100% solar heated house for cold climate

I have posted to my web site a document describing a novel thermal scheme for a solar heated house for a cold climate.
Drawings, graphs, and calculations may be seen at the web site. Text below.
See <http://geocities.com/davidmdelaney/thermal-cs/thermal-crawl-space-1.html
I would be grateful for comments.
David Delaney, Ottawa
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Thermosyphon solar air heater and overhead thermal crawl space for 100% solar heating
David Delaney snipped-for-privacy@sympatico.ca Ottawa, November 2004
keywords: solar air heater, thermosyphon, natural convection, flow organiser, flow organizer, thermal crawl space, thermal closet, heat store, passive solar, solar fraction, solar thermal energy, bed of stones, bin of stones, rock bed, damper
A house in Ottawa, Ontario (45.3N, 75.6W, continental climate) can get 100% of its winter space heat from a solar air heater that operates by natural convection to charge a heat store in an overhead thermal crawl space. The house uses common materials, simple components, simple control, and simple building techniques, but needs a stronger structure than an ordinary house to support the weight of the overhead thermal mass. There are no dampers requiring daily operation. The only parts that move every day are the blades of a conventional ceiling fan.
The heavily insulated thermal crawl space, lies above the living space, and extends above a thermosyphon solar air heater that forms the south facade of the house. When the sun shines, heating the air heater, air moves by natural convection from the air heater to the thermal crawl space and back. When the sun stops shining, air stops moving between the air heater and the thermal crawl space, because the air in the heater is then colder and denser than the air in the thermal crawl space above it.
The flow organizer (flow organiser) allows the sheet of hot air rising from the air heater to cross through the sheet of cool air moving south along the floor of the crawl space. The sheet of cool air eventually falls through an east-west slit in the floor of the crawl space, then falls through the air heater against the glazing, keeping the rising hot air away from the cold glazing.
A massive but relatively thin layer of small smooth river stones provides heat storage. The stones are from 1-1/2" to 2-1/2" (35 mm to 65 mm) in diameter. The stone layer is suspended one or two feet above the floor of the crawl space on a wire mesh. There is a one foot air space above the stone layer so that hot air from air heater can spread out above the stones. The stone layer extends above the whole of the habitable space below. The stones present an enormous surface area for heat transfer between stone and air. There is very little resistance to convective vertical flow through the stone bed because of its very large horizontal cross sectional area. To match the volume flow rate of air coming up from the air heater, air will move down through the stones at a volume rate equal to the volume rate of the air rising from the flow organiser. The rate of descent through the stones will be the volume rate divided by the effective duct area of the stones. The effective duct area of the stones will be approximately the product of the void fraction and the area of the top of the stone bed. Given that the stone bed extends over the whole of the living area, the velocity of air descending through the stones will not exceed about a twentieth of the velocity of the air rising by natural convection through the flow organiser. As a result, resistance to the flow through the stone bed should be extremely small. 100 lb of stone per square foot of ceiling area (490 kg/m2) is about right to produce the desired thermal capacity. 100 lb/ft2 corresponds to a 1 ft (0.3 m) depth of stone with a 40% void fraction. The crawl space extends 3 to 4 ft (0.9 to 1.2 m) from its floor to its ceiling.
A ducted ceiling fan moves hot air from above the stone layer down into the living space. A conventional 4 ft (1.2 m) diameter ceiling fan is located in the lower end of a 4.5 (1.4 m ) diameter circular duct that runs from the ceiling of the living space up through the crawl space and the stone layer to the top of the stone layer. The ceiling fan operates at reduced speed, and consumes 50 watts or less when running. It might be powered by a small area of solar photovoltaic panel. Control of the temperature of the living space can be very simple: a thermostat that turns on the fan when the living space is colder than desired.
A large solar air heater, super insulation, and thermally efficient windows that are not too large, are required to get all needed space heat from the sun in Ottawa Ontario. Ottawa has a difficult December, with 1483 F heating degree days below 64.4F, (824 C heating degree days below 18 C) (according to NASA). The average December temperature is 14F (-10C). In December, a total of 2.16 kWh per day of solar radiation falls on each square meter of a south facing vertical surface (NASA). Design calculations are currently based on the assumption that the air heater can transfer 50% of the December incident solar energy into the thermal crawl space as heat.
Dimensions and suitable R values for a small bungalow in Ottawa, Ontario: Living space: 40 ft (12.2m) east-west, 30 ft (9.1 m) north-south, 1200 square feet (112 m2). Insulation: ceiling of crawl space: R 100 (RSI 17.6); walls of crawl space: R 57 (RSI 10); walls of living space R 50 (RSI 8.8); underslab: R20 (RSI 3.5). Windows: window R-value: R 4 (RSI 0.7 ); window area: 120 square feet (11.1 m2). Fresh air: 45 ft3/min (21 l/s) The air heater must have an area of 430 ft2 (40 m2), which could be achieved with an east-west glazing 40 ft (12.2 m) long and 11 ft (3.4 m) high. These air heater dimensions are based on the assumption that the air heater can transfer 50 per cent of the energy of the solar radiation that falls on the exterior of its glazing into the crawl space. The calculations to justify these specifications, and to create the graphs below, may be seen in 100% Solar heated house for Ottawa, Ontario, with overhead thermal crawl space. (PDF)
AT 430 ft2 (40 m2) the air heater is sufficient for December space heat, but 30% larger than is needed for either November or January, the next most demanding months. The surplus heat available in the less demanding winter months might be used to heat domestic hot water. The air-water heat exchanger might be placed in the top of the thermal crawl space directly above the air heater, where it would be accessible for maintenance and repair.
A stone layer area of 1100 ft2 (102 m2) at 100 lb (45.5 kg) of stone per square foot provides a thermal capacity of 22,000 Btu/F (11.6 kWh/C). Assume a non solar heat gain of 600 W, of which 200 W is due to two human bodies. If the temperature of the stones is 100 F (38 C) and the outdoor temperature is 14 F (-10 C) when the sun ceases to shine for several days, and the fan is controlled to maintain a desired temperature of 70 F (21 C), the temperature of the habitable space will not fall below that desired temperature until after 120 hours of darkness, and will fall to 59.8F after 168 hours of darkness, and to 39.1 F (4 C) after 20 days of darkness. This calculation is quite conservative. In Ottawa, a prolonged period of no-sun days is almost always accompanied by relatively warm weather, say around 32 F (0 C). When the temperature descends to 14F ( -10 C) , as in this calculation, or lower, there is almost always some clear sky each day.
The 430 ft2 (40 m2) air heater specified above can maintain the average temperature of the heat store (the thermal crawl space) at 110 F (43 C) and the habitable space at 70 F (21 C) during an Ottawa December of infinite duration but typical temperatures and sun. (with 600 W non-solar heat gain).
If the utility electricity fails in a typical December, but there is PV power to run the fan, the temperature of the habitable space will not fall below the desired temperature unless there is a long string of no-sun days. (Assuming a 200 W non-solar heat gain, just the two human bodies). As the graph to the right shows, the heat store (the thermal crawl space) even in the absence of dark days, the temperature falls to equal (a comfortable) habitable space temperature, making it impossible to maintain this temperature during multiple dark days. Backup heat might be desired to anticipate multiple dark days during a prolonged December electrical utility failure. Backup heat would not be needed for prolonged electrical failures in other months. A wood or propane cooking stove would provide sufficient backup heat.
If there is a failure of the fan or of the electricity supply that drives the fan, a door, a window, or a special opening in the south wall of the house may be opened during the day, producing the flow pattern through the house and air heater shown to the right. The air heater will be less efficient in this configuration, and much of the benefit of the crawl space thermal mass will be lost, but substantial solar heat gain will still occur. The thermal mass will still keep the thermal crawl space hot, providing some heat at night by radiation to the living space below and eliminating heat loss from the living space through its ceiling.
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[snip]
I like the out-of-the-box thinking, but I'm concerned about efficiency gains here.
I'm sure you're aware of Kachadorian's book *The Passive Solar House* on a similar principle, but moving the air through the floor rather than the wall and ceiling.
How come you don't store the thermal mass in the floor? The expense of storing it in the ceiling [added expense of engineering] and forcing it down surely won't pay for any efficiency gains [if any]. Three-four days of no sun and you won't be running your PV fan anyway, and the mass in the ceiling won't do you any good if you heat it up in some other way, because you have to still force the air down, losing the efficiency of storage.
I'm designing a solar-gain straw bale house and am storing the thermal mass in the floor and some walls, and I won't be forcing the air around anywhere. So at first blush, your idea looks impractical from a simplicity standpoint, and you always want simplicity.
Good luck,
D
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Dano wrote:

fan blew it to the crawspace filled with large gravel. it worked alright. where i lived, the solar gain was not as good as other parts of the country. the house was built with 2x6 16 on center. double sliding doors(which was great) there was a 18 inch space between the two. outer one had a small roof over it. i believe in a well insulated house and tight doors for central ohio area. there is a map somewhere that shows areas that are great for solar gain.
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Put the rocks in the basement over natural insulation. 1 heat rises, 2 your construction load is too high. 3 orientate a thin house N to south , Maximum exposure N to S , with summer overhangs.
Alt . energy. hompowrer is a better group.
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Heat rises, attic or high storage looses the basic principle of this
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Nick
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How much fin-tube under the ceiling and how big a crawlspace tank would we need for a mythical 24x24x8' R32 building in Albany, with 1" foil-faced foamboard under the ceiling?
With a ceiling conductance of 576ft^2/R42 = 14 + 768/32 = 24 for walls, totaling 38 Btu/h-F, we might need about (65-26.5)38 = 1463 Btu/h for 8 hours per day and (50-26.5) = 893 at night, for a total of 26K Btu/day. If 0.9x700x8x24 = 121K Btu enters an R1 south wall cover over 6 hours on an average December day and 121K = 6h(T-30.7)192/R1+26K, we might get T = 113 F air out of the air heater for 6 hours.
N feet of 5 Btu/h-F-ft overhead fin-tube might keep the room air 65 F with Tmin = 65+1463/(5N) = 65+293/N F water inside, with a slow ceiling fan and a room temp thermostat. For instance, N = 32' makes Tmin = 74 F.
We need to store about 2x1463+16x893 = 17.2K Btu of overnight heat. With an infinite tank at Tmin, N' of pipe might collect 6h(113-Tmin)5N = 6(113-(65+293/N))5N = 17.2K Btu/day, which makes N = 18 feet :-)
We need 5x26K = 130K Btu for 5 cloudy days in a row. With enough thermal mass and surface aloft to ensure that a finite tank with volume V ft^3 never supplies overnight heat on an average day, its long-term steady- state temperature might be close to 113 F. If 130K = 62.3V(113-Tmin), V = 44N/(N-6) ft^3. N = 32' makes V = 54 ft^3.
We might use 2 4'x8'x1' deep underfloor tanks lined with 2 folded 6'x10' pieces of EPDM rubber, with two pipes connecting them. One might be close to 50 F for night warming, and the other might be 113 F. After fin-tube priming (the overhead vacuum can last for days on end, according to Eric Hawkins), two low-power pumps might circulate water through the fin-tubes, keeping them cool while maintaining the room temperature schedule.
Nick
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