Build your own flat panel solar thermal collector

I've seen a few different designs for solar water heaters (on this site and others) and I wanted to share my own. It is quite an efficient design since every square inch of collector surface is in direct thermal contact with the water being heated. You can easily modify the design to any size you like. I made mine 8ft long by 22" wide so that it can fit between the rafters in my attic. Tests showed that system output averaged about 530 Watts, heating 20 litres of water from 24 degrees C (75 degrees F) to 47 degrees C (117 degrees F) in one hour.

Aside: I'm in the middle of re-roofing my house and plan to build in a transparent section of roof in one area. Then I can experiment with different solar collector designs like this one and install and remove them easily from inside my attic instead of having to go out on my roof. It will make the plumbing easier too. The drawback is that if a collector springs a leak, it will leak into my home instead of into my gutter.

For information on this and other projects of mine see my website IWillTry.org.

Step 1 Concept

The collector is made from corrugated plastic sheet, commonly used for making signs. It has multiple square channels running lengthwise from end to end. When I first saw this type of sheet I immediately thought, "Wow, this would make an excellent flat panel solar collector if only there was a way to pipe water through all those little channels." Several weeks later, a method of doing so occurred to me. If a slot of the right width is cut lengthwise in some ABS pipe (so the cross section looks like a "C") then this pipe can be fit over the end of the corrugated plastic. The seams can be sealed to make everything water tight. The sheet can be painted black and viola... you have a flat panel solar collector.

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. In practice the maximum temperature is difficult to guarantee. Water may stop circulating, or may drain out completely for a number of reasons and the panel will overheat. Therefore this may not be a practical design for residential installation but it is an inexpensive, easily built experimental system that produces as much or more hot water than commercially available systems. Mine cost about $60 in materials (about $4.00 per square foot) and about 6 hours of construction time.

Comments

[jfoerster]

Are you still using this set-up? Have there been any water leaks in the joints to date? This same material was used as a heat exchange condenser plate for a desalination prototype and it did very well as compared to traditional fin-and-tube condensers. I'm about to start building a prototype using the Coraplast material (anyone besides me always read that as "chloroplast"?) as a condenser and was wondering if you have any comments and observations on the longevity of the joints you used. The actual study analyzing the performance of the material is titled "Performance of a greenhouse desalination condenser: an experimental study" by Al-Khalidi, A.A.T. Zurigat, Y.H. Dawoud, et al.

For what it's worth, the better commercial solar pool heaters run at about 80% efficiency and here's a DIY unit that hit 78%.

http://rimstar.org/renewnrg/solar_pool_heater_diy_fp.htm

[wolfkeeper]

For permanent installs, wouldn't a steel central heating radiator used as a heat exchanger, with double glazing on top, work nearly as well or better and avoid the problem of potentially melting?

[phredder]

You did a nice job on your thermal collector. I built a similar design several years ago to heat my kids pool that was 12 feet in diameter and 3.5 feet high. Mid summer the pool temperature would get up to around the 90 degree mark, without the collector the temp would be below 70. I used the full 8 x 4 black sheet if coraplast and used 1.5 inch a.b.s. for the ends. The a.b.s. ends were 4.5 feet in length. I didn't make a continues cut along the length of the a.b.s., rather I started my cut about two inches from the end, then I'd cut an 11 inch cut, leave another uncut portion of about 3/4 inch, make another 11 inch cut, leave another portion of material uncut, and so-on down the length of pipe until I got to 48 inches, I then left the last 6 inches uncut. Before I slid my coraplast in the a.b.s. I had to notch the coraplast to fit the uncut portions of the pipe. My reason for this type of slotting was to add strength to the pipe to resist the internal pressure of the water that was being pushed by a 1.5 h.p. pool pump. The hose I was using was 1.5 inch black sump pump hose. Not the most attractable pool hose but functional. My biggest concern was what adhesive to use to join the pipe and coraplast. At that time the only info I could find on coraplast was from the manufactures data sheets and modelers/hobbyists who use coraplast to build their projects. I did used silicone but next time plan to use urethane, the stuff the automotive industry uses to adhere windshields to vehicles. the was fastened to a 8x4 sheet of plywood and then the whole assemble screwed to my shed roof that face south. The collector remained on the roof for 3 years and withstood everything except the cats claws.

[napjax3000]

The process seems workable. I am seriously considering giving this a try. Hope it turns out ok.

[gearskin]

Since this plastic solar thermal collector can only reach a maximum temperature of about 80 Fahrenheit before it might melt (like you mention), do you think you could use this collector as a sort-of preheater for a metal solar thermal collector? I don't have any experience building or using them, myself, so I don't know if that would help the process at all, but with my rudimentary understanding, it makes sense :P

[macrumpton]

The plastic sheet material appears to be Coroplast, a fluted polypropylene panel available in thicknesses from 2mm to 12 mm in a wide variety of colors (including black). According to the Coroplast.com site the temperature properties of it are:

Normal temperature performance range	-17 degrees F to 160 degrees F
Melting point	162 degrees C, 324 degrees F

The one fly in the ointment is that it is virtually impossible to reliably glue anything to it, especially if there is stress on the joint or low temperatures which can make the joints very brittle. I have spent several years wrestling with this material (in a quest to make a folding boat out of it) and the best ways to attach anything structural to it are staples, pop rivets, nuts and bolts. and heat sealing other things made of polypropylene to it. Hot glue can be used if the result is not going to be stressed while cold or wet and there is no peeling force.

Your best bet for local sources are sign shops which often have 4'x8' sheets in the 4-5mm thickeness. they also have 2-3mm thick smaller panels which are used for real estate and election signs.

[iwilltry]

Hi bell017, That's 80 degrees C (176 degrees Fahrenheit) and that is plenty hot enough for most applications. This issue isn't that a hotter temperature is desirable. Panels are more efficient at lower temperatures so generally you don't want the panel to get much hotter than the desired final temperature of the water (ex not over 50 degrees C for residential water heating). The issue is that there's no safeguard to prevent hotter temperatures from being reached occasionally. During it's lifetime there are any number of reasons the water could stop circulating. If that happens the panel will overheat very quickly and you have meltdown ;-). The same is true even if it is just used as a preheater. So... while it's a great experimental unit, it's not a good choice for permanent installation. That is, unless you install it in open air without glazing in which case it probably will not overheat (that's the way many commercially available pool heating panels are constructed).

[gearskin]

Ah, I guess I should have read that again before I replied. 80 Celsius is plenty hot enough for me :) I've been interested in solar thermal energy for a year or so now, but I've either not had the time or the money to get something started. What are some of the big reasons that water might stop circulating?

[iwilltry]

Some reasons water could stop circulating: 1. System develops a leak and the water leaks out. 2. Air gets into a thermosyphoning system causing vapour lock. 3. Pump failure in a non-thermosyphoning system (could happen due to power outage, faulty temperature sensor, bad connection, etc). It's unlikely any solar hot water installation (even commercially available ones) will operate without one of these happening occasionally.

[carlos1w]

This is very nice.  Exactly what I need for improving my last project:  http://www.instructables.com/id/Hybrid-solar-panel-photovoltaic-and-thermal/.  Do you think that this corrugated plastic exists that is somewhat rigid?

[devonfletch]

Polycarbonate 'structured' roofing 'Polygal' and other brand names is pretty stiff, and comes in various thicknesses 6, 8, 10, 16mm (1/4" to 5/8"). But it's dear as poison, and for experiments, best to source offcuts or second-hand. Note that this product should be sealed with non-acetic roofing/plumbing silicon only, (it is allergic to polyurethane and other sealants). Also, it is UV-protected on one side only: if you install it upside-down, it will die in 6 months. But it would be a good base for solar cells, and it' easy to cut by score-and snap, or jigsaw, and can be heat-bent with care.

[carlos1w]

I know that pc leeches BPA, but I would not call it deadly.  Could you point me to some reference on why it is "dear as poison" (I would certainly want to know!).  THANK YOU.

[devonfletch]

I installed this roofing product for 10 years, and it was the most expensive roof (except titanium!) available, $50-60/sq m here in Oz. Perhaps 'dear as poison' is a local expression. It means inordinately expensive, maybe it is less so in other parts of the world.  In any case, it is very versatile, due to its impact-resistance, insulation properties and easy handling.
Another handy material is the stainless steel sheet, salvaged from dead clothes dryers (the drum is often SS), and commercial (and increasingly, domestic) refrigerators and other appliances.

[carlos1w]

Excellent, thank you!  I was not aware of the "australianism" :)

[lensam69]

Yes, They make a rigid version that is sandwiched between two aluminum plates. Call sign supply companies (i.e. Grimco)

[carlos1w]

The site at grimco has lots of interesting plates that would work nicely.  I also found out that it is not good to mix polycarbonate with water, unless you keep a strict separation of the medium and the water you will actually use in your home (BPA leeches out of the polycarbonate).

[fbujold]

Build the same concept several years ago out of black coroplast and the only weak link was the PL adhesive sealant. Tried at least 7 or 8 different product and the final champion was marine goop. (http://www.eclecticproducts.com/ag_adhesives.htm) Available at home depot and other fine hardware strores. Make sure to sand the coroplast and pipe surface for improved adhesion.

[weldor]

I believe that what is being referred to as "thermal siphoning" is actually a natural type of circulation called convection. It is also called "ebullient cooling" when the process is used to transfer heat out and a way from (as a method of cooling a piece of machinery (typically an engine).

To avoid air bubbles it is a good idea to install your fill point at the lowest point of the system. This is because air always rises to the top. The expansion tank should also have a safety relief valve from a water heater installed. Better yet, use a water heater as the expansion tank.

[paladin42]

neat concept. would like to know how to apply this to home use. can you use your existing hot water heater as a storage tank and can you hook it up directly to your water line. if yes to the former how do you keep the stored water hot until use?

[iwilltry]

You could use your existing hot water heater as a storage tank. You could not put this system directly in line though. City water pressure would burst the joints. You would required a heat exchanger in the tank (do a web search on Solar Wand) and a pump to circulate the water when it is sunny. The tank keeps the water hot till use (it's insulated). During non-sunny periods you can run the tank normally. A well built system would switch automatically between normal and solar operation as needed.

[votecoffee]

You could use this with an existing system, but you are right in that you would need a check valve on the output and a pressure regulator on the input side that would greatly reduce the pressure.  If you can not find a pressure regulator that will bring the pressure down sufficiently, you could use a storage tank and float valve to fill the tank to a set height.  Either way, you will need a pump to get the water pressure up higher than house pressure for the check valve to open and let water in.

[paborralho]

I forgot the important comment, I think it is better if the "in" and "out" of the panel are opposed, to avoid water circulating by the shortest path. Paulo Borralho "There are lots of Yesterdays and Tomorrows"

[paborralho]

paulo_do_campo match
p b x p t o @ g m a i l . c o m
sin espacios

[iwilltry]

Agreed. My bad. Someone else pointed it out too. It's especially important for a pump operated system. For a thermosyphoning system it is not as important since there is a negative feedback in that faster flowing channels heat up less, causing the flow rate to decrease, while slower moving channels heat up more causing the flow rate to increase.

[jhearty]

OK, so my wife and I finally got one of these built and tested.  We built it as a drainback system and used plain water dyed black using pond dye.  We did not paint the panel.  We got frosted tempered glass panes from Craigslist to build this and the next ones.  We used an old hot water circulator pump also from Craigslist.

We did a 4 hour test on a clear day, readjusting the panel angle a few times during the test.  The full spreadsheet is available but I was not sure if it could be posted here, so I just posted a screen shot of it.  It is a 1.814 square meter panel with 37.85 liters (10 gal) of water in the system.  We used a 55 gallon plastic drum for the tank.  The tank and hoses were not insulated.

At the 1 hour mark average power was 951 watts, 52% efficient.  At 2 hours, 768 watts, 42%.  At 3 hours, 593 watts, 33%.  At 4 hours, 464 watts, 26%.  We also did a stagnation test with no water in it, and it got up to 152 degrees F on a 45 degree day.  We are looking forward to mounting it permanently and testing reliability/longevity.  One thing we still need to do is get UV clear paint to help protect the panels from UV breakdown, and see if that affects the efficiency much.

[lucindanl]

Does the length of the hose make a difference? I want to build something to keep a worm composting bin from freezing this winter. My bin is a disused cement water cistern w/o access to AC power. It’s about 6’ x 6’ x 3’ and lined with plastic. I plan to surround it with hay and cover it with insulated panels, but I’ll need a heat source, beyond the composting itself, as I live in the northeast where we have substantial freezing. I’m wondering if the thermal siphoning would work if I used substantially more hose – like 75’ to 100’ – coiled on the bottom of the bin and buried under the compost. The tank and panel could be placed as high as needed as long as the hose could be lower. Do you think this would work? Also, would freezing during the night destroy the panel?

[avid0g]

This sound really great.  If the collector is above the cistern, you will need a pump, so I hope you have a downslope to place the solar collector.  The length of hose will slow down the siphon but the biggest drag is elbows and collector heat loss; avoid them.  You need a low-pressure check valve to prevent back-flow and a water storage tank/bladder above the compost.

The cistern sounds like a great way of having Thermal Mass.  If you can keep the soil Under it away from moving ground water, sink the insulation deeper into the soil around it and use the additional mass of the soil underneath to ride through cloudy days.  The insulation and surrounding soil may need to be covered with tarp to drain away moisture.  It may take a while to heat the soil underneath, but the solar collector would eventually get there.  

Rigid hose under the compost sounds great.  Of course the plumbing and collector would need substantial insulation, since it will be out in the cold.  The water is in a closed loop and not used for drinking or for moisture, so you can load it with a non-toxic antifreeze and black dye for improved heat absorption.  (We are on a web page with a transparent absorber!)  

It is also a good idea to provide some heating from above.  This could be rubber hose and/or bladder on top of soil and/or transparent insulation (bubble-wrap or white closed-foam) above the rest to let in light.  

Good Luck!

[Dilynda]

I have a roof solar panel installed and a huge holding tank for hot water usage in my home can I use this system to heat my pool and if so what adjustments would be needed to incorporate the pool?

[avid0g]

 I recommend that you add more collectors if you want to heat your pool.  It is usually not necessary to reach a high temperature for pools so the new collectors can be dedicated to the pool, and be less expensive since they need less insulation and can use cheaper plastics.  It is always a good idea to insulate the pool surface when not in use.

The pool heating may overload your current hot water system so that it cannot ride through cloudy and/or cold days; then you are using fossil fuel.  :-(

If your home is well insulated, you may be able to use some of that domestic hot water energy for air heating or radiant floor heating.  ;-)  This is also a good use for excessive hot water from the solar pool heaters. They can provide preheated water before a final boost from the domestic hot water system.  This loop would Not use potable water.  It would run on a separate loop requiring a pump, heat exchanger(s) and second thermostat.  

[lucastro]

  I am a plastics fabricator and had a similar idea a while back.. Great to see it works! I would, however, strongly recommend using a polycarbonate twin wall (coreflute) sheet instead of what is used for signage. It can be bought in varying thicknesses and has a UV protective coating. also being polycarb, it will resist higher temperatures and be alot stronger... What I had in mind was to use twinwall as not only the water 'membrane' but as the insulation as well, laminated together 'cross ply' front and back to make it super strong... you can also buy capping for these sheets that will perform the same function as your 'c' tubes. Nice work!

[alex-sharetskiy]

wouldn't it be better if you painted the collector black?

[iwilltry]

It is painted black. See Page 3, item 17. You can also see it painted in one of the images on Page 4.

[alex-sharetskiy]

sorry i didn't notice that

[adrian.ccs]

Hi iwilltry, thanks for the guide but one question, is the corrugated panel double so that water pass right through it??? or how does this work?

[iwilltry]

The corrugated panel has square channels that run through it so water can pass right through.

[vinyard]

Where do you get the corrugated plastic sheet?

[iwilltry]

I got mine from Home Depot. $17 for an 8' x 4' sheet.

[madrasi]

If you are worried about it springing a leak there is an easy fix for that. Since it fits inside the rafters you could build a box around it using some old car tire tubes, cut them up to where you can line the inside of the box with them since you can glue them together to fit your need. Then put in a big enough hose to drain the water to an outlet of your choosing,maybe to the gutter system. From there you can put in a on/off valve just before it turns to go out through the roof. All you need is a few more 2x4's and plywood which you might already have and the inner tubes should be free from the junk yard and your still doing it all on the cheap. Good luck

[davey703]

I've been planing on doing this ideas myself with a car radiator, I'm just a bit confused about the thermo siphoning, or how it is judged, in your design once inplaced at its angle, is the cold water feed entering from the top or the bottom??

[iwilltry]

The cold water should exit from the bottom of the storage container and feed into the bottom of your collector (radiator in your case). The hot water should exit the top of your collector and feed into the top of the storage container. The collector should be lower than the storage container for effective thermo-syphoning.