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re: passive solar heating for cold frame -- long
4 nov 2001
sylvia steiger   wrote:

>we're about to put our 14x70' mobile home on some land we own outside of
>town, and one way i plan to both keep heat under the house (to keep
>pipes from freezing) and try to grow veggies in winter is to build cold
>frames running the whole 70' on the front of the house... i am totally
>open as to how far they should extend from the mobile home and how tall
>they should be, so i'm looking for recommendations...

i suggest a lean-to, plastic-film-covered, quarter-cylindrical greenhouse
across the front of the mobile home, with roughly a 10' radius, curving
from the roof down to the ground 10' to the south, and 70' long. you could
build this in less than a week. 

or make it cover the roof if you like, with an 18' radius, and grow plants
on the roof and/or cover it with a trough near a foil-faced foamboard north
attic wall. the trough might have a desiccant solution for heat storage on
top of a transparent cover, with hot water for showers beneath the cover. 

you might make a shallow outdoor pond to the south of this sunspace to
reflect about 30% more solar heat inside when it is frozen. pond ripples
make nice light patterns inside when it is not frozen and only reflects
6% of the sun.

>1. i have access to 30-gallon and an occasional 55-gallon drum which i
>plan on painting black and filling with water to use as a passive solar
>source to keep the plants from freezing... should i place the barrels
>towards the outside, where they would get more sunlight and get warmer, 
>or towards the inside where they would have less danger of freezing...

you might put some vertical drums near the south wall of the home,
behind another a layer of south glazing. make a long useful shelf,
with 2 rows of drums on the ground and 1 partial row above and north
of that to hold down 4x8 sheets of plywood. 

nrel says january is the worst-case month for solar heating in cheyenne,
with an average 26.5 f, 37.7 daily max, 1290 btu/ft^2 of sun that falls
on a south wall (340 of that diffuse) and 690 on a horizontal surface.
you might fill the space between two layers of glazing with a 10' radius
with r20 soap bubble foam at night to collect about 0.8x10x70(1290+690)
= 1.1 million btu/day and lose about 6h(80-33)1100ft^2/r1.8 = 172k with
80 f sunspace air, for a net gain of 936k btu/day :-) 

you need about 7x24h(65-26.5)1100ft^2/r20 = 336k btu for a cloudy week.
this might come from 336k/(120-80) = 8893 pounds or 1100 gallons or 140
ft^3 of water that cools from 120 f to 80 f, eg an 4'x16'x2' deep pond
under 2 plastic film covers, with a reflector behind it. 

>2. i'm thinking of 2x4 wood frames with clear or white plastic sheets
>screwed to them, and either windows or clear plastic screwed to more 2x4
>frames for the lids...

i've made 16' bows on 4' centers by screwing 4" 1x3 spacers to a 16' 
1x3 every 2', then bending it to a 10' radius, then screwing another
1x3 on top. dry-bending a 10' radius shouldn't be hard, using 1x3s
without many knots. i snapped about 30% of the ones i bent to an 8'
radius. they became spacers and smaller pieces. you can make bows with 
smaller 1x3s if top and bottom joints are in different places. for a
"foundation," i drilled 5/8" x 3.5" holes in a pressure-treated 2x4
perimeter every 8' and laid the 1.5" side on the ground and hammered
2' pieces of 1/2" rebar into the holes. 

>what kind of automatic vent system would be appropriate to keep it
>from overheating in the full sun?

temperature-operated greenhouse vents that open sashes? you can also
use the soap bubble foam for shading. ideally, you'd store extra solar
heat in the winter. you may also use venting for dehumidification,
although that wastes energy compared to indoor condensation. 

>3. i'm planning on filling the cold frames with horse manure mixed with
>sawdust, to which i also have unlimited access... how long should i let
>it compost before i can plant veggies?

i've stunted tomato plants with less than a year...

>any way of accelerating the composting process?

an aerated pile with a cover... a 25:1 c:n mix, 50% moisture, at least
10% oxygen, 120 f... you might control this naturally, with some 4" pvc
pipes under a pile, or build a box with a oxygen sensor and a blower
and a humidistat and a soaker hose. 

>i'm going to put in a drip irrigation system to keep the plants watered
>and could easily put it in before planting if that would help it compost.
 
don't forget to cover the ground with a vapor barrier, eg black plastic
film over the drip irrigation tubes. 

nick




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