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re: charge fridge(with ice) not batteries?
24 jun 2004
rtrolll  wrote:

>how would door openings/air changes effect the 9.3 hrs? (say 4 per night)

...9.3 days. a door opening might let 16.6(9ft^2)sqrt(6x(70-32)) = 2300 cfm
flow through the fridge, eg 188 ft^3 with a 5 sec opening. a pound of air at
70 f and 50% rh has 0.00792 pounds of water vapor, and a cubic foot of 70 f
air weighs about 0.075 pounds. a pound of 32 f 100% rh air has about 0.00379 
pounds of water, so each opening might condense 188x0.075(0.00792-0.00379)
= 0.058 pounds of water out of the 70 f air, adding about 1000x0.058 = 58
btu of heat, like 12.7 minutes with the door closed.

>would aluminum cans or other sizes plastics effect ice melt rate?

not much.

>can similiar type fridge be used ie side by side? with 12vdc computer fan?
>on tstat? to circulate cool air?

sure, altho that's more complicated. then again, it could make the fridge
temp closer to 32 f, compared to a naturally thermosyphoning system with 
some holes in the freezer floor. the fridge might have a thermal chimney
inside near the back wall to help thermosyphoning. a side-by-side fridge/
freezer might have holes at the top and bottom of the wall between them.

>can ice melt too slow? 

not exactly. you want lots of ice container surface in order to freeze
it quickly and get the coolth back out into the air. a single 2'x3'x2'
ice cube would take a long time to freeze. with only 32 ft^2 of surface
and about 1.5x32 = 48 btu/h-f of airfilm conductance, it could supply
274 btu/h with a 274/48 = 5.7 temp rise, ie the fridge would be about
37.7 vs 32 f, or warmer, on a 90 f day...

            t = 32+i/48 = 39.6 f
     1/7.2  |   1/48
90 ---www---*---www--- 32

        <------ i = (90-32)/(1/7.2+1/48) = 363 btu/h

with 192 1-liter bottles and 170 ft^2 of surface and a 250 btu/h-f
conductance, the fridge would be about 32+274/250 = 33.1 in 70 f air
or 33.6 on a 90 f day, with a "stiffer cold source."

>i guess a fan could be used with a tstat
>to bring the temperature back down after air dumps out on door open?

that's a useless complication, since the air in the fridge will warm when
the door opens, but the food temp will hardly change, and the air has
little thermal mass, so it will quickly cool when the door closes.

>whats better?
>a. using 200 watts to pour 60 f. water over fridge on a 90f day(under or over
>insualtion).(pump specs say 1.2 gal per min at 600' depth)


>b  using 200 watts through battery/invertor etc for compresor time?. (which
>does'nt go far with stardard fridge)

about 6 hours, if your fridge uses 1 kwh/day. you don't want to freeze
the food in the fridge while you are making ice in the freezer, which
is another reason to have lots of ice surface and some unfrozen water at
the end of the freezing cycle, or a single 2'x3'x2' ice cube with only
the bottom surface exposed to fridge air, or a few "regulator" water
bottles in the cold airstream near the top of the fridge...

the boat us catalog also says

    freeze or not freeze

    while most products other than ice chests and thermoelectric coolers
    can freeze ice cubes or keep your haagen das from melting, you might
    want to consider a solution that has greater freezer capacity if you're
    going to be out for a few days. this is especially important if you
    entertain, and you don't want to be serving beverages without ice!
    nothing is more embarrassing for a sailor than to have to borrow ice
    cubes from a nearby sportfisher, or vice-versa. 


need an answer or a method for figuring which window, insulation, heating
or refrigeration system to use? ask us.

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for an all-day workshop on new solar house heating and natural cooling
strategies ("hvac nonsense") on july 9 in portland, or--see page 25 of

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