Freezer conundrum - physics question

[motco]

According to Southern Electric, who say in a flyer that you must keep your freezer as full as possible, this statement is true:

Southern Electric said:

...it takes more energy to freeze air than it does to freeze solids and liquids, so you'll actually save money whilst planning your feasts!

How can this be true? The insulation standard of the freezer is independent of the contents, surely, and there is no exothermic or endothermic reaction taking place. Once steady-state is achieved I would have thought that the running costs of an empty freezer would be the same as a full one.

Edited by motco on Saturday 12th December 12:25

[mrmr96]

motco said:

I would have thought that the running costs of an empty freezer would be the same as a full one.

Only if you never open the door. As soon as you open the door an amount of cold air is replaced by warm air which then needs to be cooled again. By keeping the freezer full of food it reduces the amount of cool/warm air exchanged when you open the door and therefore minimises the work done to cool it again.

This is my understanding of why a full freezer uses less electricity, but it doesn't appear to fit with exactly the words in that press release. But you need to bear in mind that the 'marketing' people are about as far removed from scientists as you could ever be - so it wouldn't surprise me if they were given the reason, got the wrong end of the stick and did that botched write up.

[robinhood21]

If, for no other reason; no opening and closing of door to get stuff out.

ETA: Must type faster.

Edited by robinhood21 on Saturday 12th December 12:31

[HiRich]

mrmr96 said:

Only if you never open the door.

This.
Also why a chest freezer can be more efficient than an upright (less air spills out) and why it can be effective to fill a near empty freezer with scrunched up newspaper (retains pockets of chilled air)

[motco]

That doesn't properly address the assertion because if it were empty you wouldn't need to open the door! I suspect the Marketing-Speak answer is the right one