Super efficient fridge - need math advice
 

So I am seriously considering doing a conversion like this one :
DIY, Super-Efficient Fridge Uses .1 kWH a Day | EcoRenovator.org

Rather than spend over a thousand dollars on the freezer used in the article, I plan to use one of these :

Holiday at Lowe's: 5.0 Cu. Ft. Chest Freezer (Color: White)

or this one :
Holiday at Lowe's: 7.0 Cu. Ft. Chest Freezer (Color: White)

Neither are Energy Star rated :mad:.

The 5 cubic foot model uses 240 kWh per year, and the 7 cu.ft. model uses 277 kWh per year.
For comparison, the VestFrost used in the article was rated at around 197 kWh and has just over 8 cu.ft. of space.

Dividing the kWh used by the square footage of the freezer seems to indicate that the larger that a freezer is, the more efficient.
Is this true ?
Example : The 5 cu.ft model uses 48 kWh per year for each square footage of space used, the 7 cu.ft. model uses 39.57 , and the VestFrost around 24 kWh per sq.ft. ( unmodified )

However, I notice that when I divide the kWh used by the sq.footage of some Energy Star upright refigerators, the math seems to indicate that even a massive 25.5 cu.ft. model uses around 19 kWh per sq.ft. of space in the fridge. This seems to contradict things, since even Energy Star upright refrigerators are not going to be as efficient as an Energy Star chest freezer.
So is kWh used per square footage of space not really a good indicator of a frides efficiency ? Did I just do the math wrong ?

Check into other things, like the type of pump, location/size of the radiator (wrong name, but you know what I mean), shape of the freezing area, type of insulation used, etc.

Sometimes, it can just be a matter of how long the controller lets the pump run for, or the number of cycles it uses.

There are a lot more than just KWh/CuFt to determine how efficient a freezer is.

Also, remember that if you want to save energy, don't get something bigger than you need. That's one thing about many households... they want the biggest fridge out there, but they only keep milk and eggs in it.

Also - CuFt and SqFt are two completely different measurements.

Cd: cu ft are volume, sq ft are area. (I'm assuming you know that, but I'll write it out just in case.)
Suppose you have a box which is 1 ft x 2 ft x 3 ft. The volume is 1x2x3=6 cu ft, while the surface area is 2x(1x2 + 2x3 + 1x3)= 22 sq ft.
The volume of a fridge/freezer tells you how much stuff you can put in. It also tells you how much air you will have to cool down after closing the door (upright fridge case). If you use a chest freezer, the door is the top, so you don't lose cold air when opening (that's one of the reasons it's more efficient). This means that you should look at heat transfer through the sides. The amount of heat gain (or cold loss) is proportional to the surface area. So, what you want is a unit with the smallest surface area for a given volume. This would be sphere, but since they don't make spherical fridges, the closest thing is a cube, i.e. height, width and depth are the same.

Next are the coils (the radiator, as Christ called it, since that's what it really is). A lot of new refridgerators have coils at the bottom, under the unit. This is bad, since the warm air rises off of the radiator straight onto the fridge. A radiator on the back of the unit is much better. (Recently I read a thread on convection, conduction, radiation, either here or on EcoRenovator, but I can't find it now.)

All other things being equal, pick the compressor which draws less power. This may mean that each cycle will take slightly longer before switching off, but this is good for efficiency. Especially if you want to a freezer-to-fridge conversion, since then the compressor will only turn on for a very short time. With a more powerful compressor, the on-time may even be below a minute, which isn't too healthy for it.

Next is insulation. Thicker walls don't always translate into better insulation, but a freezer is always better insulated than a fridge.

Last, placement. Make sure the unit isn't placed in a warm place, like next to a stove, oven, heating vent, or where the sun will shine on it. A basement with year-round cool temps is ideal.

Hope that helps.

--Adam

The amount of heat gain (or cold loss) is proportional to the surface area. So, what you want is a unit with the smallest surface area for a given volume. This would be sphere, but since they don't make spherical fridges, the closest thing is a cube, i.e. height, width and depth are the same.

So all things being equal, the 5 cubic foot model is more efficient than the 7 cubic foot one, since it has less surface area, and a smaller area within the freezer to cool ?
The 5 cubic foot model is sqare shaped, and the 7 cu.ft. model is more rectangular.



Next are the coils (the radiator, as Christ called it, since that's what it really is). A lot of new refridgerators have coils at the bottom, under the unit. This is bad, since the warm air rises off of the radiator straight onto the fridge. A radiator on the back of the unit is much better. (Recently I read a thread on convection, conduction, radiation, either here or on EcoRenovator, but I can't find it now.)

( Not much traffic at EcoRenovator, which is why I'm posting here.) I'll search for that thread as well.

So what are the coils buried within the walls of refrigerators and freezers ? Is this the same type of coil that you are talking about ? I notice that the sides of my mini-refrigerator get very hot. I insulated the walls of my minifridge with 1.5" thick styrofoam, yet it did nothing - it even seemed to increase the power useage at one point. I guess I was just sealing in all that heat.
Since coils can be mounted outside of the walls of the refrigerator, would it help the efficiency to rip out the coils within the walls and place them on top of something like my mini-fridge ? I was under the impression that the coils need to wrap around the walls of the 'fridge to extract the heat.

With a more powerful compressor, the on-time may even be below a minute, which isn't too healthy for it.

Can you please explain that last statement further ? So if I have the compressor kicking on for less than a minute, this might damage it ?


Last, placement. Make sure the unit isn't placed in a warm place, like next to a stove, oven, heating vent, or where the sun will shine on it. A basement with year-round cool temps is ideal.

I plan to place the chest freezer in my laundry closet, which has sliding doors and the right type of outlet. Would a small closet just trap in the heat generated by the fridge ? ( I should add that the closet is empty - so no washing and drying will take place there )

Hope that helps

( Very much so ! Thank you ! )

One last very important question - one that my decision hangs on actually, is whether or not parts of the chest fridge can be used to store frozen food.

It would be crazy to have to run another machine just to store a few frozen items !

If I convert the freezer to a refrigerator, will I lose my frozen items ??

Has anyone here done one of these chest freezer conversions ?

If the doors are not solid and allow a lot of air circulation, then it won't be that bad. Just make sure that if the radiator coils are in the back that they don't touch the wall. I know someone who kept his computer in a closet and was always angry that it kept overheating and switching off.
Most likely you will have shelves in the chest, and since warm air rises, the temperature closer to the top will be above freezing, while at the bottom you might get below. This all depends on where you place the temp sensor and how you set it, but the temperature difference between top and bottom won't be more than a few degrees, so don't count on deep freezing anything. Barely frozen would be a better term.

I'll write more after the weekend.

Barely frozen actually is preferred since it takes less energy to warm the food back up. :thumbup:

Thank you Piwoslaw & Christ for you advice. Get out and have fun this weekend :) .

Barely frozen is no good for Ice Cream, though. ;)

On the contrary. I have to leave ice cream out on the counter for as much as a half hour to be able to scoop it, this with my freezer at -10F.

Jeez, that's when I think it's best! I love it when the stuff just won't scoop... gives me longer to eat it out of the box before I have to put it back in the freezer. LOL

Although, I do agree that maybe a setting at about 20*F would be great to have a freezer compartment in the "fridge", especially if only the lower section (where there's usually a step-in for the motor housing and such) was the "freezer" section.