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Build your own flat panel solar thermal collectorby iwilltry on August 18, 2007

Table of Contents

License:   Attribution Non-commercial Share Alike (by-nc-sa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Intro:   Build your own flat panel solar thermal collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

step 1:   Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

step 2:   Tools and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

step 3:   Build the collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

step 4:   Build the frame and assemble the panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

step 5:   Fill the panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

step 6:   Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

step 7:   Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

step 8:   Why does this collector design work so well? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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License:   Attribution Non-commercial Share Alike (by-nc-sa)

Intro:  Build your own flat panel solar thermal collectorI'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 ofcollector 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 itcan 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 47degrees 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 designslike 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 acollector 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 .

Image Notes1. Here I'm measuring the exit temperature from the panel about 1 minute afterfilling the system with water. Most of the water in the tank is still around 23°C, butthe temperature at exit is 50.7°C (123°F) indicating that thermo-siphoning is takingplace.

step 1: ConceptThe 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 firstsaw 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 littlechannels." 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... youhave 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 (176degrees 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 maydrain 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 squarefoot) and about 6 hours of construction time.

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Image Notes1. The basic concept is to use ABS tubing with a slot cut in the side to pipe water through narrow channels inside the plastic sheet. The tube end will be capped. Theseams will be sealed. The plastic sheet will be painted black. And the whole assembly will be placed in an insulated enclosure with transparent lid.

step 2: Tools and MaterialsTools

Table SawHand SawDrill pressPower Drill3/4" drill bit1" hole sawExacto knifeTape measureScrew driverDigital thermometerCaulking gun for silicone adhesiveCoarse round file

Materials for collector (shown in image below)

1 - sheet of corrugated plastic (4'x8') cut to 22"x90" - $8.501 - 4' of 1 1/4" ABS tubing - $64 - 1 1/4" ABS caps - $102 - threaded 1/2" hose nipples - $1.001 - cartridge of silicone adhesive/sealant suitable for plastic - $3.501 - can of flat black spray paint - $5.00

Materials for frame

1 - 1/2" sheet of plywood (4'x8') cut to 24"x8' - $8.001 - 3/4" sheet of polystyrene (2'x8') cut to 22"x87.5" - $2.503 - 2x3 x 8' - $8.00 used.1 - at least 4'x10' of transparent plastic sheet - basically $0misc screws and staples

Materials for tank / water circulation

1 - cooler (or other water tank, preferably insulated) - $20 but I had one already1 - 15ft of 5/8" garden hose - $5.502 - 1/2" hose clamps - $1.50

Total cost of materials excluding tank = $59.50

Image Notes1. 1 1/4" ABS tubing x 4ft2. 1 1/4" ABS caps3. Silicone adhesive/sealant. Note that brand shown is not what I actually ended up using. I used a product designed for sealing gutters and flashing rated for up to200 degreesC.4. 1/2" hose nipples5. 1/2" hose clamps6. 15ft garden hose7. Flat black spray paint suitable for plastics.8. Corrugated plastic sheet, typically used for sign making.

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step 3: Build the collector1. Use an exacto knife to cut the corrugated plastic sheet to 22"x90". When cutting lengthwise, be sure to cut in a single channel for the whole length.

2. Cut the ABS pipe into two lengths, each 20.25" long. Check that when a cap is placed on either end, the total length is 22". I picked this width so it would fit betweenthe roof rafters in my attic.

3. Drill a 3/4" hole in the side of two of the ABS caps. This will be easier if you pre-drill with a smaller bit and gradually increase the size.

4. Enlarge the holes with a coarse round file until you can just thread in the nipples. I did not have a tap of the right thread, so I planned to just glue the nipples in place.

5. Drill a 3/4 diameter semicircular notch in the end of each ABS tube. This is easiest if you clamp them end to end in a vise. Alternatively you could drill this hole in theABS tube before cutting it, and then just cut through the center of the hole to make the notches. These notches fit around the nipple end when the ABS caps are in place.

6. Using a table saw with a fence, carefully rip a slot down the full length of each ABS tube. The resulting cross section should look like a "C". The ABS tube tends tocompress as you cut, so that when you are done, the slot will not be as wide as the width of your saw blade. Feed each tube through the saw a second time to clean upthe cut for a consistent width.

7. Repeat the slot cutting process with the ABS caps, keeping in mind what direction you want the nipples to be pointing when the panel is fully assembled.

8. Do a dry fit, assembling the ABS tubes, caps, and hose nipples. You may need to carve a bit out of the notch to get the slot in the tube to line up with the slot in thecap.

9. Repeat the dry fit on the end of the corrugated plastic sheet. Carve up the ABS as needed to get a nice fit everywhere.

10. After everything fits nicely, repeat the assembly, applying silicone adhesive to all mating surfaces before assembly, and applying a bead of silicone to all seams afterassembly.

11. Repeat for the other end of the corrugated plastic.

12. Allow to dry for at least 24 hours.

13. After drying, cut the garden hose in half and clamp the cut ends to the nipples.

14. Fill the panel with water (just connect the garden hose to a tap on your house) and check for leaks.

15. If there are any leaks, drain the panel, dry the area around the leak thoroughly and seal with more silicone adhesive, allowing another 24 hours to dry.

16. If you are interested in calculating the efficiency of your collector later, you need to know its volume. This is a good time to drain it into a bucket and measure thevolume (including the hoses). Mine contained 7.2 litres.

17. Once any leaks have been sealed, paint the surface of the collector black and set it somewhere to dry.

Image Notes1. Drilling a 3/4" hole in one of the ABS caps.2. It's easier and safer if you drill in steps, starting small and graduallyincreasing the hole size.

Image Notes1. Drilling notches in the ends of two ABS tubes simultaneously. This keeps thedrill bit balanced. The notch is required to fit around the hose nipple when theABS end caps are in place.

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Image Notes1. Here you can see the notch drilled in the previous image. It fits around thehose nipple when the cap is attached.2. After enlarging the hole a bit more than 3/4" using a file, you should be ableto thread in the hose nipple. I did not have a tap for this thread so I just gluedthem in.3. Dry-fitting the end cap assembly to make sure it all goes together well.

Image Notes1. Here I am about to cut a slot in the ABS tube. Position the fence so the bladeis centered in the tube.2. Aside: This is a handy table saw I made from a cheap circular saw screwed tothe underside of a circular plywood insert in the top of my workbench. Thedesign is modular so I can swap out the saw insert for a router insert that I alsomade.

Image Notes1. Always wear ear and eye protection when operating power tools.

Image Notes1. Finished slots cut in the ABS tubes. Note that they are oriented so that whenplaced on either end of the corrugated plastic, the hose nipples will be pointing inthe same direction.

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Image Notes1. Slots cut in the ABS end caps. Slots are cut in the other two end caps(without hose nipples) as well.

Image Notes1. Dry-fitting the ABS tubing to the end of the corrugated sheet. Looks good.

Image Notes1. After dry-fitting, I disassembled everything and put the pieces back togetherone by one, applying adhesive to all contacting surfaces, and finishing off with abead of adhesive over all seams.

Image Notes1. Repeat the process at the other end of the corrugated plastic and viola.... youhave a finished collector. Set it somewhere to dry for at least 24 hours.

Image Notes1. After filling the panel, I stuck the end of the hose up here to increase the

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water pressure.2. Testing the collector for leaks. This is important to do before painting it blacksince additional adhesive, if needed, will not stick as well to a painted surface.3. I had one small leak.

step 4: Build the frame and assemble the panelYou can use the collector as is. Just lay it out in the sun and pump water through it. However, much more heat can be captured by building an insulated enclosure for it.

1. Cut one 2x3 to two lengths of 22.25" for the ends of the frame. Screw the other 2 2x3s into the ends to make a rectangular frame.

2. Wrap the transparent plastic around this frame and tape in place to make a transparent lid to fit over the collector. In my case this is for test purposes only, since Iintend to eventually install the collector underneath transparent roofing material in my attic. The 22" panel width will fit between existing rafters which will provide a ready-made frame.

3. Cut the plywood to 24"x8'.

4. Cut the polystyrene sheet to 7'4" by 3'9" and place it centered on the plywood. This will be the insulation for the back of the panel.

5. Test-fit the collector and drill two holes in the plywood large enough for the hoses to easily fit through (about 1" should be good). Make one of these holes into a slot bydrilling another hole right beside it and cutting away the wood between them. This is to allow for thermal expansion of the corrugated plastic sheet. Plastics typically havea high coefficient of thermal expansion. If you restrict the panel from expanding, it may warp and cause a leak.

6. Now stack the whole works together: First the plywood, then polystyrene, then the collector, then the transparent cover.

7. Secure the transparent cover to the plywood back with several clamps (or you can screw it on, but initially you might want to be able to remove it easily for access tothe collector)

Image Notes1. The finished cover. It's just a frame of 2x3s with some transparent (though notas transparent as I'd like) plastic. This was just for a quick test or I would havemade it nicer. More rugged transparent roofing will already be present in the finalattic installation.

Image Notes1. Hole for top hose nipple to poke through the back.2. Slot for bottom hose nipple to poke through the back. A slot is preferable toallow for thermal expansion of the panel.3. 1/2" plywood cut to size for the back.

Image Notes Image Notes

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1. Polystyrene insulation goes between the plywood back and the collector. 1. The painted collector in place on top of the plywood back and polystyrenesheet.

Image Notes1. You can see one of the hose nipples protruding from the plywood back here.2. The assembled panel. I used clamps to temporarily hold the cover against theback. I wanted to be able to remove the cover easily to inspect the collector incase of leaks. Note that the covering plastic sheet is more transparent than itappears in the image.

step 5: Fill the panelFilling the panel in such a way that you get all the air bubbles out is easier said than done unless you use a few simple tricks.

1. Lift one end of the panel and rest it on a chair or other object (I used my fence). Rest the other end on a couple blocks of wood so that the bottom hose will haveclearance from the ground (remember I eventually want to install this on the underside of a roof, between rafters, which is why I made the hoses connect through theback instead of the sides).

2. Mount your storage tank higher than the panel and stick the top hose in it.

3. Connect the bottom hose to a tap on your house and turn on the water gently.

4. Watch as the panel fills. When water starts coming out of the top hose, let it continue and fill the tank.

5. As the tank is filling, temporarily tilt the panel so the corner where the top nipple exits is the highest point. This forces any air in the system to move towards the exitnipple where it will be expelled.

6. Once you stop seeing air coming out of the top hose, return the panel to its previous position.

7. Turn off the tap. Introduce a kink in the bottom hose to keep the water from flowing out. Then remove the hose from the tap.

8. Keep the bottom hose kinked, and the top hose under water in the tank. Raise the end of the bottom hose above the water level in the tank and release the kink.Slowly lower the end of the hose until water starts coming out, then plug it with your thumb and quickly stick the end under water in the tank creating a sealed system withas little air in it as possible.

9. Orient the hoses so that the bottom hose draws water from the bottom of the tank and the top hose delivers water to the top of the tank. Whatever you do, be careful toalways keep both hose ends under water or you will "break the seal" and introduce air into the system which will prevent circulation by thermo-siphoning.

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Image Notes1. Bottom hose, located to draw cold water from the bottom of the tank.2. Top hose, located to deliver warm water to the top of the tank.3. I made a mark on the inside of the tank indicating the correct water level for atotal of 20 litres of water in the system (7.2 litres in the panel and hoses, and 12.8litres in the tank).

step 6: TestingIf you have removed all the air and have a sealed system and there is enough sunlight hitting the panel, it should start thermo-siphoning almost instantly.

1. Turn the panel towards the sun and raise or lower the top end of the panel to better aim it towards the sun. One end of the panel must be raised higher than the otherin order for thermo-siphoning to work. The storage tank must also be kept higher than the top end of the panel.

2. Feel the top hose where it exits the panel. It should be hot if your setup is thermo-siphoning. The bottom hose should still be cool. If this isn't the case, it probablymeans you have a vapor lock (air bubbles) somewhere preventing the water from circulating. Connect the bottom hose to your tap again and repeat the filling process,attempting to remove all the air bubbles.

3. Once thermo-siphoning starts, use a digital thermometer with probe to measure the water temperature. By sticking the temperature probe inside the ends of the hoses,you can measure the inlet and exit temperatures of the collector. It took me about a minute after filling before I had my thermometer set up. At that time the inlettemperature was 23 degrees C (basically the initial temperature of the water) and the exit temperature was 50.7 degrees C (123 degrees F).

4. Measure the inlet temperature over a period of an hour or so (or till the temperature stabilizes). The inlet temperature should always be the lowest temperature in thesystem. Measuring here will give conservative results when calculating the amount of energy transfered to the water.

Image Notes1. Here I'm measuring the exit temperature from the panel about 1 minute after filling the system with water. Most of the water in the tank is still around 23°C, but thetemperature at exit is 50.7°C (123°F) indicating that thermo-siphoning is taking place.

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step 7: ResultsSee the image below for a plot of temperature vs time.

Thermo-Siphon Flow RateThe hoses are setup such that the bottom hose draws cold water from the bottom of the tank and the top hose delivers hot water to the top of the tank. The water in thetank does not mix much due to the low flow rate. Therefore the water drawn into the bottom hose stays at almost a constant temperature (the original water temperature)until all the water in the tank has been drawn out and been replaced by warm water that has passed through the collector. Dividing the tank volume by the time till thetemperature starts to rise gives a rough approximation of the flow rate through the collector.

Tank volume: 12.8 litres(Note: I filled it this much so the total volume in the system including the panel and hoses would be an even 20 litres)Time to empty: 25 minutesCalculated thermo-siphon flow rate: 0.8 litres per minute

Note that the thermo-siphon flow rate decreases as all the water heats up and the temperature difference between the tank and the panel is less.

Power CalculationThe temperature change I was able to achieve was about 23 degreesC over a period of 1 hour. The heat capacity of water is 4.18 kJ/kg/degreeC. There were 20kg ofwater in the system. Given this information it is possible to calculate the average power that was actually input into the water:

Power = 4.18 kJ/kg/degreesC * 20 kg * 23 degreesC / 3600 seconds = 0.53 kW or 530 Watts.

Efficiency CalculationThe collector area is about 1.4 m2. Energy available from sunlight is about 1000 W/m2. Therefore the panel receives about 1400 W of incoming power when aimeddirectly towards the sun. The efficiency is simply the power actually extracted divided by the power available.

Efficiency = 530 Watts / 1400 Watts = 0.378 or 38%.

This is quite comparable to commercially available solar collectors. However, I'm doing this in my back yard with uninsulated hoses, a non-air tight panel, a single plasticpane that's slightly opaque, an open topped tank and no pump. The fact that I can achieve commercial level efficiencies with this setup is a testament to the design andindicates there is plenty of room for improvement in the industry.

Image Notes1. This is the point at which all the original water in the tank has drained into the collector and we start to see warm water that has already passed through thecollector.2. The temperature peaked at around 50°C at 90 minutes.

step 8: Why does this collector design work so well?Most home brew and commercial solar collector designs I have seen use metal (usually copper) tubing to carry the water through the panel. Metal fins are attached to thecopper tubing. The fins are painted black. The fins heat up and conduct the heat to the tubing. Metal is a good conductor, but the heat has to travel a long way through athin cross-section to reach the tubing. In my design, I used plastic which is a poor conductor, but the heat only has to travel about 0.3mm through a very large cross-section from the front surface of the panel to the water. I'll illustrate why this is better.

There is a property of any thermal system called thermal conductance that indicates how much heat (power) can be transfered from point 'a' to point 'b' for a giventemperature differential. The formula is:

Thermal Conductance = K * A / Lwhere:K = thermal conductivity (a physical property of the material)A = cross-sectional area through which heat must travelL = distance heat must travel (the distance from 'a' to 'b').

Lets calculate the thermal conductance of a typical flat panel collector.

Assume the panel is 2'x8' with 4 copper tubes running lengthwise and fins sticking out 3" on either side of every tube (6" per tube x 4 tubes fills our 2' width). Suppose thefins are 1mm thick and also made of copper. When the fins heat up, the cross sectional area through which this heat must be conducted to reach the tubes is 1mm * 8 ft *8 fins = 2438 mm2. The average distance the heat must be conducted is 1/2 the fin width or 1.5" = 38 mm. The conductivity of copper is about 0.4 W/mm/degreeC.

Therefore the thermal conductance from the collector surface to the water is 0.4 W/mm/degreeC * 2438 mm2 / 38 mm = 25W/degreeC. In other words, a 1 degreeCtemperature difference between the water and the fin will result in 25W of heat transfer. But the panel is receiving something on the order of 1400 W of incoming power

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from sunlight. To transfer all that power to the water by conduction alone the fins would have to heat up to 56 degrees C higher than the water temperature.

Now repeat the calculation for the corrugated plastic panel.

The cross sectional area through which heat must be conducted is the receiving area of the panel itself (2' * 8' = 1486448 mm2). The distance the heat must travel toreach the water is just the thickness of the plastic wall or about 0.3mm. The conductivity of plastic is about 0.0001 W/mm/degreeC. Note that it is over 1000 times lowerthan copper which makes sense since plastic is general thought of as an insulator, not a conductor.

Therefore the thermal conductance of the system is 0.0001 W/mm/degreeC * 1486448 mm2 / 0.3 mm = 495 W/degreeC. In other words, a 1 degreeC temperaturedifference between the water and the collector surface will result in 495 W of heat transfer into the water. To transfer 1400W, the panel surface only needs to heat upabout 3 degreesC hotter than the water.

Of course in practice, not all of that 1400W goes into the water. The conductance from the collector surface to the water is in parallel with another conductance from thecollector surface to the outside air. The relative values of those two conductances determines how much heat goes where (Aside: this is analogous to current in anelectrical circuit with two resistors in parallel.)

ConclusionYou can see that in spite of the much lower thermal conductivity of plastic, using a corrugated plastic sheet as a collector achieves 20 times higher conductance betweenthe collector surface and the water when compared to a typical tube-and-fin design.

If a corrugated collector could be made from copper, the results would be even better, but not very much better, for reasons I won't go into because I can already feeleveryone's eyes glazing over.

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

Image Notes1. The typical tube-fin design used in most flat panel collectors. The heat must be conducted a long distance through a thin cross-section.2. My design. Heat is conducted a very short distance through a very large cross-section to reach the water. The improved geometry gives much better heat transferin spite of the lower conductivity of plastic.

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Cheap and EasyPassive SolarWater Heater foryour Home.($300) (video) byjaketeater

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Comments50 comments Add Comment view all 128 comments

 napjax3000 says:  Jul 11, 2010. 4:59 PM  REPLYThe process seems workable. I am seriously considering giving this a try. Hope it turns out ok.

 bell017 says:  Aug 6, 2008. 1:13 PM  REPLYSince this plastic solar thermal collector can only reach a maximum temperature of about 80 Fahrenheit before it might melt (like you mention), do you thinkyou 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'tknow if that would help the process at all, but with my rudimentary understanding, it makes sense :P

 macrumpton says:  May 18, 2010. 3:43 PM  REPLYThe 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 FMelting point162 degrees C, 324 degrees F The one fly in the ointment is that it isvirtually 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 havespent 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 arestaples, 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 stressedwhile 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 whichare used for real estate and election signs.

 iwilltry says:  Aug 6, 2008. 4:21 PM  REPLYHi 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 isdesirable. Panels are more efficient at lower temperatures so generally you don't want the panel to get much hotter than the desired final temperature ofthe water (ex not over 50 degrees C for residential water heating). The issue is that there's no safeguard to prevent hotter temperatures from beingreached occasionally. During it's lifetime there are any number of reasons the water could stop circulating. If that happens the panel will overheat veryquickly 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 choicefor 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 manycommercially available pool heating panels are constructed).

 bell017 says:  Aug 7, 2008. 4:58 AM  REPLYAh, 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 ayear 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 stopcirculating?

 iwilltry says:  Aug 7, 2008. 3:23 PM  REPLYSome reasons water could stop circulating: 1. System develops a leak and the water leaks out. 2. Air gets into a thermosyphoning systemcausing vapour lock. 3. Pump failure in a non-thermosyphoning system (could happen due to power outage, faulty temperature sensor, badconnection, etc). It's unlikely any solar hot water installation (even commercially available ones) will operate without one of these happeningoccasionally.

 carlos1w says:  May 7, 2010. 8:26 AM  REPLYThis is very nice.  Exactly what I need for improving my last project:  http://www.instructables.com/id/Hybrid-solar-panel-photovoltaic-and-thermal/.  Do youthink that this corrugated plastic exists that is somewhat rigid?

 devonfletch says:  May 17, 2010. 4:50 AM  REPLYPolycarbonate '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'sdear as poison, and for experiments, best to source offcuts or second-hand. Note that this product should be sealed with non-acetic roofing/plumbingsilicon 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 says:  May 17, 2010. 7:20 AM  REPLYI 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 wantto know!).  THANK YOU.

 devonfletch says:  May 17, 2010. 10:16 PM  REPLYI 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 veryversatile, 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.

http://www.instructables.com/id/Build-your-own-flat-panel-solar-thermal-collector/

 carlos1w says:  May 18, 2010. 6:38 AM  REPLYExcellent, thank you!  I was not aware of the "australianism" :)

 lensam69 says:  May 16, 2010. 7:08 PM  REPLYYes, They make a rigid version that is sandwiched between two aluminum plates. Call sign supply companies (i.e. Grimco)

 carlos1w says:  May 17, 2010. 7:19 AM  REPLYThe 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 youkeep a strict separation of the medium and the water you will actually use in your home (BPA leeches out of the polycarbonate).

 fbujold says:  May 16, 2010. 4:42 PM  REPLYBuild 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 productand 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 says:  May 16, 2010. 2:08 PM  REPLYI 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 tankshould also have a safety relief valve from a water heater installed. Better yet, use a water heater as the expansion tank.

 paladin42 says:  Jun 24, 2008. 12:35 PM  REPLYneat 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 directlyto your water line. if yes to the former how do you keep the stored water hot until use?

 iwilltry says:  Jun 24, 2008. 4:47 PM  REPLYYou 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 thejoints. 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 tankkeeps the water hot till use (it's insulated). During non-sunny periods you can run the tank normally. A well built system would switch automaticallybetween normal and solar operation as needed.

 votecoffee says:  May 16, 2010. 8:42 AM  REPLYYou 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 inputside that would greatly reduce the pressure.  If you can not find a pressure regulator that will bring the pressure down sufficiently, you could use astorage 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 pressurefor the check valve to open and let water in.

 paborralho says:  May 8, 2008. 10:41 AM  REPLYI 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 says:  Apr 20, 2010. 9:19 PM  REPLYpaulo_do_campo matchp b x p t o @ g m a i l . c o msin espacios

 iwilltry says:  May 8, 2008. 3:28 PM  REPLYAgreed. My bad. Someone else pointed it out too. It's especially important for a pump operated system. For a thermosyphoning system it is not asimportant since there is a negative feedback in that faster flowing channels heat up less, causing the flow rate to decrease, while slower moving channelsheat up more causing the flow rate to increase.

 kelebe says:  Apr 9, 2010. 10:09 PM  REPLYWe are menufactory of solar water heater,solar flat panel in China,our products are also exported to many countries, such as America, Mexico, Spain, Chile,South Africa, Romania and Bulgaria etc. more details pls. vist our company's website :http://www.expsolar.com

 jhearty says:  Mar 28, 2010. 2:02 PM  REPLYOK, 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 notpaint 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 fromCraigslist.

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 beposted 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 gallonplastic 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%.  Wealso 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 andtesting 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 theefficiency much.

http://www.instructables.com/id/Build-your-own-flat-panel-solar-thermal-collector/

 lucindanl says:  Aug 27, 2009. 11:03 AM  REPLYDoes 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 disusedcement 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 thermalsiphoning 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 andpanel 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 thepanel?

 avid0g says:  Mar 7, 2010. 8:42 PM  REPLYThis 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.  Thelength 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 valveto 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 insulationdeeper into the soil around it and use the additional mass of the soil underneath to ride through cloudy days.  The insulation and surrounding soil mayneed 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.  Thewater 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 heatabsorption.  (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 says:  Apr 29, 2009. 6:19 AM  REPLYI 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 adjustmentswould be needed to incorporate the pool?

 avid0g says:  Mar 7, 2010. 7:43 PM  REPLY 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 newcollectors 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 toinsulate 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 agood 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 says:  Oct 9, 2009. 1:19 AM  REPLY  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 willresist 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 says:  Jul 5, 2009. 5:41 PM  REPLYwouldn't it be better if you painted the collector black?

 iwilltry says:  Jul 6, 2009. 11:29 AM  REPLYIt is painted black. See Page 3, item 17. You can also see it painted in one of the images on Page 4.

 alex-sharetskiy says:  Jul 6, 2009. 11:42 AM  REPLYsorry i didn't notice that

 adrian.ccs says:  Jun 28, 2009. 7:41 PM  REPLYHi 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 says:  Jul 6, 2009. 11:30 AM  REPLYThe corrugated panel has square channels that run through it so water can pass right through.

http://www.instructables.com/id/Build-your-own-flat-panel-solar-thermal-collector/

 vinyard says:  Jun 24, 2009. 3:37 PM  REPLYWhere do you get the corrugated plastic sheet?

 iwilltry says:  Jun 28, 2009. 10:46 AM  REPLYI got mine from Home Depot. $17 for an 8' x 4' sheet.

 madrasi says:  Jun 22, 2009. 6:37 PM  REPLYIf 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 tiretubes, 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 todrain 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 theroof. 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 itall on the cheap. Good luck

 davey703 says:  Jun 4, 2009. 3:51 AM  REPLYI'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 onceinplaced at its angle, is the cold water feed entering from the top or the bottom??

 iwilltry says:  Jun 4, 2009. 4:07 PM  REPLYThe 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 watershould 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 effectivethermo-syphoning.

 davey703 says:  Jun 8, 2009. 9:19 AM  REPLYI'm using a car radiator, a small copper cylinder and extension tank(if the water overheats) all connected by copper pipes, what would be the maincauses if the system doesn't work, or the thermo-siphoning doesn't work....in diagram 1 to 10

 iwilltry says:  Jun 8, 2009. 10:43 AM  REPLYWithout seeing how you've set up your system, it's difficult to say, but the usual cause of thermo-siphoning issues are:

air lockscollector not low enough relative to storage tankconnecting tubes not large enough in diameterconnecting tubes not providing a continuously rising (for hot tube) or continuously falling (for cold tube) path for the water to follow

I don't know what diagrams you are referring to.

 bi018792 says:  Jun 4, 2009. 10:14 AM  REPLYgreat idea."i will try" one myself

 UK_Westy says:  Jun 1, 2009. 11:19 AM  REPLYIf the storage container was below the height of the solar collector, I assume a pump could be used to pump it back up to the top and let gravity take it'scause ? Do you think a drill pump fitted to a rechargable hand drill running off a solar panel work ?

 iwilltry says:  Jun 1, 2009. 10:53 PM  REPLYYes. It is usually much more convenient for the solar collector to be mounted higher than the storage container. Therefore a pump must be used tocirculate the water. A drill pump may work, but I don't know if they are designed for continuous duty, and I don't think they are very efficient (ie you mayrequire a bigger solar panel than you think). If you have access to AC power, I would recommend using that instead of adding photovoltaics. You mighttry an AC aquarium pump or a sump pump. But then you either have to turn it on and off manually, or design a thermostat to turn it on when the collectoris hotter than the storage container.

 UK_Westy says:  Jun 1, 2009. 10:26 AM  REPLYHi, This is great I have been looking at using solar power to heat water for a child's paddling pool. Basically, heat water one day and use the following. I wasthinking of using a solar powered pump, but thermo-siphoning costs nothing ! If I used a double glazed unit (I have a couple lying around) instead of theplastic sheeting would this create too much heat and indeed melt the corrugated plastic sheeting ? Also, if the system was plumbed into a supply and aswater was drained out, and fresh water replaced by the use of a ball valve, if the whole system was at an angle as you suggested when filling, wouldn't itenable the air to be removed constantly ?

 iwilltry says:  Jun 1, 2009. 10:44 PM  REPLYIf you use double glazing you probably will get too hot for the plastic parts inside. It is possible to design a system that will expel air automatically. Yousimply need to ensure that at any point in the system there is a path that rises continuously from there to the highest point (highest water level) in thesystem, and that the highest point is the surface of the water in the storage container which is exposed to the atmosphere. For me that would have meantdrilling holes in the side of my cooler (one at the bottom for the exit and one at the top for the inlet). Then air bubbles would simply rise through thesystem and be released to the atmosphere. I did not want to drill holes in my cooler so I opted to bring the exit and inlet tubes in from the top whichintroduces the problem of trapping air and makes it necessary to take steps to remove the air in order to get thermo-syphoning to work.

http://www.instructables.com/id/Build-your-own-flat-panel-solar-thermal-collector/

 onebitpixel says:  Jun 10, 2008. 1:55 PM  REPLYI wonder if this could this be utilized within a pool system that would help keep it heated as well as filtering... while it is moving through the hose and throughthe filter assembly...

 AbelK89 says:  May 19, 2009. 9:52 PM  REPLYI'm been planning on trying this idea... and will post any ideas upon trying.

 Jalloy says:  May 17, 2009. 9:15 PM (removed by author or community request)

 iwilltry says:  May 18, 2009. 1:15 AM  REPLYHi Jalloy,You are welcome to add it. If you like, please also check my website www.iwilltry.org where I have some other solar projects. Cheers.

 dasdew2 says:  Mar 21, 2009. 5:10 PM  REPLYWHAT DO I NEED

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