Hi Mike,
"I took it as data on a possible retrofit. at 3.9+- kwh/day(7000+ btu/day) from essentially one set of rafters(trusses)(41sqft). "
I think if you live in a warmish climate with fairly high ambient temps, and you can use relatively low collector output temps, that might be true, but the lower your ambient and higher your collection temperature, the less you will get. If go to the efficiency equation they provide, it is:
efic = 19.1 - 525(Tinlet - Tambient)/Isun
As (Ti - Ta) goes up, the efficiency drops. So, what is the maximum Ti-Ta you can get out of the collector before the efficiency drops all the way down to zero?
If you assume 800 watt/m^2 sun (about of 80% of full sun), then your efficiency would drop to zero (ie no heat output) when Ti - Ta = 29C or 52F -- that is:
efic = 0 = 19.1 - 525 (Ti - Ta)/800
(Ti - Ta) = 19.1
0.656 = 29C (or 52F)
So, if its 30F outside (an average mid-winter HIGH where I live :-)), then the max output temp would be 80F, and you would get precious little of that (since this is the zero efficiency point). Not too many things you can do with 80F water or air?
This does not work for me, but it might work for you if you live in a warm climate.
Or, maybe I am missing something?
"I have been doing some internet digging into Nick Pine's solar thermal closet storage techniques and trying to put some costs together for an even lower tech version of this. radiant barrier, polyisocyanurate foam board box full of 2-litre bottles. "
I guess the 2 litre bottles are OK for life? I tried a little experiment with adding some thermal mass to my air thermosyphon collector using gallon bottles of water from WalMart, and they were all falling apart in a few months.
"I should be able to safely put about 55gallons worth over a four truss area ( I design trusses for a living and checked the set on my house). Looks like the water would need to go at the top of the 'collector'- in a closed box (thermo-siphon type closet) and then forced air to the house from the 'solar closet'. in thinking about it, there's a few other oddball items to add--- attic water heater drain pan, hacked automatic foundation vent (open at 150F?)(bottles get deformable above 140 to 150F), duct booster fan and flex duct, temperature controlled relay (24v control wiring in house plus box temp set point(short cycle the standard hvac unit with additional solar heat).
ther is bound to be something I missed.
"The pex tubing is fairly expensive($1/ft?) "
I think the half inch is more like 40 cents a foot.
"and eliminating it would also solve the reshingling nails problem. "
Had not thought about that.
"a foil radiant barrier would pay for itself in a few years from AC savings
Yes, that's a nice double benefit.
"and with the 'closet' box at the top closed would heat the water (probably only a few hours of space heat after dark). I have enough trusses in my attic for the functional equivalent of five 55gal. drums of water (35000 btu/day). admittedly you would get at least twice the heat gain from a glazed one(if not more). The booster fan(250cfm) and associated hardware per 'closet box' is somehwere around the 150$ mark, add radiant barrier, drain pan and foam (and 2-litre bottles-last I heard you could get people to pay you to take them). Much like one of the window box units or the r1 low tech collector without it being visible (about half the efficiency, but a lot cheaper). anyway my .02$."
Again, I think the only problem with the scheme is if you have low ambient temps and/or want high storage temps - I think either kills this scheme by lowering the collection efficiency to nothing. But, again, maybe I am missing something.
Maybe a small test panel would be in order - you could just do one bay with the radiant barrier and foam (you could even skip the foam board on the test panel), and see what kind of flow you get. You can do a half way descent job of calculating output with a couple cheap thermometers (one at the input end and one at the output end), and one of these $25 air velocity meters.
http://www.dwyer-inst.com/htdocs/airvelocity/Series480Price.cfm
(a really helpful instrument for solar air people)
Output = (Tout - Tin)(Duct Area) (Duct Air Velocity)(Air Density)(Air Specific Heat)
Eg
Output = (110F - 60F)(1 ft^2)(90 ft/min)(0.07 lb/ft^3)(0.24 BTU/lb-F) (60 min/hr) = 4500 BTU/hr.
You can even estimate efficiency of the collector by doing the test on a good clear day, and looking up the solar input in a book, or use the RadiationOnCollector program at my site. Then: efic = (Output from above)(Collector area)(Isun)(Cos (IncidAngle))
If you do this, please post the results you get.
Another thought would be to bag the storage, and just use the collector as a way to collect warm air to go directly to the house. This way, the relatively low collection temps would not mater too much - it would still be warmer than the house air. The house thermal mass will store some heat if you let the temperature drift up to 75F or so. Nick Pine likes ceiling mass that has a low emissivity surface facing the room.
Gary
Here is another of the no glazing roof collectors:
http://www.dawnsolar.com/
The Dawn Solar collector was tested by either the FSEC or the SRCC, and the results published. As I recall they were similar to the one in the paper - OK for collecting luke warm water in warm climates.
