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solar pond and imitation big fin water heaters
28 sep 2005
dj  wrote:

>i was told "...your shift is over when tank three hits 40c..." ;-).
>faster flow was not better in that case ;-).

a faster fluid velocity always increases the heat transfer rate, since
the boundary layer between the fluid and the pipe wall is thinner, with
more conductance. that's basic, but it may not get you home earlier :-)

there's a tradeoff in solar pond and big fin and greywater pipe coil heat
exchangers with pressurized dhw inside and unpressurized still heat store
water outside: copper is more conductive but holds less water than plastic,
for the same money, which makes a plastic pipe with a volume comparable to
an average hot water burst more efficient than copper by day's end, even
with slower flow and less conductance.  

we now have two victi... err, beta test sites in the philadelphia area
for full-size solar pond and ersatz big fin solar heaters.

clint's pond might be 5'x8'x2' deep, with a large flat elliptical 1" pipe
spiral under the 2" cover and epdm over the cover and a 5 watt submersible
pump moving water over the cover during the day. a local greenhouse supplier
has dynaglas polycarb roofing in standard 4'x12' pieces for about $50, so
the pond might have 2 4'x6' pieces as the upper glazing. the demo unit is
happily humming away at clint's house now :-)

on another front, 2 cheap big fins (2' widths of 0.018" brown-painted al
coil stock attached to 3 1/2" copper pipes) might be in a 4'x12'x3" thick box
on the south wall of moise's house, glued to polyiso board, with dynaglas
over that and an unpressurized 6'x6'x2' deep box tank upstairs in the house
and a flat coil in the box. in phila, with no overhang, the wall unit would
produce about 500 btu/ft^2-day in january and 1500 in july. a 2.7' overhang
above a 1.3' separation would shade it completely on 6/21. how much overhang
would limit this to 1000 btu/ft^2-day in july? overtemp and freeze protection
could come from a $5 fountain pump in the tank that turns on when the fins 
cool to 40 f and also turns on when the tank water is greater than 140 f,
with 2 $10 thermostats, like this: 

      on if box <40 f          
   |  e38 "easy heat"       |    
   |  $10.99 at lowes       |   
   |                        |  
   |                        |
   |  on if tank >140 f     |    ------
-------------x------------------|      |
      grainger 2e365 (spdt)     |      |
                                | 5 w  |
 120 v                          | pump |
                                |      |
--------------------------------|      |

another 2e365 might turn on a fan when the tank is warmer than 140 f
and the house needs heat. if the fan's ever running at dawn, the pump
may need replacement.

home depot (sku 125467) and lowes (sku 17376) both sell amerimax home
products 24"x50'x0.018" brown-painted al coil stock for about $63. make
a jig by screwing 2 5' 1x3s to a 2x8 5/8" apart, place flashing over
the groove with 1/2" copper pipe over that, pound for 30 seconds with
a big rubber mallet, remove the pipe, spread silicone caulk in the round
groove, place the fin over a 10' pipe, glue it down to 2" polyiso board
with more silicone with the brown side and the pipe groove up, al-foil
tape the board edges, add polyiso perimeter spacers and closure strips
and the 4'x12' dynaglas and screw it to the south wall of the house. 

20 pi=4*atn(1)
30 k=95'al thermal conductivity (btu/h-f-ft^2)
40 t=.018/12'al coil stock thickness (feet)
50 d=2'al coil stock width (feet)
60 od=5/8/12'cu pipe od (feet)
70 l=d/6-pi*od/4-od/2'half-fin width (feet)
80 hb=1.5/2'one-sided airfilm conductance (btu/h-f-ft^2)
90 nl=sqr(2*hb/(k*t))*l'fin efficiency term
100 tanhnl=(exp(nl)-exp(-nl))/(exp(nl)+exp(-nl))'tanh(nl)
110 feff=100*tanhnl/nl'fin efficiency (%)
120 w=72*(l+od/2)'tip-to-tip fin width (inches)
130 print w,feff'fin width (in) and efficiency (%)
140 ac=2*w/12*10'collector area (ft^2)
150 aceff=ac*feff/100'effective collector area (ft^2)
160 ag=48'total glazed area (ft^2)
170 sunpower=.9*250'full-sun heat into box (btu/h-ft^2)
180 finpower=aceff*sunpower'full-sun into fins (btu/h)
190 boxpower=ag*sunpower'full-sun heat into box (btu/h)
200 ta=34'average daytime outdoor temp in phila in january (f)
210 boxnet=boxpower-finpower'net global sun into box (btu/h)
220 boxtt=ta+boxnet/ag'thevenin equivalent box temp (f)
230 dhwtemp=100'average collector water temp (f)
240 dhwpower=finpower+(boxtt-dhwtemp)*1.5*aceff'water heating (btu/h)
250 boxtemp=ta+(boxpower-dhwpower)*1/ag'air temp in box (f)
260 sunhours=4'full-sun hours in january in phila 
270 dhwenergy=sunhours*dhwpower'dhw energy (btu/day)
280 cost=2*ag+2*ac'water heater materials cost ($)
290 print ac,boxtemp,dhwenergy,cost

fin width     efficiency

21.05476"     80.81405%

fin area      box air       daily heat       cost

35.09126ft^2  102.9664f     29958.45btu      $166.1825


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