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a power tower inside a sunspace
1 apr 1997
a sunspace and solar closet combination allows the sunspace to collect a large
amount of daily house heat at a lower and more efficient temperature than the
closet, allowing the closet can be used only for cloudy day heat, in a house
with lots of interior thermal mass, eg masonry walls and exterior insulation,
or a slab with some 4" tubes on 1' centers and a ceiling fan with an airshaft
or a low-speed blower to move ceiling air under this solar hypocaust.

but what to do with a low-thermal mass house, like most new houses, which
seems to require a closet glazing almost as large as its sunspace glazing, so
the sunspace glazing becomes thermally useless and uneconomical, and the
sunspace tends to be hot, making it uncomfortable for people, and a less-
efficient solar collector? the closet also needs lots of thermal mass surface
and airflow to quickly gather and store house heat for every average winter
day, vs slowly storing heat for rarer cloudy days. and it can be difficult to
arrange that most of the sun that falls on the sunspace wall hits the closet
glazing without much shading. it's easier to just make the interior surface of
the sunspace a dark color, or hang a dark mesh in the sunspace near the house
wall, to fill the back of the sunspace with warm air when the sun shines.

how about some thermal storage in the sunspace, for a low-thermal-mass house?
a collection of 55 gallon drums etc, in a tall insulated unglazed box? putting
the warmstore in the sunspace conserves more expensive living space, with only
a small thermal penalty, since it's cooler than the closet, but it loses more
heat through the sides exposed to cool sunspace air at night. this box can
also cool the house in summertime, if it's vented to the outside at night.

suppose we make the box 4'x4'x16' tall (for a 2-story sunspace), with r11
insulation outside, filled with 16 steel 55 gallon drums filled with 1200
soda bottles and 9 15' x 4" pvc sewer pipes full of water. the drums might sit
on 3 courses of 8" hollow concrete blocks with the holes aligned horizontally,
and the top might have 2' of vertical clearance to the south for a motorized
foamboard damper or a one-way plastic film damper to allow only downward flow
of sunspace air. the north side of the box would have an opening to allow
upwards warm airflow into the low-thermal-mass house. this leaves 12' of
vertical space for 4 layers of 4 vertical 55 gallon drums, each drum filled
with 75 vertical 2-liter plastic soda bottles in 3 circular layers. each drum
might be topless, with 3 projecting tabs around the upper perimeter to center
the one above it, and 36 3" circular holes cut in the bottom to support bottles
while allowing airflow. the drums seem plenty strong enough to hold up 1200
pounds of bottles, a 1/16" x 6' sidewall strip of 50k psi drum steel might
support 113 tons (at most) before buckling. 

the space between the 4 drums might be filled with 5 15' x 4" vertical sewer
pipes resting on the ground, and the inside corners of the box might have 4
more sewer pipes. the box would contain 5040 pounds of water in bottles with
a surface area of about 1440 ft^2 and 750 pounds of water in pipes with about
130 ft^2 of surface, a total of 6000 btu/f of thermal mass with 1600 ft^2 of
surface, counting the concrete blocks. 

the base of this structure might contain 2 20" window fans with thermostats
exhausting a total of something like 2,000 cfm of air from the box into the
sunspace. what kind of natural airflow would the structure have? the bottles
and pipes occupy a cross-sectional area of (25x4+9)pi(2/12)^2 = 9.5 ft^2 out
of the 4x4' box area, leaving vent area av = 6.5 ft^2 for airflow. the formula
cfm = 16.6 av sqrt(h dt) gives 1930 cfm for a 20 f difference in temperature
dt from top to bottom, so the fans at the bottom might not be needed, altho
higher velocity air would make heat collection more efficient. the natural
air velocity might be about 1930 ft^3/min/6.5 ft^2 = 300 linear feet per
minute, ie about 3 mph, making the thermal mass air film conductance about
2+3/2 = 3 btu/hr-f-ft^2, ie a total of about 1,600x3 = 4,800 btu/hr-f, so a 
tower like this might store about 96,000 btu/hr from 96k/200 btu/hr = 480 ft^2
of r2 sunspace glazing in full sun, with a 20 f delta t. 

if the top of the sunspace were 115 f on an average day, the mass might store
(115f-75f)6,000 btu/f = 240k btu of useful house heat on an average day. a
square foot of r2 sunspace glazing with 80% solar transmission in december
with an average of 600 btu/ft^2/day of sun that falls on a south wall and an
average daily high of 35 f might collect about 480 btu/day and lose about
6(100-35)1ft^2/r2 = 285 btu net, so collecting 240k btu/day with this little
sun would require about 240k/285 = 842 ft^2 of r2 glazing. making the air next
to the sunspace glazing 68 f would raise the net sunspace gain to 380 btu/ft^2
and reduce the needed area to 630 ft^2.

nick




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