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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 |