Refrigerator power vs. dissipation

[Pengwuino]

OK someoone on a different forum said his refrigerator is an 80 watt refrigerator (small lil college fridge). As in it takes 80 watts from the wall. Isnt it true that this isn't the same amount of heat that can be taken out of the contents inside?

I remember someone talking about an airconditioner using 2000watts of power while dissipating like 2500 watts... any help would be ... helpful :D

 

[brewnog]

Just as with the air conditioner thread a week or so back, think of a refrigerator as a device which moves heat, rather than producing it.

The amount of power consumed to 'move' the heat from the refrigerator cabinet to the surroundings isn't necessarily equal to the amount of heat removed from the cabinet.

Just think of carrying a block of metal at 50000 degrees across a room. If you're wearing gloves, it doesn't matter that the block is hot. You're just moving heat.

 

[Pengwuino]

Ok good.. thought so... didnt want to give these guys incorrect info.

 

[Andrew Mason]

OK someoone on a different forum said his refrigerator is an 80 watt refrigerator (small lil college fridge). As in it takes 80 watts from the wall. Isnt it true that this isn't the same amount of heat that can be taken out of the contents inside?

A refrigerator performance is measured by the co-efficient of performance: Heat removed from the cold reservoir (ie inside) divided by the Work input:

$$CP_{ref} = \frac{Q_c}{W}$$

The CP depends on the temperature difference but is typically about 2 or 3 for air conditioners and a little higher for refrigerators - ie. 2 or 3 times as much heat transferred as energy input.

Refrigerators are also rated in terms of 'cooling power' or heat removed / time divided by power input (x BTU/hr cooling/y watts input) but since the units are odd you can't really tell how efficient it is in terms of heat out/input.

AM

 

[russ_watters]

AFAIK, refrigerator specs don't give cooling capacity, but often times, air conditioner specs give both cooling capacity and electrical usage. http://www.sears.com/sr/javasr/prod...vertical=APPL&pid=04273055000&tab=spe#tablink is an example. It says it has a capacity of 5600 BTU/hr and consumes 510 watts of energy to provide it. There are 3.4 BTU per watt, so that's 1647 watts of cooling. The heat rejected out the back will be the sum of the two. Using A_M's forumla, that's a CoP of 3.2.