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Posted by nicksanspam on March 21, 2008, 2:32 pm
Marilyn seeks solar adaptions for

NREL says 820 Btu/ft^2 of sun falls on a south wall on an average 29 F
January day in Boise. North, east, and west walls get 180, 340, and 350.

Working backwards, a $00 cloudy-day heat storage tank and a $5 1000 Btu/h-F
used car radiator with its 12V fans (36 watts, in series) can keep a house
with a G Btu/h-F thermal conductance 70 F on a 21.6 F morning with a min
water temp Tmin = 70+(70-21.6)G/1000. With 2 foil-polyiso foamboard dampers
with limit switches and $0 windshield wiper motors, the radiator can also
collect and store heat from a sunspace.

If cloudy days are coin flips and we store heat for 5 days, the house can
be at most 100(1-2^-5) = 97% solar-heated. If a frugal 600 kWh/mo indoor
electrical use provides 68.2K Btu/day and a 4'x8'x3'-tall plywood tank with
a folded EPDM liner (and a 1/2% ACI-100 non-toxic corrosion inhibitor from
DW Davies) supplies 4x8x3x62.33(140-Tmin) = 5d(24h(65-29)G-68.2K) Btu to
the 65 F average house, G = 165 Btu/h-F max.

A 48'x48'x8' house with 180 ft^2 of R4 windows and no air leaks and a 30 cfm
80% air-air heat exchanger and an R40 ceiling and R30 walls would have
180/R4+48^2/R40+(1-0.8)30+(4x48x8-180)/R20 = 165 Btu/h-F, approximately.

If 20, 40, 40, and 80 ft^2 of north, east, west, and south windows with
50% solar transmission collect 0.5x20(180+2(340+350)+4x820) = 48.4K Btu
and electrical use adds 68.2K Btu and the house loses 8h(65-29)G = 47.5K
and the 140 F tank with R30 walls loses 8h(140-65)104/30 = 2.1K during
an average 8 hour solar collection day, the house mass needs to store
48.4+68.2-47.5+2.1 = 71.2K Btu of overnight heat, eg in C = 7120 Btu/F
(or less, with a waterwall near the south windows) with a 10 F temp swing
from 60 F at dawn to 70 at dusk.

RC = C/G = 43 hours lets the house cool to 60 F in -43ln(60-29)/(70-29))
= 12.1 hours, so it needs (16h-12.1h)(60-29)G = 20K Btu more night heat
from the tank and radiator, which can collect 22K Btu at 2.8K Btu/h from
a sunspace or a solar attic or a 140+2.8K/1000 = 143 F airspace inside
A ft^2 of R2 $/ft^2 polycarbonate "solar siding" with 80% transmission
if 0.8x820A = 8h(143-29)A/2+22K, ie A = 110 ft^2. A $0 1"x300' 13-gallon
PE plastic water pipe coil in the tank can provide hot water for showers,
with a few 4"x10' PVC pipes under the floor as a greywater heat exchanger.

Is this "passive"? Is it "green building"? Do we care? :-)


Posted by Morris Dovey on March 21, 2008, 1:41 pm
nicksanspam@ece.villanova.edu wrote:


I'm curious about Marilyn's response to that. ;-)

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by nicksanspam on March 23, 2008, 11:15 am

Haven't heard back from her, which is typical developer behavior :-)

Meanwhilst, check out Riverdale, one of Canada's first 12 Net Zero
Energy Houses. The powerpoint connected to this page is very informative.

For more information from CMHC on the EQuilibrium Housing Project:



Posted by Kitep on May 10, 2008, 9:54 pm

Nothing like responding to a post 1-1/2 months later :)

I noticed the above web site says Alberta pays $.67/kWh.  I assume that's
Canadian, so it's only $.66/kWh US.  I know it's a peak rate, but still
seems like a place where solar may be cost effective.

Posted by You on May 10, 2008, 11:10 pm

Hmmm, I would doubt that anyone on a BIG GRID is paying 67 Cents
($.67/kWh) a KwH.... Juneau, Alaska had it's Hydro Power cut
by an avalanche last month, and had to go on purely Diesel Generated
Power and they went from $.11/KwH, to a whopping $.52/KwH.  The locals
are looking to let some Blood Flow, from the Politico's who handed the
local Power system over to a Private Company, a few years back, now that
their rates have gone up 4 fold. $.67/kWh is way out of line for a
Grid Tied Power System... I would expect more in the range of $.067/KwH.
Maybe you slipped a Decimal Point, or something.

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