|
|
will, will, will... (was: nick, nick, nick....)
25 may 1996
william r stewart wrote:
>[followups set to alt.solar.thermal so as not to bore the rest
>of the free world...]
you don't want to bore people, eh will? :-)
>nick pine wrote:
>> william r stewart wrote:
>> >nick pine wrote;
>> : here's a big hint: if a "solar house" has a heat loss of 150 btu/hr-f
>assuming that you are alluding to my place, show me a 2250 ft^2 house with
>150 btu/hr-f loss...
that seems like a non-seqitur... i'd say your place will be a very energy
efficient home, and that's good, and not easy to accomplish...
150 btu/hr-f was about what we got for its thermal conductance
with the window insulation in place, as i recall.
:and
:a thermal capacity of about 8,000 btu/f, so rc = 8,000/150 = 53 hours, and
:the house starts out with an interior temperature of 68 f when it's -10 f
:outside, with no electric or wood heating, and no sun, the interior temp after
: a couple of days will be approximately - 10 + (68-(-10))exp(-48/53) = 31.5 f.
>this would be fine for a house that undergoes these conditions. my house is in
>an area that goes below 16 f exactly 1% of the time.
>(reference: "statistical
>abstract of the united states", 92nd edition, us bureau of the census)
that's a very nice reference, will.
a good and powerful reference...
but where will your house be?
the 1993 ashrae hof lists the 99th percentile winter design dry bulb
temperature for andrews air force base (is that near your house?) as 10 f.
the 97.5% number is 14 f, ie the winter temp there will be less than 14 f
less than 2.5% of the time.
1% of 6 months is about 40 hours. what would the interior temperature of
your "solar house" be after 40 hours, if your house and its 3,300 pounds
of water walls and 3,000 btu/f of drywall, concrete furniture, etc, start
out at 66 f one fine cloudy morning with the window insulation in place,
when it's 16 f outside, and you are not burning wood or using electricity?
rc = 6300/150 = 42 hours, so t(40) = 16 + (66-16)exp(-40/42) = 35.3 f.
that wasn't hard, was it?
why call this a solar house, if 56% of its heat comes from the electricity
you use and your bodies, and a lot of the rest will come from burning wood?
and who cares whether it's a solar house? it's an energy-efficient house...
that's good. but it seems to me that it could be vastly improved and made
less expensive in the way it uses the sun for heat and hot water.
>i wouldn't have electricity off for a couple of days (will have photovoltaic panels
>and battery storage).
good for you... what does all that cost?
>i will have an energy efficient fireplace and will fire it up from time to time,
>especially when extreme weather conditions occur.
good for you... bad for the earth, but nicely nostalgic.
>> >if the solar house you describe does not have a direct gain thermal storage
>> >capability, aside from room temperature walls and floors, then you could see
>> >something like the above.
>>
>> i see that _with_ your water walls...
>
>you push heaps of numbers around, but you fail to understand essential aspects
>of thermodynamics. it's no wonder you don't see...
i wonder where you would find an error in the tiny heap of numbers above...
never mind... my vision is unlike yours.
>> >however, if one *does* have thermal storage, such as 4000 lbs of water walls
>> >that are heated by direct solar gain, then these will rise to a higher
>> >temperature than the ambient indoor temperature without raising the indoor
>> >temperature uncomfortably, and will also provide heat by convection and
>> >radiation.
i thought that was 3,253 pounds of water walls, by your last calculation.
i suppose these water walls will not be insulated, and they were 4" thick,
(and cost $2k, + how much per square foot of floorspace?), so they might
have a volume of 50 ft^3, and a surface area of 150 ft^2, on each side,
and the us r-value of a still air film is about 2/3, so they would have
their own rc time constant of 2/3/300ft^2x3253=7.2 hours. not very long
compared to the time constant of the house, so i don't think their time
constant will matter much as your house cools. if we combined these time
constants by squaring them, adding them, and taking the square root, it
would add another half hour to make a 42.6 hour overall rms time constant
for your house.
another simple way to get a feeling for this is to see what the water wall
temperature will be in your 66 f house after 40 hours, if the house somehow
stayed at 66 f the whole time and the water walls were initially 10 f warmer
than the house. (how do these water walls 4" away from the windows affect
the view?) anyhow... t(40) = 66 + (76-66)exp(-40/7.2) = 66.04 f, not much
warmer than the house... such is the magic of time constants. they are really
quite useful and simple to use for things that cool. find the r, find the c,
multiply them together and use that simple formula with the "e-to-the-x"
button on your $20 calculator (this is the inverse of the natural log button
on my casio fx-991h, which i bought for $19.95 at k-mart.)
>> r-values include convection and radiation.
>
>(this last line is a keeper, rabbits or no rabbits)
>
>i rest my case.....
here's a quote from pages 79-80 of _from the walls in_, little-brown, 1979,
by charles wing, phd, physics, formerly an apollo lunar experimenter at mit:
h = a x delta t/r... thermal resistance, r, is actually defined by
this equation. in the laboratory the two faces of a slab of insulative
material are maintained delta t degrees apart, the amount of energy
per hour required to maintain delta t is measured, and r calculated
as a result. of course no house consists entirely of solid uniform slabs
of material. more typically a wall consists of various layers of boards
and plaster enclosing a cavity containing either air or a fibrous
insulating material [or bubbles :-)] to complicate the picture further,
there may be four surfaces facing air, each of different emissivity!
how do we calculate the total thermal resistance of such a hodgepodge?
here it is worth noting the difference between an engineer and a physicist.
the physicist wishes to "understand" physical processes. this has led him
from being a philosopher of nature (knowing a little about everything)
through the process of specialization (knowing more and more about less
and less) to his logical end (someone who knows everything about nothing.)
on the other hand, the engineer typically cares not a whit for why
something happens, only how to assign a number to it. results are what
counts! thank god for engineers.
here's how the engineer does it. first he builds a sample wall, roof, or
floor construction. then he says, "i don't care whether heat is flowing
through that thing by conduction, convection or radiation or all three at
once. the simplest formula i have is for conduction alone, so i'll use it."
he then performs the conduction experiment (illustration 48), turns the
calculator crank, and out comes apparent total thermal resistance, r.
after performing this experiment dozens of times with different assemblies
of material and air spaces he can sort out the apparent thermal resistance
contributions of air surfaces, air spaces, and solid materials. reversing
the procedure, he can then predict the total thermal resistance of any
proposed construction by adding the thermal resistances of each of the
component parts. and it works!
tables 15, 16, and 17 list the thermal resistances of surfaces, air
spaces and solid materials as discovered by engineers...
those are r-values. you can see them stamped in big letters and numbers
on rolls of insulation for sale at building supply stores. ours are much
larger than europe's. perhaps we should be proud.
cheers,
nick
|
|
