirror Alignment.
You will be holding the mirrors in place by washers sandwiched in between two nuts. This is tedious. You will begin by putting together a spreadsheet or other calculation to tell you how many mm each mirror corner should be offset from its neighbors. The basic formula for this is the formula for a parabola. y=x2. This is a parabola in 2 dimensions, however, so it's a little more complicated than that. Really, the formula is z=1/(4*f)d2 where d=sqrt(x2+y2) and f is the focal distance of the device. I chose a 72 inch focus. Don't do anything different unless you understand why you're doing it. Let me recommend that you study the math and don't try to align the mirrors by hand. It's even more difficult than measuring it and the focus isn't as good.
What happens is that when you get past the 3rd row out from the center is that your distance up each screw starts to get bigger than the screw is long. This is not really a problem. The trick is to offset all sides of a given mirror so that the lowest edge is as low as you can make it. What you get is rows of mirrors at the same angle, but not in the same plane. This is similar to a fresnel lens for those who understand optics.
If all of this is lost on you, I have attached a screenshot of the spreadsheet. This is a list of the mirror offsets for each of the mirrors as measured in millimeters. This will give you a focal distance of 72 inches. It works fairly well.
Step 6: Focal Assembly
The focal assembly is simply four angle stock pieces set up so that they are screwed to the corners of the concentrator and meet at the center. If you use 8 foot pieces, they will meet at a point about 7 feet from the center of the mirror assembly. This gives you about 1 foot between where they meet and the focal point so you can mount things easily at just about the exact focal point.
NOTE! When you're not around the concentrator make VERY SURE that it is pointed in a direction that the sun will NEVER be. It WILL catch things on fire. Think very carefully about how the sun moves and remember that it traces out an arc in the southern hemisphere (northern if you're an Aussie).
The broom below took only 30 seconds to burst into flame. I cannot stress enough that concentrated sunlight is VERY dangerous. Also much fun if you're careful.
Step 7: Completed Concentrator
In order to come closer to the goal of solar steam generation, it makes sense to build a heat-exchanger module. Next, we'll tackle that problem. It turns out to be a little simpler than the concentrator. The completed concentrator is shown here atop a makeshift box to hold it in place. It was originally intended to be a tracking frame, but it proved impractical.
Step 8: Heat Exahgner.
For the heat-exchanger, take a length of 1/2 inch copper pipe (about 8 or 10 feet). Cut it into lengths of 12 inches and use copper "T" junctions. Sweat (solder) them together using high temperature solder and a propane torch. If you've never done it before, read about how to sweat copper. Of utmost importance is to dry fit everything before you sweat your first joint. Also, keep everything very clean. Very Very clean. Use lots of flux. Otherwise, you'll have lots of leaks. Leaks are no fun.
The structure will look much like this. Note that there are gaps between the rows. You can take yet more copper and just solder them in between as shown on the right-most end of this example.
The gap filling ones don't need to be watertight. They're only there so that the light hits them and conducts the heat instead of passing through the gaps.
Step 9: Measure Performance
The whole project is now complete. You can now use the device for anything you please. You can measure the performance of the device by putting water into the heat-exchanger, mounting it in place, and measuring the temperature rise. How fast the temperature rises is an indication of the amount of energy being transferred per unit time. You can alternatively measure the amount of time it takes to boil a given amount of water or measure how much water boils per hour.
I finally abandoned this design and disassembled it mainly because I was not able to track the sun accurately enough for it to be practical. It sure was a lot of fun though.