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re: thermal mass
2 jul 1996
>(nick pine) wrote:
>...a complete dhw system (including electric back up)... wll cost about
>$43.75 per sq ft. - $3500..

so, that's 80 ft^2... or 7.4 m^2. where i live near philadelphia, the average
yearly sun that falls on a surface tilted at the latitude is 4.6 kwh/m^2/day,
ie 365 x 4.6 x 12,483 kwh fall on that collector. the average yearly temp
is 54 f. say we collecting heat at 120 f, average? is the panel losing about
8hr(120-54)80/r2 = 21k btu/day, or 365 x 21k/3410 = 2250 kwh/year for an
annual average collection efficiency of 82%? is the heat store infinite? if
not, how often does we run out of solar hot water? what's the annual solar
fraction for a system like this?

>ok, my system is installed and operating at $43.75 per sq ft. your "heat
>equivalent of oil" nonsense means nothing until you have purchased and
>installed the necessary equipment to process that oil into a useable form.

i don't know what an oil burning water heater costs, and yes, you'd need a
tank and a chimney. perhaps i should have said electricity, at 0.08 cents/kwh,
or a third of that, using a heat pump that cools the basement air a bit? or
natural gas? let's say we just use electric resistance heat. then this $3500
investment, less $300 for the electric water heater, assuming the solar heat
store is infinite, saves 10,233 kwh/year, ie about $800/year. good :-) like a
25% tax-free bond. worth doing, but are there better alternatives?

>how many gallons of oil... does your sunspace replace per foot?

1-2 gallons of oil per year per square foot of glazing. but the sunspace
doesn't heat water, just the house. the solar closet would heat water. 

>remember, getting it into the sunspace doesn't count. it's getting it into
>a place where it can be used that matters.

sure. getting warm air into the house with a motorized damper or window fan,
and getting sun into the solar closet through the inner glazing, with another
motorized damper or low power fan, then getting the heat into an electric
water heater on the floor above using, say 20' of fin-tube pipe near the solar
closet ceiling, in a thermosyphon loop, a la big fin sunspace collectors.

now there's another more economical alternative than all these rooftop solar
panels and pipes and tanks and pumps and so on, no? something like big fins in
a sunspace. might even glue some pv solar shingles on them, and add 2:1
reflectors, a la richard komp. electricity, hot water and heat for the house,
as well as more daytime floorspace...
>> >air is a pitiful heat transfer medium.
a cfm of water with a temperature difference of 1 f moves about 3720 btu/hr.
a cfm of air with a temperature difference of 1 f moves about 1 btu/hr. so? 
>> and it doesn't freeze,
>good point.
norman saunders, pe, often makes this point.

>> it wasn't long ago that one of the
>> alternative energy experts around here said "air has no mass." :-)
>compared to water, it doesn't.

but it's a matter of degree, no?

>> >how umpteen many more times heat capacity has water got than air
>> about 4000 umpteen times, by volume, 1000 times less density and 1/4 the
>> specific heat by weight. is this a problem? can planes fly? should we
>> abandon them all for boats? 

>there's an intelligent argument. should we abandon all toasters for blenders?

i don't follow your logic here, dan. can you explain further?

>lets see,  if i buy a $100 grudfos 15-42f circulator at 120v and 85 watts
>and my closed loop has the equivalent resistance of 5 ft of head then i
>can pump 15 gallons per minute. divided by 7.5 gallons per cu ft = 2 cu ft
>per minute. times 4000 umpteen = 8000 cu ft per minute. so, for every ett
>(elephand turd truckload) of heat that i can move in one minute using 85
>watts, you need to move 8000 cu ft of air.

yup. you can move lots of heat. but what's your point?

>you like grainger's stuff, here's a floor fan that can move from 6810 cfm
>to 10300 cfm on page 2898 of the 1995 catalog. it only costs $562 and only
>uses 930 watts. it may cost you something to have delivered. shipping weight
>148 lbs.

i wouldn't buy one of those. but i might buy a $12 holmes 20" window fan, or
a $60 4c588 36 watt 560 cfm 149 f max cooling fan, or a $168 4c721 130 watt
5' diameter 43,700 cfm ceiling fan, with a $15 speed controller. it weighs
34 pounds. or i might try to make the whole shebang work by natural convection,
as steve baer has been successfully doing for 30 years. i'd probably cheat
a bit and use a $50 2 watt damper motor and thermostat.

>my pump costs about $5 to ship. of course, this being a fan, it will only
>deliver that cfm where it has deliverable free air, in other words, low static
>pressure, in other words, a big open room. if you need to duct it there's
>a 22" blower on page 2929 for $1035 for a 230/460v 3hp motor. or, there's
>a 36" whole house fan on page 2993 for $319 at 690 watts, it too needs low
>s.p. to do the job.

hey dan, you might like grainger's 7f916 25 _horsepower_ 3 phase blower! :-)
page 2930, 23,000 cfm at 2" of static pressure, $2139, 325 pounds. our solar
closet has 0.05" static pressure at 500 cfm. we haven't tried to streamline it.

>> it may surprise you to learn that air may not need pipes, that one can move
>> air through whole rooms and floors and walls and ceilings sometimes, with
>> absolutely no loss of floorspace.
>at 8000 cfm ?

now where did the 8000 cfm come from? oh yes, you were moving 2 cfm of water.
i wonder why? oh, you don't have any natural way to make the hot water go
downhill... i wonder if you do copper crickets.

>why does this matter? why does this matter? nick, it's time to wake up.
>this is the entirety of the argument. your sunspace gets hot. very little
>of this heat gets into the livable area. even less gets into storage.

we've seen the the sunspace heat the little room in our outdoor structure
comfortably and controllably using only natural convection, with a motorized
damper between them, while our measurements indicated the solar closet was
capturing 55% of the incoming sun. 

>any house in the northern hemisphere with south facing windows gets a direct
>heat gain when the sun shines. any more heat gain than what you can use
>right now is waste.

that's the usual arrangement... were we talking about water heating?

>just like your sunspace. insulate those windows at
>night and forget about your plastic greenhouse. 

ah yes, the ritual curtains. that will work, but it's a bit expensive,
and not automatic, and there's no heat stored at a higher temp than the
house, for future cloudy days, and no hot water. we were talking about
water heating here? 

>> >add to that the longevity of copper pipe
>> do we also add the longevity of wood or drywall or fiberglass insulation 
>> or plastic 55 gallon drums? :-) 

>of course you do. dont forget to factor in rot. if your system worked, the
>wood, drywall, or fiberglass that is the surface between hot and cold will
>condense if given a high enough delta t.

can you be more specific, dan?

>by the way, if it leaks air, it can leak rainwater and bugs.

who says "it" leaks air? and what is the "it"? bugs are our friends.

>plastic drums wont rot, they'll just get soft and weak with repeated thermal

have you ever seen that? these drums should mostly just stay hot, in this 
trickle-charged heat battery, which is a collector with an instantaneous
solar collection efficiency of zero, unlike a rooftop collector.

>> insulation... where are your numbers, dan?
>no, nick, it's not a matter of numbers. it's a matter of common sense. it
>doesn't take an engineer to realize that any number of etts going thru
>finned copper pipes immersed in water (or air, or floor, or boot warmers)
>will put more etts into where it's needed than the same number of etts
>blown around some plactic drums, even at 8000 cfms.

i don't quite follow your logic here, dan. you can move about 75k btu/hour
with 2 cfm of water. that is impressive, but an r20 4'x 8'x 10' solar closet
only needs about 22k btu/day to stay warm, eg 360 cfm of air for 6 hours a
day with a 10 f temp difference. if it makes hot water, it may need a fan to
collect sun, or larger motorized dampers, or a motorized insulating and
reflecting cover, hinged at ground level, that folds down from the glazing
to a horizontal position in the sunspace to let sun and reflected sun shine
directly onto the thermal mass during the day. these things work best when
lightly loaded.

>nor does it take an architect to look at a sheet of 80% shade cloth (called
>that because 20% of its surface area is holes -you can see thru it) and
>decide if it will be a better absorber sheet for ir light than copper treated
>with a selective surface.

engineers are typically more interested in those things than architects, eg
norman saunders, pe, who has been using transpired collectors like this since
about 1944 in his 100% solar new england houses. he likes them because they
are 50 times less expensive than selective surfaces, and they have little
thermal mass to wake up in the morning, and they store and waste little heat
at dusk. there is more surface area to absorb the sun than with a simple dark
surface, so the average surface temperature is less, since heat loss to air
is proportional to area, and the lower temperature of the absorber surface
reradiates less ir heat back out at the glazing. glass and polycarbonate
plastic block uv and ir in both directions. norman also says that transpired
absorbers are more efficient because some of the sun shines through the holes
and hits the dark surface to the north, which heats up that surface, which
reradiates heat to the south, and the transpired absorber intercepts that
reradiation trying to heat up the glazing. he also likes them because we can
end up with relatively cool air next to the relatively cold glazing, with
sun-warmed air next to the house wall, away from the cold glazing. 

>an automobile mechanic... doesnt need a book of formulas to recognize that
>his car wont get far on the gas produced by the taco he had for lunch.

perhaps you are saying this is a matter of proportion.

>i know what a heat exchanger is for and how to use it.

heat exchangers make pretty good scrap metal...

>>>if, like most fluid based systems... the storage temps consistantly approach
>>>150f plastic drums, $25 waterbed mattresses, and cheap swimming pools are a
>>>disaster waiting to happen.

recall we've tested the plastic paint pails now. care to try the drums? 
>> so are airplanes, especially lately. engineers specialize in disasters
>> waiting to happen. the better ones avert disasters for a very long time.
>better still, if you build nothing, you can avert disaster forever.

yes, but that won't make the world a better place. or maybe it will...
bertrand russell used to say "all that is necessary for goodness to
triumph in the world is for enough good men to do nothing."

>you were right from the start.

ahh, i like to hear that :-)

>it's time that i give up.



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