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'[EE]: refrigerator calculations -- please check m'
2008\04\29@230727 by Al Shinn

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I hope this IS EE?? but some may see it as [GW] politics?

Some time ago it was mentioned that opening the door on a fridge was a
major energy expense. That seemed somehow wrong to me so I finally did
some calculations but I frequently mess such stuff up so I am asking for
a reality check from the rest of us.

The data:
sp heat of air  1 J/(gram degK)  - (wikipedia)
density air .0745lb/cuft -  (don't remember wherefrom)  = 33 g /cuft
(how's that for mixed units? :-)  )

calcs:
so we get 33 J/cu ft deg C   = 33Wsec / cuft deg C

my fridge is about 12 cuft  (33" x 23" x 26") so I get  396 Wsec/deg C,
drop the temp from 25 to 0C gives 25 x 396 = 9900 Wsec per cool down
lets make up, say, 20% efficient gives 49,500Wsec  ~~50,000Wsec
so, 50kWsec/60= 833 KWmin = 13.8 KWhr  at $.10/KWHr = $1.38 / cooldown
Can this be so?  I'm never gonna open that sucker again as long as I
live!! At least not till I need the next cold beer.


Of course not all the air escapes when you get the milk out, and there
IS other stuff in there taking up volume, and etc but still...

So the guy that converted his chest freezer DID have a valid point
(unless my calculations are messed up) I am still quite skeptical as I
doubt that the fridge uses a KW (can't find that spec on any of the
labels?) when running but even if it does, I doubt that it takes 13 hrs
to cool down.

So we could save KWHrs (= $$) by putting in little clear doors for each
shelf so only the level you are accessing is air exchanged??
--

Looking forward,
Al Shinn


2008\04\30@081612 by Tony Smith

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{Quote hidden}

That number is a tad large.  I'm too lazy to go back & check, but I think it
was under 1/4 of a cent per opening last time we worked it it out.

You could always buy a watt meter and put that on the fridge for a few days.
I was always a bit suspicious about the 'major energy loss' claims too.  I
worked out my fridge cost a few cents per day to run, so little that the
payback period if I brought a new one was over 20 years.  IIRC, it used
~100W when running, and ran 2-3 hours per day.

That said, the bloke with the chest freezer is right, a chest freezer won't
loose that cold air when the door opens, so it'll use less power.  Probably
not worth it for the average person, but he was trying to squeeze every watt
posible out of his solar system or something.  Supermarkets know this, they
don't even bother with doors (lids) for their freezers.

To claims the savings that he did, I think he just had a crap fridge to
start with.

Tony


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