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'[EE]: Thermal efficiency of Refigerators -- Air'
2008\02\20@153235 by Barry Gershenfeld

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>A fridge that takes only 0.1 kWh a day

The author states that when you open the door on a vertical refrigerator,
you lose all the cold air.

And my question is, what is the significance of this volume of air?  It
seems that the heat content of air is quite a bit lower than the heat
content of the objects inside.  Anybody want to do an estimation on how
much heat energy this is?  Maybe as a fraction compared to the
contents.  I've been suspicious about this one for years.   A manual once
stated that the fridge would work more efficiently if you kept it full of
stuff.  That got me thinking.

Barry

2008\02\20@155823 by Bob Blick

face picon face
--- Barry Gershenfeld <spam_OUTbarry_gTakeThisOuTspamzmicro.com> wrote:
> >A fridge that takes only 0.1 kWh a day
>
> The author states that when you open the door on a
> vertical refrigerator,
> you lose all the cold air.
>
> And my question is, what is the significance of this
> volume of air?  It
> seems that the heat content of air is quite a bit
> lower than the heat
> content of the objects inside.

You are quite right. Letting in some warm air does not
waste huge amounts of energy. But the food starts
cooling the warm air, so the longer the door is open,
the more energy is lost.

> years.   A manual once
> stated that the fridge would work more efficiently
> if you kept it full of
> stuff.  That got me thinking.

That might have more to do with the way thermostats
are designed. Since compressors don't have
proportional cooling, and there's no perfect location
for the temperature sensor, it basically all works
better if there's a good mass of food inside so the
variable duty cycle of operation becomes smooth
temperature control.

I've designed a couple of electronic thermostats for
refrigerators, and you have minimum on and off times
running a higher priority than temperature, so an
empty fridge will have large temperature swings.

Cheerful regards,

Bob

2008\02\20@163228 by Tony Smith

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face
> >A fridge that takes only 0.1 kWh a day
>
> The author states that when you open the door on a vertical
> refrigerator, you lose all the cold air.
>
> And my question is, what is the significance of this volume
> of air?  It seems that the heat content of air is quite a bit
> lower than the heat content of the objects inside.  Anybody
> want to do an estimation on how much heat energy this is?  
> Maybe as a fraction compared to the
> contents.  I've been suspicious about this one for years.   A
> manual once
> stated that the fridge would work more efficiently if you
> kept it full of stuff.  That got me thinking.
>
> Barry


I've wondered that too.

It's not all that hard to test, apart from needing four identical fridges.

Leave two empty, and fill the other two (bottles of water, etc).  Leave for
a day or two so their temperatures stabilise.

Set up something to open two fridges (one empty, one full) every 10 minutes
or so.  Do this for a day or so, and compare energy usage on all four.

In theory, the two that were never opened should be the same, and convential
wisdom says the empty fridge should have used more.  How much more is the
question, it shouldn't take too much energy to cool a square metre or two of
air.

I suspect with the full fridge it's more the case that the thermostat is
surrounded by cold stuff, so the incoming warm air doesn't affect it much.
Then there's the fridges that limit the run time of the compressor, so it
may not kick in even if the inside of the fridge is full of warm air.

Maybe we need another fridge, with a sole bottle of water propped up next to
the temperature sensor.
 
Tony

2008\02\20@190157 by Apptech

face
flavicon
face
>> The author states that when you open the door on a
>> vertical
>> refrigerator, you lose all the cold air.

>> And my question is, what is the significance of this
>> volume
>> of air?  It seems that the heat content of air is quite a
>> bit
>> lower than the heat content of the objects inside.
>> Anybody
>> want to do an estimation on how much heat energy this is?
>> Maybe as a fraction compared to the
>> contents.  I've been suspicious about this one for years.
>> A
>> manual once
>> stated that the fridge would work more efficiently if you
>> kept it full of stuff.  That got me thinking.


The thermal content of air is somewhat over 1000 times lower
per volume than for water. For very rough figuring you can
consider a cc of water and a litre of air equivalent as long
as you don't involve any phase changes. If the air is phase
changing in your application then you probably need other
advice :-).

So the air in a 200 litre 'fridge is about 200cc of water
equivalent. ie every time you open the door it's equivalent
to adding about a cup of water at ambient temperature. This
assumes that all the cold air is lost and that the
convection flow does not scrub more coolth off surfaces when
you leave it open longer. This shows why it's bearable to
open vertical fridges if you don't do it too often but a
good idea to do it as little as practical.

If I put a 1.5l bottle of liquid in a fridge I'd expect it
to be reasonably cool in  hour and probably close to fully
cooled at 2 hours. Longer never hurts. A PET 1.5l bottle
placed in a freezer is at risk if left for more than 1 hour
(don't try this at home if you want to drink the liquid).

A full fridge would lose less air per opening.

Heat loss per time should depend largely on delta t so, once
pumped down, the ongoing energy cost of storage per time per
mass is inversely proportional to fullness. Pump down versus
storage costs depend on insulation effectiveness and time of
storage and delta t, amongst other things :-).




   Russell McMahon.


2008\02\21@042708 by Alan B. Pearce

face picon face
>I suspect with the full fridge it's more the case that the thermostat
>is surrounded by cold stuff, so the incoming warm air doesn't affect
>it much.

In the case of my wife, it is a case of how long she has the door open while
she makes up her mind what we have for dinner ......

2008\02\21@073819 by Tony Smith

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face
{Quote hidden}

It takes 1 calorie (not food calorie) to heat 1 mL of water (1cc) by 1
degree celcius.  We need to drop 20 degrees, so 200 x 20 = 4000 calories of
coolth needed.  Say 25% efficiency, make it 16000.

That's not much, in food terms (K calorie) that's a jellybean or two.

Aparently a calorie is about 0.00000116222222kWH, more commonly known as
'not much'.

My power bill says a kilowatt hour is worth $0.12AU, so opening the fridge
door costs me $0.0022, or less than a quarter of a cent.

It appears fridge compressors are fairly crap these days, so limiting run
time wouldn't be a bad thing.

Tony

2008\02\21@093104 by Apptech

face
flavicon
face
> It takes 1 calorie (not food calorie) to heat 1 mL of
> water (1cc) by 1
> degree celcius.  We need to drop 20 degrees, so 200 x 20 =
> 4000 calories of
> coolth needed.  Say 25% efficiency, make it 16000.
>
> That's not much, in food terms (K calorie) that's a
> jellybean or two.
>
> Aparently a calorie is about 0.00000116222222kWH, more
> commonly known as
> 'not much'.
>
> My power bill says a kilowatt hour is worth $0.12AU, so
> opening the fridge
> door costs me $0.0022, or less than a quarter of a cent.

Notably he says that he uses 100 Wh/day whereas typical
fridges use kilowatt hours/day.
If he is correct then the difference between theory and
practice may, as often happens, be greater in practice than
in theory.

If door opening is the sole heat loss method (and it's
not)(although his horizontal fridge concept emphasis
suggests he things it matters) and if you and he are
correct, then the average user opens their fridge
0.12/0.0022 ~= 50 times / day.

It would be interesting to do some energy measurements (and
some formal calculations) and determine whether door opening
or decent freezer insulation predominates as the main power
saver. The results from converting a vertical freezer to
fridge use would be interesting.

Somebody could actually make some money out of this (and
gain a few [ack] carbon credits) if they could be bothered.





           Russell

2008\02\21@100100 by David VanHorn

picon face
Perhaps the consumption of the regular fridge was overestimated, or he
happened to pick a particularly bad unit?

2008\02\21@101704 by Dr Skip

picon face
I've actually done the conversion, but not for research purposes... I redid the
kitchen, and rather than pay $10k+ for a nice large built in fridge/freezer, I
bought 2 25 cu in upright freezers (blown air, light, door shelves too) and
changed the thermostat on one so it operated in the proper range. Less than
$1500 total, plus they are plain white and no exterior coils. I can face the
doors with the matching veneer of the kitchen too, as well as "build them in".

I have a digital thermometer on one (my own doing) and have observed 2 things:

- the temperature in the fridge doesn't vary but a couple of degrees with door
open and recovers almost immediately

- They stay very full all the time, which probably contributes to the thermal
mass which helps the item above, as well as worries me whenever we go to
someone else's house and I look at their teeny fridge and wonder how they can
live that way... :O

;)


Apptech wrote:
> It would be interesting to do some energy measurements (and
> some formal calculations) and determine whether door opening
> or decent freezer insulation predominates as the main power
> saver. The results from converting a vertical freezer to
> fridge use would be interesting.
>
>             Russell
>

2008\02\21@113527 by Tony Smith

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face
{Quote hidden}

We're being told that keeping a second spare fridge (aka beer fridge) is
bad, because we all know that old stuff uses more power etc etc.  If the
seals are buggered then that's fair enough.

I tested the power consumption of my fridge, (which is just a fridge, no
freezer section) and found it used less power than I expected.  The running
cost was something like 3 or 4 cents a day (< 0.3kWh/day, ~175kWh for the
year), so the payback period if I brought a new 'energy efficient' fridge
and tossed the old one would be well over 25 years.

That's well under the 1kWh/day the guy in the article was saying.  Maybe he
just picked a dud fridge, or should have got one without the icemaker.
Fridges sold in Australia have an 'energy rating' label, something I saw the
other day showed a bunch of fridges around the 200-250kWh/year mark.  I'm
not sure if that's typical.

The freezer (lid on top) used even less power, so that's due to the cool air
not falling out, and/or better insulation.  He might well be onto something,
but whether it's as significant as he claims... Well, someone will have to
bother, as Russell said.

As an aside, I put a CF bulb in the fridge to see what would happen to it.
It works fine, but there's a slight start-up delay.

Since aerogel doesn't seem to be readily available yet, let's bring back
asbestos.  My brother brought a large kiln that turned out to be lined with
asbestos, and it's fantastic.  Inside can be 200 degrees, and the outside
hasn't changed.  (Ha, the other day I was contemplating converting a fridge
into a kiln.)

Save the planet, line your converted freezer with asbestos today!

Tony

2008\02\21@132815 by Bob Blick

face picon face
--- Tony Smith <.....ajsmithKILLspamspam@spam@wix.com.au> wrote:
>
> We're being told that keeping a second spare fridge
> (aka beer fridge) is
> bad, because we all know that old stuff uses more
> power etc etc.  If the
> seals are buggered then that's fair enough.
>
> I tested the power consumption of my fridge, (which
> is just a fridge, no
> freezer section) and found it used less power than I
> expected.  The running
> cost was something like 3 or 4 cents a day (<
> 0.3kWh/day, ~175kWh for the
> year), so the payback period if I brought a new
> 'energy efficient' fridge
> and tossed the old one would be well over 25 years.
>
> That's well under the 1kWh/day the guy in the
> article was saying.

I had a 13 cubic foot International Harvester
refrigerator manufactured in 1947. It used about 150
to 200 watt-hours per day in typical use. The door
seals were not in great condition and the freezer door
didn't fit well, so I'm sure when it was new it did
better. My current fridge is a typical modern 19 cubic
foot model and uses about three times as much power.
But the dormitory-style fridge I have in my office
probably uses almost the same amount of power for 2.5
cubic feet of storage. It all depends what's important
in the design!

Cheerful regards,

Bob

2008\02\21@203438 by James Newton

face picon face
Note that the seal around the door is also leaking (slightly) on an ongoing
basis when the 'fridge is vertical and not leaking (as much) on a horizontal
unit. I'm not sure how much of a difference that actually makes, but the
point has not been considered in this thread so far.

I'll pay $50 for a well documented (pictures, description and data) study
comparing energy use on upright 'fridges when the door is being opened and
closed and when it is not. One fridge should do it, just check energy use
over a few hours with the door close and then open and close the door every
15 minutes for a few hours and done.

Another interesting study would be the temperature under the lower door seal
compared with ambient.

--
James.

{Original Message removed}

2008\02\21@221501 by Apptech

face
flavicon
face
> Note that the seal around the door is also leaking
> (slightly) on an ongoing
> basis when the 'fridge is vertical and not leaking (as
> much) on a horizontal
> unit. I'm not sure how much of a difference that actually
> makes, but the
> point has not been considered in this thread so far.

The original 100Wh/day site noted that you can leave a
horizontal freezer open with low losses. ie door seal
leakage is of low importance.


       Russell

2008\02\22@020507 by Tony Smith

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face
> Note that the seal around the door is also leaking (slightly)
> on an ongoing basis when the 'fridge is vertical and not
> leaking (as much) on a horizontal unit. I'm not sure how much
> of a difference that actually makes, but the point has not
> been considered in this thread so far.
>
> I'll pay $50 for a well documented (pictures, description and
> data) study comparing energy use on upright 'fridges when the
> door is being opened and closed and when it is not. One
> fridge should do it, just check energy use over a few hours
> with the door close and then open and close the door every
> 15 minutes for a few hours and done.
>
> Another interesting study would be the temperature under the
> lower door seal compared with ambient.


You'd also have to do it in a climate controlled area to make sure ambient
temperature was constant, an air-conditioned office would do.

I wouldn't be a bit surprised to see huge losses thru a faulty seal, it's
alarming to see how much coolth you can lose in a tiny gap.

After extensive whining ('too hot!' 'too cold!') at an old job, I placed a
number of temperature dataloggers around the office, including one in the
server room with it's own air-conditioners.

FWIW, the server room area was originally where a mainframe was, so we
already had a raised floor.  Nice.  We only needed 1/3 of the area, so that
bit was walled off, the remainder was left empty and christened the dance
floor*, complete with railing, disco ball and blinkenlights from the server
room hardware.  Anyway, we left the old labelled fuse box, the idea that a
printer would need 40 amps and a hard drive some ungodly number was
fascinating to those of us who started on Apple IIs and the like.

The server door had keypad entry, and this was a PITA.  We were a small
company then, so we used to wedge it open by dropping a tile lifter (those
vacuum things) in it.  This only cracked the door open a few millimetres
(say 1/8th to less enlightened folk), but had the effect of making the
server room temperature the same as the general office.  Hmmm.  As a bonus,
you could see when the first person came in, and when the last person left
by noting the sudden temperature change as the door was initally opened and
then eventually closed properly.  So that's why our air-conditioners kept
blowing up.  Oops.

We later realised we could brew beer in there, and lowered the temperature.
This was stopped not because we realised it was a bad-ish idea, but one guy
pointed out he was tired of turning blue after spending long periods in
there.

Tony


* We eventually lost the dance floor when the number of staff went up, at
that point keeping the server door locked became a really good idea too.
Yes, we did use it for parties.

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