A Humbling Excersize in Minimization

[trebuchet03]

I had an epiphany today. Currently, I'm running an experiment on a standard refrigerator, trying to keep in the cold and save energy. I'm trying to minimize losses.

My datum for the experiment. My refrigerator consumes 1.32kWh per day.

Experiment aside, lets do a quick benefit analysis (we're not going to even consider the cost at the moment - just all the benefits).

So, 1.32kWh per day translates to:

481.8 kWh per year - $48.18 per year @ 10 cents/kWh (mine is a tiny bit less)
~646 pounds of CO2 annually by 1999 standards (if you can find more recent data, I'd appreciate it)

So, for every .01kWh/day saved you save
3.65kWh per year - $0.37 per year @ 10 cents/kWh
~4.78 pounds of CO2
3.65kWh gets an EV (250wH/mi) ~ .9 miles

For every .5kWh/day cut... you save:
182.5kWh per year - $18.25 per year @ 10 cents/kWh
~239 pounds of CO2
182.5kWh gets an EV (250wH/mi) ~ 46 miles
This type of savings begins to be reasonable

For every 1kWh/day cut... you save
365kWh per year - $36.5 per year @ 10 cents/kWh
~478.15 pounds of CO2
365kWh gets an EV (250wH/mi) ~ 91.25 miles


Now, if I had did this basic calculation beforehand... I may not have gone forward with the experiment without doing some hand calculations. I have a strong feeling that ~40% (.5/1.32) of the energy that goes into a refrigerator does not come from opening the door considering that the door stays closed for most of the refrigerator's life...

Perhaps later, or tomorrow, I'm going to do some hand calculations to see just how much energy is lost when one opens/closes a refrigerator door - 40% very well could be reality... But here's a preliminary result.

Subjectively - I feel very little cold air hit my feet when I open my refrigerator (as compared to pre modification).
Quantitatively - we're seeing little results on the KillaWatt (although, the average goes down slightly every day - I'm resetting tonight)



As this applies to everything else.... Looking at the overall impact when you're trying to minimize a low number is very important. When you're starting off low, and try to minimize... It's difficult - and in some situations, not cost effective or worth the effort. 1 cD point (.01) saved is worth it - but an eighth of a point (0.00125) may not be depending on cost...


Doh! I put this in the wrong category (maybe)... Sorry about that :/

[Snax]

Just based on the specific heat of air vs. other solid masses, I would think that very little energy is actually lost during brief door openings.

So what are you doing to improve your refrigerator performance?

We are planning to purchase a chest freezer to be placed in our garage, but we are going to wrap it with 2-3" rigid foam which I am guessing will about double the insulation factor for it.

[trebuchet03]

Originally Posted by Snax

So what are you doing to improve your refrigerator performance?

That's exactly the question I didn't ask myself before starting....

Currently - we have a crude prototype to trap cold air and prevent it from "falling out." If anything, it's a quantitative result on how much energy is lost from opening the door

[unstable bob]

What I want to know is what kind of beer do you have in that fridge?

[cfg83]

trebuchet03 -

I heard that keeping your fridge full allows it to run more efficiently, because the food doesn't transfer the "coolth" to the outside as fast as the air does.

Assuming your standard refrigerator is not full, why not put jugs of air (cools fast) or water (stays cool longer) into the fridge in the empty parts? If you do this, there will be less volumetric air to escape, so the fridge will be able to retain the "coolth" when you open the door.

CarloSW2

[Erdrick]

cfg83: I have heard that that is actually a myth. A fridge will run more efficiently when it has a good path for the cold air to circulate around. Having a fridge tightly packed WILL retain the "coolth" when you open the door and close it after getting whatever you needed, but at the cost of killing air circulation when the door is close... which is greater than 90% of the time...

So, I am not sure what kind of conclusions you want to draw from that, but at least it is something to think about. Perhaps I will try a test with my fridge... I do have a kill-a-watt now, so it would be possible if I felt like burning some time.

[bowtieguy]

Originally Posted by cfg83

trebuchet03 -

I heard that keeping your fridge full allows it to run more efficiently, because the food doesn't transfer the "coolth" to the outside as fast as the air does.

Assuming your standard refrigerator is not full, why not put jugs of air (cools fast) or water (stays cool longer) into the fridge in the empty parts? If you do this, there will be less volumetric air to escape, so the fridge will be able to retain the "coolth" when you open the door.

CarloSW2

unsure of efficiency, but do realize that a full "container" allows the cold unit to work less. during my cold deliveries, after closing the door, a full truck(container) ALWAYS cools back down more quickly than 3/4,1/2,empty etc.

[cfg83]

bowtieguy and Erdrick -

Originally Posted by bowtieguy

unsure of efficiency, but do realize that a full "container" allows the cold unit to work less. during my cold deliveries, after closing the door, a full truck(container) ALWAYS cools back down more quickly than 3/4,1/2,empty etc.

Ok, this implies that there is some truth to it.

bowtieguy -

Does the cold truck you drive have a "home refrigerator style" refrigerator unit? Could there be a difference in the way the refrigeration works?

CarloSW2

[cfg83]

Erdrick -

Originally Posted by Erdrick

cfg83: I have heard that that is actually a myth. A fridge will run more efficiently when it has a good path for the cold air to circulate around. Having a fridge tightly packed WILL retain the "coolth" when you open the door and close it after getting whatever you needed, but at the cost of killing air circulation when the door is close... which is greater than 90% of the time...

So, I am not sure what kind of conclusions you want to draw from that, but at least it is something to think about. Perhaps I will try a test with my fridge... I do have a kill-a-watt now, so it would be possible if I felt like burning some time.

Ok, assuming it's a myth, then here is a modified version of same :

1 - Refrigerator X has volume Y
2 - You only need a subset of Volume Y.
3 - Put (non-toxic inert!!!!) insulation and/or jugs of water in one continuous volume of the refrigerator that does NOT interfere with the circulation.

For example, if the cooling was originating form the top of the interior of the refrigerator, pack the insulation into a volume at the base of the refrigerator (i.e. the salad trays). That way, the refrigerator would only have to cool a smaller "continuous interior volume".

Does this make sense?

CarloSW2

[bowtieguy]

Originally Posted by cfg83

bowtieguy and Erdrick -



Ok, this implies that there is some truth to it.

bowtieguy -

Does the cold truck you drive have a "home refrigerator style" refrigerator unit? Could there be a difference in the way the refrigeration works?

CarloSW2

the unit is Thermo King brand(unsure of website). been told its $20k because it has its own brain,starter,alt,batt,fuel tank etc. probably not very similar to a home style unit, but wouldn't the cool down concept be the same? oh,also we can use them to heat such that when necessary, keep certain products warm during winter. Erdrick brings up a great point tho--my wife asks me not to cover the fan in the freezer as per manufacturer instructions.

[trebuchet03]

Originally Posted by bowtieguy

probably not very similar to a home style unit, but wouldn't the cool down concept be the same?

Refrigeration is, in general, very similar in concept across the board. Compressor, condenser, evaporator and a circulation fluid (air, water/brine, both). The key differences being in the type of component (piston v. centrifugal v. etc. compressor et. al.).

cfg83: I have heard that that is actually a myth. A fridge will run more efficiently when it has a good path for the cold air to circulate around. Having a fridge tightly packed WILL retain the "coolth" when you open the door and close it after getting whatever you needed, but at the cost of killing air circulation when the door is close... which is greater than 90% of the time...

Circulation and thermal capacity don't need to be strictly dependent on each other You can have a refrigerator with plenty of heat sinks (water jugs etc.) and have good air circulation An interesting thing to keep in mind... Turbulent flow is much better at cooling/heat transfer than laminar flow.