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re: utility costs
9 apr 1999
williams  wrote:

>some things to keep in mind in the direct gain passive system are
>the ability of the thermal mass to collect, store and distribute heat.
>different collectors (different things used as collectors) work better
>then others.

it seems to me the thing to keep in mind is that all forms of direct gain
suck for house heating outside of the southwest, in climates with more
than a few cloudy days. considering nighttime and cloudy day losses,
solar air heaters with no thermal mass that get cold at night can be
25x more efficient for house heating. no contest, but they are rarely
used. why? partly because the national concrete masonry association and
the brick institute of america are leading dumb consumers and solar
architects by the nose, with government subsidies for their work. 

>one of the biggest problems is finding true south.

it's pretty simple to use a compass and look up the correction in your
part of the country. where i live, compasses point about 12 degrees west
of true north. and small errors don't matter much. available solar energy
decreases as the cosine of small misalignments, about 4% for a 15 degree
pointing error. 

>the earth is traveling in an elliptical orbit around the sun.

but who cares? (btw, the earth is 3% closer to the sun during the
northern hemisphere winter...) having nrel's nice solar radiation
data manual for buildings (see their website) with average amounts of
sun that fall from different directions in different months and cdroms
with hourly historical solar weather data for 246 us cities for the
last 30 years, we don't have to think much about this.

>the earth's axis is at an angle of 23 1/2 degrees. this angle makes
>the sun appear to 47 percent higher in the sky in june then in december.
>keep this in mind when considering overhangs to prevent over heating in 
>the summer.

yes, the sun has maximum altitudes of 90-lat-/+23.5 in winter and
summer, and nice overhangs admit most of the winter sun at noon and
shade most of the summer sun at noon. a simple matter of geometry,
like this (in courier font, as shown for 40 deg lat, with a 4' overhang
shading 8' of wall including 6' of window--no need to make the upper
2' of expensive heat losing window, since it never sees any sun, unless
the overhang is seasonally adjustable, eg some boards stacked up near
the house wall in the winter, and spread over some horizontal support
beams extended from the wall in the winter):

      ws            |      4'     |
            ws   ss    overhang
                   ws-------------  -- 
                         ws  a    |      ws: noon winter sun  
                     ss        ws | 2'   ss: noon summer sun
                           b      |
                        \         w --    a: 26.5 degrees
      south                       w       b: 73.5 degrees
                          ss      w
                                  w 6'
                             \    w
                                ssw -- 


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