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re: utility costs/ environment gain
12 apr 1999
james mitchell wrote:
>...many superinsulated homes have so much insulation that they do not
>require any form of aux. heat,including solar until the ambient temp.
>reaches -10 deg.c.
i first read this as -10 f :-) you know, i've heard of scandinavian homes
like that, but it seems to me that one with more than a few windows (eg 4%
of the floorspace as windows, a la boca code) can't do that without using
lots of electrical energy.
if m built a 32' cube with real r40 exterior walls and ceiling (including
thermal bridging) and 164 ft^2 of r4 windows and 0.1 air changes per hour,
most people would say she'd have a good airtight superinsulated house with
a thermal conductance of 164ft^2/r4 = 40 btu/h-f for the windows, 150 for
the walls, and 0.1x32^3/55 = 60 for air leaks, a total of 250 btu/h-f.
keeping it 70 f on a 14 f (-10c) day means (70f-14f)250 = 14k btu/h, ie it
needs an electrical power consumption of 4.1 kw, almost 3000 kwh/month,
which is hardly frugal.
otoh, she might add something like my $500 12' deep x 32' wide x 16' tall
sunspace made from standard commercial plastic film greenhouse components
over the south side, or a more dramatic 16' deep x 32' wide x 32' tall
lean-to atrium made from 9 $10 doubled curved 1x3 bows on 4' centers and
1x3 purlins :-) say she gets an average of 1,000 btu/ft^2 per day of sun
on a south wall in january, and 600 more btu fall on a square foot of
horizontal surface, and the average monthly outdoor temperature is t (f),
and the $75 1500 ft^2 of r1 4 story atrium glazing has 90% transmission
so it gathers 1000x0.9x32^2+600x0.9x16x32 = 1.2 million btu of solar heat
for 6 hours a day. then, conservatively, the house can stay 70 f until
1.2 m = 6h(70-t)1500ft^2/r1+24h(70-t)250, ie t = 70-1.2m/15k = -10 f.
where i live, the average outdoor temperature in january is 30 f, so this
cube would need 24h(70-30)250 = 240k btu/day to stay warm, or 1.2 million
btu for 5 30 f cloudy days in a row. this might come from an 8' wide x l'
long 1 story shoebox with a deck on top in the sunspace, with 120 f water
cooling to 80 f over 5 days, so 1.2 million = 8x8x64l(120-80), so l = 8',
and the shoebox is another 8' cube. a house with less insulation might
have an 8' deep x 16' wide 2 story shoebox/deck in the sunspace.
>...some designs use an air to air heat exchanger... the problem here is
>the heat exchanger. most i`ve seen for residential use are imho not worth
>the cost for what you get...
a double wall woodstove chimney might make a good air-air heat exchanger,
or a triple wall air-cooled "all-fuel" fluepipe with cold outside air
coming down the outside cavity to a point lower than the air intake for
the woodstove.
this is a "counterflow air-air heat exchanger" as on page 3-4 of the 1993
ashrae handbook of fundamentals. say house air enters the inner fluepipe at
thi = 68 f (when there is no fire) and cold outside air enters the top of
the chimney at tci = 30 f, with 50 cfm of airflow. and the inner fluepipe
is 6" in diameter and rough on the outside (wrapped with a few layers of
chicken wire), and 32' long, inside an 8" fluepipe.
the inner surface is a = pix32'x6"/12" = 50 ft^2. the u value might be
about 1.5 for the inside and 3 for the outside, with a small blower moving
down outdoor and ceiling air at 4 mph for an overall thermal conductance
u = 1/(1/uinside + 1/uoutside) = 1/(1/2 + 1/(2+4/2) = 1.33, so the number
of exchanger heat transfer units ntu = au/cmin = 50ft^2x1.33/50 = 1.33,
roughly, and e = ntu/(1+ntu) = 1.33/(1+1.33) = 0.57 = (tco-30)/(68-30), so
outdoor air warms to tco = 30 + 0.57(68-30) = 51.7 f on the way in.
with a fire with 20 cfm of 600 f fluegas, we might have ntu = 3.33, so
e = 0.77, and the outgoing flue gas temp, tho, might come from the formula
(600-tho)/(600-32) = 0.77, so tho = 600-0.77(600-32) = 163 f, and the water
in the flue gas condenses, raising the efficiency and making damp wood burn
better. the fluepipe joints would be installed "downhill," lapped so the
creosote runs back into the woodstove.
we could turn on the blower when the fluepipe gets hot, with a thermostat,
or when the humidity rises, in an airtight house.
>...i solved a similar problem by using large exhaust fans in the kitchen
>and bath. this kept the moisture down to a reasonable level...
moisture builds up in an airtight house in the winter. one might remove it
using a dessicant which adds about 1,000 btu of sensible heat to the house
as it absorbs a pound of water. the dessicant might be reconcentrated with
a solar still in the sunspace. it might store backup heat for the house
forever in an uninsulated tank 25x smaller than a water heat store. the
heat might be recovered by allowing moist air to rise up from a cool damp
crawlspace or basement floor. the basement floor would get colder, and the
house would get warmer, with this kind of absorptive heat pump. lithium
chloride costs $4 per pound, and it can absorb about ten times its weight
in water...
>...some people do a least cost alternative to make choices about when
>to stop insulating,and start solalrizing... using some oversimplification,
>you do the cheapest item among all alternatives first (the one that saves
>the most energy for the least amount of money) then the next cheapest...
this works better on paper than in real life in the sense that one can get
stuck in a local minimum via this kind optimization, and actually doing one
thing after another can mean undoing some things to do others later.
> usually,but not always,from a cost effectiveness standpoint only, solar
>is one of the last things done, if at all. this is due to the high cost of
>adding active solar of the type you envision...
there are less expensive alternatives to the usual "active solar."
>...solar heating systems are not like other types of heating systems.
>they require more involvement by their owners...
and there are more automatic and lower-maintenance versions...
nick
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