Fridge stocked or unstocked - which uses less energy?

[engineer888]

Hi all,

Bit of a random question but the answer is killing me.
Does a fridge use less energy when it is stocked or unstocked?
For example, you have one fridge full of produce and another fridge completely empty. You open and close the door of both fridges the same amount of times over the course of say a week, which uses less energy to maintain the same temperature inside?

Thanks

 

[berkeman]

Hi all,

Bit of a random question but the answer is killing me.
Does a fridge use less energy when it is stocked or unstocked?
For example, you have one fridge full of produce and another fridge completely empty. You open and close the door of both fridges the same amount of times over the course of say a week, which uses less energy to maintain the same temperature inside?

Thanks

Which one do you think uses less energy, and why?

 

[Burnerjack]

eng888: I have not tested this, but let me offer you this: The refrigeration is to remove the heat infiltration through the thermal envelope. On this point, the contents offer no change, either way. HOWEVER, when the door is opened, a massive air exchange takes place, when the envelope is reestablished, the warm air just introduced must be cooled, adding to the heat gain of the system. That being said, there is a greater volume of air exchanged in the empty refrigerator. So, if I had to guess, I'd put my chips on the empty unit. Monitoring energy use, empty vs full. with the same ambient conditions of temp and light exposure should yield you result.

 

[ModusPwnd]

eng888: I have not tested this, but let me offer you this: The refrigeration is to remove the heat infiltration through the thermal envelope. On this point, the contents offer no change, either way. HOWEVER, when the door is opened, a massive air exchange takes place, when the envelope is reestablished, the warm air just introduced must be cooled, adding to the heat gain of the system. That being said, there is a greater volume of air exchanged in the empty refrigerator. So, if I had to guess, I'd put my chips on the empty unit. Monitoring energy use, empty vs full. with the same ambient conditions of temp and light exposure should yield you result.

Thats a good line of thinking. But I think otherwise...

I think the heat capacity of a full fridge is much higher. When you move into a new place and open the fridge, sometimes you wonder if its working because it feels warm as soon as you open it. Similarly, I believe, it would cool down quickly because there is only air to cool down.

A full fridge on the other hand feels cool for a while while open. This is because the food and water continue to absorb heat from the air. Because the food/water/air combo absorbs more heat while open than the empty fridge's air alone it will need to work more to expel that heat once closed.

 

[Burnerjack]

Modus, after further review, without testing, I must concur. Greater heat absorption with the higher specific density. Good job! Not only greater density but I would suspect greater surface area for said exposure.

 

[sophiecentaur]

A fridge full of food will allow less air circulation internally. This should reduce the rate of heat loss because the outer, slightly warmer, layers of air in the cavity will not reach the core and the thermostat so the compressor should work less frequently. Another way to look at this could be in terms of temperature gradient which will be less near the walls - hence less heat transfer.

 

[Lsos]

A full fridge also has more stored energy (is this the correct way to describe it?), and more surface area. This means that it can potentially lose more energy than an empty fridge. An empty fridge will only lose the energy contained in its cold air. A full fridge can lose much more than that, depending on how long the door is open and how effectively the heat convents from its contents to the outside air. I would therefore say that 1) it depends, and 2) probably a full fridge will cost more to maintain. On review, I think this is inline with Modus's thinking.

On the other hand, an empty fridge allows (maybe) for more losses when the fridge is closed, since the air circulates more freely. These two factors, and probably more, point to an answer of "it depends".

Perhaps a test should be conducted.

 

[sophiecentaur]

When the fridge is switched off, it will stay cold (in the core of the food) longer when full than when it's empty, even with the simplest model. ( more heat energy to disperse from inside with the same temperature difference).
In most designs (with motor running), the heat loss will be less if the fridge is full of food because the cooling panel is at the back and most heat is lost via the door. The intervening food is a thermal barrier.
But it would be possible to arrange for a design with a circulating fan which could have the effect of directing its cooling air right onto the door if the cavity was filled in an unfortunate way.

PS I like the concept of the 'negative Energy' that's stored in a fridge. A bit dodgy, perhaps but it has legs.