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re: house cooling
24 may 2001
anthony matonak wrote:
>i'm still intrigued by the concept of seasonal storage. i.e. ice house.
>if your location gets cold enough in the winter to make ice at night
>by simply letting tanks of water freeze then you could store large
>quantities of that, essentially free, ice in a super insulated structure
>for the summer months. cooling your house using this ice could be very
>simple using either ducted air or water (and antifreeze) filled radiators.
icemaking can be a limitation. (a pessimist might say this kind of
cooling only works in climates where cooling isn't required :-) nrel
says january is the coldest month in phila, with a 30-year average
30.4 f outdoor temp and an average daily min and max of 22.8 and 37.9,
so t(t) = 30.4+7.5sin(15t) = 32 when t = 0.8 and 11.2, ie it's above
freezing for 10.4 hours on an average day, and the average temp is
27.4 for the other 13.6 hours, so the average number of "freezing
degree-hours" (fdh) is about 13.6(32-27.4) = 63 per day.
december has 28.1, 35.8 and 43.4 min/avg/max, with about 8 30 f hours
per day or 16 fdh. february has 24.8, 33.0, 41.0, and 11 hours at 28.4,
ie 40 fdh. the other 9 months have average min temps above 32. so
we have a total of about 90days(16+63+40) = 10.7k fdh per winter.
with lots of airflow and a tank airfilm surface conductance of about
1.5 btu/h-f-ft^2, an fdh can freeze 1.5 btu of ice per square foot of
tank surface, ie 1.5/144 = 0.0104 pounds of ice, ie 111 pounds (about
2 ft^3) per winter, ie 16k btu of coolth. the tanks might be 4' thick,
with freezing air on both sides and a layer of ice on top. ice has a
thermal resistance of r0.065 per inch (vs r0.26 for water and r6 for
air at 32 f, with downward heatflow), so a 10" ice layer would double
the thermal resistance of the top surface, but the sides below the ice
would continue to gather coolth efficiently.
phila has 1101 (65 f) cooling degree-days per year. a modest airtight
mansion with superinsulation and 300 btu/h-f of thermal conductance
would require 24hx1101x300 = 8 million btu of coolth or 8m/16kx111
= 55k lb or 920 ft^3 of ice, eg a 10 foot cube, if the icehouse were
perfectly insulated (how many "thawing degree-days" in a year?), but
that only has 400 ft^2 of surface, excluding the top and bottom, and
we need about 8m/16k = 500 ft^2, with no ice freezing resistance.
>my thoughts on construction of such an ice house would be to use a cheap
>steel building as the shell and lots of sprayed on foam insulation though
>there may be cheaper forms of construction.
how about a 20x20x10' tall steel barn with 3 4x16x8' tall tanks inside,
with 2' of airspace between each tank and a strawbale floor and sides?
the tanks might be 36 4x8 sheets of plywood lined with 3 20'x33' pieces
of epdm rubber from a $600 20'x100' roll. we could make 1536 ft^3 of ice,
with no freezing resistance. the building might be a "cool air trap,"
with holes at the top but no holes at the bottom. it would freeze more
efficiently with some bottom louvers that opened below 32 f.
or, we might use a 16' cube with lots of fans and a snowmaking machine...
>anyone know if there is a kind of mirror finish or reflective paint that
>could be applied to the outside of such a building, especially the roof,
>that would keep it from absorbing sunlight and heating up from it?
maybe white paint, with a radiant barrier underneath.
nick
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