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passive seasonal icemaking
6 may 1997
maybe it's time to talk again about passive refrigerators, or storing winter
"coolth" as ice for summer cooling. here's a start, based on the national
renewable energy laboratory's tmy2 ("typical meteorological year") hourly
weather data for philadelphia, one of 239 us cities listed at

10 open "tmycool" for input as #1
30 for h=1 to 8760'read ambient temperature every hour for a year
40 line input#1, s$'input a line from tmy2 file
50 temp=val(mid$(s$,27,4))'ambient temp (f)
60 atempt=atempt+temp'accumulate average yearly temperature
70 if temp < 32 then fdht=fdht+32-temp'accumulate freezing degree-hours
80 if temp > 32 then mdht=mdht+temp-32'accumulate melting degree-hours
90 next h
100 print "average yearly temperature:";atempt/8760
110 print "freezing-degree days:";fdht/24
120 print "melting-degree days:";mdht/24
130 close 1

this fine little basic program yields:

average yearly temperature: 53.62591 (f)
freezing-degree days: 339.4584
melting-degree days: 8232.917

like heating and cooling degree days (f) for 32 f houses. the number of
"freezing degree days" (fdd?) is the total equivalent number of days in
which the outdoor temperature is less than freezing during a typical year,
times the difference in degrees between 32 f and the outdoor temperature,
on each of those days. 

fdd for a particular climate measure how easy it is to make ice in the winter.
it takes about 144 btu to freeze a pound of water at 32 f, and a water surface
with a thin cover has a thermal conductance of about 1.5 btu/h-f-ft^2 in still
air, and a cubic foot of water weighs about 64 pounds, ie about 5 pounds per
inch of depth, so it looks like 1 fdd can freeze an ice layer 24hx1.5/(144x5.3)
= 0.05" thick, on top of some 32 f water (ice is a good thermal conductor,
about 100 times better than water, for downward heatflow.) over a year's time,
in philadelphia, we might freeze a layer of ice about 339x0.05 = 16" thick
on some sort of specially-designed icemaking pond... 

melting degree-days determine how big an icehouse needs to be to stay cold
for a whole year (bigger icehouses work better.) a icecube l feet on a side
surrounded by r40 insulation contains about 64l^3 pounds of water, ignoring
the insulation thickness, and initially stores about 9200l^3 btu of "coolth," 
which might equal its yearly heat gain of about 24hx8200x6l^2/r40 btu/year in
phila, which leads to a minimum size of l = 3 feet. a 5x5x5 = 125ft^3 icecube
inside a 10' strawbale cube might stay partially frozen all year, if it were
frozen solid to start with and it only had to keep itself and some vegetables
cool, vs say, air-conditioning a nearby building.

freezing a 5' cube takes about 5^3x64x144 = 1.15 million btu of cooling, eg
some sort of roofpond with an area a that makes 24x339x1.5a = 1.15 million, 
so a = 94 ft^2, eg a 10' x 10' (or half that area, with ambient airflow
underneath) shaded roofpond covered with a thin layer of salt water under a
piece of polyethylene film in freezing weather, and otherwise dry. (outdoor
air isn't still, especially in the winter. moving air and night sky radiation
would cool the pond more, as would evaporation, without the poly film cover.)

a tiny low-power pump or some sort of passive plumbing arrangement might move
the salt water around through a heat exchanger loop in an 8'x8'x4' thick water
wall inside a 10' cubical outdoor refrigerator. this 256 ft^3 water wall might
be a single $150 piece of epdm rubber roofing material 20' wide x 24' long,
folded up into a box with no seams, like a chinese food take-out box.

or... an air-antifreeze heat exchanger and fan might replace the roof pond.
something like the $35 used 1984 dodge omni auto radiator with its 12v fan in
my living room floor, or a fan-coil unit like magicaire's $150 all-copper
2'x2' shw 2347 duct heat exchanger, which can transfer 45k btu/hour between 
125 f water and 68 f air (a thermal conductance of about 800 btu/h-f) at 1400
cfm (like a window fan) with a very low airflow resistance, ie an air pressure
drop equivalent to the weight of a column of water 0.1" tall. a 100 watt fan
with this heat exchanger would have to run about 2.3million/(800x(32-25)) =
410 hours at 25 f to freeze that 256ft^3 water wall, consuming 41 kwh/year, ie
about $4/year of electrical power at 10 cents/kwh, while providing 256 ft^3 of
year-round refrigerated space inside the 512 ft^3 box, equivalent to about
17 home-sized fridges. is it time for neighborhood icehouses yet? fill them
with beer and lawn chairs to facilitate apres-lawn-mowing-male-bonding? and
what will we do about lawns, after the oil runs out? 


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