|
|
re: improving performance of dhw panels?
13 oct 2005
brian hall wrote:
>given the huge rise in heating bills expected this winter (i use natural
>gas for the furnace & water heater, when the solar is insufficient),
>i am wondering what i can do to improve the efficiency of the system
>in winter (in summer i get more than i can use- i am at 6200' in colorado
>springs, co)... about 4" of rock wool insulating the ceiling, don't know
>about the walls. cathedral ceiling over one end of the split level...
nrel says 800 btu/ft^2 falls on the ground and 1450 falls on a south wall
on an average january day in colorado springs with a 35 f daytime temp.
if 0.9x1450 = 1305 btu day enters a square foot of r1 vertical sunspace
glazing with 90% solar transmission and 6h(70-35)1ft^2/r1 = 210 leaves,
the net gain is 1095 :-)
>i just cleaned the solar panels this past weekend. i also cut down
>a tree that was shading the panels in the winter. there are other
>trees (and a house up the hill!) blocking my sun in the winter,
>can't get rid of them :-( wish co had "solar easement" law...
if you can't find unshaded space for a sunspace on the ground to the south
or east or west of the house, you might add a well-insulated ceiling and
replace the cathedral ceiling with a clear layer of dynaglas polycarbonate
and move air down into the house during the day as in soldier's grove, wi or
put lots of fin-tube pipe under the ridge, with insulation above, and pump
water up through the fin-tube during the day from the bottom of a big tank
on the ground and let thermosyphoning keep it from freezing at night.
frozen condensation can be a problem with a single-glazed sunspace. before
i put a good vapor barrier on the ground (black plastic under shredded oak
playground mulch), i could walk out into my cold 32'x12'x16' tall sunspace
with frozen condensation inside the poly film glazing and bright sun outside.
the frost reflected most of the sun, so the sunspace didn't heat up until
i removed it with difficulty, using a rubber window cleaning squeegee on
a long telescoping aluminum pole. we might melt condensation on the inside
of a single-glazed sunspace automatically with a house-sunspace circulation
fan that starts when the house needs heat and sun shines into a glazed box
with a 130 f snap disc thermostat, outside the sunspace, even though sunspace
air isn't warm yet. david delaney suggested a simpler solution, a pattern of
dark dots stenciled inside the glazing, eg 1/4" dots on 6" centers.
a house with humid wintertime air would lose heat to unfrozen condensation
inside single glazing, even with a good vapor barrier on the ground.
can we keep a sunspace full of 70 f air for more human comfort and solar
collection efficiency, with less-than-full-sun on people? full-sun is
uncomfortable, even in 70 f air, and makes it hard to see a wireless laptop
screen. if we drape 80% shadecloth inside the glazing, with a large gap g
between it and the glazing, hot air will rise, as in norman saunders' cliff
house. we might have a foil or suspended ceiling which doesn't radiate much
heat back down into the space, something like this, viewed in a fixed font:
|
| <-south house wall
|
| . . .
| big .
w fins? .
i .
n another .
d sunspace? .
o .
w .
s .
| .
| .
| .
| white surface .
|--------------------------------------
/.<==fan foil? 100 f? . |
/ . . |
/ . . |
| . |
| . |
| . |
| . |
| 70 f? . g | 8' s->
| . |
| . |
| . |
| . |
| . |
. \ . |
. \ |
. | white surface
---------------------------------------------------------
one of norman's patents says hot air rises to the right of the shadecloth
and falls to the left of the glazing if g is large, and it tends to rise near
both if g is small, so there's an optimal gap size that avoids convection
currents and lowers heat loss in the space between shadecloth and glazing.
we might vary the distance and do a smoke or hobo experiment...
lasko's 2155a 16" window fan ($53 at ace hardware stores) moves 2470 cfm
with 90 watts (at 27 cfm/w) at high speed. it has 3 speeds and a thermostat.
grainger sells lasko's $73 4tm66 air king "industrial" version with a 3-prong
plug :-) if all else fails and we have solar-warmed air near 8'x16' of glazing
with 128x225 = 28.8k btu/h entering in full-sun = (t-35)128ft^2/r1+(t-70)cfm,
two of these fans could keep the sunspace air temp t = 74.8 f while collecting
4.8x2x2470 = 23.7k btu/h at 74% efficiency, or more, if the house is cooler.
airsealing a sunspace isn't very important, compared to airsealing a living
space. outdoor creatures made holes near the ground in my poly film glazing,
but that wasn't so bad, since it was well-sealed above. once they made a few
holes, they had no urge to make more.
stuppy (800) 733-5025 sells 80% sundown shadecloth (item # 23691) for about
14 cents/ft^2 in various widths, eg 6' and 12', with 17 cent "poly clips"
(item # 1380) to grab and suspend it from a wire or cable. you can see out
but not in through it, like a window screen. you can also buy custom-sewn
pieces with grommets, with extra cost and delay. not worth it, imo.
nick
|
|
