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Simpler solar attics

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Posted by nicksanspam on August 21, 2007, 5:31 pm
 
Laren Corie writes:


Maybe no windows at all, just plastic film glazing.


Historically, most people tire of moving insulation twice a day. Twice a year
seems OK. Or automatically filling a glazing cavity with soap bubble foam
at night. My favorite "movable insulation" is a big fan with 2 thermostats
in an insulated wall between a sunspace and a living space.


We might rethink how we use spaces. People seldom look out windows at night.
They cover black holes with curtains. A living space might only have 1-2% of
the floorspace as windows for small views. Picture a core living space behind
enclosed porches, or "viewspaces" with lots of glazing for large views. During
the day, move into a viewspace and steal some heat or AC from the living space
with an occupancy sensor and a thermostat and a fan.

A 32x32x8' tall living space with 16'-deep SE and SW viewspaces and
a 48'x48' footprint might have 24ft^2 of R4 windows with 6 Btu/h-F. An R40
ceiling and R30 walls would add 32x32/40 = 26 and 33, with 30 more for
30 cfm of air leaks, if it's tight, for a total conductance of 95 Btu/h-F.

With 4 American Craftsman 6068-2 6'x80" U0.48 sliding glass doors ($69
each at Home Depot) or 320 ft^2 of R4 windows, a 16'x48' SE viewspace
would have a 123 Btu/h-F conductance. Two more doors would give a 16'x32'
SW space 61 Btu/h-F. The glazing might have overhangs to reduce summer
sun and dark mesh curtains to reduce light levels for people, eg 80%
greenhouse shadecloth, which preserves views, like a dark window screen.

If the average living space temp is 65 F and we spend 4 hours per day in
each 70 F viewspace (Henry Mercer built bonfires on the roof and moved from
desk to desk as the sun moved in his 6-story concrete castle in Doylestown
PA) on an average 30 F January day in Phila, the house needs 24h(65-30)95
+ 4h(70-34)123 + 4h(70-34)61 = 79.8K + 17.7K + 8.8K = 106K Btu/day of heat.
With 34.1K from 300 kWh/mo of frugal indoor electrical use, we need 72K
more solar heat, which might come from a solar attic.

The solar attics of Soldiers Grove (see http://www.ece.villanova.edu/~nick )
can be improved. They blow warm air down into a building during the day,
with a motorized damper to let the attic stay cool at night. Some have
rock bed or hypocaust stores, but few store heat for more than 1 day.

A new attic might have a $/ft^2 corrugated R1 Dynaglas polycarbonate
20-year south roof with a 60 degree slope and 90% solar transmission.
NREL says 620 Btu/ft^2 falls on the ground and 1000 falls on a south wall
on an average January day in Phila, so 1 ft^2 of roof would collect
0.9(1000sin(60)+620cos(60)) = 1058 Btu/day.

Nathan Hurst's "Low-cost active heat storage" story in the July-September
2007 Issue 100 of ReNew (http:www.ata.org.au) shows how to collect solar heat
with a Mazda car radiator and its 16 watt electric fan. (I have a $5 1984
Dodge Omni radiator below my living room floor) With an 800 Btu/h-F air-water
thermal conductance like MagicAire's 2'x2' SHW2347 duct heat exchanger, we
could store 0.75x72K/6h = 9K Btu in 140 F water in 6 hours on an average day
with a 140+9K/800 = 151 F attic air temp. A radiator in a box below an attic
floor can both store and distribute heat, like this, viewed in a fixed font:
                  
       upper                                   g
       attic                                    l
   |           |                                 a
   |           |                                  z
   ~           ~                                   i     south -->
   |           |                                    n
   |  vertical |       motorized   /                 g
   |    duct   |          damper  /
   |           |                 /
   |           |            day /
   |           |               /
   |           |              /
   |           |             /   night                    attic floor
---|            -------------............----------------------------------
   |           .            r            .
   |           . d    room  a           d.
   |           .  a    air  d f         a. f
   |           .   m   out  i           m.
   |    ==>    .    p       a a ==>     p. a <== room air in
   |           .     e      t           e.  
   |           .      r     o n         r. n
   |           .            r            .
    -------------------------------------
               |            |
               | duct to    |
               | room floor |
               |            |
               |            |
               ~            ~

To collect heat, open the motorized damper and run the radiator fan.
They typically last 3-4K hours at 225 F. If the fan lifetime doubles with
every 10 C decrease, it might last 70K hours at 150 F. To distribute heat,
close the motorized damper and run the room fan. The passive dampers could
be plastic film over hardware cloth, aka "the 7-cent solution" invented by
Doug Kelbaugh (now Dean of the UMich Architecture school) in Princeton in
1973. The motorized damper could be polyiso foamboard with an auto windshield
wiper motor and limit switches or Honeywell's $0 6161B1000 damper actuator,
which only uses 2 watts as it moves up to 45 in-lb. The room air outlet would
also have a passive damper that opens out of the page into another vertical
duct or closet to move warm air down into the room. The floor might have more
motorized dampers over polycarbonate film to bounce light and heat down into
rooms during the day.

If 1 ft^2 of glazing gains 1058 Btu/day and loses 6h(151-34)1ft^2/R1,
the net gain is 356, so we might need 50.4K/356 = 142 ft^2 of glazing.
A 4'x48' strip would do. At 140 F, we could make hot water for showers
with a $0 1"x300' piece of pressurized black PE pipe in a heat storage
tank and a simple graywater heat exchanger (eg 2 uninsulated 55 gallon
plastic drums) to add heat to the house.

On an average day, with an 800 Btu/h-F radiator conductance, we can heat
the living space with 70 + (70-30)95/800 = 75 F water. If the viewspace
use patterns don't change on cloudy days, we can store 5x72K = 360K Btu
for 5 cloudy days in a row in 360K/(140-75) = 5538 pounds of water, ie
665 gallons, in an STSS tank or a 4'x8'x3'-tall plywood box lined with
a single folded 10'x16' piece of EPDM rubber.

Nick


Posted by ransley on August 21, 2007, 6:07 pm
 
On Aug 21, 10:31 am, nicksans...@ece.villanova.edu wrote:

 Name anybody you know that moves insulation twice a day or twice a
year for that matter, Soap bubbles, I think you have had to many
bubbles nick, we need more math to confirm this idea of yours.


Posted by Steve O'Hara-Smith on August 21, 2007, 10:00 pm
 On Tue, 21 Aug 2007 11:07:52 -0700


    That would be anybody who closes curtains at night.

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Posted by Torrey Hills on August 21, 2007, 6:37 pm
 On Aug 21, 10:31 am, nicksans...@ece.villanova.edu wrote:

Wow, thank you for the posting. Really enjoyed it.

Ken


Opportunities are never lost. The other fellow takes those you miss.


| Torrey Hills Technologies, LLC    |
| www.threerollmill.com                |
| www.torreyhillstech.com             |



Posted by Jim on August 21, 2007, 9:23 pm
     We did a bit recently on storing energy at the phase change of Glauber's
Salt (Sodium sulfate decahydrate) which melts at 90*F and when it cools
releases a tremendous amount of energy as Heat of Fusion as it solidifies.
It is less than $/lb, and a barrel of it (dry) in a 55 gallon drum with
water circulating around it will be a major component of my new system.



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