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.
<p>thank you so much, I love this project</p>
I love the mechanism for holding the corners of the mirrors. Sheer genius. <br> <br>I was thinking of making something similar, but using the long full length mirrors that you can get for mounting on doors. I have seen them for around $8 for a 5' long one. The advantage of using the long mirrors would be that fewer adjustment points would be required, and that the curvature would let you do a tighter focus, at least in one dimension.
<p>Using ordinary mirrors has the problem of excess weight. Aluminium sheets are light and cheap (you can even use the inner surface of coke cans) which is best for the tracking system. Using long reflectors is suitable for cylinder-parabolic concentrator. </p>
<p>I think it is too big for a sun tracking system. There's no need for such a complex structure and focus doesn't need to be that far. Just adjust the reflectors to converge closer. https://www.instructables.com/id/How-to-build-a-strikeheliostatstrike-paraboli/</p>
I really like this solar boiler project. I especially like your method of adjusting the mirrors to focus them. I've wanted to build one of these for a long time now. Unfortunately I live in an apartment and don't have anywhere to build it. I was wondering what your cost was for the aluminum angle pieces. Although it might make it slightly more difficult in drilling the holes, I was wondering if steel conduit might work better for providing strength and be cheaper as well (as little as $1.96 per 10ft). Also painting the heat exchanger flat black would help it's efficiency.
I'm most gratified that you liked the project. Thank you. It was a most interesting and rewarding project. I don't have much room myself. That's why I built it so that it can be disassembled and stored when not in use. My wife is particularly happy that it's not taking up permanent room in the back yard. I don't recall exactly how much the aluminum angle braces cost. The most significant cost was the chrome plated mirror pieces. All in all, I believe I recall that I spent some $300 on the project. More accurately than that, I simply don't recall and didn't keep very good records. As far as painting the heat exchanger, I recall doing some experiments with that. It is a common misconception that simply painting something like this black will make it more efficient. It does indeed help for low temperature devices but works not at all for higher temperatures. It turns out that ordinary paint from the hardware store will burn readily under the power of a solar concentrator. Basically, it just burns up in the first 10 minutes or so leaving the copper bare again anyway. It seems that a better way to improve efficiency is to add an insulator to the back side. Something like fiberglass works well. Of course, the paper backing of the fiberglass should be removed to avoid fire. What you're aiming for is not allowing the excess heat to radiate from the back side.
One of the best <em>selective absorbers</em> is oxidized nickel. &nbsp;Since stainless steel contains a good bit of nickel, it turns out that it does a fine job itself. &nbsp;You simply make the absorber out of stainless steel, heat it until it turns a dark purple, and you are done: a selective absorber that has an&nbsp;absorption/emission ratio of about 11 - excellent! &nbsp;And, of course, it does not blister or peel. &nbsp;Bare copper has an&nbsp;absorption/emission&nbsp;ratio of 3. &nbsp;There are oxidizing chemical treatments that will turn copper black - and they are not paints. &nbsp;They do not have good abrasion resistance, but that is not an issue here, yes?<br /> <br /> If the absorber surface temperature stays low - like at or below the boiling point of water - it does not need to be a selective absorber as boiling hot objects do not radiate much infrared energy. &nbsp;On the other hand, selective absorbers do not <em>absorb</em> infrared energy as well as non-selective absorbers do. &nbsp;Since sunlight at the surface of the earth has about 40%-45% of the thermal energy in the infrared range, you typically <em>want</em> a normal absorber like carbon black.<br /> <br /> If you are dealing with temperatures close to the boiling point of water you have many choices for paint, but if you need really high temperature paint, try Pyromark High Temperature Paint. It is available in 800, 1200 and 2000 degree F ratings.<br /> <br /> One of the inefficiencies of heat transfer is the temperature drop across the thickness of the metal barrier; another is mixing the fluid enough to bring all of it in contact with the hot media. &nbsp;I feel that the best, most efficient way to solar heat water, especially with concentrator mirrors, is to <strong>place the absorber </strong><strong>in the water.</strong>&nbsp;&nbsp;That is, pump the water in transparent tubes into the absorber surface at the focal point. &nbsp;One excellent media is an open-cell black ceramic sponge (looks like an aquarium aerator stone); another is to put a very concentrated (stable) black dye in the water. &nbsp;(maybe use both techniques?)<br /> <br />
Great point about placing the absorber in the water. That is definitely the way to go. <br><br>I would add to the absorber discussion that the emissivity values are for thermal radiation and emissivity varies with both frequency and temperature. For solar power from most reflective concentrators, the majority of radiation is visible, not thermal. So glass is a great transmitter of visible radiation but very poor for thermal radiation. A glass mirror that looks shiny bright in the visible range looks almost completely black when viewed (with a thermal imager) in the thermal range. For concentrators that use glass mirror reflectors, nearly 99% of the reflected power is in the visible frequency range because the mirrors absorb most of the thermal radiation. However, this design uses metal reflectors and these are efficient in both visible and thermal bands. Ideally, the translucent material should transmit both visible and thermal frequencies. In addition, the receiving face of the translucent material should be flat and normal to the incoming beam. One possible solution would be to use a multichannel polycarbonate panel used for greenhouse walls. The polycarbonate has excellent visible and thermal transmission. Its important to get the window-grade polycarbonate.
The material they use in big sized solar concentrators is carbon. although will probably not be able to get your hands on big carbon tubes etc, you could try coating the copper in carbon by putting the copper tubes under and oxygen deficient flame. I have seen that acetylene produces a lot of black deposit on things when no oxygen is mixed with it, maybe you could just use a candle to coat it? The only thing i suspect may happen with this is that once the carbon gets hot enough, it will combine with the oxygen in the surrounding air and simply float of in the form of co2. still, worth a try?
Another focus method :Cover all facets with pieces of paper. Suspend a fixed target at the intended focus point. Uncover one facet at a time and adjust until reflected light is cast onto target. when finished, turn assembly away from sun and remove all the paper sheets. No math, no spreadsheet and very accurate. Not my idea (found it on the web) , but simple and effective.
I think that you would have to be very quick to set each panel and maybe reference to the first panels focus (by moving your hole frame) every five or so minutes to allow for the suns movement in the sky @ 15 degrees per hour. If you were taking 3 hours to set up, the last panel would have a very different focal point.
1 Select one panel as a reference panel (ideally one centered on the desired focus point with the selected light source.) Use a large flat target to see the beam cast by the reference panel.<br>2. Cover up all the reflectors EXCEPT the reference panel and the adjustment panel.<br>3. Adjust the screws on the adjustment panel so that the light of its beam center coincides with the beam center cast by the reference panel.<br>4. Once both beams are aligned, cover the panel adjusted and move on to the next panel, removing its cover.<br>5. Repeat steps 2 to 4 until all panels are aligned.<br>Reasoning: since the light of the reference panel moves as the sun tracks across the sky, adjusting all the other panels to it create a focus that is referenced to both reference panel focus and sun angle. Since the reference panel points to target, the entire frame can be moved to track sun. All other panels will always point to reference panel beam focus. Use of reference panel allows alignment procedure to no longer require an adjustment for sun movement.<br>
The focal point is set by the parabola formula. It does not have to be pointed at the sun for an accurate focus to be achieved. The system was designed to be mounted on a tracking device that moves the entire assembly to track the sun. I agree that if I was aligning the focus by moving the mirrors and aligning them to the sun, that would be highly impractical. This is why I decided to calculate the height of each corner based on a parabola and aligned it in my workshop in the shade. Only when it was complete, did I take it into the sunlight to test the quality of the focus and make minor adjustments.
hey guys am eston,currently a final year engineering student in jomo kenyatta university in kenya.I wish to develop a project on a solar powered steam soil sterilizer to assist in control of micro-organisms in seedbeds.However am not sure whether to use parabolic dish concentrators or trough concentrators.i wanted to try the latter but am not sure how to go about it and what materials 4 the troughs would be best suitable.any ideas pls? your assistance will be greatly appreciated.u can reach me on stnmacharia@gmail.com
I did something by accident one time that you could probably use .<br>I laid a sheet of black rubber roofing membrane that was about 1.15mm or .045inches thick on a bright sunny day .<br>It got soooo hot beneath the rubber sheet that it literally killed everything that was growing in the soil at that time even the weeds and thistles.<br>I had to reseed my grass lawn to get it green again .<br>Depending on how long it is left on the soil and how hot it got ,it might make a very worthwhile weed control system.<br>I should do another experiment like this again and this time install some temperature probes at different depths in the soil beneath the black rubber sheet like right on the surface and then every 1mm or so and record the results.<br>I believe to be effective ,the heat needs to get down to about 3 inches or 7-8mm in depth to kill all spores ,roots ,nodes etc.<br>Again you would have to rely on the sun for the heat but it would be very clean as there would not be any chemicals involved .<br>The rubber sheet could be moved in the mornings or evenings to a new location once it cools enough to hold onto .<br>
That is a pretty good idea. I have weeds etc. but I'm just not into poisoning my property. Just seems wrong. This sounds like a good alternative.
Nice idea to use solar concentrators for soil sterilization. Did you proceed with the project. That said, with the solar potential in Kenya, Just using plastic for a month or two on a seed bed would probably be better. Building a solar system to create steam to use on a seed bed only every so often would be expensive.
Sorry Jarney1 I was directing my thoughts to Burnerjack01 and his method, I forgot to address it properly. I fully agree with the 'measure first' then if thats not quite good enough correct it practically way.
You could place the concentrator on a flat surface (like a garage floor) and hang a laser pointer on a plumb-line from the ceiling. Then you could adjust the mirrors by sliding the concentrator around. The direction of the incoming laser beam would remain constant so the focus would be accurate, no matter how long it took to set it up. This way you could also do it when it's cloudy or dark.
Ha , <br>I had the same idea using the garage floor to level the mirrored reflector so it is pointing exactly strait up.<br>I wanted to use pulleys , small steel cables and electrical metallic conduit for the frame.<br>Think etcha-sketch toy for kids.<br>I could then move the laser pointer around via 2 knobs in an Y and X coordinate type of arrangement.<br>You could then aim each of the small mirrors individually on the face of the dish at the exact focus point.<br>I even bought one of those party fog machine kits which will help find the laser light beam when I can't see it.<br>I have a project in the works that uses 7 of the old C-band satellite TVRO dishes.<br>I want to recycle the dishes and heat my house with the collected heat .<br>4 of them are 8 feet in diameter and 3 of them are 10 feet in diameter.<br>That is why I need a more rigid system to aim them with before I install them.<br>The nice thing is that I can do this inside the garage where the weather or sun is not a factor.<br> <br>
With dishes that big, wind loading should be figured into your structure.<br>That's a lot of power at your command. Remember to use you power for good.<br>Hey Mech, let me(us) know how it woks out, OK?<br>
I installed the basic Passive Solar Heater on the south-east side of my house (glass in front, black metal in back, vents in top &amp; bottom) (completely south facing wall not available)<br> I then realized the wall I installed it on faces more east than I initially thought and is not heating enough to be of use. Could I just put 2 long mirrors on the north side of the heater to curve around and reflect the light in? How do I figure out what angles to put them? I appreciate your help.
didn't they have an image of this in popsci once?
Yes, sort of. My father and I worked on a design project for the fellow from Vashon Island, WA who built a parabolic dish solar furnace about 24' in diameter. I think this is the one you might be thinking of on the cover of Popular Science magazine. Unfortunately, the fellow was very difficult to work with and in my opinion had just two good ideas to his credit; Idea #1 - thin mirrors are actually flexible enough that you can epoxy to a spot on the back of them and pull the mirror section into a slight compound curve. Try it yourself, it does work. Think how flexible an inexpensive glass mirror designed to hang on the back of a door is, glass actually flexes reasonably well. Once you have a way to curve an inexpensive mirror in two axis then dish concentrator arrays are quite a lot easier. The multi-mirror array structure is still a difficult manufacturing/engineering challenge as the good designer of this "Multifacet parabolic solar concentrator" instructable has mentioned already. Idea #2 - His deep dish focal point (below the dish rim - more on why this was necessary later) heated the outside of a spiral wound tube structure (if I remember it correctly?) which contained water flowing in good contact with the tubing walls due to centrifugal force. As the water flowed in a tight series of tube circles, it presses against the outside wall of the tubes. Think of a coil of garden hose. As the water is pushed along inside it presses against the outer portion of the inside wall of the hose due to centrifugal force. That's it. Although he liked to think his other great idea was to use a geodesic dome for the solar concentrator dish backstructure, that is why he needed to keep the focal point below the dish rim horizon. A geodesic dome must have it's flat side either tied firmly to the ground or use some kind of tensile/compression structure in that plane to hold the bottom ring of the geodesic in a rigid geometry. He never got it threw his head that this geodesic structure was completely mis-applied to the task he needed. It was overly complex and a real pain. My father and I designed a much better structure, but he wasn't interested. We had done work for Boeing Solar Energy to build doubly curved, high accuracy sheet metal mirror substrates for one of their projects using our special metal forming process. Later we developed a proprietary inexpensive process to manufacture sheet steel or aluminum compound curved dish gore sections for 12' and 4m diameter microwave antennas complete with backstructures. Our process had a "floor to floor" time of 70 seconds per concentrating dish gore section including forming the side flanges. With 12 sections per dish in about 15 minutes we were producing the entire skin surface for each microwave dish. Our favorite material was 22 gauge galv-annealed mild steel, but 0.050" thick aluminum was great too. We produced (700)+ 12' diameter microwave antennas for two companies in California, until the bottom fell out of the large dish antenna market in the mid-1980's. My dad developed a very special type of metal forming technology called "axial load bulge forming" and started an aerospace company (Exotic Metals Forming Inc.) here in my home town - Seattle. Parabolic concentrating dishes are actually inexpensive with the right manufacturing technology, though R&D companies working on $multi-million dollar DOE contracts have a vested interest in convincing the government and the rest of us that they are...really expensive instead. Polar mounts require only one actuator and track nicely too. I must agree with jarney1 though; parabolic troughs are even better for solar concentrating systems from a cost to heat output basis. In my opinion Ausra Inc. with their multi-trough segment (linear fresnel lenses?) concept is going to do very well in the parabolic trough solar energy marketplace, though I think I have a design which might give them some competition....we will see.
I am looking for designers for making all parts of a Solar Heliostat in Parabolic and flat Mirror shape. Please Email me suresh.singlewatts@gmail.com
dear god, that must be the longest reply i have ever seen. Thanks for all the info.
You are welcome. Sometimes I'm a bit too chatty, however my long reply was admittedly a cheap way to provide some solar design info to this good online community here without doing the work of writing a real instructable. I think many would agree, jarney1, the author of "Multifacet Parabolic Solar Concentrator" has done some good work. I hope more people get a chance to see it.
Thank you for your kind words. I did work hard on the design. The construction was simple and can be carried out in any manufacturing facility with off-the-shelf components and requires very little time or skill once designed. I moved on to the question of energy transduction once I established that the concentrator array could be built simply and cheaply. The two most difficult components to build simply and cheaply are the tracking frame and the engine. I've been experimenting with thermoacoustics and the so-called "lamina flow" or "thermal lag" engine because of its potential to reach the Stirling cycle for efficiency and the simplicity of the design and construction. Unfortunately, my efforts there have been less than spectacular and so I have not posted anything about them specifically.
Hello All I'm not sure what is ment by &quot;solder&quot; these joints. If this &quot;boiler&quot; gets to the temp. as implied, I'm not sure this will whole together. Could blow apart. Instead of solder I would suggest using &quot;silfas&quot; ( not sure of spelling). This is what A/C repair men use to connect copper tubing in A/C work. The pipe might burst, but the joint want.. Just a though. As for as an engine goes: use a 5hp. B&amp;S eng. Remove the head- discard. Make another head from a block of alum. Bore a hole paralell with the crankshaft, make a rotary valves &amp; turn it 1 to 1 with crankshaft. You can find rotary valve set up on the net. This enging will have untold HP. according to steam pressure your using.. catch you later: STC
I wounder how to avoid letting the shade from the heat exchanger block the sun shinning on the reflectors. i.e. the focal point needs to be located where it will not obstruct the sun rays arriving on to any part of the reflector(s).<br />
&nbsp;It really is not necessary to build parabolic dish reflectors that are fully symmetric. &nbsp;For example, the new&nbsp;satellite receivers are not fully symmetric, so the single arm and receiver do not cast a &quot;shadow&quot; on the dish. &nbsp;Speaking of shadows, that is how you pick where to omit the mirrors. &nbsp;
I realize that your solar concentrator has already been taken down, but, if you ever build another one, I have a computer program at the link below which should help with the tracking. (It's free and open source by the way)<br/><a rel="nofollow" href="http://www.cerebralmeltdown.com/projects/suntrackprogram/default.htm">http://www.cerebralmeltdown.com/projects/suntrackprogram/default.htm</a><br/><br/>It works by first calculating the position of the sun and then sending signals out through the parallel port for controlling stepper motors.<br/><br/>I wrote the program for controlling my <a rel="nofollow" href="http://www.cerebralmeltdown.com/heliostatprojects/">heliostats</a>, so there are some features that you won't need. Otherwise though, it should work fine for something like this. Just choose &quot;Point at Sun,&quot; and the program will track the sun instead of reflecting it to a single target like with a heliostat.<br/><br/>I added the sun tracking feature to this program because I noticed that there are a lot of people who were building solar concentrators and getting stuck on the tracking. <br/><br/>I would really like to build one of these myself someday, but I don't have the time right now. Hopefully, my program will be of use to somebody so that they can more easily build their own. That way, I can live precariously through them. :)<br/><br/>Awesome instructable by the way. I'll have to keep that chrome-plated aluminum sheeting in mind for my future projects.<br/>
Oh, and I forgot to say that if anybody has any trouble with the program, let me know and I'll see if I can fix it. It is still beta after all.
I once mounted about 1200 1" mirrors like this. It was a pain to adjust but when it was all aimed it would melt an aluminum can in about 15 seconds. Very impressive.
when i don't boil water for $300 i flash airplanes.
Wouldn't it be easier to find an old satelite dish and use that for your parabola?
parabolic, not parabola. Parabolas are used in Algebra =D<br/>
get a large water tank and heat your tap water with this thing! ( I'm not as creative as you guys)
there is a solar tracking system that uses some inert gas.
instead of a boiler, try attaching an alpha or beta Stirling engine
I was gonna say the same thing, until I saw you comment. Much more efficient than boilers.
you can use a clock mechanism to track the sun cheaply. This works because of the relationship between time and space (minutes and seconds of latitude and longitude!) The mechanism works like a cuckoo clock, with a large weight that is used as the force to make the clock tick. At the end of the day the dish is simply pushed back across and this raises the weight and therefore &quot;winds&quot; the clock ready for the next day :)<br/><br/>The other pretty cool thing about the dish in question is that it is shaped to only use part of the parabolic shape, This means that although the dish rotates to track the sun, the focus point stays static, therefore there is no need to have an arm with a collector!!<br/><br/>I have seen the dish on a documentary, and it is being manufactured with simple tools at low costin 3rd world countries. Cant find the original site, but this one <a rel="nofollow" href="http://www.inforse.org/asia/M_III_solarcook.htm">http://www.inforse.org/asia/M_III_solarcook.htm</a> has the dish, with (small) diagrams of the dish focus point stuff. Scroll down a bit :)<br/>
Sorry, me again!<br/> <a href="https://www.instructables.com/id/Tracking-parabolic-Barbecue-the-wave-of-the-futur">https://www.instructables.com/id/Tracking-parabolic-Barbecue-the-wave-of-the-futur</a><br/>is a video of mine. I bring it up because it has a different way of attaching the array to the equatorial mount than the way it is done in community solar cookers.<br/>In community solar cookers the dish gets deformed through the year to keep it correctly aligned.<br/>(I believe the way I suggest is much simpler) <br/>Perhaps you could adapt your array of mirrors to work with this method? Brian<br/>
Thank you for the link. You have your dish attached to an equatorial mount (an axis pointing at the north star in northern hemisphere) and point it at the sun in the morning. As long as it turns on the axis at 15 degrees per hour, it stays pointed at the sun all day! (Same speed as the hour hand on a 24 hr clock). If you have the focal point also on that axis, the focal point never moves all day and that is super good because you can keep your food or energy absorber detached from the dish. (seasons are the reason you should not use a full dish to track the sun). It is easier to adjust a part of a dish and maintain a focus at one point.
try <a rel="nofollow" href="https://www.instructables.com/id/Robotic-Solar-Powered-Sunflower/">this</a><br/>
I think this idea has tons of merit! What about using a giant fresnel lens pointing down on top of a steam system. Seems like a lightweight lens on top and the heavy steam components on the bottom would be easier to move in a tracking system.
This is essentially a giant Fresnel reflector.
I can not get metal in 6 inch increments. I can get it with 20cm (1.2 x 2.4 m) (48) I was thinking about using a empty spot in the centre. Would x and y thus be 70,50,30,10,10,30,50,70 ?
The actual size does not matter. With 20cm squares, you would have each (x,y) as:<br/><br/>(70,70) (70,50) (70,30) (70,10) (70, -10) (70,...<br/>(50,70) (50, 50) (50, 30), (50, 10), (70, -10), ...<br/>(30, 70) (30, 50) (30, 30), (30, 10), (30, -10), ...<br/>(10, 70) (10, 50) (10, 30), (10, 10), (10, -10), ...<br/>...<br/>and so on with each pair of numbers corresponding to the x and y of the corner of a mirror.<br/><br/>Then for each of these corners, calculate h=sqrt(x<sup>2</sup>+y<sup>2</sup>)<br/><br/>and z=h<sup>2</sup>/4f where f is your focal length in centimeters.<br/><br/>Here, each z will be the distance out from the back plane of each of the corners of the mirror.<br/><br/>I hope that's clear enough.<br/>
Go to your library and research Mother Earth mag. July/August 1978 for "Low Cost Solar Furnace" Uses 12 inch square mirrors - same principle as yours - excellent! Shows steam recepticle and hook-up. Next, check the January/February 1979 issue for the "Solar Tracking System" for the Solar Furnace. Really liked your instructable. Ken

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Bio: I like to build stuff and write code. I'm pretty tired of code though, and want to do more building.
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