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re: clean sheet solar design
22 dec 1999
johnnyvee wrote:
>...we are planning on a solar powered home, building site is located
>in southern maine.
the worst-case month for solar house heating in portland is december,
when the average outdoor temp is 26.5 f and the average daily max is
35.1. nrel says 150, 340, 340 and 940 btu/ft^2 of solar heat falls on
news walls on an average day, an ew-axis tracking south-facing solar
trough can concentrate up to 777 btu/day of solar heat, 480 falls on
a level surface, and the average yearly (deep ground) temp is 45.4 f.
the shore may be a lot cloudier than say, 3 miles inland...
>any advancements made in the last twenty years regarding passive design?
sure, there are better techniques, materials, and weather data, although
these haven't been used much yet, since "passive solar houses" are more
of a cut-and-dried hobby than a necessity, more a matter of look and feel
("picture windows") than performance.
>anyone have any info on the "solar slab"??
it can partially solar heat a house, say 20% in december, max. let's do
an example: suppose a 32'x32' 2-story house with real r24 sip (gumboard-
foam sandwich) walls and ceiling has 176 ft^2 of r4 windows (8.6% of the
floorspace) with 50% solar transmission and fairly low air leakage, say
0.2 ach (house volumes per hour), ie 55 cfm on an average winter day.
then its total thermal conductance is 176ft^2/r4 = 44 btu/h-f for the
windows plus 78 for the walls plus 43 for the ceiling plus about 55 for
air leaks, ie 220 btu/h-f, so it needs about (70f-26.5f)220 = 9.6k btu/h
or 230k btu/day to stay warm in the winter, above-ground. on an 80 f
slab with r10 insulation beneath (2" of styrofoam) and 6' deep perimeter
insulation, it also loses about 24h(80f-45.4f)1024ft^2/r20 = 43k btu/day
to the (r10) soil, for a total of 273k btu/day.
with a frugal 300kwh/month (34k btu/day) of indoor electricity usage,
it needs to store about 1.2 million btu of solar heat for 5 cloudy days
in a row. storing that heat in a t-foot-thick 25 btu/f-ft^3 slab with a
10f temperature swing requires tx1024ft^2x25btu/f-ft^3x10f = 1.2 million,
so t = 4.7 feet(!)
if 10, 15, 25 and 50% of the windows face nwes, they would collect 55k
btu/day of sun and might provide 20% of the home's heat on an average
day with an average amount of sun. we might add enough passive air heaters
to the south wall to collect all the heat the house needs on an average
day in the form of warm air (we could also do this by adding more windows,
but that means the slab has to be even thicker.)
the house needs about 60k btu to stay warm for 6 hours on an average
december day, which might come from the windows, with no heat storage
needed. on an average day, it needs to store about 180k btu of heat in
the slab over about 6 hours, at a rate of about 30k btu/h.
moving lots of 85 f air through lots of pipes in the slab with an airfilm
conductance of 1.5 btu/h-f-ft^2 and a ft^2 of heat transfer surface makes
30k = (85f-80f)1.5a, so a = 4,000 ft^2, eg 1,000ft^2 of top surface
(with no rugs) plus 3,000 ft^2 of tube surface, eg 100 4"x30' pvc pipes
(or should we use 1,000 harder to clean but environmentally-friendlier
hollow concrete blocks?) in the slab, say an array of 99 horizontal tubes,
33 wide and 3 deep, with 1' of horizontal and vertical spacing... but
those pipes are too large in diameter, since the heat must also flow
through the r0.6 thermal resistance of 6" of concrete. we might try 400
2"x30' pipes, with a 5' manifold trench on 2 sides of the slab...
>...simple/inexpensive/idiot-proof/stone-reliable solutions to heat
>and hot water would be appreciated.
sorry, this 180 cubic yard floorslab knerpwhistle won't make hot water,
but you might get a christmas card from the concrete contractor... :-)
let's see... simple/inexpensive/idiot-proof/stone-reliable...
i'd put a simple inexpensive reflective trough in a solar attic in
order to concentrate solar heat in water with no moving parts except
a small low-power pump, but maybe that's not straightforward enough.
how about a few big poly water tanks for heat and rainwater storage
(rich komp in jonesport, me uses rainwater for drinking (vs a deep well
through rock)) a 4" hydronic floorslab for heat distribution, and some
air-water heat exchangers (like auto radiators) for heat collection?
we might put the heat exchangers in the house or in a warmer low-thermal
mass sunspace or solar attic. higher air temps make heat collection and
storage less expensive, and sunspace and attic glazing can be a lot less
expensive than r4 windows, with higher solar transmission, since it doesn't
have to stop heatflow at night or on cloudy days.
so, we need to collect and store 180k btu/day. let's start with the attic,
since houses need roofs anyhow. we can always add on a sunspace to collect
more solar heat. a 90% transparent r1 polycarbonate south roof with a 1:1
pitch can collect 0.9x480x16'x32' = 221k btu of overhead sun and 433k of
south sun, a total of 654k btu (or 192 kwh) per day. looks good... (this
might also work with a shallower pitch and a 4' stemwall.) insulating the
north roof and attic walls, we lose about 6h(t-31f)724ft^2/r1 btu/day
through the south roof, making t 31+654k/(6x724) = 182 f, max.
if we store 1.2 million btu in 2 $400 1,500 gallon water tanks (rich has
never run his 1,000 gallon rainwater tank dry, with moderate conservation)
cooling from t to 80 f, then t = 130 f. good. hot water for showers, too,
with a graywater heat exchanger, eg a 55 gallon drum near a basement drain
with an overflow outlet.
we might make the attic 140 f (which also reduces the ceiling heat loss)
and store 30k btu/h with 30k/10 = 3k btu/h of thermal conductance, eg 4
$150 magicaire all-copper 2'x2' shw 2347 duct heat exchangers. each one
transfers 45k btu/hour between 125 f water and 68 f air at 1400 cfm with
a 0.1" h20 pressure drop. these might be mounted below one of grainger's
$109 86 watt 21k cfm 4c853 48" 315 rpm ceiling fans moving 5600 cfm with
about 86(5.6/21)^3 = 1.6 watts(?) how long would the fan last under these
conditions? maybe we need a higher temp dutch multifan. eur van aurdel
may have a better solution using low-power fine-wire heat exchangers.
the concentrator seems simpler, but then we can't store lots of junk in
the attic. either way, we'd probably want a few flat skylights in the
insulated attic floor with foil-faced foamboard covers that automatically
close at night.
>...so far we have a septic design, everything else in this project
>is up for grabs.
putting the septic tank next to the rainwater tanks would store more
solar heat and speed up the sewage treatment process by a factor of 30
or so in december. the rate doubles with every 10 c temperature rise.
nick
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