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:
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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.
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.
## 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.
## 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.
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.
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.
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.
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 !
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)
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 ?
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.
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.
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.
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.
Very true.
Even if it does, it's a naff way to do it. Dead battery = lost food.
Sounds like the go.
Gotta agree!
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.
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.
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.
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.
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
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