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  #1  
Old 11-05-2007
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Refrigeration power consumption reduction

Hypothetically as I donít have a fridge yet, but the boat I get will most probably have one.
If the Fridge cabinet is say 6 cubic feet and it uses X Amp/H, would making the internal size physically smaller by wedging in 3 cubic feet of polystyrene foam reduce the Amp usage by half of would it turn into a freezer. The reduction of size would be a one off, for short time say weeks rather than months, in an attempt to limit/save Amp consumption.
I hope this made sense!
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Old 11-05-2007
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You mean or turn it into a freezer? It depends on what you set the thermostat to.
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Old 11-05-2007
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The reduction would not be significant because it's about heat loss. The block would not reduce heat transfer and therefore not reduce amp hour consumption. If you were to use the styrafoam as additional insulation that would reduce heat transfer and amp hours
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The best is to have a 'box' type with top-lid campared to a cabinet with fron door.
Depending of boat design etc concidder it as an option using part of the bench top as lid.
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Old 11-05-2007
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The volume plays no role (well it does as it affect the surface area) in the heat /cold transfer.
There are 3 major areas of loss:
1) All of the surfaces of the box. So, for given volume the beast form would be a sphere, but it is not practical. So cube is usually a close you can get.
Better use of your foam would be to use it on all walls of the fridge (from inside) including bottom and top, but they must be tight.
2) The lid gasket: Often the lid itself is insulated the worst and in addition there is some loss at the lid gasket. it may be more loss there than in all other surfaces combined. Specially if you have a drain, which is open - then you get cold air from the fridge escaping from the drain and fresh "hot" air is getting in through the bad gasket.
3) opening of the fridge and/or adding new stuff in: Every time you open the fridge you create big loss as some cold air escapes and is replaced with hot air. Top opening fridges are much better in this respect as cold air stays down. Removing items, leaving them out for a while and retuning them back in is bad, also adding items which are not cold. Adding a few 6 packs of beer which were brought by a dinghy on hot day will use power and time to cold them.

Adding a foam in the bottom half of the fridge will reduce the power usage, but not by 50%.
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Old 11-05-2007
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I think it would depend a lot on whether you can get the insulation inserted all around the cooling element inside the refrigerator cabinet. Typically the two sides of this are in accessible. But otherwise yes, the smaller the volume that has to be cooled, the less work the pump has to do. You would have to consider where the thermostat sensor is located too.
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Old 11-05-2007
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Actually, just inserting a large block of polystyrene foam would probably make the refrigerator work harder, not easier. A refrigerator needs to be kept full of food for it to work efficiently. A block of polystyrene foam has a much lower ability to retain cold than say a several one-gallon bottles of water. If you had water there instead of foam, it would help stabilize the temperature of the refrigerator. IIRC, most refrigerators work the hardest when they are almost empty.

If you want to reduce the refrigerator's energy usage, you really need to add insulation to the exterior. Adding some heat reflective insulation around the refrigerator would probably help far more than half-filing the thing with foam, unless the foam is spread out and layered to reduce heat loss. Also, adding foam to the interior might cause it to work harder, since it might insulate the cooling coils from the interior of the refrigerator, reducing the refrigerator's ability to remove heat efficiently. The insulation really has to be added outside the cooling coils IMHO.
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Yes, insulate the outside. It's from there that the heat gets in. I think that putting big chunks of foam in there will change very little.
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Agreed, adding extra external insulation is always good - if the space isn't used for something else - but that was not the original question, which was about adding insulation inside the refrigerator cabinet.

SD - I don't understand your physics. I see it like this:

If you take an empty closed refrigerator, at ambient temperature as the starting point, and switch it on, the pump must cool down the cooling element and the air inside and then compensate for the heat leaking in.

Now repeat the experiment with a jug of water inside. The pump must now cool the water (but less air) and compensate for the heat leaking in. The cooling element is a constant in the comparison.

I think that water has a much higher specific heat than air, so more electricity will be needed to cool the water than to cool the empty refrigerator.

Once cooled, in both cases, the pump must compensate for the same heat leakage into the cabinet.

The value of extra insulation to reduce electricity demand, whether inside or outside the cabinet, is to be judged on its ability to lower the heat leakage. If an extra few centimetres could be added on the inside uniformly - so that the cooling element stays within the insulated volume - then the heat leakage is likely to be reduced.

Everything that is put into the cabinet has to be cooled, the more that goes in, the more that has to cooled. If less can get in, then less has to be cooled. So reducing the internal volume with insulation will reduce the need for cooling and electricity required.
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Old 11-05-2007
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The idea is that when the refrigerator is filled with items that have a high specific gravity the cooling mechanism will be triggered less often but work longer to keep a temperature range. If the fridge is empty the cooling must turn on and off much more often - theoretically for shorter periods. But there is a lot of energy loss involved in starting the pumps and getting the condensation/evaporation cycle in the element functioning. Therefore it is usually recommended to at least partially fill a refrigerator.
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