This is a question for someone with a background in physics :

In steady state, will a refrigerator full of beer cans
consume  more electricity than an "empty" refrigerator ? 
Why ?

I know that the "full" refrigerator may cosume
more electricity in the beginning, when the beer
needs to be cooled. However, once the cans have been
cooled, and things are in steady state, will the fridge
consume more electricity per-day than when it was empty ?

yesdi-ga,

It will make no difference once the beer has been cooled to the
steady-state temperature of the refrigerator. The only energy consumed
by the refrigerator will be the energy required to maintain the
temperature due to the heat loss through the insulation around the
refrigerator. Assuming that the temperature is the same as the
non-beer-filled refridgerator the heat loss will be identical (due to
mostly natural convection around the refrigerator).

Having read the above... consider these little tidbits.

1) Getting all that beer down to temperature will require quite a bit
of energy initially, just as you have pointed out. Depending on your
refrigerator, this may take a few hours or a few days. Judge by how
long it takes to get the beer nice and cool :)  taste test my friend,
taste test

2) If you actually -have beer- in the refrigerator your more likely to
be opening and closing the door getting beer out.


krobert-ga

---

Clarification of Answer by krobert-ga on 02 May 2003 14:07 PDT

Sorry... forgot to add this to number 2:

If you keep opening and closing the door, the refrigerator is
definitely going to be using more energy.

krobert-ga

---

Request for Answer Clarification by yesdi-ga on 02 May 2003 15:18 PDT

Krobert,

I have a strong science background and I 
know the basic "energy required = energy lost" 
principles.

There's more to this than meets the eye.

My basic google search had turned up links such as :
http://www.psnh.com/Residential/ReduceBill/quicktips.asp
which says that : "A full freezer is more efficient - it takes more 
energy to cool empty space."

I'm looking for an answer that digs a little deeper into the
details of refrigeration.




2)

---

Clarification of Answer by krobert-ga on 02 May 2003 16:02 PDT

With reference to the link that you provided, what they are talking
about is the temperature stabilizing ability of foodstuffs in a
refrigerator.

Say you keep single bottle of beer in the refrigerator. If you open
and close the door once a day to access this one bottle, the action of
opening and closing the door moves a tremendous amount of heat -into-
the refrigerator which must now be cooled back down.

Now lets go to the scenario of a refrigerator full of beer, and
opening and closing the door once a day (once a day... with that much
beer, yeah right!). Much less heat is allowed into the cooled space of
the refrigerator because not much air is able to be exchanged with the
outside world.

What the problem boils down to is how often you need to access your
beer. Your linked article mentions having an "extra" large
refrigerator in you basement or garage. If you have one of these large
units or have just ever seen one, think about how they are designed.
Usually, they open upwards like a car trunk... why design it like
this? It keeps the cool, dense air from spilling out into the
surroundings.

The energy that is lost from an empty refrigerator is lost in this
cool air that escapes when you open the door. Believe it or not, but
it takes quite a bit of energy to cool down -air-.

Let me know if this answers your question. If you'd like me to, I
could run through a calculation of how much energy it takes to cool
room temperature air down to near near freezing.

Best to you,

krobert-ga

---

Request for Answer Clarification by yesdi-ga on 02 May 2003 18:17 PDT

The link I posted does not say that the reason that 
a full refrigerator is more energy efficient is 
because of heat-loss during opening and closing.
Is that own personal belief ? If so, please substantiate why 
you think so (links would be helpful).

It could be very well be that having a refriegerator full of
solids or liquids affects the heat-transfer rate
through the insulating material (walls). In which case, it might
be cheaper to run a "full refriegerator" even if you never
open it.

---

Clarification of Answer by krobert-ga on 02 May 2003 19:39 PDT

I was assuming a few things when constructing the above scenarios
(there's no other way to do it). I guess you could say that it is my
personal belief, but you -are- getting an expert opinion (BS Aerospace
Engineering, MS Metallurgical and Materials Engineering, Professional
Engineer, state of Indiana).

It is physically impossible for the heat GAIN (thanks kemlo) through
the walls of the refrigerator to change due to the changed contents.
All heat transfer happens through the insulation in the walls of the
refrigerator. There is heat convection outside the refrigerator,
conduction through the walls, and convection again on the inside.
There can be conduction on the inside due to contact of food with the
walls, but unless you have your refrigerator absolutely stuffed this
is unlikely and probably doesn't make much of a difference anyway.
Basically, this is due to the first law of thermodynamics. You have
one temperature on the inside, one on the outside and a linear range
of temperatures through the insulation. All it has to do with is the
rate of heat transfer into the refrigerator from the outside.

So... to sum up, whether you have one beer or 100 beers, once the
inside of the refrigerator has reached it's specified cool
temperature, energy usage is the same assuming you never open the
door.

I hope this clarify's things for you,

krobert-ga


Here's those links for ya:

Adobe Energy Audits
http://www.adobeenergymanagement.com/audits.html
Search for "full refrigerator", sorry it's kind of buried in the
document.

What are your ideas to lower energy costs?
http://ths.gardenweb.com/forums/load/saving/msg0600022023263.html
Same thing, search for "full refrigerator". The explanation is at the
second "hit" for the phrase.
This one supports the statement I made earlier about the cool air
transfer out of the refrigerator being the culprit.

Thanks for the links krobert-ga. They were helpful.

I disagree with your statement that :
"It is physically impossible for the heat GAIN (thanks kemlo) through
the walls of the refrigerator to change due to the changed contents"

Consider the "content" I put into the refrigerator being a solid
block of insulating material that fits perfectly into the rectangular
area of the fridge. Or for that matter just a rectangular shell of
insulating material that fits into the fridge and effectively doubles
the thickness of the insulating walls. This would be an unusual 
"content" but it should decrease the heat GAIN into the fridge and
thereby reduce energy cost.

I do agree that beer cans, even when fully stocked in the bridge, 
will not have quite the same effect (and quite possibly
have no effect on the heat-transfer rate) :-)

To Krobert 
I think you have gone slightly wrong , you dont get heat LOSS through
the insulation you get heat GAIN.  Anyway the most gain will be thru
the door seal.
Regards Kemlo

Good point.... :-)

That's what I meant, just not what I wrote!

krobert-ga

Concerning yesdi-ga's final comments: having content in the fridge
that itself acts as extra insulation will not make the fridge more
efficient as the thermostat (and thus the device that controls how
much energy the fridge uses) would be outside the extra insulator. It
would be trying to maintain the 'gap' between this and the walls at a
constant temperature.

Another thought: if the fridge used a feedback system to measure
temperature where it allowed the temperature to fluctuate rhythmically
(ie. cooler switches on for a bit, cools it to 4 degrees C, then
switches off until it heats above, say, 6 degrees C). In that case
then contents with a high heat capacity (ie beer cans) would fluctuate
more slowly. You would think that this would cost the same amount of
energy in the long run, but maybe the sensor can only pick up a slow
change in heat and so for contents that cool down more slowly (ie beer
cans) it would detect when the temperature had lowered before it had
got too cold. For example, maybe having lots of beer cans in the
freezer means it only oscillates between 5 and 6 degrees C. In this
case then on average there would be less of a temperature difference
between inside and outside the fridge and so the fridge would use
slightly less energy.

This is just based on my experience as a med student (the body works
like this) and you would probably need to do some experiments to see,
but it is the only way I can think of using different amounts of
energy without breaking the laws of thermodynamics.

It seems to me that the contents of the fridge *will* have a (very
small) impact on the rate of heat gain - as krobert-ga said, one of
the heat transfer mechanisms involved is convection on the inside of
the fridge. If you stock the cans such that the convective flow near
the door is reduced, this should lead to a slightly higher temperature
around the door, and thus a slower rate of heat entering the fridge
(since the temperature difference between the door and the outside
will be marginally less, and some heat transfer mechanisms have rates
more or less proportional to this difference).

-- 
The Scarlet Manuka