I have been trying to figure out how to make a cheap solar thermal panel.  Like this instructable, I settled on plastic.  There is one BIG problem with making a solar thermal panel out of plastic. 


The problem, put simply, is that plastic solar thermal panels, if built like traditional solar thermal panels, cannot turn themselves off if they start to overheat. You cant just paint a plastic panel black and slap some glazing on it.  If you stop removing the heat from the panel, and the panel is built from plastic, it will soften or melt eventually. This was recognized by Rob in his instructable:

"because the whole collector is made of plastic, it is important that the temperature doesn’t get too high or it will soften and possibly spring a leak. 80 degrees C (176 degrees F) is about the limit. Don’t think it can get that hot? Think again. ...Therefore this may not be a practical design for residential installation"

The problem is figuring out how to make a solar panel turn off.  If you can figure this out, then solar thermal panels could be built of plastic and the cost reduced to a point where it is affordable to everybody. (i.e. $1000 per panel to $200) Why is this important? Because heating our homes (air and water) accounts for the majority of energy we consume in our homes.  If solar thermal panels were cheap, we could heat our house with them and save billions every year.

So this is where I am coming from.  I want to find a way to build a solar thermal panel from commonly available materials and have it compete with commercial systems.  The design I have come up with may be the answer, or it may not, but if the instructable community would work on this problem with me, the benfits to our society could be huge.

check out the blog I created for this idea for more info and for updates as I have them.

Blue Skies,


Step 1: Use Polypropylene, Not Polycarbonate

Until I have a chance to redo the figures (maybe in a week or so), to any users out there who wants to build this panel:

Don't use polycarbonate!

Turns out that this plastic degrades in long exposure to hot water.  Following up on some user comments, I checked out Coroplast, which is actually polypropylene plastic. long story short, coroplast is MUCH cheaper, MUCH more water-resistant (its used for plumbing) and easily recyclable.  Not even any question about what is the better option.

However, you will have to heat-weld the manifold because polypropylene does not glue easily. Read about it at Wikipedia.  Until I have the time to test this, and unless you want to give it a try, I recommend just thinking about this idea and coming up with more ways to make it better.  There is obviously a lot we can do!

HI, I'm from philippines and energy cost is quite expensive. It's good to know that this kind of technology is growing. However, here in my country doesn't seem too interested on this. I want to build or make one for my own. I need your ideas about this, and I reckon if you could help me on this one. http://build-home-solar-panel.com/<br><br>I saw this site and I need your inputs. I want to have at least one guide for now. Thanks in advance!<br>
In essence you need a door like object that is set to close when the internal temperature is too high. The door will shield the unit from sunlight thus allowing the unit to cool. I do warn people however that metal is to be trusted in building collectors. The flash point for wood could easily be exceeded in a solar collector and a fire on a roof is to be avoided at all costs.
Here's a simple solution: Black tubing can stand in the field in the hottest summers for 10 years. Polygal [http://www.google.com/search?ie=UTF-8&amp;q=polygal+images] is built as roofing material, is low cost, has a good insulation for the lower and higher layers, so that heat is not lost to the roof base or to the air above. Insert the tubing in the middle, and you have exactly what you wanted.
What material exactly are you thinking for the black tubing? It's important that it be able to withstand the high stagnation temperatures associated with insulation, and Polygal, being polycarbonate, cannot. Since the black tubing will absorb heat continuously, this idea runs the risk of deformation of the polycarbonate panels or the black tubing (i'm assuming polypropylene). The stagnation temperatures of insulated panels can reach 150C. The upper working temperature of polycarbonate (Polygal) is about 120C and the melting temp of polypropylene is about 150C. I think your solution would work in most cases, but its the rare stagnation events that will melt it. If you lowered the insulation it could work, but then you get low efficiencies in the winter. Since im trying to make solar panels for space heat, this is a problem.
In fact, there is an Israeli company that built for a large industrial project in Brazil, with EXACTLY Polygal. The hot water passes in standard (for Israel) pipes, and this has been field tested for over 25 years!! No risk of deformation whatsoever. (Polygal factory has roofs even older than that, never changed, and still with original form. The old clear ones get a bit &quot;yellowish&quot; over the years. The new ones are treated so that they should stay clear for 10 years and more. (Meanwhile 7 years strong). Tubing from Netafim - the guys that invented irrigation, because they didn't have enough pressure to work the sprinklers. Then discovered that it works much better... http://www.netafim.com/success-stories or Metzerplas from Kibbutz Metzer who copied the system: http://www.metzerplas.com/en-US/61/870/ This stuff is out in the Israeli sun (and freezing cold in winter in the mountain and north areas) in the fields on all parts of Israel. Lasts at least 15 years. The black pipes sometimes reach almost boiling heat, pressure is given off through the irrigation holes. So simply add a few pressure guages that automatically open if the pressure is too high. Also use connectors at the bends, so that they serve as a &quot;fuse&quot; or circuit breaker in case of failure. You can easily fix that by inserting the connector back in.
Thank you for the background. I have no doubt that polygal has been field tested, but I do question the conditions. If they have been used in thermal panels, then the panels were not well insulated. It is not simply a matter of opening valves to relieve pressure. Stagnation occurs when there is no water to pump and hence nothing to remove the heat out of the panels. The heat is &quot;stagnant&quot;. Other brands of polycarbonate panels would likely work just as well. Its not a matter of brand, its a matter of the physics of plastic and solar thermal panels. If you want high efficiency in the winter, you run the risk of high stagnation temps. If you are seeing solar panels built in this manner then they are likely low-efficiency (or non-functional) in winter. The industry is looking at a few ways to solve the overheating problem, including manufacturing special plastics, transparent plastics that turn opaque when the exceed a temperature, interesting designs using the refractive properties of water, advanced controls systems and vents, etc. These links may help. http://www.iea-shc.org/task39/index.html http://cat.inist.fr/?aModele=afficheN&amp;cpsidt=17036823 http://www.springerlink.com/content/t632m867672l8w46/ Again, thank you for the comments.
Its hard to read because instructables don't leave the spacing or formatting. [b] Let me try[/b] [br] or this <b>another try </b><br> I don't need you to show me the problem. Please understand that I <b>ACTUALLY MANUFACTURED</b> a system like this, after talking to the ENGINEERS at two companies, one of the Polygal (a two hour drive out to Kibbutz Ramat Hashofet). <br><br> So let me explain once again: The Polygal is only the structure and insulation. The black piping is what takes the heat, and YES it has worked without failure for over 10 years! I'm not sure what happens when there is no water in the pipe. Seems to me the air heats up, builds up pressure and moves out the pipe, thus cooling it down.<br><br> Your right its not a matter of brand, but that company treats its PC so that it is ready for roofing and roof conditions, and, as I stated, it worked already, and works for at least one company for many years. BTW take a look at this - no insulation at all... http://www.youtube.com/watch?v=sBAi_TVNaiM
The inside of the PC is presumably in contact with the heat being generated from the black hose, which is stagnate if the water is not taking the heat away. Since heat rises, your venting mechanism is problematic if the panels are on the roof. Your previous post states that the hose reached almost boiling. Is this with or without insulation? I am assuming its without, since its irrigation...correct? As you say, the system in the video link you sent does not have insulation. Such a system may work in mild climates (its basically a pool heater..read about it on wikipedia), but its not going to work in the winter in northern latitudes for space heating, and definitely not DHW. Too much of the heat will be lost back to the atmosphere. Unless 1-5% efficiency is ok, which it is not for me. I am aiming for a system with 50% or greater efficiency. You can read a little about it here: http://www.daviddarling.info/encyclopedia/S/AE_solar_collector.html You have not addressed the overheating problem. You seem to have it all figured out in your head. I would very much recommend you build your system and do two things. 1) measure the efficiency of the panels in the winter and 2) measure the stagnation temperature in the summer. If you really have solved the problem, there is a tremendous market for solar space heaters. Best of luck, and blue skies.
Yes. I misread. I posted a second reply sending us to continue from ur blog.
OK, sorry, I read the blog. Let's continue there. Your right about the winter (not much of a problem in my country, and the amount of insulation<br> <br> Just as a side, I was considering clear pipes and adding particles to the flow (there's a company that does exactly that - adding particles to (and removing them from) liquid flow for various industrial needs.
This looks really interesting - great instructable! have you experimented with the minimum angle the panels can be held at before the particles settle on the sides of the channels instead of sinking to the bottom?<br />
Yeah, 45 degrees is about the limit. Any lower and the particles will just stick to the backside of the panel rather then sink down.&nbsp; For my intended application, heating in the winter, this is ok. However for domestic hot water in equatorial latitudes, this clearly will not work.<br /> <br /> <br />
Oh that's not so bad; I live 54 degrees north :-)<br />
I'm not bound to build a panel using this method...<br /> So far it's been much easier using dorm room fridge radiators, and window glass.<br /> <br /> That being said, I do have an idea for your 'ible readers.<br /> I've recently built a CNC Guinea Pig cage.<br /> I used a white coroplast. however the sign shop I bout it from also carries the same material in clear.<br /> Advantages are, CHEAP... I think I picked up a 4x8 sheet for $25(though online can be much cheaper if you can talk your way out of shipping). Temp resistant. Well, it'll withstand 60PSI at 200F! So as long as you're using water to move the heat, all should be well.(melting is somewhere over 350F)<br /> <br /> <a href="http://corrugated.ws/StormPanels.html" rel="nofollow">This Place</a> has some 'good stuff' rated for hurricane country. they even sell end caps, premade!<br />
that's a very good point.&nbsp; Do you know if coroplast can withstand long term exposure to UV?&nbsp; Anyway, definitely need to look into this.<br /> <br /> The reason i am attempting this is because I want to find a way to make a cheap solar panel that can be manufactured.&nbsp; Smart tinkerers like yourself will always be ahead of the curve.&nbsp; Its all the other people out there who cant do that, don't have the time, don't think they can, or just don't want to... but would go solar if it was cheap and easy.<br />
I&nbsp;don't have any long-term data on hand... but in the 3-4 year range, yes. ANY coroplast should resist UV just fine. Year old political signs still cut/weld/work like new stock...and that's with Chicago smog and salt!<br /> Long term, it MAY develop brittleness... not an issue if it's overglazed, like in your design.<br /> <br /> If it's a big concern, one of the optional coatings is UV resistance. don't remember which site, or which product, but the number was 98% light transmission, with &quot;High&quot; uva/uvb resistance. <br /> <br /> Just on a side note... Sonic welding is expensive, but best.<br /> Thermal welding works well too though.<br /> I&nbsp;mention this, cause you can cut a few inches on the bottom of the panel in half, Remove the top portion, sand the bottom smooth, form, and weld.<br /> If you're willing to sacrifice the outer most 2 channels... you can make your entire panel from just the one sheet! and if you do the thermal/sonic welding well, it'll never come apart!<br /> If that doesn't make enough sense... I could draw up a cut /weld diagram... but you probably got what I meant :-)<br />
that seems like a great idea.&nbsp; How could I get that wire mesh in there?&nbsp; <br /> <br /> as a side note a potentially better route could be to eliminate the wire mesh altogether.&nbsp; It would be best if a separate box housed a filter, say right at the input and output ports.&nbsp; The problem here is insuring that the particles mix.&nbsp; The reason it works with the channels is because the flow pushes the particle to the top of the channel, causing an increase in resistance and decreased flow.&nbsp; The flow is then diverted to another channel, increasing the flow and raising the particles.&nbsp; meanwhile the previous channel has low flow, so the particle start to fall...&nbsp; The whole thing make the particles spread out.<br /> <br /> The best solution I think is a design that lets the particles spread out while eliminating the problems with the manifold (via your technique, for example) and eliminating the wire mesh.<br /> <br /> <br />
&quot;How could I get that wire mesh in there? &quot;<br /> <br /> That is an issue I hadn't thought of...<br /> My solar heaters don't use an in-water heat absorber like yours does.<br /> <br /> I think it may be possible to change the U shape a little and still imbed it.<br /> <br /> <br />
this is a great idea.&nbsp; <em>anything</em> that can be done to simplify the manifold reduces the cost and time of the design.&nbsp; I guess the plus side is one less weld and the minus side is you have to sand off the cross-ribbing.&nbsp; <br /> <br /> Actually, this is making me think that the thickness of the plastic I used in the manifold was super over-kill.&nbsp; 1/32 would probably do.&nbsp; And that is already flexible.&nbsp; I could bend it around and thermal/sonic weld it, then use polyester resin to fill the sides (while also puting an input and output port in).<br /> <br /> You seem to have some experience with sonic/thermal welding.&nbsp; Any advice/pointers/urls I could check out?&nbsp; I have set aside some money to develop this idea (not a lot, but certainly enough to invest in some reasonably-priced tools)<br />
I wouldn't use &quot;experienced&quot; to describe what I've done. :-)<br /> <br /> My experience to date is NOT&nbsp;with waterproof welding. Just structural.<br /> That being said, my work hasn't let the dust from pet bedding through, and I didn's SEE any leaks last time I washed the pan... So it MIGHT be waterproof. Just don't know.<br /> <br /> my <a href="http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=96712" rel="nofollow">&quot;plastic welding torch&quot;</a>. Was the cheapest piece of junk I could find that would still do the job, with my skills. If you have the cash, go for <a href="http://www.urethanesupply.com/5600ht.php" rel="nofollow">one of these</a>.<br /> <br /> If you're budget is REALLY low, <a href="http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41592" rel="nofollow">this</a> was the cheapest one I've ever seen. If you're willing to practice and looks aren't too important, an old junky solder iron should do a passable job, as long as it has temperature control. The cheapest of the cheap iron-style plastic welders are set up like that. So much so, in fact, that most of them look like the seam irons that Hobby Lobby sells, for seamstresses.<br /> <br /> <br />
so i just looked into it a bit.&nbsp; Turns out coroplast is polypropylene, and it is by far and away a better plastic then polycarbonate for this use.&nbsp; Its also dirt cheap. Thanks for the heads up.&nbsp; I added a little warning to the instructable in step two to this effect.&nbsp; <br /> <br /> thanks!<br />
<span class="long_text" id="result_box"><span title="Perdona que haga preguntas que tal vez ya est&aacute;n respondidas, el caso es que no hablo ingl&eacute;s sino espa&ntilde;ol, y me cuesta leer y entender todos los comentarios.">Sorry to ask questions that perhaps are already answered, the fact is I don't speak English but Spanish, and I can hardly read and understand all the comments. <br /> <br /> </span><span>I have two questions: <br /> <br /> 1) Why you use silicon carbonate, not just coal particles, which is much cheaper? <br /> </span><span>2) Do you think is enough to stop the flow of water to stop heating?<br /> <br /> Finally, EXCELLENT&nbsp;INSTRUCTABLE!!!<br /> </span></span>
Saludos. <br /> <br /> En respuesta a sus preguntas:<br /> <br /> 1- Silicon carbonate fue la unica particula negra que pude encontrar de 60um y en pequena cantidad. Cualquier particula que no se disuelva en el liquido y que a la vez sea negra y suficientemente pequena puede servir, por ejemplo el 'coal slag'.<br /> <br /> 2- Si-- toma un minuto para que bajen las particulas, y despues de eso el liquido deja de absober luz (calor).<br />
Gracias por tu amable respuesta. Como suele ocurrir, una respuesta ocasiona otra pregunta: &iquest;Por qu&eacute; 60um? &iquest;No podr&iacute;an ser de 1mm o 2, o de cualquier otra medida?<br /> <br /> For the Instructables<span class="short_text" id="result_box"><span style="background-color: rgb(235,239,249);" title="angl&oacute;fono"> Anglophone community: </span></span><span class="medium_text" id="result_box"><span style="background-color: rgb(255,255,255);" title="Gracias por tu amable respuesta.">Thanks for your kind reply. </span><span style="background-color: rgb(255,255,255);">As often happens, an answer causes another question: Why 60um? </span><span style="background-color: rgb(255,255,255);">Could not be 1mm or 2, or </span></span>whatever else measure<span class="medium_text" id="result_box"><span style="background-color: rgb(255,255,255);">?</span></span><br />
excellent question!<br /> <br /> as the particles get smaller, the force of the water relative to the force of gravity gets bigger.&nbsp; This means that if you use bigger particles you will have to increase the flow rate to keep the particles up.&nbsp; If you have a small circulation pump, then you will need smaller particles.&nbsp; If you have a very large pump or lots of water pressure, then you can use bigger particles.<br /> <br /> let me know if you need a translation and I can ask my mother.&nbsp; I was born in Mexico but move to the U.S when I was a baby.&nbsp; My whole family speaks Spanish but not me. ;(
<span class="short_text" id="result_box"><span style="background-color: rgb(255,255,255);" title="Gracias por la explicaci&oacute;n, suena muy convincente">Thanks for the explanation, it sounds very convincing. <br /> <br /> I can read English almost perfectly, aided with Google translator, if the text is wrote in English as yours. I say this because there are others that are wrote in slang.<br /> </span></span>
&nbsp;Awesome instructable, nicely done. &nbsp;Appears to work pretty well. &nbsp;Do you have any data on it yet as to how many BTU's you can get out of it for given light levels? &nbsp;There is an ible on here somewhere on a light meter, you can get the rest by taking readings of input water temp, output water temp and light levels and flow, or you can take light level readings and set it up with an insulated reservoir and a known volume of water and see how many degrees it is raised over the time of your readings. &nbsp;From either of those you can get us some good efficiency data. &nbsp;Great job.<br /> <br /> Fuzzy<br />
I did take some measurements, but they come with warnings.&nbsp; I measured the input and output temp differential (with my multimeter thermocouple) and also the output flow rate (dumped the output of the panel into a container for a minute and then measured how much was there), and using the heat capacity of the water...&nbsp; I&nbsp;came out with 750-950W/M^2 power.&nbsp; I even got one measurement above 1kW (but threw it out).&nbsp; Before you laugh out loud...<br /> <br /> This was in novemeber and the water was coming from my well.&nbsp; It was not terribly cold outside, so the ambient was warmer then the water temp.&nbsp; Also, its possible that the water temperature from the well fluctuated during the measurement.&nbsp; <br /> <br /> So, bottom line is I need to run some closed-loop tests with a circulation pump.<br /> Until I do that (or somebody else out there builds one and does it) I would venture to say that its going to turn out well.&nbsp; The &quot;absorber&quot; plate is more like an &quot;absorber engine&quot;, since the particles form a convention current with turbulent flow bringing them into the sun and back into the liquid in a cycle.&nbsp; Its a total-new type of absorber with totally new physics.&nbsp; All aspects of the design can be controlled. For example, by using larger particles you can increase the flow rate because the particles sink faster which would probably increase the efficiency. &nbsp; <br /> <br /> <br />
Surely step 14 should read &quot;When you turn off the water, the particles should settle and the panel should go CLEAR&quot;. That's how the panel&quot;turns off&quot;.<br /> <br /> Great idea.<br /> <br /> Cheers<br /> <br /> Mark<br /> <br /> <br />
nice.&nbsp; Well, what I am showing and what I am thinking are two different things.&nbsp; ;)<br /> <br /> What I am thinking is that in a production panel, the whole thing would be insulated.&nbsp; The back would have foam and the front a panel of polycarbonate.&nbsp; I would put some reflective foil back there to reflect the light out of the panel.<br />
&nbsp;If your setup is enclosed in an insulated box with a clear front, a portion of the light entering the collector will be reduced in frequency to the infra-red=(heat). &nbsp;Glazing tends to reflect ifra-red to some extent while it passes visible light. &nbsp;It may be&nbsp;necessary&nbsp;to open a sort of vent should the collector box overheat. Once possibility that comes to mind for an inexpensive actuator which would be simple, cheap and with few moving parts would be one using low temp muscle wire activated by the heat of the collector instead of an electrical current. &nbsp;LT or (low temp) muscle wire actuates at about 70 degrees C or around 160 degrees farenheit which would be about ideal to open a vent to allow the panel to cool. &nbsp;By placing the actuator lower within the panel one could (fine tune) when it actuates since the upper portion of the panel would be hotter than the lower portion. &nbsp;Just a thought and my two cents worth.<br /> <br />
I definitely see your point. My hope is that by turning the water off and having the particles fall, a reflective mirror (mylar or some such material) could be used to reflect the light out, and that would prove sufficient. &nbsp; This remains to be seen, however, and your idea could prove just the added element of protection against overheating.&nbsp; Indeed, 160F is just about perfect.&nbsp; Very cool.<br /> <br /> <a href="http://cat.inist.fr/?aModele=afficheN&amp;cpsidt=17036823" rel="nofollow">this paper</a> is interesting, and apparently looked into using vents to cool a plastic plate absorber, to no avail.&nbsp; The combination of a vent with an absorber plate that can turn off should put a nail in the coffin of the overheating problem.<br /> <br /> thanks fuzvulf! (and great username)<br />
Silicon <em>carbide</em> - novel use for it I guess?<br /> <br /> L<br />
I think so!&nbsp; It was the only thing I could find that was pre-sifted and black and available for less then 2000lbs.;)<br /> <br /> In my searching, it looks like coal slag (byproduct of coal fired metal plants) would work better (certainly cheaper and in abundant supply), but it needs to be sifted.<br />
Like this stuff?<br /> <a href="http://www.usmix.com/black_beauty_coal_slag_idf.phtml" rel="nofollow">www.usmix.com/black_beauty_coal_slag_idf.phtml</a><br /> <br /> L<br />
exactly. Now if I could get that pre-sifted to 60 microns (or smaller), that would be great!&nbsp; There was a very interesting comment by a <a href="http://www.blog.thesietch.org/2006/06/22/new-solar-thermal-panel-project/#comments" rel="nofollow">guy in another forum </a>regarding the particles needing to have a selective absorption, with low emissivity in the infrared.&nbsp; Scroll all the way to the bottom of the comments.<br /> <br /> <br />
60um - that's <em>dust</em>. Powdered charcoal?<br /> <br /> L<br />
its very very fine.&nbsp; think of the grit that is used on fine sandpaper.&nbsp; This is the raw material.&nbsp; The metal mesh is even smaller.&nbsp; I had no idea they made metal mesh so fine (at such a reasonable cost).&nbsp; Actually, as soon as you get smaller then 40um the mesh starts to get really expensive.<br />
I've used test sieves down to 20um - with <em>polystyrene</em> - no joke, that was a drag...<br /> <br /> L<br />
To make it turn off, could you use a low voltage sprinkler valve for the flow, and a temperature sensor attached to some point on the panel?<br /> <br /> Perhaps a simple thermistor circuit or even a slightly more complex sensor and IC circuit could be used?<br /> <br /> Not entirely sure if that is what you mean by &quot;turn off&quot; but it is just an idea.<br /> <br /> <br /> So now that you have hot water running to your house, how do you convert that to space heating on the cheap?<br /> <br />
The idea is just that you could use a temperature sensor to detect how hot the water is.&nbsp; If it get too hot, you stop the pumps.&nbsp; (basically just what you said).&nbsp; Also, when the panels are not being used (the pumps are off), the panels go reflective and they do not over heat (which could causes the plastic to melt).<br /> <br /> I plan on tapping into my existing radient heat system!&nbsp; Go figure...i circuilate hot water through my floor right now anyway, only it powered with a gas furnace.<br /> <br /> Alternately, you could just get a barrel of water and put it in a room.&nbsp; It would radiate that heat into the room.&nbsp; It would be ugly though.&nbsp; Maybe a &quot;heat mass sculpture&quot; is more like it.<br />
Cool Idea!&nbsp; <br /> I wonder, when you turn it off, and the particles of silicone carbide settle to the bottom, this is what prevents overheating right?&nbsp; <br /> Might there not be a problem with localised heating, just in that bottom section?&nbsp; <br /> I suppose you could cover that small section with something so that it is in the shade... <br /> <br /> Thanks for this - very interesting. <br />
Exactly.&nbsp; When the particles settle the panel stops absorbing heat.&nbsp; In a product design Iwould probably shade the bottom part like you said.<br />

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