long term heat storage for unusual and inexpensive houses SOLAR.KnowledgePublications.com

long term heat storage for unusual and inexpensive houses
12 oct 1996
which is less expensive or more convenient for space heating, more solar
glazing and thermal mass, or more insulation? and how little backup fuel do
we want to use? a direct gain house can easily save 20% of an oil bill, but
to save or 90 or 100%, we need to change techniques. steve baer says it's
easy to make a 100% solar heated house. just turn off the backup system.
he also says the greatest discovery of solar investigators has been that
if we use lots and lots of insulation, we need very little heat for a house,
from the sun or any other source. but superinsulation doesn't heat water.

and when david boyer and i put 200 free 55 gallon drums full of water in his
local $1000, 20x100' r1 plastic film greenhouse last winter, a structure the
size of a house, with 1/20 the usual wall insulation, his propane heating
bill changed from $8,000 the winter before to $1,500 last winter. that was
a better first step than insulation. and i only used 25 gallons of oil last
winter to heat my old 4 bedroom stone house with 2 r10 and 2 r2 walls. 

we might build a 100% solar house in the seattle that has a very large time
constant, and no solar glazing at all. open it up in the summer, and let the
air heat it up, and close it up more in the winter, when it's cold outside.
how about a monolithic dome, half full of water, with a houseboat inside? :-)

an indoor pool, perhaps with fish... a water supply and heat sink and sewage
treatment system? monolithic has been building these domes for 20 years now,
all over the world, and some of them are very large, eg 260' hemispheres,
and they have been used as water tanks, with no extra waterproofing. so...
hire someone to build the dome in the normal way, for $25/ft^2 of base area,
then build a house inside, without much weatherproofing, then seal up the
dome door below and flood it? no need to worry about floods :-)

or hold up the house with beams or cables into the concrete walls, in a more
normal way, and excavate first, so the pool is underground, and the house is
at ground level... in that case, the foam and concrete might not need to be
so thick. how would we warm or cool the water? perhaps let air flow in over
the water through some holes in the dome perimeter at ground level, with a
vapor barrier floating on top of the water. 

how big would the dome have to be to get through a 6,000 degree day f winter
with a temperature change of less than 10 f, without using any solar glazing
at all? we need a time constant of about 1 year, roughly 10,000 hours. a
standard monolithic dome hemisphere has 3" of r20 exterior foam with 2" of
reinforced concrete inside. filling it up with water completely makes
rc = 20/(2pir^2) x 2/3pir^3x64 lb/ft^3 = 427 r = 10,000, so r = 10,000/427
= 24 feet. if the 300 tons of water inside were 78 f in the fall, and it
lost 24x6000x1152ft^2/r20 = 8 million btu of heat over the winter, the water
would be 65 f in the spring. expand the dome to 40', a more normal size for
a house, use less water, add some solar glazing, eg some windows above, or
a transparent skirt around the lower south quarter, outside of the foam, with
an air gap between, and a northwest winter should be no problem.

how about building a house on top of a strawbale pond foundation, inside a
$1/ft^2 commercial plastic film greenhouse? the ponds might be 2 bales deep,
about 32" tall, inside a standard 30' wide greenhouse like this: 

     ---                  .
                   .             .           
               .                     .          
     15'     .                         .     
           .                             .
          .                               .   
         .                                 . ----
        .b                b                b. 32" 
     ....b................b................b.................
        |                30'                |

        .....................................
        . . . . . . . . . . . . . . . . . . . ---
        . .               . .     13'     . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . . 62'         . . 64'
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . .               . .             . .
        . . . . . . . . . . . . . . . . . . .
        ..................................... ---

the 2 ponds might be 13' wide x 62' long x 32" deep inside, made with about
170 bales and a few 2x4s and dacron rope and ground stakes, and 2 pieces of 
epdm rubber, 20' wide x 70' long. they would each hold about 17k gallons of
water, enough for a good water supply. and enough to hold the structure
down without using any ground stakes, eg on a flat city roof.

the house might sit on a $3/ft^2 corruform concrete slab, poured on top of
currugated metal, as in office construction, or some thin reinforced 15'x 8'
concrete slabs, with a rib along the bottom. the concrete would act as a
vapor barrier above, and we might add a vapor barrier under the concrete,
eg a floating vapor barrier on the water, some sort of plastic film or oil.

build the house out of strawbales, too? unweatherproofed ones, since this
house is out of the weather. with an arched unweatherproofed roof with bales
running lengthwise south to north, on top of a vapor barrier, with 2x4s on
18" centers running east and west under that, attached to some flat arched
2x4s under that, and a few horizontal north-south tension members under that?

as to thermal performance, strawbales are about r50, so we have (roughly)
rc = r50/2000 ft^2x32"/12x2x13x62x64 = 6877 hours, or 287 days or 41 weeks
or 9.4 months. not quite passive annual heat storage, but this stucture has
a lot of glazing, and it could have a ceiling fan blowing some warm air down
under the length of the floor. if we start off with this house at 68 f on
december 1, where i live, where the average december temperature is 36 f,
it might cool to 36 + (68-36)exp(-30/287) = 64.8 f after a cloudy month.
we rarely get 30 cloudy days in a row. perhaps that happens more often in
seattle, where the average december temperature is 40.5 f, and the average 
amount of sun that falls on a south wall in december is 420 btu/ft^2/day.

how warm could we make this house in seattle? 420x15x64x0.9 = 363k of sun
might get into the greenhouse, on an average 6 hour solar collection day,
and if the strawbale roof touched the peak of the greenhouse, so not much
heat was lost from the north side during the day, 6hr(t-40.5)960ft^2/r0.8
= 7200(t-40.5) btu/day would be lost through a single layer of poly film,
and another 18hr(t-40.5)2000ft^2/r50 = 720(t-40.5) btu would be lost during
the night, so 7920(t-40.5) = 363k, ie t = 40.5 + 363k/7.9k = 86 f.

sounds good to me, especially since we didn't count the sun that falls on a
horizontal surface in december. or in june. but cooling is another story. 

nick