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Running a Refrigerator Efficiently


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I do not claim to be a scientist, and don't know why what I'm going to tell you is true, but it is.

 

We are subject to prolonged power outages here from hurricanes, ice storms or just high winds because we have overhead power lines and live in a heavily wooded area. When my freezers and fridges are full, the contents stay colder much longer without power than they do when their is little in them.

 

This also helps with the quality of storage in the freezers which cycle on and off on defrost phase, and when they are full, but with still plenty of room for air circulation, the food lasts longer, because it doesn't thaw as much on the defrost phase.

 

It might take more electricity to cool and freeze things in a full fridge and freezer, but for my situation, I think I'm sticking with it. I've started putting empty seltzer bottles filled with tap water in the freezers and always keep a large supply of refrigerated water.  Our Southern construction standards only call for the water pipes to be buried 18", and the cold water runs warm on summer days. The full fridge/freezer plan works for me, and I'm a very thrifty person. Your situation may be different.

> ^ . . ^ <

 

 

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5 hours ago, paulraphael said:

 

That's the folk wisdom, but it doesn't turn out to be a big deal. The amount of thermal mass of the air in the fridge is minute ... it takes very little energy to chill a few cubic feet of air from room temperature to 30-something. 

If you calculate the thermal mass, of say, 20 cubic feet of air, and look at the amount of energy it takes to raise it from 4°C to 22°C, it's equal  to 0.0004 kilowatt hours. If you assume a refrigerator isn't that efficient, and takes twice as much energy to cool that air back the other way, you've got .0008 kwh. At our current price of 19.2 cents per kwh in NYC, this is 0.015¢ to open the door and let out every molecule of cold air.

 

So it should go without saying that it makes no difference if you open the door for 5 seconds or half a minute. You use more energy putting in a jug of room temperature water and letting the fridge cool it.

 

 

 

That is the long way around to what I was saying , but yes the time to really save on energy when it comes to fridges and freezers, is when you buy them.  Buy the most efficient unit you can afford, and after that just use it  whatever way you normally would.   https://www.energystar.gov/products/appliances/refrigerators

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"Why is the rum always gone?"

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10 hours ago, Thanks for the Crepes said:

I do not claim to be a scientist, and don't know why what I'm going to tell you is true, but it is.

 

We are subject to prolonged power outages here from hurricanes, ice storms or just high winds because we have overhead power lines and live in a heavily wooded area. When my freezers and fridges are full, the contents stay colder much longer without power than they do when their is little in them.

 

This makes sense, but doesn't have anything to do with efficiency or with the amount of air in the fridge. You've just got more thermal mass in there in the form of food, adding to the total thermal mass (which includes the interior of the fridge itself). This will absolutely keep things cold longer when the power goes out, but it won't affect efficiency at all.

 

As far as how this affects the defrost cycle in the freezer, I have no idea. And I never get a chance to observe changes to this, because my freezer is always too full. But it shouldn't be an influence on efficiency. Defrost cycles are usually just follow a timer.

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20 hours ago, dcarch said:

 

Every time you accelerate, you pay a price in time. The car is slowed down by the extra weight. and you pay a price in extra gasoline to move the extra weight.

Every time you slow down, you pay a price in more wearing of your brakes.

 

 

Well, sure, that was my thinking.  But, if if you accelerate once and drive two hours at a constant speed before braking and parking, surely there would be a break-even point.

 

But I suspect it's more complex than that, and maintaining the cruise also becomes more expensive due to road friction or something.

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Specifically what's going on is that when you have a lot of thermal mass, losing X amount of energy results in a small temperature loss. If you have less thermal mass, losing that same amount of energy results in a larger temperature loss. Which is why the full fridge stays cold longer when the power goes out. But it's the same amount of energy being lost in either case. Since it's temperature and not energy that triggers the thermostat, the only real difference you'd expect is the frequency of the compressor cycling on and off. Theoretically you might see differences in efficiency due to longer / shorter compressor cycles, but in practice this doesn't seem to make much difference.

 

Anyone have tips on how to clean a decade of cat hair off a fridge that has coils on the bottom? I think this will make a difference ...

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On 14/04/2017 at 11:20 AM, paulraphael said:

Anyone have tips on how to clean a decade of cat hair off a fridge that has coils on the bottom? I think this will make a difference ...

We used a combination of the vacuum cleaner's crevice tool, its brush tool, and a long-handled bottle brush. I don't remember how many times we emptied the vacuum before we were done, and there was a lot to sweep up as well.  It's amazing how it builds up. 

 

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“Who loves a garden, loves a greenhouse too.” - William Cowper, The Task, Book Three

 

"Not knowing the scope of your own ignorance is part of the human condition...The first rule of the Dunning-Kruger club is you don’t know you’re a member of the Dunning-Kruger club.” - psychologist David Dunning

 

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On 14/04/2017 at 10:20 AM, paulraphael said:

Specifically what's going on is that when you have a lot of thermal mass, losing X amount of energy results in a small temperature loss. If you have less thermal mass, losing that same amount of energy results in a larger temperature loss. Which is why the full fridge stays cold longer when the power goes out. But it's the same amount of energy being lost in either case. Since it's temperature and not energy that triggers the thermostat, the only real difference you'd expect is the frequency of the compressor cycling on and off. Theoretically you might see differences in efficiency due to longer / shorter compressor cycles, but in practice this doesn't seem to make much difference.

 

Anyone have tips on how to clean a decade of cat hair off a fridge that has coils on the bottom? I think this will make a difference ...

 

 

lots of time and a good shop vac with a skinny attachment. Look for  embedded Condenser coils on the next unit you buy if you continue to be a pet owner.   Basically the coils are attached right beneath the outer skin of the cabinet, the outside of the fridge will generally feel warm to the touch if you have a unit like this.  It would still be a good idea to clean the compressor area one or twice a year though.   

"Why is the rum always gone?"

Captain Jack Sparrow

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On 4/13/2017 at 2:14 PM, IndyRob said:

 

Well, sure, that was my thinking.  But, if if you accelerate once and drive two hours at a constant speed before braking and parking, surely there would be a break-even point.

 

But I suspect it's more complex than that, and maintaining the cruise also becomes more expensive due to road friction or something.

 

I'm going to take the position that aircraft engineering takes, which is that every pound of weight takes energy both to take off from earth and to maintain airspeed. Even if the road you were on were perfectly flat you still have to expend energy to combat air drag. Add in even minor changes in elevation as you travel and that adds to fuel consumption. It does add to the fuel costs to carry extra weight in vehicles.

 

I, however, will admit to at times not removing weight that I could. I know better.

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The Once and Future Cook

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@paulraphael Is basically correct. I would explain it a little differently.

If we assume the fridge is well insulated, the major heat gain would be from convection when the door is opened.

You open the door and cold air flows out and warm air flows in.

If I have just one bottle of water in the fridge the temperature change from the warm air would be more than one bottle of water can absorb.

If you have a couple of cases of water and open the door for the same amount of time, the water can absorb much of the heat

with out a major change in temperature.

Ether way the fridge needs to expel the additional heat. It just might not need to cycle on as often with the larger mass.

There seems to me to be a secondary effect of reduced air flow if the fridge is full.

 

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I've been perusing the web, reading other discussions about this subject, and am as undecided as ever.  But the bottom line may be that it really doesn't make much of a difference either way.

 

Yes, when you open the door the cold air will make a break for it.  But cooling air is not energy intensive.  And if we are worried about it, how about filling the fridge with empty (but closed) gallon milk jugs.  Now most of that air can't go anywhere.

 

Another factor pertains to how the refrigerator is set to operate.  Most likely, it has a range of acceptable temps that it drops through before cooling begins.  What that actual range is is going to be critical.  If it allows a drop in temp of the entire mass, it's going to have recoup all of that before it turns off.

 

I think we've been ignoring the cost the initial cooldown on the assumption we'll get it back over the long haul.  But will we really?  Certainly, if we were to do a test of this, we would take two identical units and fill one with frozen water jugs and another with just one water jug.  As soon as the flag drops, one of the samples is way behind in energy consumption.

 

But in a practical setting, how much food inventory is being dedicated to icebox duty?  Are we stuffing extra food dollars in there to save energy?  Do we have to calculate the cost of taking our some frozen broccoli for use?

 

Are there better conservation methods we might explore?  For those of us who experience that thing called 'winter', do any of us have a provision for using that abundant coldness to cool the refrigerator box that we've placed inside of our artificially heated house box?

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I'm going to take the position that aircraft engineering takes, which is that every pound of weight takes energy both to take off from earth and to maintain airspeed. Even if the road you were on were perfectly flat you still have to expend energy to combat air drag. Add in even minor changes in elevation as you travel and that adds to fuel consumption. It does add to the fuel costs to carry extra weight in vehicles.

 

This isn't an accurate analogy, because it takes a ton of energy to keep mass in the air in an airplane. But the efficiency of a refrigerator's insulation (and the energy losses) have basically nothing to do with the mass of the contents. It does take significant energy to cool the contents in the beginning, for sure. But the mass just stops being an important factor after that. 

 

You open the door and cold air flows out and warm air flows in. If I have just one bottle of water in the fridge the temperature change from the warm air would be more than one bottle of water can absorb.

 

 

This is still presuming that the amount of energy that goes into cooling the warm air that's come in is significant. It's in fact hardly anything compared with the heat absorbed through the insulation on all sides that has to be pumped out. If you opened the fridge door every 5 minutes in perpetuity, it would only make a small difference in energy used, regardless of the fridge being full or empty.

 

But the bottom line may be that it really doesn't make much of a difference either way.

 

This.

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1 hour ago, paulraphael said:

 

 

 

This is still presuming that the amount of energy that goes into cooling the warm air that's come in is significant. It's in fact hardly anything compared with the heat absorbed through the insulation on all sides that has to be pumped out. If you opened the fridge door every 5 minutes in perpetuity, it would only make a small difference in energy used, regardless of the fridge being full or empty.

 

I used to run a convenience store with a very large cooler (20 something merchandising doors, plus about 5 feet behind that for back stock storage).  There were thermometers around the cooler, with sensors to go with them.  They were largely in air, but one was in a fluid filled capsule.  The air temperature would drop pretty fast when a door was left open, at least near the door.  When the door closed, a fan would come on, and circulate the air, and the temperature would go down again.  The compressor only came on when the temperature didn't go back down, and it could run for a longish period. 

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3 hours ago, paulraphael said:

 If you opened the fridge door every 5 minutes in perpetuity, it would only make a small difference in energy used, regardless of the fridge being full or empty.

 

But the bottom line may be that it really doesn't make much of a difference either way.

 

This.

 However, this is precisely where lightbulb efficiency would be important...

(in my current fridge the original bulb was just a bit smaller than a regular old lightbulb and used 20 watts of electricity, I put in an LED to improve performance)

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5 hours ago, paulraphael said:

 

I'm going to take the position that aircraft engineering takes, which is that every pound of weight takes energy both to take off from earth and to maintain airspeed. Even if the road you were on were perfectly flat you still have to expend energy to combat air drag. Add in even minor changes in elevation as you travel and that adds to fuel consumption. It does add to the fuel costs to carry extra weight in vehicles.

 

This isn't an accurate analogy, because it takes a ton of energy to keep mass in the air in an airplane. But the efficiency of a refrigerator's insulation (and the energy losses) have basically nothing to do with the mass of the contents. It does take significant energy to cool the contents in the beginning, for sure. But the mass just stops being an important factor after that. 

 

You open the door and cold air flows out and warm air flows in. If I have just one bottle of water in the fridge the temperature change from the warm air would be more than one bottle of water can absorb.

 

 

This is still presuming that the amount of energy that goes into cooling the warm air that's come in is significant. It's in fact hardly anything compared with the heat absorbed through the insulation on all sides that has to be pumped out. If you opened the fridge door every 5 minutes in perpetuity, it would only make a small difference in energy used, regardless of the fridge being full or empty.

 

But the bottom line may be that it really doesn't make much of a difference either way.

 

This.

 

 

The first point is one that some people might not appreciate.   A fridge, freezer , air conditioner  etc, is basically a pump, it is pumping heat instead of air or water or what have you.  Once you have pumped out the heat energy of whatever item you place in it to achieve the desired temp, that item's load on the system is finished. The unit just has to maintain against any added heat -  E.g.   opening door, bad gaskets, whatever heat gets through the insulation , the heat in whatever new items are added . 

 

7 hours ago, IndyRob said:


Yes, when you open the door the cold air will make a break for it.  But cooling air is not energy intensive.  And if we are worried about it, how about filling the fridge with empty (but closed) gallon milk jugs.  Now most of that air can't go anywhere.


 

 

They don't even need to be closed.  Cold air is heavier than the warm air coming in, the cold air will just stay in the jug  unless you have the fridge door open for a really long time and the air inside warms up.  A small amount might get pulled out in a venturi effect with the cold air of the cabinet flowing downward pulling warm air in from above but that would be negligible.

"Why is the rum always gone?"

Captain Jack Sparrow

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