Freezer to Fridge Conversion project in ReNew

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There is a "freezer to fridge" conversion project in this months ReNew
magazine which claims to convert a freezer into a supper efficient
0.1kWh/day fridge.
It's basically a thermostat which switches the freezer mains power off
and on to regulate its temperature.
It's a follow up to an existing article which I haven't seen, so
doesn't have much detail on how it works or how the measurements were
taken.

Although I don't know much about fridges or freezers, several things
raised my "What the?" detector immediately:
1) 0.1kWh/day is 36.5kWh/year, that's about 1/10th the power
consumption of a regular fridge. Something ain't right there... what's
the catch?
2) The circuit is hardly "micro-power" as claimed, not that it really
matters.
3) The use of a battery to power the circuit when the freezer
compressor is not on. WHY?. Presumably the author thinks it lowers the
power consumption?. Looks like he's designed a Free Energy device!

Here is the circuit:
http://www.alternatezone.com/electronics/images/FreezerConversion.jpg

Can a freezer be made into a more efficient fridge using this
technique? My gut feeling is "maybe", but the amount claimed seems very
unlikely. I suspect the author has goofed the measurements.

Comments please...

Dave :)


Re: Freezer to Fridge Conversion project in ReNew


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On second thought it is obvious that the author thinks that charging a
battery and using that instead of having the mains transformer
permanately powered is going to save some power. Possibly, have to go
through the numbers I guess. The transformer has losses, but so does
the battery and charging.
The obvious thing is that if the battery fails your fridge dies. Then
when it does die you have manually kick-start the thing again. Seems a
silly way to go about it just to save some quiescent power.

Dave :)


Re: Freezer to Fridge Conversion project in ReNew


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## I would think the compressor would require just as much power to cool as
a normal fridge. Perhaps it is the physical design of the freezer, where the
bucket shape and better insulation holds the cool in better.

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## Looks like SW1 has two settings - one to run the circuit off mains (to
charge the battery) and the other to just run off the battery. Probably to
reduce power consumption by limiting the use of the transformer.

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How often does he use the fridge? I reckon that is the main factor in fridge
power consumption. Or pehaps he is comparing a freezer with an efficient
compressor to a dodgy old kelvinator.

Ross



Re: Freezer to Fridge Conversion project in ReNew



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it all depends on the size of the fridge (and this wasnt specified in
your post), and how good the insulation is on it.  The little "beer
fridge" we have in our workplace (made circa 1987-88) draws 153w
according to the specifications.  I presume that this is the power the
compressor draws when it is running, so if you assume that even if
this compressor never turns off - its going to draw .153 Kwh.
Depending on how often it gets opened, it probably would come close to
the 0.1 kwh average power over a 12 month period.  
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I think what these people are working on is the fact that a typical
freezer is going to have a MUCH better insulation than a typical
fridge, so if its run as a fridge (higher temperature than a freezer)
the temperature will take longer to raise, due to the better
insulation, and the much less time the compressor will have to run in
the average day.
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As an added bonus, if they are talking about "tucker box" freezers
(with the top mounted lid), then when they are opened, very little
cold will "spill" out, whereas with a standard fridge with the hinged
front door, the cold air will "pour" out very quickly when opened -
wasting a lot of energy  !
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In a theoretical fridge that had a perfect seal with no heat leakage,
and perfect insulation that the heat could NEVER pass through, and no
metal parts going from the inside to the outside (to pass heat inside)
the fridge would only have to be powered for long enough to bring it
down to the desired temperature, and then it would stay at that
temperature eternally, as long as it wasnt opened, and nothing in
there gave off heat (food eventaully rotting etc)

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no.  If they wanted to save power, and just had to have this
"electronic control" they wouldn't be stepping down a 12vac secondary
(more like 17vdc when rectified) to +5v via 2 series regulators, and
they also wouldn't be using a relay (surely a triac would have less
overall losses ?
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Save running the transformer all day, and all the other lossy crap in
there. (mentioned above) Whether it saves any significant power is
debatable though.


------------------
If they are that worried about power wastage, then why not stick to
the standard mechanical thermostat (used for decades without
problems), and simply modify it (if needed) to achieve the desired
temperature range !  it uses ZERO electrical power to operate !!

There are millions of fridges out there at recycling centres, that you
could take the thermostats from for this project, if the existing
freezer one cant be set high enough.


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I think it could be, but NOT because of the electronics used.

If you are going to do this, I would start with the most efficient
freezer you can buy, not some old thing.
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Re: Freezer to Fridge Conversion project in ReNew


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I agree, in fact why not have both 'stats installed with a changeover
switch?
Or I guess by using the 'off' setting on the freezer 'stat, the changeover
switch would not be neccessary.

--
Regards ......... Rheilly Phoull



Re: Freezer to Fridge Conversion project in ReNew
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Yeah, a small fridge might I guess.
My fridge (500L or whatever) is rated at 500kWh/year, that's
1.36kWh/day, and that is one of the lowest consumption ones on the
market for that size.

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Very true.


Even if it does, it's a naff way to do it. Dead battery = lost food.

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Sounds like the go.

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Gotta agree!

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No plans to do it myself.

I figure it's all probably a load of bollocks in a practical sense. To
get any sort of direct comparison figure you'd have to have a fridge
and freezer of the same size and construction, being opened the same
amount of time every day.
If freezer mechanisms were somehow more efficient then every
manufacturer would be using then in fridges.
The electronics in the article is doing nothing a normal fridge
thermostat doesn't do (except waste some extra power).

The idea of converting a chest freezer might have some merit if you
were going to store long term stuff in there for instance. Bit useless
for practical day to day use though.

A job for Mythbusters!

Dave :)


Re: Freezer to Fridge Conversion project in ReNew


Just thought I'd mention that Silicon Chip Mag desribed a similar project -
turning a fridge or freezer into a wine 'celler' I think - but same
principal, different temps - might be worth a look

David

KLR wrote:

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Re: Freezer to Fridge Conversion project in ReNew


This kit is available from JAYCAR. Kit number is KC-5413 and actually looks
pretty good!

Is a revamp of the ReNew project.

JD

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Re: Freezer to Fridge Conversion project in ReNew



One possible efficiency boost a chest freezer would have over
a fridge is  you retain most of the cold air inside when you
open the lid, opening a fridge door would lose nearly all of
the cold air inside.

It remains to be seen just how much BTU's can be removed from
room temp air with a given humidity.

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Assuming a 500L air volume at room temp I get a figure of 600
Joules per degree. So assuming that's correct 600 x 25= 15 Kj
(30 deg ambient) lost each time you open the door.

Re: Freezer to Fridge Conversion project in ReNew


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I haven't seen either of these issues of ReNew, but my guess is that the
battery is there because the owneer uses solar panels for most of his
power, but is on the grid if he needs it (and isn't using one of those
two-way meters that means he's always on the grid). That way, he can use
the solar panels, via the battery, for most of the time. But his lettuce
doesn't go mushy if the solar panels/battery fail as he can switch the
fridge back to the grid.

Cheers!
Rick Meaham

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