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tank sizing

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Posted by Jeff on March 22, 2008, 6:26 am
   I'll need to build a tank sometime soon. I'll have 120SF (roof mount,
near azimuth) of collector (I still need to paint (solkote) and
install), this has been enough of a job that I don't expect to expand
for some time.

   So what's the wisdom on collector capacity to tank capacity?

   Sunny days often have as cold or colder nights as overcast days (no
cloud cover to keep in the heat). Why try to save for a rainy day when
it's spent on clear nights?

   It seems to me that the tank capacity should be roughly the same as
the daily collector output (on a good day).

   So say I'm getting about a therm. I should try to operate at moderate
temperatures to get better efficiency, perhaps cycling from 80F to 120F.
One therm at 40F would be about 40 cubic feet of water. I could add
another therm at less efficiency by going up to 160F.

   I'm all electric resistance (yes, spaceheaters) now (and the air
solar). I average 25 kWhr/day in January, about 30 kWHr (~ 1 therm) in
December and the latest was 18KWHr. Yearly minimum is about 12 kWHr/day.
That would imply to me that I'll have maybe a quarter to a half a therm
leftover to either heat more of the house or save for another day or
heat the DHW.

   Does this seem right?

   I'd probably be in bad shape if I lived in a colder climate!


Posted by ROBERT ALLAN \"BOBBIE\" BEAR.M on March 22, 2008, 9:39 am
On Sat, 22 Mar 2008 02:26:35 -0400, Jeff wrote:

Sir, tanks are illegal to build or own unless you are militarily
affiliated. Caution, Sir.
Dr. Robert Bear, MD, FRCPC, (ex Vice Dean, Not James)
American Board of Internal Medicine & Nephrology Certification
Clinical trials, renal soiling, DEPENDS research,
Dialysis population, no boners research
Euthanasia Certified; Uncontrolled Pissing Expert
Telephone 780-407-7239; Fax 780-407-7771

Posted by Robert Scott on March 22, 2008, 11:04 am
 I am more curious as to how you intend to use stored hot water down to 80F.
Normal hot water baseboard heat uses about 140F water or higher, right?
Assuming a room temperature of 70F, then dropping from 140F to even 90F would
mean that the baseboard heat delivery would be 28% of what it was at 140F.  So
you would need about 3.5 times as much baseboard fin tube.  Or are you using a
forced-air heat exchanger?  This type of consideration may shed some more light
on what tank capacity should be.

Robert Scott
Ypsilanti, Michigan

Posted by Jeff on March 22, 2008, 4:30 pm
 Robert Scott wrote:

In my heart of hearts, I'd like to do staple up radiant.

But I think I'll start with some auto AC condensors, or perhaps a small
radiator, with a small fan.

   I don't have a wife here, so I can get away with stuff you guys can't!

   I don't know what range I'll be cycling over. If I build up a
surplus, it will certainly be higher!


   Or are you using a

Posted by nicksanspam on March 22, 2008, 7:17 pm

Here's John Siegenthaler's equation for the heat output Q of 1' of baseboard
radiator fin tube with low water-air temperature differences:

Q=G^.04*Nom*9.6865E-04*(Tw-Ta)^1.4172 Btu/h-ft,

where G is the flow rate in gpm, Nom is the nominal heat output in Btu/h-ft
with 200 F water and 65 F air, and Tw and Ta (F) are water and air temps.

So Argo Technology Lo Trim fin tube (about $/ft) that makes 690 Btu/h-ft
with 1 gpm of water at an average temp of 200 F in 65 F air with a thermal
conductance of 690/(200-65) = 5.1 Btu/h-F-ft would make 280 Btu/h-ft with
1 gpm of 140 F water and 70 F air at 280/(140-70) = 4 Btu/h-F-ft, and
47 Btu/h-ft with 90 F water and 70 F air at 47/(90-70) = 2.3 Btu/h-ft.


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