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metal roof/tubing solar house design
16 jul 2003
m writes: 

>my location is roughly denver colorado.

nrel data say that january is the worst-case month, with 1370 btu/ft^2 on
a south wall and 750 on a horizontal surface on an average 29.7 f day with
an average daily high of 43.2. the average daytime temp would be about 
(29.7+43.2)/2 = 36.5 f over an average 6 hour heat collection day.

>the house is 47'x25' with a 12:12 pitch metal roof and bedrooms in the
>attic.  it will have r24 walls and r44 roof using structural insulated
>panels and the long end faces south.

why not squarer, with less materials and a lower surface to volume ratio?
how many square feet of windows on each wall, with u0.25 thermal conductance
and 50% solar transmission? how many kwh/mo of indoor electrical energy usage?
a typical us house uses 800 kwh/mo.
 
>the solar design is to have the "sunspace" occur between the metal roofing
>and the r44 panel.

sounds like the roofing is not transparent to solar heat :-) you might
consider a single layer of polycarbonate instead, like my 32x20' roof.

>the south roof area is about 47 feet long by 15 feet tall.

with 47x19.5 = 918 ft^2 of surface with an atan(15/12.5) = 50.2 degree
tilt up from horizontal? this would receive about 47(15'x1370+12.5'x750)
= 1.4 million btu/day of sun in january. lots of heat, but i'm afraid most
of it will be lost to the outdoors with the scheme you have in mind.

>the metal would be raised off the panel by about 1 inch with wood spacers.

sounds quite insufficient for free airflow over the 19.5". as i recall,
steve baer suggests a 19.5/15 = 1.3' air gap. 

>the sides and top would be sealed so that heat can rise an accumulate near
>the ridge.  the north roof area would be vented and not be connected to
>the south roof area.

hmmm.

>the heat would then be transferred into plastic pex tubing (best choice ??)
>that has a glycol/water solution.  i'm thinking that laying out tubing along
>the top 3 feet near the ridge and along the entire length of the roof would
>provide sufficient heat exchange area.

so you might have 1/2" pex tubing on 1" centers, about 47x36 = 1692 feet
of it with 221 ft^2 of surface and 332 btu/h-f of water-to-slow-moving air
conductance. steve baer/zomeworks are developing a roofing system with
tubing under standing seams and heatflow by direct conduction vs air-water. 
 
>the pump would be located in the solar closet which is under the house in a
>crawl space.  the solar closet would be long and narrow to facilitate
>getting the heat to the house.  maybe 30' long by 4' wide by 4' tall.

doesn't sound like a solar closet (tm) with its own glazing inside a sunspace.
as a heat storage tank, it sounds inefficient, with such a large heat-losing
surface to heat-storage volume ratio. 

>the closet would have sufficient water tanks to hold enough heat for a week or
>so of cloudy winter weather.  the tubing from the roof would first go to an
>electric water heater (located in the solar closet) as the first priority.

seems like that would be a smaller and higher temp need than space heating.
lower temps are more efficient for solar collection and less efficient for
heat storage and heat distribution. 

>the tubing would then go into the water heat storage tanks to transfer the
>remaining heat.

heat exchanger tubing or (more efficiently) water goes into the tanks?

>a simple method of getting the heat from the solar closet into the house
>would be to let it rise up into the various rooms above through registers
>that could be closed or open based on the heating demand.

manually? sounds like an everyday pain.

>to make the heat delivery a more controllable one could use fans or have
>another pump and tubing for a radiant floor heat system or room radiators.

less painful.

>excess heat in the summer could be used in a pool or the roof could vented
>by manually venting the roof near the ridge.  a system of wires and pulleys
>up to the ridge could make the manually venting possible from ground level.

perhaps easier said than done.

>do you think this system would work?

not well.

>the cost would be low since the sunspace is pretty much no extra cost,
>the solar closet would be relatively cheap, the water storage is cheap,
>one pump is cheap, and the tubing is cheap.  it's attractive in that
>it wouldn't need solar panels or a greenhouse to fit into the architecture.
>also, the solar closest isn't taking up valuable southern exposure from
>other rooms.

yes...

>the biggest question mark to me is whether the roof would provide enough
>heat in the winter.  maybe insulating the metal roofing along upper 6' near
>the ridge would allow the heat to buildup to a higher temperature.  solar
>panels could also be used instead of (or maybe in addition to) the sunspace
>described to collect the heat needed.

with 100 f water, you might have something like this (viewed in a fixed font):

          36.5 f
            |
            w 
            w r0.5/918ft^2
   1.4m/6h  w
     ---    |   1/332
|---|-->|---*-x--www--- 100 f
     ---         -->
 = 233k btu/h     i

disconnecting at x makes this (thevenin) equivalent circuit:

       0.5/918  1/332
       --www--x--www--- 100 f
      |          -->
      |           i = (163.6-100)/(0.5/918+1/332) = 17.9k btu/h
      |
      |  163.6 f = 36.5+233kx0.5/918.
     ---
      -
      |
      -

you might collect 6hx17.9k = 107k btu/day with a 107k/1.4m = 8% efficiency. 

how about a sunspace and shelfbox instead?

nick




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