Your opinions please

[wildbill hickup]

I'm am building 3 solar heat collectors and I would like your opinions on a choice between sheet metal roofing (painted black of course) and a layer of black asphalt shingles as the collector.

Planed construction will be as follows

Box built from 2X6's with plywood backing, width and hieght approx 3'X 5'.

Layers from back to face

1st 1" thermax on back and sides R6.5

second R11 fiberglass unfaced

third 1/2 thermax R1.5 (i think)

fourth layer of 1/2" copper pipe (spaced 3" to 4" apart) with inlets&outlest on bottom and top respectivley.

fifth(collector) Metal or asphalt?? recessed to withing 1" of glazing.

sixth double pane clear glazing in wooden frames.

At first I had concidered metal roofing(galvanised, corrigated) only but this would be difficult to get a good thermal bond to copper pipe other than random touch. Asphalt shingles would tend to mold to the copper and surface transfer would be more constant. Also I figure that shingle material will help insulate against cooling at night better that metal. Someone on the board I believe used rubber roofing (thanks for the idea). I don't have any of that so I concidered asphalt shingle material, however I wonder if temps inside the panel will cause these to melt. For the time being these panels will be used in the winter for constant radiant heat to a floor measuring 14'X12'.Possibly expanded incase of to much heat (in my wildest dreams). Panels will be on a closed system with antifreeze heating exchanger to radiant sysetem, for possible expansion(domestic hot water). A extra gas hot water heater will be used in the radiant system for a heat reserve tank and if need be suppliment durring cold nights. Valves will be places in heat exchanger line to keep panels from cooling water in water tank at night if needed.

Well there it is, let me have it. I await your opinions.

Wildbill

 

[ghurd]

The shingles are on top of the pipe? That isn't good.

Most of the heat will be between the glass and the 'black', the pipes should be there collecting heat before it radiates out.

Galvanised idea... run the ribs up and down. The pipe can lay in the valleys, making more surface area touching.

There was some talk about why 3/4" was better than 1/2". Don't remember, but it sounded good at the time.

G-

[mikey ny]

  High temps could be a problem with the asphalt material. My panel (still needs improvement and completion) use's the epdm rubber. The temp in the panel gets to about 280 f. I used some foil faced styrofoam around the sides of the panel on the inside ( the kind used behind vynl siding) and it melted down to just about nothing. Apparently this type of styrofoam is not for high temp applications. In my panel I used a layer of rubber and adhered the pipe to it useing strips of rubber and then covered the entire heat exchanger with another layer of rubber kind of like a zip lock bag. So far I have only achieved water temps of 110 degrees with about 45 gal. of water in a closed loop. I think the asphalt may be ok if you only use it as the layer under the pipe. You may try stapling strips of 15 or 30 lb. felt paper over each of the pipes to help with a thermal bond to the heat sink (asphalt). You may be able to google a particular brand of asphalt shingle ( by the way there is a lot of fiberglass in most roofing material) and get the specs on the temp ranges they are designed to withstand. I hope this info helps.

                                                   Mike

[mikey ny]

  High temps could be a problem with the asphalt material. My panel (still needs improvement and completion) use's the epdm rubber. The temp in the panel gets to about 280 f. I used some foil faced styrofoam around the sides of the panel on the inside ( the kind used behind vynl siding) and it melted down to just about nothing. Apparently this type of styrofoam is not for high temp applications. In my panel I used a layer of rubber and adhered the pipe to it useing strips of rubber and then covered the entire heat exchanger with another layer of rubber kind of like a zip lock bag. So far I have only achieved water temps of 110 degrees with about 45 gal. of water in a closed loop. I think the asphalt may be ok if you only use it as the layer under the pipe. You may try stapling strips of 15 or 30 lb. felt paper over each of the pipes to help with a thermal bond to the heat sink (asphalt). You may be able to google a particular brand of asphalt shingle ( by the way there is a lot of fiberglass in most roofing material) and get the specs on the temp ranges they are designed to withstand. I hope this info helps.

                                                   Mike

[GaryGary]

Hi,

"Box built from 2X6's with plywood backing, width and hieght approx 3'X 5'.

Layers from back to face

1st 1" thermax on back and sides R6.5

second R11 fiberglass unfaced

third 1/2 thermax R1.5 (i think)"

Thats way way more insulation than you need on the back of the collector.  The single layer of one inch thermax is enough.  The insulation prevents heat loss out the back of the collector, but remember that most of the heat lost from the collector will be out the front, which is only going to be about R2 depending on what kind of glazing you are using.  If the collector backs up to a heated part of the building, then back insulation is even less critical, since the loss goes to a heated area.

Thermaz or any form of the Polyisocynate insulation is good -- don't use the polystyrene (the pink, blue of recycled coffee cup foams) as they won't stand up to the heat.

Consider whether you really need the plywood back or not -- my collector has no back --it just goes right against the siding and works very well.

fourth layer of 1/2" copper pipe (spaced 3" to 4" apart) with inlets&outlest on bottom and top respectivley.

"fifth(collector) Metal or asphalt?? recessed to withing 1" of glazing."

The asphalt shingles would absorb OK, but I'm not sure they would hold up well to the high temps inside the collector.  How would you thermally bond the copper to the shingles?

You need some way to thermally bond the copper to the absorber -- otherwise, the absorber will run very hot, and the heat losses out the glazing will be large, and the efficiency low.  

"sixth double pane clear glazing in wooden frames."

Glass is good, you might also consider Polycarbonate plastic -- it is good up to 270F, and should work fine.  Home Depot sells corrugated Polycarbonate glazing for 1 dollar per squarefoot.  Make sure its Polycarbonate and not acrylic (Plexiglass) or PVC, as they won't take the temps.

"At first I had concidered metal roofing(galvanised, corrigated) only but this would be difficult to get a good thermal bond to copper pipe other than random touch. Asphalt shingles would tend to mold to the copper and surface transfer would be more constant. Also I figure that shingle material will help insulate against cooling at night better that metal. "

The collector will cool off at night rapidly -- it radiates well to the cold sky temperature -- there is really nothing you can do to prevent the cool down.  Its best just to keep the thermal mass of the absorber as low as possbile, so that when the sun shines on it, it will heat up quickly.

"Someone on the board I believe used rubber roofing (thanks for the idea). I don't have any of that so I concidered asphalt shingle material, however I wonder if temps inside the panel will cause these to melt. For the time being these panels will be used in the winter for constant radiant heat to a floor measuring 14'X12'.Possibly expanded incase of to much heat (in my wildest dreams). Panels will be on a closed system with antifreeze heating exchanger to radiant sysetem, for possible expansion(domestic hot water). A extra gas hot water heater will be used in the radiant system for a heat reserve tank and if need be suppliment durring cold nights. Valves will be places in heat exchanger line to keep panels from cooling water in water tank at night if needed."

Could you build the panels larger?  Panel area is the biggest factor in how much useful heat you will get out.  It is not that much more work to build a 4 ft by 10 ft than a 3 ft by 5 ft, and you will get three times as much heat from it.

"Well there it is, let me have it. I await your opinions."

There is a lot of material on building collectors on my site:

www.BuildItSolar.com    

Take a look at "Projects" -> "Space Heating"

Also take a look in the "Experimental" tab.

Just as a thought, have you considered building air collectors rather than water?

The air collectors could work in parallel with your radiant floor.  Air collectors are simplier, cheaper, and don't make a mess when they leak :-)

Gary

[GaryGary]

Hi,

A couple afterthoughts:

The traditional way to get a good thermal bond between the copper tubes and the absorber is to solder them in place.  This works well, but is time consuming, and you have to an absorber metal that can be soldered to.

If you want to try avoiding the soldering, you might take a look here:

http://www.builditsolar.com/Projects/WaterHeating/msbatchsys.htm

And, go down to the Maine Solar Primer for Closed Loop Water Heater.  I think that this is a pretty good design for a DIY water collector.  The alum plates that are used as absorbers can be made for less money by constructing a groove or channel using plywood that is just a bit larger than the copper tube you are using -- then take the alum flashing sheet that they sell at Home Depot and lay it over the groove -- then using a mallet, pound the tube into the groove -- this makes a groove in the alum that fits the copper tube.  Take the tube out, put a bead of silicone in it, and put the tube back in the groove and let the silicone set.  Make the tube to alum fit as close a possible -- this silicone will conduct heat well if it very thin, but  not if its thick.

I've heard that this technique works well, but I've not actually used it.  

Another option might be to buy the absorber plates, and build the rest.  SunRay makes some pretty inexpensive absorber plates.  I think they are at:  http://members.cox.net/sunraysolar/

Gary

[TERRYWGIPE]

As a roofer, I can say for certain that the asphalt shingles will melt at much lower temps than the rubber. we dry off rubber with a torch. on hot summer days we start earlier because the asphalt shingles melt in the sun(not contained).

 Terry

[Catch66]

 Hi:

Have you considered "perforated,"coregations. 4x8 sheets that are used in metal buildings under, to hold insulated bating's.The only draw back is that they are pre.. painted and would be difficult to solder onto.

Good luck

Catch66