Hi,
Just to start with what I think would be your best bet if you can:
- Separate the two 21 pipe assemblies so that you can lay them out next to each other rather than in front and behind. This basically doubles your collection area, and is a hands down winner over anything else you could do (I think). All things being equal, it will double the hot water output of the system.
- If you only have one of those sliding door panels, use a utility knife to cut the edge seal and make two single panels that can be used to glaze your now 6 ft by 6 ft collector.
- The R40 is overkill. You have R1 or R2 glazing, so you really don't need the sides and back to be much bette than (say) R7 (one inch of polyiso insulation board) to get the heat loss through the back and sides to very quite small compared to what you are going to lose out the front no matter what you do. That is, when you have 10 BTU going out the front for every 1 BTU going out the back, you don't gain much of anything by cutting that 1 BTU out the back down to half a BTU with very thick insulation -- you are still losing nearly all the heat out the glazing. This will make your box thinner and more managable (and cheaper).
- Be sure you use high temperature insulation inside the collector -- the white, pink, or blue insulation board (polystyrene) will melt in collectors. Use Polyisocyanurate (polyiso) -- sounds exotic, but most lumber yards have -- they may not know it by than name, so go look at the panels -- they will say polyiso on the panels.
You are kind of designing this like a batch solar water heater:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#BatchBut, that does not make sense, because it only has about a gallon and a half of water in it -- better to think of it as a collector -- like these:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Example1KSystemsYou can do the copper coil wrap around the tank -- here is Doug's system that does that:
http://www.builditsolar.com/Projects/WaterHeating/DougThermosyphon.htmI think that it works pretty well for him -- I'm sure its not as effective as a coil inside, but it may be good enough.
Another option is to do the system like this one:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htmBasically, there is a big, unpressurized tank that stores the solar heated water. When you are not pumping fluid through the collector, the water in the collector drains back to this storage tank.
To get the heated water into your regular hot water tank, the system uses a single pass heat exchanger immersed in the big non-pressurized solar storage tank. The incoming cold water passes through this single pass heat exchanger on its way to the regular hot wate tank.
I use a 300 ft coil of 1 inch PEX for the single pass heat exchanger -- others have used copper.
Anyway, if none of that is feasible and you just want to continue with the radiator the way it is:
- I'd say inlet to the back coil and outlet from the front coil -- but I'm not really sure.
- Painting black inside or using reflector is about the same by tests done years ago in batch heaters.
- R40 is still way more insulation than you need.
- I think you want to run the pump much more often -- If you have about 18 sqft of glazed area, the 1.5 gallons of water in the radiator is going to heat up (very roughly) at 3 F per minute -- so, in 10 minutes it will be up 30F. You don't want to leave in in much longer than that or the radiator will get so hot that it will just lose a lot of heat out the glazing. It would be better to run a small pump all the time. A good pumping rate would be 0.04 gpm/sqft or about 0.7 gpm for 18 sqft.
Have you thought about how you are going to protect the collector from freezing? Or, maybe that is not an issue?
Gary