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re: a spencerator
2 mar 2005
daestrom  wrote:
 
> wrote:

>>nrel says the average daily november temp in massena ny is 35.3 f, with an
>>average daily min of 27.7. december has 20.2 f and an 11.7 min. january 
>>has 14.3 and 4.3. february has 16.5 and 6.5. march has 28.3 and a 19.1 f min.

averages can help predict fuel consumption, altho some days and months and
years are not average...

>> a 2'x2'x8' tall r20 34 f fridge in a 70 f room needs (70-34)80ft^2/r20 = 
>> 136 btu/h or 3456 btu/day of coolth...
>>
>> the ice box might be 2'x2'x8' tall, with a 55 gallon drum packed with 100
>> 2 liter water bottles surrounded by an antifreeze solution thermosyphoning
>> through an old 800 btu/h-f auto radiator above with an 8' chimney above.
>> cold air might enter the radiator and flow out at the top of the chimney.
>>
>> with 800 btu/h-f of conductance from 32 f water to outdoor air at temp t,
>> (32-t)800 = 3456 btu makes t = 32-3456/800 = 27.7 for 1-hour per day 
>> cooling and t = 32-3456/1600 = 29.8 for 2-hour cooling, and so on.
>>
>> the crude tmy2 (typical meteorological year) simulation below indicates
>> that a fridge like this could work for 241 days (2/3) of a typical year
>> in massena, from september 23 through may 22...
>
>...i found that as early as april 7, there are very few hours that get down
>to 32 f.  i counted only 58 hours between 4/7 and 5/22 (46 days)... for
>a total of only 231.5 'fridge-degrees hours' (#hours * (32-t)).

well, 231.5x800 = 185,200 btu, 16% more than the 46 day cooling requirement
of 46x3456 = 158,976 btu, and this period may have begun with 144x440/3456
= 18 day's worth of perfectly-insulated ice storage. 
 
>your program seems to take credit for any temperature below 32, no matter 
>how slight...

and ignores coolth collected from air between 32 and 36 f... 

>it doesn't consider that at least a slight temperature 
>difference must exist to circulate the antifreeze.

how do low concentrations of antifreeze or salt water thermosyphon when cold?

i figured cold water weighs about 62.67 - 0.003t lb/ft^3, with t in degrees f.
the density difference caused by the temperature difference between up and down
pipes causes a pressure difference proportional to the height of the water 
column, making a flow through the resistance of the pipe loop. with 8' of
height and a dt temperature difference, dp = 0.024dt lb/ft^2.

a pipe with radius r and length l in feet and pressure diff dp has laminar
flow q = pir^4dp/(8mul) ft^3/s. viscosity mu is 6x10^-7 lb-s/ft^2 at 32 f.
with 16' of 1/2" pipe, 8' up and down, q = pi(0.25/12)^4dp/(8x6x10^-7x16')
= 0.0077dp ft^3/s or 1729dt lb/h, which might move 1729dt^2 btu/h. we might
collect 3917 btu of coolth in 2 hours with a dt = sqrt(1958/1729) = 1.1 f
thermosyphoning temperature difference, which might be ignored, given the
radiator conductance.

someone who'd like to try this might do a more accurate simulation or use
a couple of pumps and fans to start with...

nick




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