Passive solar help

[wildbill hickup]

With the price of fuel oil going the way it is I have decided to build a solar/radiant system for part of the house. I've got most of it figured out, but have a question on the panel design.

I have a bunch of 1/2" copper pipe (free) that I'm going to use to make the collector with aand I'm not sure which design would actually collect/tranfer more heat to water/glycol circulating through the pipe. Here are the two choices

(1)Get a piece of sheet steal lay flat in insulated box,paint it flat black and then fasten lenths of copper pipe to it flush to collect the heat.

(2)Instead of sheet steel for backing use a mirrored surface for backing and raise pipe 2 or 3 inches off the surface.

In both cases I have enough pipe to make a 3'X 5' collector with the pipes spaced 6" from each other. Pipe would also be painted flat black.

I would think that the second method would collect more as the pipe would be getting either direct or reflected sunlight from all sides. Also if I cradled the pipe in lets say 1" angle iron from the back side would the increased surface area help? Angle iron would also be black

Is my thinking along the right lines or would it make that much difference? The second method would be more costly and also more time consuming, so I'm wondering if the extra work will pay off.

Thank in advance Wildbill

[Vtbsr]

hello wild bill, I would use the steel adsorber plate. Try to find some sliding glass door glass, for the top cover. These standard sizes are about 50% less than ordered size. Make the collector the size of glass you find. You may find a throw away unit of glass.  If the glass seal is bad that is ok. Take a utility knife and cut the rubber seal to separate the alum spacer and glass. This will give you two tempered 1/8" glass covers then clean. I would get a sheet of copper for the adsorber plate. With copper you can spot solder the 1/2 pipe to the adsorber sheet. If you use the steel, paint it first with high temp flat black.You need a layer of fiber glass insulation behind the adsorber plate or all your heat will run out the back. If the steel and copper are in contact the steel will erode by galv. action. If you used the mirror idea I think you will reflect the solar right back out of the collector and will not get the heat. One more thing is the direction of flow in the collector. If the flow is in, on the bottom left side, then have the flow out on the upper right corner. If you dont the antifreze will just take the shortest run of copper pipe in the collector. I found that if you make 4 of these you can get all your hot water, or make it into a preheat system for your hot water heater.

[ghurd]

There are / were over 100 for sale in the classified section of this board.

Plenty cheap enough, if you are in the Carolinas area.

Don't use the mirror!

[Chagrin]

If pricing is reasonable use aluminum instead of steel and you'll get better heat transfer throughout the sheet (aluminum is five times more thermally conductive than steel) but you won't be able to solder it AFAIK.

Instead of fiberglass use the sheet foam. I assume this is what Vtbsr meant.

[LEXX]

Aluminum sounds good, it's cheaper than copper and lighter and more heat conductive than steel.  Foam insulation sounds bad, no experience with it in this type of situation but it seems that it should melt.  I used a piece of that hotwater heater wrap insulation (silver side toward aluminum backing) and then another piece of insulation behind that worked well.

LEXX

[Ungrounded Lightning Rod]

You don't want to use a reflector.  The total amount of heat available to your device will be the same regardless of whether you use a concentrator or a broad absorber.  A concentrator just changes the temperature at which you collect the heat (and bounces some back out).  A black surface in an insulating box under NON-tinted dual-pane window glass gets adequately hot for building heat or hot water.  It's simpler to build, doesn't require critical alignment, and isn't subject to extreme thermal stresses (and potential steam explosion and molten metal issues) from focused sunlight.

Copper-steel won't corrode if it's kept dry (which you can't count on in a sealed box).  But it also expands differentially with temperature changes.  You don't want that humping up and down once a day - the flexing will soon destroy the solder joints, collector plate, or (worst) the pipe (dumping your coolant where it can damage your house).  Spend (or scrounge) a bit and use a copper collector plate with copper pipe.  (It's a better conductor of both heat and electricity than aluminum, anyhow.)

[Vtbsr]

No i ment the fiberglass. If your antifreeze stops circulating the temp.will quickly rise to the melting point of the foam.  I do like foam for other applications.

[BT Humble]

I know I'm playing semantics, but what you're proposing is Active solar heating, not Passive. ;-)

Anyay, you'll probably want to have a look at this:

http://www4.tpgi.com.au/users/eafc/solarm/pdf/80_Solar_Heating_DIY.pdf

BTH

[Chagrin]

You're right - I hand't considered melting the foam.

I did some research and found that "polyisocyanurate" foam is appropriate for this purpose, however, as it has a 300 deg. F service limit. Johns Manville sells this under the name "AP" foil-faced sheathing and Dow Chemical apparently sells this under the brand name "TUFF-R". Obviously if your water hits boiling you've got larger problems than melted insulation, and antifreeze apparently turns acidic if left over 240 degrees (resulting in increased corrosion).

This is my first little bit of research on this - feel free to bash me with any other input.