I saved a copy of your notes. I'll have to print them out and run
through the exercises.
I see. I guess I would be wise to try a small scale experiment before
investing large sums of money. Maybe I can try something with the old
run down mobile home or the small shed out back. Too late this year,
everything is already closed up for the winter. In the mean time I
will continue to moniter this sight and work on improving my knowledge
of things solar.
If you want the cottage to heat up quickly when you get there, you do
not want to have a lot of thermal mass in the *living space*, because
you are going to have to heat up all that mass when you arrive for the
However, if you want a solar collector to keep the cottage from
freezing overnight, you'll need some thermal storage somewhere to
supply heat during the night. This storage should be isolated from
the living space so that you do not have to heat it when you are there
on the weekends.
Air is a terrible heat transfer fluid.
- You need to move a lot of it. If you pick up 500 BTU/ft^2/day
off a 300 ft^2 collector with a 10F temperature rise, you'll need
to move 750,000 cubic feet of air a day. That's about 3000 cfm,
which is not going to happen with natural convection.
- You need big ducts to move all that air, which are big enough to
have internal convection. It is this internal convection that
makes it so hard to get an air thermosyphon to move the heat
where you want it to go.
- Air doesn't store heat, so you need to transfer the heat from
the collector to the air, then from the air to the store, and
then back to the air in the cottage so it can leak out at night.
I know you don't want to hear it, but water is a much better fluid
for this application.
Would it be possible to orient the cottage so that one eave of the
roof faces south? If the cottage is 26' east-west, you might build a
30' wide by 18' high trickle collector on the south slope, using
corrugated aluminum painted black with double-wall polycarbonate over
it. Use double wall for the strength rather than the insulation,
because you want it to deal with snow loads and winds without flexing
and rattling around so much that it develops stress fractures.
I would avoid glass at first. I'd want to actually see the polycarb
fail before I tried something so much more expensive. Note that the
polycarb you'll be using is more usually employed in greenhouses which
have very high inside humidity and temperatures above 60F. That's
pretty similar to the conditions you'll put it in.
In particular, I'd shoot for a water temp rise from 70F to 90F during
the day. Your collector will see maybe 800 BTU/ft^2/day insolation,
transmit 600 BTU/ft^2/day when it's dirty, and lose 160 during 4-5
hours of collection. So it'll put 240,000 BTU/day into the heat
store. Now you're cooking with gas. If you can get the whole cottage
down to an average of R-10 (x 1700 ft^2 area) the trickle collector
could keep the thing at 65 degrees in February. (But that'd require
a hydronic floor.)
My understanding is you'll use concrete block construction for the
basement, which will be 20' x 26'. Build a low wall 3 feet tall
across the short distance, perhaps 12 feet from the side of the
basement. Install 4 inches of EPS foam on the bottom and against the
basement sides (2 layers of 2" boards). Don't insulate the side
facing the rest of the basement. It would probably be better for this
insulation to be between the basement and the dirt, but that gets into
a bunch of details I don't want to consider.
Cover the resulting space with a 30' x 20' EPDM sheet, "folded up like
a Chinese take-out box" (this, like everything else, is taken from
Nick Pine's playbook). Fill it with water, cover with another 20' x
10' EPDM sheet. That's your 4500 gallon tank -- big enough to store a
few of nights worth of heat. It'll heat the house by conducting
heat to the air in the basement across that top face. You might need
more heat flow than that, but you can tinker with it after you move
in. I tend to like oversize tanks.
To keep the collector from baking the polycarb to an early death each
summer, you'll need to vent that collector to the air, top and bottom,
but only when you're not running the trickle pump, of course.
- 20' x 26' cottage
- 8mm 2-wall R-value 1.7, 81% transmission
- 30' x 18' trickle collector
- 6 gpm pump, 40' head: 1/8 HP pump, on for 4-5 hours a day
(electric cost: about $2 for a 160-day heating season)
- 20' x 12' x 2.5' insulated tank: 4500 gallons