Solar heat project ineffective.

[C & E]

Well, I tried an admittadly cheezy solar heating experiment. I painted a piece of flashing flat black and backed it with a piece of corrugated cardboard. I then mounted it in a garage-shop window, 2" from the insulated panes in a double hung sash (roughly 24 x 44). A 2"opening on top and another on the bottom and the thing will bring the temp up 4* in a 336 sq ft area. Evening temps are in the teens and daytime temps in the low to mid

30's, mostly sunny, but as the days cloud over I'm slowly surrendering to the thermal flywheel of the equipment. So, what's the consensus? Not enough mass in the flashing? Too much glare from the glass (not low-E)? I had planned on a thermosiphon protruding from the bottom of the window at an angle about equal to our latitude ( Unfortunately, time, money and health have me doing this poor substitute. TIA for your thoughts!

[m Ransley]

How was it any different , it was behind a window glass right. So the suns energy came in anyway.

[C & E]

I was of the supposition that the angle of incidence of the sun rays to a more perpendicular glass surface would reflect more of the light than if the glass was inclined, causing the all of the light to pass straight through to the black flashing.

[rider89]

it sounds like it is pretty effective, actually, considering you have less than 8 s.f. in a vertical opening. To usefully heat the space you'll probably need about 40 s.f. at the proper sum angle. This is hard to do in common structures, which is why you see various style collectors on roofs.

The other problem with wall glazing is that it's a significant heat loss when the sun is not shining. If you're just heating a workshop during the day, it's not an issue.

bill

[nicksanspam]

Where I live near Phila, 40 ft^2 of R2 south windows with 80% solar trasmission might admit 32K Btu and lose 6h(70-30)40ft^2/R2 = 4.8K on an average 30 F January day, for a net gain of 27.2K Btu, enough to keep

336 ft^2 with 1000 ft^2 of exterior walls and ceiling 70 F for 24 hours if 27.2K = 24h(70-30)1000/Rv with Rv = 35 walls and ceiling, given night insulation for the window/collectors and enough thermal mass.

No. Vertical south glazing is fine for winter heating.

Unlike a window, an air heater might have insulation behind the absorber with no airflow nor heat loss at night :-)

That could work on an average day with R18 walls and ceiling.

Nick

[rider89]

the cost of doing all that to a shop space would heat it conventionally for decades.

[nicksanspam]

Nope. Dynaglas corrugated polycarbonate costs $1/ft^2...

Nick

[yourname]

In the north east, clear day radiation is worth ~64 btu per square foot of south facing glass. now, figure my well insulated 1000sq ft great room runs 40000 btus on a cold night, you do the math.

As far as sloping the collector, in the winter, with the sun low, it is not that important. For max year round efficiency it is probably better.

What might be fun with the oil/gas so high is to build a little outside collector, maybe a 4x8 plywood box with a little pump and some copper coils to collect and a car radiator or somesuch on the inside and a little fan. Might cost a hundred bucks to make, if you can scrounge some stuff. math says it would put out more than one of those plug in 1500 watt heaters, free.

[nicksanspam]

What does that mean??? NREL says 1880 Btu/ft^2 falls on a south wall on an clear January day in Phila.

BTDT. The OPs room was 336 ft^2. I assumed 1000 ft^2 of exterior surface.

How vague. Vertical is fine.

Just a toy. Do a whole wall, with thermosyphoning air vs water.

Nick

[yourname]

I am not arguing with you, just the numbers from the book 'from the ground up' from some years ago. Also fits my seat of the pants for my barn built then. 50 sq ft of south facing glass did not come close to what 200 gallons of heating oil did in a season.

Again, I am not arguing with you, merely stating what my room needs, and how unreasonable it is you expect that amount of heat from a small window.

Ummm, my point exactly

Not fer a hundred bucks you won't....

[Goedjn]

Just build a greenhouse/lean-to against the entire south side of the house, circulate the air when it's colder in the house than both (A) the greenhouse, and (B) you want it. In the daytime, when it's helping. you can use the greenhouse as extra living space, and even when it's too cold for that, it's still keeping that side of the house warmer than it would otherwise be.

7' is all you need. If you're going to invest in a big expanse of glass, you might as well enclose living space in it.

[nicksanspam]

Nice book. I can't find my copy, altho I've still got some copies of "From the Walls In," having given away lots of them. If that's an accurage quote, what did Charlie Wing mean by "64 Btu per square foot"? Full sun (AM2) is about 250 Btu/h-ft^2. I've heard Wing's books and lectures and TV show and Cornerstones school did such a good job teaching people how to save energy that Exxon bought him out after a couple of years, and now he spends most of his time sailing around the world on his large yacht :-)

Where I live near Phila, each $1 square foot of polycarbonate solar siding might collect the heat equivalent of 1 gallon of oil per year...

Mon temp sun gain loss net

Oct 56.4 1150 1035 142 893 temp is the 24 hour vs daytime average. Nov 46.4 990 891 202 689 sun is average Btu/ft^2-day on a south wall Dec 35.8 900 810 265 545 gain is 0.9xsun Jan 30.4 1000 900 298 602 loss is 6h(80-temp)1ft^2/R1 Feb 33.0 1080 972 282 690 net is gain - loss Mar 42.4 1070 963 226 737 Apr 52.4 950 855 166 689 ---- 4224 Btu/day x 30 days = 126720 Btu/year.

It would be unreasonable to expect that.

Nick

[yourname]

I did find my copy, and the number was more like 60, and it is actually more like the average net available btus, IOW, taking into account cloudyness etc, not sunny day as I had stated. It is however the number one would use to calculate heating. Also it was for Boston/Portland ME, Philly is somewhat further south, thus more available sun.

I did not have my reading glasses, nor the energy to fight thru the actual math, but there is a bunch of it there.