re: thermal mass/passive solar information
16 mar 2002
>jr wrote re:
>> "the passive solar house" (j. kachadorian)...
>the "solar slab" as presented in the book is a reasonable idea, and
>if it's your intention to have 40% or so of your heat supplied by solar
>(maybe 70% if you live in the southwest of the us), and you absolutely
>*must* have a 100% passive design, go for it...
i seem to recall that this slab uses a fan. it might make more than a
40% heating fraction, with lots of airflow and slab channels (ie heat
transfer surface) and a carpet on top of some foamboard over the slab and
a low thermal mass sunspace with separate 100 f air ducts between the
sunspace and the slab and lots of house insulation, eg 12" r48 sips.
then again, we might make the house walls hollow block with the holes
lined up so air can naturally flow vertically through the walls, with
lots of insulation (eg dri-vit) outside the block.
but either way, we seem to have lots of little inaccessible nooks and
crannies to attract dust and spiders and varmints.
>without backup heat, a passive solar design will get too
>cold in (at *most*) 3 days, and only one day without going
>to herculanean measure for both insulation and mass...
storing heat for 5 cloudy 30 f days in a house that cools from 75 to 65 f
means 65=30+(75-30)exp(-120h/rc), so rc = -120/ln((65-30)/(75-30)) = 477 h.
a 48'x48'x8' house with an 8" 25 btu/f-ft^3 slab with c = 48x48x8/12x25
= 38.4k btu/f needs a max thermal conductance g = 38.4k/477 = 80.5 btu/h-f,
or 57.5 for 3748 ft^2 of r65 walls and ceiling, after subtracting 4% of
the floorspace as r4 windows, with no air leaks or internal heat gain.
making the house 32x32x16' tall with a 17k btu/f slab and 2048 ft^2 of
5 btu/f-ft^2 block walls makes c = 27.2k btu/h-f, so g = 27.2k/477 = 57,
or 36.5 for 2990 ft^2 of r82 walls, with no air leaks. more herculean.
>the solution is to have a separate heat store insulated from
>the living space. most probably that will be water, since
>it has the highest thermal storage density of common materials
>(more than double that of concrete)...
sounds good to me, with a hydronic slab.
>two quick ideas for heat collection...
>1. hot water panels on the roof. if you really want 100% solar
>through the winter months (and you don't live in the deep sw),
>they pretty much have to be thermomax or equivalent...
harry thomason trickled water over a roof under a layer of plastic and
a layer of glass, with a 2/3 solar heating fraction for the us customs
house in richford, vt. trickling a 50% 2:1 cacl2+licl solution over a
dark asphalt shingle roof under a single layer of corrugated polycarbonate
plastic might be a painless retrofit, with 4x4 sleepers. the desiccant
solution could store more cloudy-day heat than the same volume of water,
with less thermal loss due to condensation under the glazing.
>a better idea is to enclose them (particularly for less expensive standard
>collectors), which increases their efficiency.
sounds good. big fins in a sunspace, with "thermal cogeneration," as the
sun heats the panels and their "waste heat" (warm air) heats the house.
>2. an idea that's been thrown around this group quite a
>bit is a concentrating solar "mirror" in an attic with a
>transparent south-facing roof. this is quite a cheap
>way to go, but there is also a bit of engineering to do,
duane johnson mentioned a recent patent for a "clfr" (concentrating linear
fresnel reflector?) imo, this kind of thing requires more tinkering than
engineering. cutting kerfs in 2x4s, trying to avoid meltdown, with some
pvs under 3 suns in the shallow water pool...
>...there are no books explaining it as far as i know. i've done the
>numbers on this for several house designs (in mo and co), and it
>definitely works (although quite a bit easier in co !).
somebody should build one and write a book. how about you?
you might do a licl house and a bubblewall house as well.
>...there seems to be a conspiracy on the lack of good fundamental books
>on this subject...
solar house heating is pretty ho-hum for most people these days.
letting oil prices rise or making house heating a competitive sport
like america's cup racing would help (no outboard motors allowed :-)