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Re: How we hijack this group???? and moderate it?? - Page 3

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Posted by gary on April 27, 2008, 3:39 pm
 
On Apr 7, 7:08 am, nicksans...@ece.villanova.edu wrote:

Hi Nick,
I'll take a look.

I have observed that there is definitely a tendency for the hot air
from the exit vents to keep the first couple joist cavities very
warm.  Under sunny conditions the entry air temp tends to run about
60F and the exit about 120F -- a paint can full of water and hung in
the first joist cavity reaches about 110F by the end of the day if I
am recalling correctly.

I guess I would be inclined not to include the extra wall that returns
air to the inlet vent.  It seems like the some kind of tray in the
area of the first two joist cavities with the skirt and a reflective
coating on the tray might be a good thing to try?  I have about 20 ft
of collector length to work with.

I did try a sort of closet arrangement on one bay of the collector.
It had an inside insulated wall to return exit air to the inlet, and
gallon bottles hung in this cavity -- maybe 30 gallons in all for the
one bay.  The inside "wall" was hinged so that you could close it
during the day to heat the water bottles, and open it later when heat
was wanted.  It did not work very well -- flow around the loop was
weak, and only the top couple rows of bottles heated up.  I think that
mass near the ceiling will work better.  I'm a bit skeptical that you
can get good flow around the loop -- thats the basis for wanting to
get rid of the inner wall, even though it would give higher
temperatures.

Any thoughts/advice on this?
--
A friend here in Bozeman installed a 14 ft high version of the
thermosyhon air heating collector -- I've not seen it yet, but plan to
go over and measure the flow rate and temperature rise.  Seems like it
should be pretty good.  He is using it to heat a room used to make
biodiesel from veggie oil.





Posted by nicksanspam on April 27, 2008, 10:08 pm
 


Sounds promising. A shallow water tray would have more surface/volume.


Ceiling mass with a slow ceiling fan still seems good to me, but after
a calculation, letting hot air slide out from some shiny mass to allow
a non-shiny ceiling to radiate downwards seems less good than a slow
ceiling fan or a vertical mass wall with thermosyphoning airflow into
the room. An 8' R15 cube with a 4'x8' mass and a 4'x8' non-shiny ceiling
needs a 92 F (min) mass to stay 70 F on a 30 F day, vs 78 for an 8'x8'
2-sided wall with separate overnight and cloudy day heat stores.


I remember that.


Sure, but how do we keep it hot without overheating the room?


Maybe not so good is still OK. ("Le mieux est l'ennemi du bien.") More mass
surface is good. A 100 F ceiling mass would not accept any more heat from
a 100 F air heater, but 7' of 100 F wall above 1' of 90 F wall would still
accept some heat, with an average cool column air temp that's less than
the average hot column air temp.

20 AC=6*8*8'cube surface (ft^2)
30 RV'US R-value of cube walls
40 GC/RV'cube conductance (Btu/h-F)
50 TA0'outdoor temp (F)
60 SUN00'average sun on south wall (Btu/ft^2-day)
70 CH=8'solar collection hours
80 TDp'day room temp (F)
90 TN`'night room temp (F)
100 PD=(TD-TA)*GC'daytime heatflow (Btu/h)
110 QH=8*(TD-TA)*GC'daytime heat (Btu)
120 PN=(TN-TA)*GC'night heatflow (Btu/h)
130 QN*PN'overnight heat (Btu)
140 QD=QH+QN'daily heatflow (Btu)
150 AS=8*8'south glazing area (ft^2)
160 RS=1'south glazing resistance (per layer)
170 TS=.92'south glazing transmittance (per layer)
180 SSS=TS^2*AS*SUN'sun entering south glazing (Btu/day)
190 TCA=TA+RS*(SSS-QD)/CH/AS'average cool cavity temp (F)
200 AV=8*8/12'vent area (ft^2)
210 H=8'vent height difference (ft)
220 DT=(PN/11.7/AV/SQR(H))^(2/3)'night thermosyphoning dT
230 AN=8*8'1-sided overnight heat store area (ft^2)
240 UA=1.5'slow airfilm conductance (Btu/h-F-ft^2)
250 TMINN=TN+DT+PN/(2*UA*AN)'predawn heat store temp (F)
260 TMAX=2*TCA-TMINN'max cool cavity temp (F)
270 CN=QN/(TMAX-TMINN)'overnight heat capacity
280 DN*CN/AN/62.33'overnight store depth (inches)
290 PRINT TMINN,TMAX,DN
300 DT=(PD/11.7/AV/SQR(H))^(2/3)'dawn thermosyphoning dT
310 TMIN=TD+DT+PD/(2*UA*AN)'dawn heat store temp (F)
320 THA=TMAX+SSS*RS/AS/CH'hot store temp (F)
330 CC=5*QD/(THA-TMIN)'hot store cap (Btu/F)
340 DH*CC/AN/62.33'hot store depth (inches)
350 PRINT TMIN,THA,DH

66.66535      124.9347      .6343741

78.56218      230.7347      2.024268

A 60 F night setback seems to help a lot, with the same average outer glazing
cavity temp but a higher overnight mass swing than with a constant 70 F day/
night temp. This would also allow using the inherent mass of the cube to store
some overnight heat, vs having to store all overnight heat in the cool store,
if there's no night setback.

Nick


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