Refrigerator Efficiency

[Xema]

A friend asked me recently to comment on a dispute he'd been in: Is a loaded refrigerator more efficient (i.e. uses less energy to stay at a set temperature below ambient) than an empty refrigerator?

His view (and mine) is that the efficiency is the same in both cases, depending only on the insulation and basic thermal efficiency of the refrigerator in question - it's gonna exchange heat with its environment at the same rate no matter what's inside. (There could be a "second-order" effect in that the empty refrigerator has less thermal mass and will cycle on more frequently, for a shorter time each cycle). But he was talking with a guy who claimed it was "common knowledge" that an empty refrigerator uses substantially more energy.

I figured that this would be a well-handled theme, and that Cecil had probably covered it. But various searches turned up nothing. I'm assuming I've missed something obvious and am now betting some Dopers can put me on the path to enlightenment, as they so often do.

[KneadToKnow]

The theory is, as I understand it, both liquids and solids change temperature more slowly than gases. Therefore, the more liquids and solids you have in there, the less gas, the slower the overall temperature change will be within the unit (as long as the door stays closed), and the less need there will be for the unit to cycle on periodically to cool things back down.

As to whether or not it works, all I can say is I've heard all my life that you should keep a freezer full, even if it's just with bottles of water. With a refrigerator, I think the constant opening and closing of the door would make any difference almost too small to measure.

[Xema]

KTK wrote:
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The theory is, as I understand it, both liquids and solids change temperature more slowly than gases. Therefore, the more liquids and solids you have in there, the less gas, the slower the overall temperature change will be within the unit (as long as the door stays closed), and the less need there will be for the unit to cycle on periodically to cool things back down.
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Right - when the thermal mass is large (e.g. freezer full of frozen pot roasts) the thing will cycle on less frequently. But when it does, it will run longer to get things cooled to the shut-off temperature. So it winds up representing the same total energy consumption as if it were turning on more frequently, for a shorter time each cycle.

[billy]

You're correct in the rare case of a freezer that never gets open.

IANA refrigerator specialist but with the door sealed tight and not opened, then there should be no difference at all after equalibrium is reached. The whole "heat out" = "heat in" thing and all that is same through the walls of the thing no matter what is inside.

If you open the door however, of course, the more stuff you have inside that can't fall out when the door is open, the better. With an upright freezer, opening the door causes all the cold air to fall out, to be replaced with warm air that needs re-cooling. If it is full of icecream, there's no air to fall out and the ice cream doesn't change temperature very fast so you don't have re-cool things much.

Plugging in a warm(new) freezer and measuring the energy intake before equalibrium is reached will show you that an empty freezer is much easier to GET cold, than one full of water or icecream.

[Xema]

I probably should have said that my friend's original dispute concerned a refrigerator left running while people were away on vacation - in other words, we can assume the doors are left closed.

I think billy is obviously right - if you're opening the door, a big question for efficiency is how much cold air is lost. A full refrig or freezer has less air to lose.

[Una Persson]

I think that billy and Xema are both correct too, and I am, in one sense, sort of a refrigerator expert.

[scr4]

Originally posted by Xema
my friend's original dispute concerned a refrigerator left running while people were away on vacation

If he's that concerned about energy efficiency, why doesn't he unplug the fridge while on vacation?

[Xema]

scr4 wrote:
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If he's that concerned about energy efficiency, why doesn't he unplug the fridge while on vacation?
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That would clearly be the most economical of all choices. But the assumption was that there's food in there he'd prefer not to throw away. So the question became, should he buy more food and pack it full so it would be more efficient? My friend felt it made no difference.

If anyone knows of a relevant URL, I'd be grateful for it.

[partly_warmer]

I'm ashamed, but I haven't recovered from having my message count reduced to less than 100 in the great data loss of 2002.

Whether a fridge takes more energy full or empty depends on what's in it. If it's full of batteries or Brittany Spores trying to get out after being shut in by an indignant music-lover, then the full fridge is gonna take more to keep cool. Until the batteries or Brittany run down. Pound-for-Pound, it's Eveready. Less lead, more energy, less vapid artistry.

[sailor]

As somebody who travels for weeks or months I have often faced this question. #1- If you leave the fridge running it is going to make close to no difference whether it is full or empty. (BTW, buying more food would be crazy as you would lose it in a power outage. If you want to fill the refrigerator use water or whatever). #2- If you are away long enough it may be cheaper and easier to get rid of the food anyway. If it is a short time away then we are wasting time discussing this. If it is a long absence then why would he have tons of frozen stuff left over? That would be poor planning.

On a couple of ocassions I have had some frozen meat left over. Suppose it is worth $5. I am not going to leave the refrigerator running for 6 weeks even if it costs less than $5 to do so. I just take the meat, put it in a jar with a tight lid and cook it in the pressure cooker. That way it is like canned food, it will keep for months and years with no refrigeration.

[David Simmons]

Originally posted by Xema
KTK wrote:
------------------------------------------------------------------------------------
The theory is, as I understand it, both liquids and solids change temperature more slowly than gases. Therefore, the more liquids and solids you have in there, the less gas, the slower the overall temperature change will be within the unit (as long as the door stays closed), and the less need there will be for the unit to cycle on periodically to cool things back down.
------------------------------------------------------------------------------------

Right - when the thermal mass is large (e.g. freezer full of frozen pot roasts) the thing will cycle on less frequently. But when it does, it will run longer to get things cooled to the shut-off temperature. So it winds up representing the same total energy consumption as if it were turning on more frequently, for a shorter time each cycle.

I've got to agree with Xema. The refrigerator runs only to remove the heat that leaks in from the outside. This leakage is the same whether or not the refrigerater is full. So the amount of energy required should be the same in all cases.

Disagreement is allowed. On rare occasions (extremely rare, that is), people who disagree with me are right.