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re: deployable doubt dispellers
25 feb 2006
m ransley  wrote:

>all you need to prove it will or won`t work is 100$.  build a box 2 x 2
>x 2 feet, use r 7.2 foamboard single layer walls and floor and 4 layers
>top to to simulate nominally used r values. install  appropriately
>proportioned glass of your specifications needed.

it's easy to see that won't work near phila: if the thermal conductance
of 3 sides and the top is 4x4ft^2/r7.2 + 4ft^2/r28.8 = 1.8 btu/h-f and
it's 80 f on a 30 f january day, it needs a 2'x2' window with (say) 50%
solar transmission, where 0.5x1000x4 = 2k btu = 24h(80-30)g, so g = 1.7,
so the window's r-value is 4ft^2/(1.7-1.8) = minus 40 :-) r1 glazing with
90% transmission would make 0.9x1000x4 = 24h(t-30)(1.8+4), so t = 56 f... 

>from what i have read you will never reach 100%, but may achieve 80%.

that's a popular misunderstanding spread by the sustainable building
industry council's guidelines for passive solar house design (with usdoe
support and 239 customized regional editions) and the books by mazria,
chiras, and kachadorian, among others. but pe norman saunders has been
engineering houses that are very close to 100% solar-heated in cold cloudy
new england since 1946. some have long track records with digital data
loggers. norman's writings are hard to read. his "solar basics" (6th edition,
1974) is inscrutable. a lot is handwritten, with illegible differential
equations. he says people will likely find his notes for professionals
developing solar designs "remote and incomprehensible" :-)

>... 100% heating even with full sun is never possible where i live.

nonsense. with enough insulation, you don't even need the sun.

anthony matonak   wrote:

>... i don't bother to follow the math most of the time

the math can be interesting. for instance, the smallest indirect gain d-cube
might be l' on a side with fl feet of r40 per inch vacuum aerogel insulation
around a (1-f)l water cube, with g = 6l^2/(480fl) btu/h-f and c = 62((1-f)l)^3
btu/f and rc = c/g = 538 hours = 4987(1-f)^3l^2, ie (1-f)^3fl^2 = 0.108. if
d(rc)/df = 0, 3(1-f)^2(-1)f+(1-f)^3 = 0, so -3f + 1-f = 0, ie f = 1/4, ie no
matter what the cube size or what kind of insulation or mass we use, making
the cube wall thickness 1/4 of the cube edge length maximizes rc. with f=1/4,
(1-1/4)^3(1/4)l^2 = 0.105l^2 = 0.108 makes l = 1.01 feet.

>if i understand this thread correctly, the point is that nick is
>suggesting demonstration units to show that solar heating works in
>places where most folks don't believe it will.

sure. it works everywhere, altho some climates are easier.

>my suggestion is that if you want people to believe they can solar
>heat their houses then a demonstration house is what they could
>easily understand. lots of folks make portable structures on wheels
>for construction shacks and the like. make your demo built on such
>a trailer, give it the look of a small mobile home with carpeting
>and a couch and let people walk through it. maybe you could let them
>sit for a while and watch a recorded tv presentation inside.

i'd prefer something that unbolts and stacks in the back of a pickup truck,
with on-site water for thermal mass. a "house" would be nice, but that's not
a pure play. lots of people know how to make beautiful houses. let's make
heatflow concepts perfectly clear and invite those people to make larger
more comfortable and more attractive structures while preserving the high
solar heat fraction. given conventional wisdom and many low-solar-fraction
houses (ie "engineering failures" :-), the first step seems to be reproving
that high solar fractions can be achieved at all :-)

it's like an existance proof in mathematics or physics or chemistry.
once you prove that something can exist, people start looking for it. 
f they believe it cannot exist, they don't look for it.
 
>then again, perhaps this is all overkill. if you want to convince
>folks that they can heat with the sun then maybe all you need is
>a solar air heating panel. set it up outside so it vents hot air
>at a single spot where people can warm their hands on bitter cold
>days. place a clear and very simple diagram with details about how
>the panel works next to the thing. put it in places where people
>are going to be cold and standing around.

i'd like people to see that they can be warm even after a cloudy week.

nick




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