Absorber plate for diy flate-plate collector

[mbeland]

I am looking at the possibility of building my own flate-plate collectors to heat water  for both domestic water use and radiant floor heating for my house. I live in northern New Brunswick Canada. I plan to have a closed-loop system with 2 storage tanks and heat-exchangers in the basement. I recently had a quote from a solar heating company for about 8000$ for the whole system (with 4 panels of 20 evacuated tubes) uninstalled. I would like to find ways to lower this cost without lowering the capacity of the system.

I have a fairly good idea of how to make these collectors:
- Aluminum channel
- Back panel made of plywood or aluminum sheet metal
- insulation in the back and side:
- glazing: double-glazed glass
- tubing: flexible copper pipes
- some sort of scellant and alu flat stock held with screws
- heatresistent flat black paint

It seems like the absorber plate is the most difficult part of the collector to design so to have:
- good thermal contact with the copper tubes
- no galvanic reaction with copper tubes
- good lifespan
- no thermal deformation
- little mass
- little thermal mass

I need ideas of how to make this cheap (meaning preferably not out of copper) but efficient.

On the web I have seen places where they just attach the tubing to a flat sheet metal plate with wire. I wonder if this would give enough thermal contact.

On another website, I have seen some guy bending sheet metal alu with a press. That seems interesting but I don't have access to equipment to do that.

Thanks for your help.

[A6D9]

I local NBer...

I'm located in Fredericton.

I think your idea will not work.  Reason being, NB climat is nto great for flat plate.  I looked into this myself.  In this area according to Gov Canada research their is not enought sun to be able to heat it int he winter months without needing a huge set up.  Big set up big cost.

This is why the Solar company you are deallign with wants to go evacuated tube.

You never mentioned how many square feet your house is.  But I find it hard to even believe that 20 tubes would be enought for you.

I did some calculations and I don;t have the #'s here now.  sorry.  but i calculated to need at the very least 30 tubes and I was goign to have a coil in my wood stove as a back up heatign sys "in case"

It's discouraging, but don;t let it.  think of it, if you get this set up, you won;t have another heatign bill ever again.

My plan is to build straw baile house, with radiant floor heattign as well.   next step after that would be windmill to supply my electricity and then bio diesel in the garage.

once I am done that, I would mostly be self sufficient.

Good luck

[GaryGary]

Hi,

Double glazing may not be worth the extra expense and trouble.  Almost all commercial collectors are single glazed with high transmission tempered glass.

Double glazing is a two edged sword -- you reduce the heat losses out the glazing, but you also cut down on the light transmitted to the absorber.

As you say, for a DIY panel the hardest part is coming up with a good way to attach the risers to the fins.  The wiring the risers to the absorber plate is not going to work well.

Using copper fins soldered to copper risers is thermally efficient, but is expensive and labor intensive.

My 2 cents would be that the best of the DIY techniques is to form 6 to 8 inch wide aluminum sheet to the shape of the riser tube, and attach the alum sheets over the riser tubes with some silicone caulk between the two to improve heat transfer.  I think these would perfrom pretty well, and this techniques has been successfully used (see the Maine Solar link below).  There are some silicone caulks that have fillers to improve conductivity, but they are hard to buy in small quantity.

There are a couple fairly easy ways to form the sheet to fit the risers -- see links below.

Another option is to buy the absorber plates premade, and build the rest yourself.

Absorber plates using Maine Solar technique:

http://www.builditsolar.com/Projects/WaterHeating/MSClosedLoop.pdf

My "press":

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/House.htm

(this was for heat spreader plates, but same idea)

Another alternative is to use the extruded heat spreader plates that are sold for radiant floors -- these are pricey, but snap on the pipe with zero effort:

http://www.builditsolar.com/Experimental/CPVCCollector/cpvccollector.htm

This is the collector I built into my garden shed wall that uses premade absorber plates:

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/collectors.htm

By integrating the wall and collector you can save some money -- the net cost of the collector was only $8 per sqft (may be more now with current copper prices).

Some more ideas here:

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm

Just as a final thought, have you thought about making some or all of the collectors be air heaters instead of water heaters?  These are a ton easier and cheaper to build.  The downside is that there is no easy way to store the heat, but until you have quite a bit of collector area, storage is not really an issue -- you get some storage for free as the thermal mass or your house heats up.

Gary

[mbeland]

Thanks all,

My house is 28 x 40 basement, two main floors and an inhabited attic. The house is currently heated with a radiant floor hydronic system using electric boiler. That is why I considered water collectors instead of air collectors.

We also have a wood stove for back up.

I know that evacuated tubes are more efficient but lends themselves less to diy.

Martin

[mikeyny]

   My diy system uses epdm roofing rubber for a heat sink and it works very well. I am pretty sure I posted info on it on this site, maybe even with pics. It was cheap for me since I used scrap rubber left over from a job. you may also check your local scrap yard for parts. Where I live I can have anything I want out of the scrap yard. I just pay by the pound.

                                            MIke

[GaryGary]

I don't want to start this whole discussion on evac tubes vs flat plates up again, but for typical winter applications they have essentially equal performance.  

You can look up the numbers on the SRCC site, or use this calculator, and plug in your conditions to see which does the best for your situation:

http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm#efic

That said, if you don't go with the premade absorber sheets, its hard to DIY a collector that is as good as a good commercial flat plate.  But, its not really efficiency that matters as much as dollars per BTU -- if the collectors are cheap enough you can just use more area to offset the slightly lower efficiency.

Gary

[mbeland]

Excuse my ignorance but what is epdm roofing rubber? By heat sink do you mean absorber plate? If so, how do you assure good thermal contact with the piping? It seams that rubber is not a very efficient thermal conductor so it wood need to conduct the heat through the air by convection inside the collector which is not what we want from and absorber plate. Did I miss something in your post?

Martin

[mbeland]

Thanks for your comment. I have looked briefly at srcc ratings but I don't know the brand names and make of flate-plate collectors vs evacuated tube collectors. Nevertheless, these ratings seem to be for hot water systems (am I right?), not for water space heating. The ratings would then consider the global performance year round, whereas for space heating the most part of the demand would be in the winter where evacuated tubes are theoretically better. On the other hand, since space heating with radiant hydronic systems use water at temperatures that are lower than domestic water heating, there may be a trade-off here. Will the calculator show me where is the tradoff?

Martin

[mbeland]

Thank,
Among all the nice ideas that you propose for absorber plates, the one I prefer is the use of soffit alu sheets that are fitted to the tubes with a home-made press. I was surprised to see on your photos that you used plastic pipes inside the collectors. It sure would be cheaper than copper tubes but I would be scary of leaks and of low resistance to high temperatures. What is your experience?

Martin

[GaryGary]

Hi,

On the SRCC site they have ratings for whole systems, but they also have ratings for each individual collector they test.  On this page, the link is labeled: "Directory of SRCC Certified Collector Ratings".

http://www.solar-rating.org/RATINGS/RATINGS.HTM

They give collector output for Categories A through E  -- categories C and D would be typical of winter space heating applications.

When comparing two different collectors, you have to be careful to divide the heat output listed by the collector area to get an output per sqft.

The section on each collector labeled "Collector Materials" makes it clear what kind of collector it is.

A bit more on using SRCC stuff:

http://www.builditsolar.com/References/Ratings/SRCCRating.htm

The calculator that I mentioned in the post above allows you to enter a specific set of  weather conditions, and and a particular collector -- it then gives you the heat output per sqft and the efficiency.  You can run it for different types of collectors to see how well each works.  The data it uses is right out of the SRCC ratings directory mentioned above.

When you do these comparisons, look at weather for your whole heating season.  In our area, a significant fraction of the benefit comes in the fall and spring when we are still heating, but the conditions are milder.

Gary

[GaryGary]

Hi,

The CPVC pipes were really just an experiment.  Nice as they are to work with, I'm not sure I would try them it for a real collector.  CPVC has limited temperature capability and the performance takes a hit because of the low tube wall conductance.

If a person were to give it a serious try, I would limit it to a single glazed vertical collector with some form of overhang above the collector to protect it from summer overheating.  Protecting the CPVC pipe from UV exposure would probably also be a good idea.  

I've thought about using PEX, which is somewhat "tougher" all around than CPVC, and the tube wall conductance is better.  For vertical space heating collectors, a serpentine tube PEX collector that uses the formed aluminum fins with the fins placed between the sun and the PEX, and something like single glazed SunTuf (which cuts out UV) might hold up for a good long time.  It would not be as efficient as commercial copper/copper collectors, but it would be so cheap you could make a lot of it.  PEX-AL-PEX has better burst pressure, and once you push it into a bend it stays holds that position -- this should make it easy to do a serpentine collector with no joints.  Somebody should try this

Gary

[fcfcfc]

Hi Mbeland:

Hi also GaryGary... Mbe, you mention 4 collectors of 20 evac tubes, total 80 tubes. What are the sizes of these tubes..??.. Are they the double wall 58mm x 1800mm or the 47mm x 1500mm or some larger single wall tube..??.. Also what specifically are you getting for that $8K figure since labor is not included..??.. Please be specific...

[mbeland]

Hi fcfcfc,

I have received only very limited information so far about this quote which is rather informal.

It stated:

4 indirect panels of each 20 evacuated tubes panels which will generated a load of 100,000 btus on a good day with 2 80 gallons storage tank with double heat coils. Cost of 4 panels, 2 storage tanks workstation is approx $8000.00Can
FOB(Toronto) shipping extra  without installations. Design will included overheating during summer as system will heat domestic hot water and space heating during winter

Do you think this is a fair price? I guess it depends what these tubes are...

Martin

[mbeland]

For use in heating in winter, shouldn't I go for the most efficient that would be doable as a DIY in order do have any significant dent in my heating bill?

Also I have read somewhere on the board that pex would suffer from the heat at the joints. I know you said no joints but between panels, there would be joints,... no? Would the temperature be low enough outside the panel to make it safe?

Very interesting discussion.

Martin

[fcfcfc]

Hi Martin:

Well, lets say the evacs are 1K each, I.E. $4000 leaving $4.000 for two 80 gallon tanks with heat exchangers and some pumps, controller maybe. That seems a bit steep. You could pick up two 80 gallon Whirlpool electric water heaters at Lowes for about $750 total (don't connect electrically), an external plate exchanger for $200 or less, two circ. pumps one cast iron and one bronze and solar diff controller for $350 or so, then fill valves, expansion tank, pressure/temp gauge, pressure relief valve, other misc for $500 or less. Then just your piping and insulation and some other small misc.. so we are at $5800 including the collectors.