re: starting over with passive design
12 nov 2002
david delaney wrote:
>for several years i have been planning to build a solar house in
>ottawa, ontario. (6800f/3800c degree-days, 45.3n, 25% sun in december).
burlington, vt looks similar, with 7771 f dd, 23.0 f in december,
30.4 average daily max, and 630 btu/ft^2-day of sun on a south wall.
>the goal is to get 100% of space heat and a good chunk of dhw heat
>from the sun.
challenging... 100% solar-heated houses can benefit from 2 heating systems,
a low-temp system with lots of efficient solar collection area for an
average winter day (and night) and a higher temp system with a small
collector and a large heat store for a few cloudy days in a row...
>for the last year or so i had settled on a minor
>variation of norman saunders's cliff house
>(high temperature hotstore in attic (10 tonnes of water) charged by
>thermosyphon from a hot air collector...
as i recall, it seldom gets over 109 f.
>low temperature warmstore (100 tonnes of small stones) under concrete
>first floor slab for radiant transfer to habitable space through the floor.
haven't looked at your web site yet, but i don't seem to recall radiant
floor heat in the cliff house, just fan power. why 100 tonnes? you might
use less, with more insulation.
>bin of stones warmstore charges by fan-driven down duct from attic hotstore.
i think norman's rationale for the stones is a) a 100% solar-heated house
is as likely to require cooling as heating at any particular instant in
wintertime, and it's better to save that extra heat energy rather than
dump it to the outdoors on a suddenly warm winter day, and b) a cool store
provides another "active source" to control the house temp better, as
in a cmos circuit or a push-pull amplifier. (norman is an ee by training.)
>this fall i got very close to committing to building such a house in the
>spring of 2003. ( i've bought the land.) but i sat back and looked at my
>plans and became dissatisfied.
the dangers of introspection... :-) the best is the enemy of the good.
>to get to a workable plan based on saunders's design i
>had given up one of my main goals: a house that could be kept
>comfortable with no purchased energy even if every electrical
>thing in it broke. the down duct fan, its motor, and its control require
>just too much engineering and are too dependent on electricity,
>even if the electricity could be solar. if these things break the house
>just does not work until they have been repaired. so i've thrown
>all my plans out.
sounds like the great northeast ice storm a few years ago. toppled towers,
people without power for weeks at a time. redundancy can make a system
more reliable (available), given electrical power. and pvs can provide
another source of electrical power...
with purchased power or non-zero interest rates, a 100% solar-heated house
with lots of fans might end up costing more to operate than the annual fuel
bill for a simpler superinsulated house without all the mass and glass.
there are lots of heroic one-of-a-kind solar experiments that never became
popular because they were "uneconomical." you might consider whether your
design is likely to be replicated many times because it fits what most
people need, or not. a woodstove or propane heater might provide heat
with no electricity...
>i now believe that i have to have a basically passive house. "basically"
>means that i can have fans and controls to maintain tight automatic
>control on temperature, but if they break, manual control can keep
>the house comfortable without having to light a fire.
norman's own house ("experimental manor") is all manually-controlled...
>i believe this necessitates dedicating the (very thick) south wall of the
>house to a very large collector (600 ft^2) and thermal closet
>(20 tonnes water-equivalant). the thermal closet is charged
>entirely by thermosiphon, and discharged by thermostatically
>controlled fans, as long as the fan works, and by thermosiphon
>through manually controlled dampers if the fan does not work.
sounds reliable. a warmstore in a wall vs attic can both charge and
discharge passively, and the mass can be easier to support, given
earthquakes and psychological unease.
why such a large collector and heat store? got arithmetic?
is there a way to have a passive 2-temp system? i think so, altho that
may be less economical than a system with fans. maybe the average-day
system could be passive, and the cloudy-day system could be active...
say our 8' cube with r48 walls and conductance g = 5x8'x8'/r48 = 6.66 btu/h-f
for the non-south walls has a south mass wall with an r1 layer of glazing
with 90% solar transmission. on an average day, it gains 64ft^2x0.9x630
= 36,288 btu. at an average collector temp t, it loses about 6h(t-27)64/r1
= 384(t-27) btu/day to the outdoors. at night, with r48 insulation, it loses
18h(t-23)64/r48 = 24(t-23) btu. at 65 f, the rest of the cube surface loses
24h(65-23)g = 6240 btu... 36288 = 384(t-27)+24(t-23)+6240 makes t = 100 f
(vs 166 with 2 layers of glazing.) assuming the mass has infinite surface,
it needs to be 65 f at dawn, which makes it 135 at dusk, and it needs to
store 18h(65-23)g = 5040 btu at night, ie (135-65)c = 5040, so c = 72 btu/f.
the cube needs about 5x24(65-23)6x64/r48 = 40.3k btu for 5 cloudy 23 f days
in a row. if the mass is 65 f at the end of 5 days and starts at t f on the
first day, c(100-65) = 40.3k, approximately, so c = 1138 btu/f.
now suppose we have 2 heat stores, 72 btu/f for an average day, with
a 135-65 f temp swing, and c for 5 cloudy days, starting at t (f).
the average sunspace temp is still 100 f (we can't afford higher,
without more cube insulation or another layer of glazing), and 0.8x630
= 510 btu/ft^2-day shines through an r1 closet glazing inside the sunspace.
if the closet is perfectly insulated, 510 = 6h(t-100)/r1, so t = 185.
probably optimistic, with no selective surface. let's say 130 f (i've
seen 157, in experiments.) then c(130-65) = 40.3k, and c = 620 btu/f.
so it looks like two stores can save significant thermal mass (692 vs
1210 btu/f) over one, even with infinite mass area. in the real world,
the cloudy day mass can have a lower surface to volume ratio, eg larger
water containers, which can be cheaper, with simpler refilling every
few years to make up for evaporation.
>(the charging backdraft damper is a house-wide cold-side plastic film
>at the bottom of the wall between the thermal mass and the collector. )
the damper might be more sensitive if it were on the warm-side,
and hinged at a point on the wall well above the slot, but i suppose
that means more heat loss at the bottom. multiple dampers could be more
reliable. norman says they need inspecting every 2 weeks or so for rips,
folds, stickiness, and so on. this "house-wide damper" could be more
expensive and lose more heat than a system with smaller airpaths and
several fans powered by individual pv panels. would it be more reliable?
>i want to be able to leave the house unoccupied with fans off and
>the dampers at a suitable setting. when i return after a month,
>i want to be confident that the house will be comfy, perhaps a
>little too warm or cool, but not cold, and with lots of spare stored heat
>to warm it up by opening the dampers further or turning on the fan control.
sounds doable in principle. you don't want the pipes to freeze.
have you looked at passive thermostatic dampers for greenhouses
and "automatic foundation vents" for masonry walls?
>the dampers and flow through the thermal closet can be arranged so
>that hot air can be delivered passively during heat collection to both the
>living space and the thermal mass, with a reversal of air flow through
>the thermal mass happening at night for discharge. (in other words
>i can get heat passively during the day without disrupting the flow
>of air charging the closet.)
sure. that's cheap solar heat, requiring no storage.
>i have been getting some quotes for the materials for a suitable thermal
>mass for the closet. i want a heat capacity of roughly 20 tonnes
>(22 tons) of water. but water is a lot of trouble. it can leak,
>and it requires a lot of supporting structure for the many
>small containers required to get sufficient surface area for
>efficient air-water heat transfer.
the average day store might be a collection of vertical 4" pipes. the cloudy
day mass can have a lower s/v ratio. it might be some 1' thick waterwalls
made with plastic film liners for 55 gallon drums on shelves made from 2x4s
and 2"x4" welded-wire fencing, with a few inches of air between the walls.
those 9x9x13" tall ropak 4 gallon plastic tubs with tight-fitting lids can
stack 20' high. made in fullerton, ca, and used for soap, and dried fruit
from the cherry central cooperative in traverse city, mi 48684. i got about
1,000 free from my local recycling center.
>my prime candidates are a bin-of stones wall...
...2-3x more volume than water, with more airflow resistance. have you
looked at section 3.16, "heat transfer and pressure drop in packed beds,"
pp 176-178 of duffie and beckman's 1991 solar engineering? also section
8.5, pp 393-400, and table 13.2.1, p. 517? figured out how that will
here's a typical calc:
a cubic foot of d" rocks has about 12^3/d^3 rocks with about 12pi/d ft^2
of surface, eg 12pi/(3/4) = 50 ft^2/ft^3 for 3/4" clean (not "modified")
stone. you might lay 6 4"x8' perforated pipes with an up-elbow at one end
on the bin bottom (for easy airflow), cover them to about 9" depth with
4x8x9/12-6x8xpi(2/12^2) = 20 ft^3 of stone with about 1000 ft^2 of surface,
then stack the 108 tubs on top. a low-power fan might move 500 cfm of air
through them with a velocity of 1000 lfm in each pipe. this would flow up
through the rocks at 16 lfm, ie 0.08 m/s, with 60 c air weighing 1.06 kg/m^3,
which makes the mass velocity go = 0.084 kg/m^2-s.
the dunkle and ellul correlation says the air pressure drop through the
rock bed is dp = lgo^2/(rhod)(21+1750mu/(god)) pa, where l is the bed
length in the direction of flow (8" is 0.2m), rho is the air density,
mu is air viscosity (2x10^-5 pa-s at 60 c), and d is the rock diameter
(3/4" is 0.019m), so dp = 0.0703(21+2.08) = 1.6 pascals or 0.0065 "h20.
>i did the estimating for 100 tons of stones or concrete blocks.
>i can get very nice washed river stone in ottawa 1.5" to 2.5",
it's nice they are already washed...
>void fraction 0.4, for cdn $60 per yard delivered plus
>cdn$40 per 13 yard truckload. at a density
>of 2400 kg/m^3 for stone, the density of the river stones is
>(0.765 yd^3/m^3)*(2400kg.m^3)*(1-0.4)=1101 kg/yd^3,
>requiring 91 yards for 100 tonnes, at a total delivered cost
>of (91*60)+(91/13*40)=cdn $5740 + 15% tax.
>by the way cdn $1 = us $ 0.637 today, november 11, 2002.
so $5740 + 15% is $4205 us. you might save the 15% by delivering them
across the border and smuggling them back in one by one. remember that
guy in california who collected rocks? he moved out of his apartment
one night and the landlord discovered it was packed with rocks from
floor to ceiling all over, except for a few narrow paths from the front
door to the kitchen sink, from the kitchen sink to the bathroom, etc.
>the design of a dry stacked concrete block wall seems to be delightfully
norman's simple dry-stacked basement walls tipped over...
>the concrete blocks (nominal 8"*8"*16") weigh 42 lbs per block and
>cost cdn $1.30 + 15% tax each delivered. 100 tonnes requires
>1e5*2.2/42=5238 concrete blocks at cdn$1.30 = cdn $6810 + 15% tax.
thinking of stacking them up makes my back ache already...
>the concrete blocks take more space too, which is more expensive
>than the material. (42 lbs each, 8"x8"x16").
>a four foot cube of carefully stacked blocks requires
>3x6x6=108 blocks and weighs almost exactly 2 tonnes (2.062 tonnes to
>be precise). so 100 tonnes is 50*64ft^3/27=118 yds as compared to
>91 yards of stones.
or 20,000/15 = 1,333 15 liter ropak tubs at about 1333 ft^3, ie 49 yd^3.
>this is deceptive however, because the bin of stones requires an
>engineered structure to contain it, which adds considerable volume,
>while the block wall can be dry stacked without any structure
>to hold it together.
harry thomason's rock bins were compact, with steel cables across the top.
>the cores of the blocks would be aligned vertically to give 36 vertical
>air pipes for every 64 ft^2 of horizontal area of the wall, and lots of heat
>transfer surface between the air and concrete.
you might build a dry stacked block wall with vertical channels, then
thread 4" water pipes through the channels, although you could squeeze
in more pipes with 1x3 spacers vs blocks.
>it is a bit tricky to arrange a cold air plenum at the bottom of the
>block wall. i'd have to get a structural engineer to design a steel pallet
>structure for the wall to stand on. does anyone have any better ideas?
sideways blocks under some sort of wire mesh, with gaps between the blocks?