min-fridge v full-size fridge: which is most efficient?

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ken

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Aug 3, 2007, 5:45:12 AM8/3/07
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I shop so often that I've been able to do without a fridge, but it's a
bit inconvenient. Recently, I was given one of those 'cool box' mini
fridges, a 10-litre job that can run on either mains or 12v battery,
but I'm wondering if it's a false economy in terms of power
consumption.

I've found that, unlike a 'proper' fridge, it doesn't have a
thermostat, so it's working all the time. I tried using it without
anything else switched on, and found it uses more than two untits a
day - that's about 35p per day.

I don't know anything about full-size fridges, so could someone tell
me if a small 'proper' fridge would go through less electricity?
Thanks.

meow...@care2.com

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Aug 3, 2007, 6:07:08 AM8/3/07
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Mini fridges are much lower efficiency than the bigger compressor
fridges. Frequent shopping trips usually consume far more energy and
cost than a fridge.

35p a day is £127 a year. Thats many times what a proper fridge would
consume. An extra £100 a year would amount to £2000 extra cost over a
20 year appliance lifetime.

Then theres the extra cost of spoiled food due to incorrect storage
temp and increased risk of food poisoning.

Finally if youre paying 17p per unit I'd shop around.


NT

d...@gglz.com

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Aug 3, 2007, 6:28:16 AM8/3/07
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Wow - that's about 2-3 units a day just for a coolbox! I use about 3.5
units a day for everything - including a fullsize Bosch A-rated
fridge.

I think those coolboxes use Peltier cooling (a weird diode that gets
cool one side and hot the other when a current is passed through it).
Peltiers are very inefficient. http://en.wikipedia.org/wiki/Thermoelectric_cooling#Performance
- says conventional refrigeration is many times more efficient.

Owain

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Aug 3, 2007, 10:22:53 AM8/3/07
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ken wrote:
> I shop so often that I've been able to do without a fridge, but it's a
> bit inconvenient. Recently, I was given one of those 'cool box' mini
> fridges, a 10-litre job that can run on either mains or 12v battery,
> but I'm wondering if it's a false economy in terms of power
> consumption.
> I've found that, unlike a 'proper' fridge, it doesn't have a
> thermostat, so it's working all the time. I tried using it without
> anything else switched on, and found it uses more than two untits a
> day - that's about 35p per day.

My new full-size Lec fridge uses 153kW a year according to its energy
efficiency label.

Those Peltier coolers aren't proper fridges in the sense of preserving
food.

Owain

Steve Firth

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Aug 3, 2007, 11:20:07 AM8/3/07
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ken <kenneth2...@hotmail.com> wrote:

> Recently, I was given one of those 'cool box' mini fridges, a 10-litre job
> that can run on either mains or 12v battery, but I'm wondering if it's a
> false economy in terms of power consumption.

It's grossly inefficient hence a false economy. If you must use a small
fridge like that then an Electrolux caravan/boat fridge is more
efficient. And the Electrolux is inefficient compared to a modern fridge
with a compressor.

Also the larger a fridge is, the greater its relative efficiency. Sadly
modern fridge makers still don't use enough insulation, because of the
need to make fridges seem large inside compared to the outside. However
even given current poor insulation standards, the larger a fridge is the
better the ratio of surface area (through which heat is lost) to
internal volume.

All fridges necessarily have poor design features such as the front door
which spills all the cold air onto the floor when the door is opened.
That is where a big fridge wastes energy compared to a small one - more
cold air lost when you open the door.

If you want a small fridge then a "cube" counter-top unit would be
better than the coolbox. If you want the best balance between
efficiency, cost and size simply browse the units in a store. They all
have efficiency ratings and estimates of annual electricity consumption.

dennis@home

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Aug 3, 2007, 2:45:45 PM8/3/07
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"ken" <kenneth2...@hotmail.com> wrote in message
news:1186134312.8...@d55g2000hsg.googlegroups.com...

They are not fridges.. they are peltier effect coolers.
They are very inefficient and you don't want one.
A Ł50 beer cooler from ASDA would be far better.


Derek Geldard

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Aug 3, 2007, 4:15:38 PM8/3/07
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On Fri, 03 Aug 2007 10:28:16 -0000, "d...@gglz.com" <d...@gglz.com>
wrote:

The reason they are inefficient is mainly that the diode proper is
very thin and heat from the hot side leaks back to the cool side,
simply by conduction. However I don't know what their thermodynamic
efficiency is like.

Their performance gets rapidly worse if the heat is not removed from
the hot side effectively enough, the temperature can rapidly go up and
cause melting of a cheap Chinese tat coolbox if the vents are covered
up in a car boot and it does nothing for your ice cream. With better
cooling (implies a more powerful noisier fan than the 12v plastic one
in a CCT Coolbox.) useful results can be obtained.

In serious applications they are handy for cooling small components
which aren't of themselves generating heat. An instrument we sold Ca.
1970 used them with success to cool an antibody chamber in an
Automated RadioImmunoassay Anayser.

DG

Andrew Gabriel

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Aug 3, 2007, 5:09:17 PM8/3/07
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In article <aq17b3d84uer485ue...@4ax.com>,

Derek Geldard <d...@miniac.demon.co.uk> writes:
> On Fri, 03 Aug 2007 10:28:16 -0000, "d...@gglz.com" <d...@gglz.com>
> wrote:
>
>>Wow - that's about 2-3 units a day just for a coolbox! I use about 3.5
>>units a day for everything - including a fullsize Bosch A-rated
>>fridge.
>>
>>I think those coolboxes use Peltier cooling (a weird diode that gets
>>cool one side and hot the other when a current is passed through it).
>>Peltiers are very inefficient. http://en.wikipedia.org/wiki/Thermoelectric_cooling#Performance
>>- says conventional refrigeration is many times more efficient.
>
> The reason they are inefficient is mainly that the diode proper is
> very thin and heat from the hot side leaks back to the cool side,
> simply by conduction. However I don't know what their thermodynamic
> efficiency is like.

At the temperature differentials you normally find in a fridge,
they consume about the same power that they pump, i.e. a 50W
peltier will pump about 50W too (giving off 100W from the hot side).
Compressor based refrigeration does very much better than this.
The inefficiency is not so much due to heat conduction as you imply,
just a 5-10% duty cycle will maintain the temperature difference when
there is no heat to pump across, so only 5-10% of the power is lost
to conduction.

BTW, it's not a diode -- peltier devices are a series of semiconductor
junctions which can be powered either way around, depending which way
you want it to pump heat. Many cool boxes have a heating option which
simply powers the peltier device the over way round.

> Their performance gets rapidly worse if the heat is not removed from
> the hot side effectively enough, the temperature can rapidly go up and
> cause melting of a cheap Chinese tat coolbox if the vents are covered
> up in a car boot and it does nothing for your ice cream. With better
> cooling (implies a more powerful noisier fan than the 12v plastic one
> in a CCT Coolbox.) useful results can be obtained.

I've had a couple of peltier cool boxes since 1981. I ran them
continuously which they weren't designed for, and the fan motor
brushes died after a few months. Replaced them with proper equipment
cooling fans, and the units lasted for years. They both still work
but aren't in constant use anymore. I repaired one peltier element
after about 10 years, and replaced the other a couple of years
later.

> In serious applications they are handy for cooling small components
> which aren't of themselves generating heat. An instrument we sold Ca.
> 1970 used them with success to cool an antibody chamber in an
> Automated RadioImmunoassay Anayser.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Graham.

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Aug 3, 2007, 5:27:34 PM8/3/07
to

>An instrument we sold Ca.
> 1970 used them with success to cool an antibody chamber in an
> Automated RadioImmunoassay Anayser.


She'll no take any more captain!

Sorry, I'll get me coat.

--

Graham.
%Profound_observation%


Derek Geldard

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Aug 4, 2007, 5:31:58 AM8/4/07
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On 03 Aug 2007 21:09:17 GMT, and...@cucumber.demon.co.uk (Andrew
Gabriel) wrote:


>>
>> The reason they are inefficient is mainly that the diode proper is
>> very thin and heat from the hot side leaks back to the cool side,
>> simply by conduction. However I don't know what their thermodynamic
>> efficiency is like.
>
>At the temperature differentials you normally find in a fridge,
>they consume about the same power that they pump, i.e. a 50W
>peltier will pump about 50W too (giving off 100W from the hot side).
>Compressor based refrigeration does very much better than this.
>The inefficiency is not so much due to heat conduction as you imply,
>just a 5-10% duty cycle will maintain the temperature difference when
>there is no heat to pump across, so only 5-10% of the power is lost
>to conduction.

Hmmmm,

>
>BTW, it's not a diode -- peltier devices are a series of semiconductor
>junctions

I bet they're not monodes ;-)

DG

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