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one way to build a high-performance passive solar house
24 dec 1996
step 1.

look up the average outdoor temperature in january, where you live.

the national renewable energy laboratory's free _solar radiation data manual
for buildings_ has this information for 239 us locations. nrel's phone number
is (303) 275-4099.

where i live, near philadelphia, the average january temperature is about
30 f or -1 c, and nrel's manual says that 1,000 btu/day or 3.3 kwh/m^2 of
sun falls on a south wall here on an average january day, with a ground
reflectance of about 0.2. a reflective surface in front of the wall like
ice or snow or white paint might add 30% to the solar power that falls
on the wall.

step 2.

estimate how much energy your house needs to stay warm on an average jan day.

for example, a very-well-insulated 30' x 30' (10mx10m) 2-story house with
about 2,000 square feet (200 m^2) of average us r20 (metric r3.5) walls and
1,000 ft^2 of r40 ceiling (100 m^2 of metric r7 ceiling) needs approximately
2,000ft^2/r20 + 1000ft^2/r40 = 125 btu per hour per degree f or 37 watts to
stay 68 f inside when it's 67 f outside. the energy needed to keep a house
warm is 24 (hours) times the product of (1) the difference between the
average indoor and outdoor temperatures and (2) the thermal conductance of
the house, ie the sum of each exterior surface area divided by its r-value.
the number of kwh/day needed to keep a house warm is 3400 times less than
the number of btu, using the same formula with different units.

this example house needs 24 hr x (68 f - 30 f) x 125 btu/hr-f = 114,000 btu
or 33 kwh to stay warm on an average january day, the approximate heat
equivalent of a gallon of oil.

step 3.

calculate how large a sunspace the house needs to stay warm, ie how much
vertical south glass or plastic film glazing area a low-thermal-mass sunspace
needs to gather enough solar heat to keep the house warm on an average jan
day with an average amount of sun.

if the low-thermal-mass sunspace has an insulated low-thermal-mass wall
between it and the house, ie a non-masonry floor and a non-masonry wall, with
no rocks nor bricks nor water containers nor collections of scrap iron inside
the sunspace, with a window fan to move most of the warm air into the house
during the day, the sunspace will be about 68 f (20 c) during the day. if we
let the sunspace get icy cold at night, the heat lost from the glazing over
an average 6 hour jan day will be about 6x(68f-30f)1 ft^2/usr1 = 228 btu/ft^2
or 6x(20c-(-1c))1m^2/r0.176 = 716 wh/day, so the sunspace glazing can provide
about 1000-228 = 772 btu/ft^2 or 3.3-0.716 = 2.6 kwh to the house on an
average jan day, in this example.

the example house needs about 114,000,000/772 = 150 sq. feet or 33/2.6 = 12.7
m^2 of sunspace glazing to keep it warm on an average day. say, a 16' high x
16' wide x 12' deep (5m x 5m x 4m) lean-to plastic film greenhouse, made from
standard commercial greenhouse hardware, including 5 long curved galvanized
pipes costing $35 each, with a lightweight gravel floor over plastic film on
the ground.

step 4.

estimate how many cloudy days in a row there are in january, where you live,
and what the outdoor temperature is during those days. in some places, cloudy
days and nights are warmer than days and nights in sunny weather, because
the clouds act as insulation.

(us residents can be more precise about this by buying a $130 cd rom from
nrel/noaa which includes 30 year's worth of _hourly_ solar weather data for
their locations, and looking over the data for long sunless periods with low
air temperatures, ie "cloudy degree day" periods, or running a very simple
computer simulation of a particular solar house to estimate the interior
temperature every hour for 20 years and predict daily temperature swings.)

suppose this example house is in a climate in which we expect at most 5 cloudy
jan days in a row with 99% confidence, with an average outdoor temperature
during those days of 30 f (-1 c), ie we expect colder and cloudier periods
to occur only every 100 years.

step 5.

calculate how many sealed 55 gallon or 200 liter plastic drums full of water
(or 5 gallon pails or 2 liter soda bottles or canned goods on shelves) are
needed inside the closet to keep the house warm for that cloudy period.

the example house needs 5x114k = 570k btu or 167 kwh to stay warm for 5
cloudy days. if the water in the drums is, say 130 f (54 c), and the drums
can keep the house warm until the water cools to, say, 80 f (27 c), then
each drum stores about 25k btu or 6 kwh of useful heat, and the house needs
570k/25k = 23 55 gallon or 167/6 = 28 200 liter drums.

we might keep the drums at 54 c by building an "solar closet," ie a box that
is completely surrounded by insulation, behind the sunspace, ideally inside
the house, with an air heater as part of the insulated wall between the
sunspace and the house, using some transparent "solar siding," eg home
depot's "paltough" corrugated polycarbonate plastic, costing about $1/ft^2,
or replex's ((800) 726-5151) clear flat polycarbonate plastic, which costs
about $1.25 per square foot ($13/m^2) and comes in long rolls, 49 inches wide.

the transparent siding might have some black aluminum window screen or
greenhouse shadecloth to the north and behind it, with an air gap on each side
of the shadecloth, to reduce reradiation and increase the solar collection
efficiency of the closet. 80% carbon-filled polypropylene shadecloth costs
about 15 cents per square foot or $2/m^2 and should last many years out of
the weather. shadecloth comes in various colors. we might have a 1" (3 cm)
air gap between the siding and the shadecloth, and another 1" gap between
the shadecloth and 3 1/2" (10 cm) of fiberglass insulation in a 6" (15 cm)
wall, with some small vents (about 1% of the closet glazing area, eg 1 square
foot or 0.1 m^2 in the example house) at the top and bottom of this air
heater, to allow cooler air from the solar closet to flow into the outside
air gap through the vent hole at the bottom of the air heater, which would
then flow horizontally through the shadecloth from south to north, becoming
warmer, and rise up and flow back through the upper vent hole and back into
the closet. the vent holes might have plastic-film backdraft dampers to keep
air from flowing when the sun is not shining. these require checking every
week or so, since they can stick open, and if they are large enough to pass
air well, they lose significant heat through the us r1 plastic film. they
might be made from chicken wire and the plastic film used for dry cleaner
bags. using a small fan can reduce thermal losses and raise solar collection

the inside walls of the closet could simply be fiberglass insulation, covered
with plastic film. the floor might be earth, covered with more plastic film.
the spaces between the sealed containers of water allow air to circulate around
them, heating or cooling them. we need a room-temperature-sensitive vent (eg
a $12 automatic foundation vent with its bimetallic spring reversed to open
some louver as temperature drops) or a fan that turns on between the closet
and house on cool cloudy days, and a return air path from the house to the
closet near the floor.

the example house might have 24 2' diameter x 3' high drums stacked 2 high in
2 rows of 6 drums making the solar closet about 8' high x 12' long x 4' deep.
ie about 3m high x 4m long x 1m deep.

we might make it 6' (2m) longer, and use the empty space for a sauna, or
a place to dry clothes. the closet might have 3 1/2" (10 cm) of fiberglass
insulation in its ceiling, ie the second floor of the house, as well as
in the other walls inside the house. most of the "waste heat" from this
closet ends up in the house, and it provides very little heat for the house
on an average winter day, with some sun. on such a day, the house is
almost entirely heated by the warm air from the sunspace.

if the closet were 2 stories tall, 1 or 2 plastic drums with threaded bungs
at the top might be plumbed in series to make a low-pressure gravity-fed hot
water system using a float valve or rainwater from the roof to keep the drums
full. a 1-story solar closet might have a fan-coil unit or about 20' (6m) of
baseboard radiator pipe with fins near the ceiling to make an air-water heat
exchanger connected to a warm-water thermosyphoning loop with some insulated
pipe through an ordinary water heater on the second floor with a heating
element that rarely turns on. in either case, the sunspace and closet need
about 64 ft^2 (6m^2) more solar glazing.

the sauna area in the closet might have a small woodstove, for burning
newspapers, junk mail, old paper towels, college committee recommendations,
letters from congressmen, and press releases announcing amazing new price
breakthroughs in photovoltaic technology.


notes: 1. air infiltration robs houses of heat, and electrical power use adds
       heat to houses, as do south windows. these things tend to cancel out,
       conservatively-speaking, so they aren't mentioned above. other helpful
       factors not mentioned above are that daytime temperatures inside and
       outside the house are higher than nighttime temperatures, and that the
       part of the south wall of the house that is covered by the sunspace
       needs no heat on an average day.

       2. sunspace airflow volume increases with the square root of the height.
       a quote from the energy crafted homes spec: "for optimal heat flow into
       the house from the sunspace, install sliding or french doors between
       the two. natural air flow through an open door can be as high as 1000
       cfm... most effective if a complete loop through the house is possible-
       two-story sunspaces can be tremendously effective at heating a house
       for this reason." a two-story sunspace probably needs no fan. it might
       operate automatically with a 2 watt $50 honeywell 6161b1000 damper
       motor in series with two thermostats, opening and closing a clear
       plastic damper in an upstairs window, with air returning to the
       sunspace via a downstairs window with a plastic film backdraft damper,
       ie a dry cleaner bag hung over the outside of the window.

       3. a large piece of greenhouse shadecloth hanging inside the sunspace
       near the south wall of the house in the winter can help the sunspace
       be cooler and more efficient at gathering heat, by keeping sun warmed
       air nearer the house wall, as described in the closet air heater above.
       hanging the shadecloth over the outside can be helpful in summer to
       keep the sunspace and house cooler and prolong the life of the uv poly
       film greenhouse glazing beyond the 3 or 4 year guarantee period. (does
       anyone know where to buy large pieces (eg 24'x32') of very clear mylar
       (polyester) film with built-in uv inhibitors, like the film once used
       in japanese solar greenhouses?)

       4. solar closets should be fairly airtight, and plastic film on the
       ground needs well-sealed edges to reduce water vapor transmission.
       air leaks between the sunspace and the outdoors are less important.

       5. solar closets work best when the ratio of thermal mass surface
       to glazing area is large, eg 10:1. closets with fans and faster-
       moving air can have smaller glass/mass area ratios.


doug lepine are not in the housebuilding business. perhaps you would like
to contribute a little money to this non-profit enterprise.

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