re: straw bale construction
26 sep 1998
>minnesota (ne, st louis cty, and other ne counties)...
not a great place for solar heating. nrel says the most difficult month
in duluth is december, with a 24-hour average outdoor temp of 12.8 f,
an average daily max of 20.7, 780 btu/day of sun that falls on a square
foot of south wall and 390 that falls on the ground. international falls
is worse in december, with an average daily temp of 7.2 f and 750 btu
of sun on a south wall, ie 750/(70f-7.2f) = 11.9 of "sun per degree-day."
(albuquerque has about 50...)
>i am designing a 16x20 cabin w/attached workshop, chicken/rabbit coop;
hope it has lots of insulation and chickens and rabbits (with a heat
output of about 5 btu per hour per pound.) anna edy's new "solviva"
book describes how she used chickens and rabbits and sheep and compost
to heat her cape cod greenhouse. she included a composting air filter
to remove some of the animal ammonia output, which was detrimental to
the plants in the greenhouse. their co2 output was beneficial.
a 16x20' airtight cabin with 320 ft^2 of us r40 ceiling with a thermal
conductance of 320ft^2/r40 = 8 btu/h-f and 576ft^2/r20 = 29 btu/h-f of
average r20 walls (a total of 37 btu/h-f) needs (70f-12.8f)37btu/h-f
= 2,100 btu/h of heat on an average jan day in duluth, which might come
from 140 3-pound chickens in 1 ft^2 cages on a 16' entertainment wall.
dry compost has about the same heating value as wood by weight, about
10k btu/lb, although it "burns" more slowly. a "composting furnace"
might treat animal waste and a sawdust toilet and greywater in a 4' or
8' cube with insulation around it and a small blower to keep the o2
content at least 10% and some greywater gathered up at the bottom and
sprinkled over the top with a small pump and filter and humidistat in
series with a heating thermostat in the house to keep the moisture
content of the warm air leaving the pile about 50%. under such ideal
conditions, 15% of the compost might disappear in 24 hours. the pile
might be 130 f inside. you might recover most of the heat and distill
the greywater with a condensing counterflow air-air heat exchanger,
preheating air going into the pile with outgoing warm moist air.
pasteurizing vs distilling the greywater would make things like the
heat exchanger a lot smaller, but that wouldn't allow reuse. storing
heat for 5 cloudy days in a row would require about 250kbtu/(130-80)
= 5,000 pounds or 625 gallons of 130 f water.
>...primary heating thermal mass solar (attached greenhouse) with
if you filled up the greenhouse with lots of thermal mass that lost
heat to the outdoors at night through the low thermal resistance of,
say, r2 greenhouse glazing with 80% solar transmission, a square foot
of vertical glazing with a perfectly insulated greenhouse behind it
would collect about 780x80% = 624 btu on an average january day and
lose 24h(t-12.8f)1ft^2/r2 btu of heat at a fairly constant indoor
temperature t, so if the solar energy that flowed into the greenhouse
were equal to the energy that flowed out (with no extra heat for the
attached house), t = 12.8 + 624x2/24 = 65 f. cloudy days would be
thermal disasters. wood would be the primary house heating source.
this might work a lot better with a low-thermal-mass sunspace which is
allowed to get cold at night so it doesn't lose much heat to the outdoors
at night, even though it has lots of glazing. a square foot of that kind
of sunspace might be 80 f over a 6 hour winter solar collection day and
12.8 f at night and provide q btu/day of heat for the attached house,
where 624 = q + 6h(80-12.8f)1ft^2/r2, so q = 422 btu per square foot
per day and that cabin would need about 120 ft^2 of vertical sunspace
glazing for 100% solar heating. for water heating, we might make the
sunspace larger, with a transparent roof, and some bare water heating
solar collectors under the roof, and a pressurized warm-water convection
loop through a conventional water heater in the attic.
sounds like some part of the greenhouse has to be above freezing at night
in the spring. you might start the plants in a smaller space over a bench
with some solar-warmed thermal mass underneath (eg a few dark-colored
55 gallon drums with a poly film skirt on the south side and insulation
on the north) and a translucent thermal blanket draped over large wickets
over the plants, eg pro-17 from american agro-fabric (about 4 cents/ft^2,
with 83% solar transmittance and about a 1 year life) or the japanese
tuff-bell 750n (17 cents, 92%, and a 10 year life?)