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

[ken]

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.

[meow2222]

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 =A3127 a year. Thats many times what a proper fridge would consume. An extra =A3100 a year would amount to =A32000 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

[dom]

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.

says conventional refrigeration is many times more efficient.

[Owain]

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]

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]

They are not fridges.. they are peltier effect coolers. They are very inefficient and you don't want one. A £50 beer cooler from ASDA would be far better.

[Derek Geldard]

- 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.

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]

>- says conventional refrigeration is many times more efficient.

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.

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.

[Graham.]

She'll no take any more captain!

Sorry, I'll get me coat.

[Derek Geldard]

Hmmmm,

I bet they're not monodes ;-)

DG