wrote in message
...
A fridge can be a large winter load for an off-grid PV house...

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.

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, or 17,280 Btu for 5 days, which might
come
from at least 17280/144 = 120 pounds of ice in an outdoor box, with a
large
water tray with a copper pipe in the freezer compartment, with free
airflow
to the fridge below and a liquid thermosyphoning loop to move coolth in
from
the box. Can the loop stop working before it freezes the fridge contents?

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...

You may want to look at that data again. 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) where temperature was below 32, for a total
of only 231.5 'fridge-degrees hours' (#hours * (32-T)).

Similarly in the fall, from September 23 to October 31, I counted only 68
hours below 32 for a total of 390. 'fridge-degree-hours'.

Frankly, living not far from Massena (I'm ~hour north of Syracuse), I find
it incredible to think that anyone could use this to freeze water reliably
from mid March through to May or from late September through mid November.
Indian-summers, freakish warm spells and all, just make it pretty iffy.
Heck, in mid January we had temperatures in the 50's and 60's for several
days. A 'green Christmas' is a real threat every year.

Your program seems to take credit for any temperature below 32, no matter
how slight. It doesn't consider that at least a slight temperature
difference must exist to circulate the antifreeze. There must be a
temperature difference to create a thermosyphon. And without circulation
you cannot move any btu/h to the barrel.

Using convection, to carry 800 BTU/h from the radiator to the barrel, the
temperature drop across the radiator would have to be 3.2 F if the
thermosyphon develops 0.5 gpm. If you don't get that much flow, it would
take even more delta T to transport 800 BTU/h. So for an *infinite*
radiator, it would still have to be 28.8 at 0.5 gpm of flow to carry 800
btu/h to a 32F sink.

A 50/50 mix of ethylene glycol and water has a density of about 68.27 -
0.01881*T lbm/ft^3 (where T is temperature in F). So a ten foot high
thermosyphon with a 5 degree temperature difference between the two legs
would have 5*0.01881 * 10 = 0.9405 lbf/ft^2 driving head (a mere 0.0065 psi,
or 3/16 inch of water gage). Raise the radiator another 10 feet (20 ft
total) and you can double the driving head for the thermosyphon (but it
doubles the length of piping).

How much flow can you get through 40 ft of large diameter tubing/piping with
0.013 psi driving head? It would have to be about 0.3 gpm to get 800 BTU/h
heat transfer. If not, you can't make '800 BTU' of ice per hour even at 27
F.

In the 'dead of winter' there is plenty of surplus 'fridge-degree days' to
make it work. But the spring and fall become pretty dicey as to just how
long you could keep the fridge cold with changing weather patterns.

daestrom