# Re: How we hijack this group???? and moderate it?? - Page 2

Posted by nicksanspam on April 7, 2008, 1:08 pm

Alternatively, the "motorized damper" could be a \$2 passive automatic
foundation vent that stays closed during the day and opens when ceiling
air drops below about 60 F at night.

It looks a single 4'x8' 500 pound water tray under a ceiling can store 50%
of the 1000 Btu/ft^2 daily sun on an air heater with 4'x8' of R2 twinwall
polycarbonate glazing with 80% solar transmission on a 30 F day. Another
tray or a 50 F night setback temp raises efficiency 5%. Doubling the air
heater size to 8'x8' without changing the store drops it by 10%.

20 TA0'outdoor temp (F)
30 SUN00'south wall sun (Btu/ft^2-day)
40 TN`'night setback temp (F)
50 AAH=4*8'air heater area (ft^2)
60 UAH=.5'glazing U-value (Btu/h-F-ft^2)
70 TAH=.8'glazing transmittance (fraction)
80 LDAY=8'solar collection hours
90 RT=1/(AAH*UAH)'Thevenin air heater resistance
100 IH*TAH*SUN/LDAY'heatflow into air heater (Btu/h)
110 TT=TA+I*RT'Thevenin air heater temp (F)
120 DEPTH=3'water depth (inches)
130 C=4*8*62.33*DEPTH/12'thermal mass (Btu/F)
140 FOR NTRAYS = 1 TO 4
150 A=4*8*NTRAYS'one-sided surface (ft^2)
160 G=3*A'2-sided thermal conductance (Btu/h-F)
170 RCD=C/G'discharge time constant (hours)
180 RCC=C*(1/G+RT)'charge time constant (hours)
190 TMIN=TN'initial min water temp (F)
200 TMAX=TT+(TMIN-TT)*EXP(-LDAY/RCC)'new max temp (F)
210 TMIN=TN+(TMAX-TN)*EXP(-(24-LDAY)/RCD)'new min temp (F)
220 IF ABS(TMAX-TMAXL)>.1 THEN TMAXL=TMAX:GOTO 200'iterate
230 HEAT=(TMAX-TMIN)*C'stored heat (Btu/day)
240 EFF0*HEAT/(AAH*SUN)'efficiency (%)
250 PRINT NTRAYS;C;TMIN;TMAX;EFF
260 NEXT

1  498.64  61.60149  94.85901  51.82354
2  498.64  60.07583  95.92481  55.86168
3  498.64  60.00356  96.70166  57.18481
4  498.64  60.00017  97.13054  57.85841

Hey Gary, how about trying this with a 12/06 MEN air heater? :-)

Nick

Posted by nicksanspam on April 7, 2008, 1:49 pm

This could be more efficient if you replace the window screen with
another layer of glazing and also warm room air during the day
with a separate airpath through the outer glazing cavity...

Nick

Posted by nicksanspam on April 7, 2008, 5:16 pm

We could ship a low-cost kit with a 4'x16'6.5" roll of 0.007" GE HP92W Lexan
that would be folded over a flat 2x4 at the top, with a tensioning spring at
the bottom. This would make a 3.5"-deep outer cavity and a 2" inner cavity.

Nick

Posted by nicksanspam on April 25, 2008, 11:38 am
8'
------------------------------------------
|                      |                   |
|                      |    -----------    |
|                      |   |           |   |
|                      |   |    ---    |   |
|                      |   |   |   | ^ | ^ |
|                      |   |   |   | | | | |
|                      |   |   |   |   |   |
|                      |   |   |   |   |c  |
|                      |   |   |   |h  |o  |
|                      |   |   |   |o  |o  |
|                      |   |   |h  |t  |l  |
|                      |   |   |o  |   |   |
|                      |Cc |Ch |u  |Tha|Tca| 8'  south -->
|                      |   |   |s  |   |   |
|                      |   |   |e  |c  |c  |
|                      |   |   |   |a  |a  |
|                      |   |   |w  |v  |v  |
|                      |   |   |a  |i  |i  |
|                      |   |   |l  |t  |t  |
|                      |   |   |l  |y  |y  |
|                      |   |   |   |   |   |
|                      |   |   |   |   |   |
|                      |   |    ---    |   |
|                      |   |           |   |
|                      |    -----------    |
|                      |                   |
------------------------------------------

We might replace the corrugated polycarbonate glazing and window screen in
Gary's nice 12/06 MEN air heater described at http://builditsolar.com  and
http://www.motherearthnews.com/multimedia/image-gallery.aspx?idt688&seq=1
with a \$4 4'x16'6.5" UPS-shippable 3-pound roll of 0.007" GE HP92W Lexan
folded over a flat 2x4 at the top, with a bottom tensioning spring to make
a 3.5"-deep outer cool cavity and a 2" inner hot cavity and solar thermal
cogeneration, as above, viewed in a fixed font.

An 8' cube with lots of simple assumptions and US Rv exterior surfaces and 2
8'x8' layers of Lexan with 92% solar transmission would have an approximate
G = 6x8'x8'/Rv Btu/h-F thermal conductance to outdoor air on a cloudy day.

Rv = 15 makes G = 25.6. If 1000 Btu/ft^2 of sun keeps the cube 70 F for 8 hours
with 8h(70-30)G = 8.2K Btu on an average Phila December day and a Cc Btu/F cool
mass supplies 16h(70-30)G = 16.4K Btu of overnight heat and 0.92^2x1000x8'x8'
= 54.2K Btu = 8.2K+16.4K+8h(Tca-30)8x8/R1, the average cool cavity temp Tca
= 88 F. If Cc is 70 F at dawn and 88+18 = 106 at dusk and 16.4K = (106-70)Cc,
Cc = 458 Btu/F, eg 458/64 = 7.2 psf of water with a 12x7.2/62.33 = 1.4"
waterwall depth.

If the hot cavity gains 54.2K/8h = 6775 Btu/h at a 106+6775xR1/64ft^2 = 212 F
max temp (boiling:-) and the hot waterwall supplies heat for 5 cloudy days in
a row with 5dx24h(70-30)G = 122.9K Btu = (212-70)Ch, Ch = 865 Btu/F, eg 13.5
psf with a 12x13.5/62.33 = 2.6" depth.

With more thermosyphoning airflow and exponential vs linear warming and finite
airfilm conductance and passive thermal control details, we might end up with
a 2x3 cool wall and a 2x4 hot wall, or thinner phase-change walls. Anna Edey
made waterwalls with 55 gallon plastic film drum liners sandwiched between
10 cent/ft^2 2"x4" welded-wire fence walls over wood frames.

Nick

Posted by nicksanspam on April 27, 2008, 12:45 pm

Looks OK so far:

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 SUN00'average sun on south wall (Btu/ft^2-day)
70 CH=8'solar collection hours
80 TRp'room temp (F)
90 QH=(TR-TA)*GC'hourly heatflow (Btu/h)
100 QN*QH'overnight heat
110 QD\$*QH'daily heatflow (Btu/day)
120 AS=8*8'south glazing area (ft^2)
130 RS=1'south glazing resistance (per layer)
140 TS=.92'south glazing transmittance (per layer)
150 SSS=TS^2*AS*SUN'sun entering south glazing (Btu/day)
160 TCA=TA+RS*(SSS-QD)/CH/AS'average cool cavity temp (F)
170 AV=8*8/12'vent area (ft^2)
180 H=8'vent height difference (ft)
190 DT=(QH/11.7/AV/SQR(H))^(2/3)
200 AN=8*8'1-sided overnight heat store area (ft^2)
210 UA=1.5'slow airfilm conductance (Btu/h-F-ft^2)
220 TMIN=TR+DT+QH/(2*UA*AN)'dawn heat store temp (F)
230 TMAX=2*TCA-TMIN'max cool cavity temp (F)
240 CN=QN/(TMAX-TMIN)'overnight heat capacity
250 DN*CN/AN/62.33'overnight store depth (inches)
260 PRINT TMIN,TMAX,DN
270 THA=TMAX+SSS*RS/AS/CH'hot store temp (F)
280 CC=5*QD/(THA-TMIN)'hot store cap (Btu/F)
290 DC*CC/AN/62.33'hot store depth (inches)
300 PRINT TMIN,THA,DC

78.56218      97.03782      2.667623

78.56218      202.8378      2.974399

Nick

•
• Subject
• Author
• Date
 Overnight air heater storage nicksanspam 04-07-2008
 Re: Overnight air heater storage nicksanspam 04-07-2008
 Re: Overnight air heater storage nicksanspam 04-07-2008
 Re: Overnight air heater storage nicksanspam 04-27-2008