Monday, July 25, 2016

insulated refrigerator freezer result negative?

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This graph from last night shows the electrical energy use of my newly insulated refrigerator freezer over about three hours with virtually nothing else on in the house.

I was surprised to find that the energy used last night was greater with the insulation I added yesterday! Until I realized that the last time July 10 I looked carefully at refrigerator usage, the temperature in the house was MUCH cooler. We are having a bit of a heat wave here so the temperature this morning in the house is 21C. You can see the little thermometer in the top right of the graph. Last time it was 12C. I would expect the fridge to be working harder when the ambient temperature is warmer. So I don't know the effect of the insulation yet. I have changed two variables and you can't do that. I'll have to repeat the measurement when the overnight temperature is cooler.

The temperature is measured outside by the Blueline Innovations device on my electricity utility power meter. Since I have the windows open, I would expect the inside and outside temperatures to be about the same. I also had the windows open on July 10.

So last night, I unplugged my small chest freezer downstairs about 10pm and plugged it back in about 5am this morning so it was not running during the recording. The fridge and the freezer have similar signatures so if I want to see one or the other, I unplug the other one for a time.

Everything in the downstairs chest freezer is still frozen solid although most of the light frost is gone from the inside walls and the plastic bags of food. There are still big chucks of ice in there on the walls after seven hours off. I have been watching this fairly carefully since I have been keeping my fridge and freezer OFF with timers during my peak period. They must keep at a safe temperature even after six to eight hours off. If you have a house full of teenagers using the fridge often, this might not work for you.

Other than the refrigerator freezer upstairs, only the network is running (which is how I get the data from my Blueline energy monitor), a couple of night lights and a phone charger. These make up the baseline load of about 70 watts. The base load is a bit lower than the previous tests because I not only turned off the computer but switched off its power bar.

Here is the graph annotated with guidelines and measurements added with CorelDraw. You can use your own favorite graphics program (or a ruler on a printout) to do the measurements and calculation. The Blueline software does not do the appliance calculation.

What I want to know is the area under the curve. The curve in this case is the square wave which represents the time (horizontal axis) that the refrigerator is running. The vertical axis shows the power demand at any particular time. The power times the time is the energy used in kWh.

Looking first at the vertical, when the fridge is ON, the house is drawing about 220 watts. I am eyeballing this by drawing a line through the middle of the tops of the square waves. It's an average, but you can see the fridge is a pretty consistent load when it is ON. When the fridge is not running, the house still draws energy (base load). Again, I am eyeballing this through the troughs of the square wave at about 70 watts. So the fridge is using the difference (220-70)= 150 watts, the same as before.

I would not expect the insulation to affect the amount of power at any time since the motor will draw the same current as before. What should happen is the motor will run less often.

Lookng at the horizontal time axis. It is hard to read exactly the Blueline graph so I am going to use some rulers. CorelDraw lets me lay down these rulers and tells me the distance. I have labeled one hour as being 3.12cm. The units of length don't matter as long as my units are all the same. I am going to use distance to compute time.

Across the top, I see that the fridge is ON an average of (0.92+0.89+0.91/3) 0.90cm or (0.9/3.12) 0.3 of an hour (about 20 minutes). I see the cycle time averages 2.15cm or (2.15/3.12) 0.7 of an hour (about 42 minutes). So I can say that the refrigerator is ON (0.3/0.7x100) 42.8% of the time.

So at this rate, based on these samples, over a whole day my fridge would use (power x time x duty cycle) (0.15x24x0.428) 1.54kWh or (1.54x364) 562kWh per year.

There is more to the story. As I was adding the insulation on the outside of the freezer compartment, I was wondering if I would have to re-balance the controls for the fridge and the freezer. At one point, several hours after finishing, I did adjust the fridge by one notch but then moved it back. I can't honestly be certain that I put it back to the same exact place. Both the fridge and the freezer controls are in the middle of their ranges.

What did surprise me this morning though, was that I had been playing with the "Exterior Moisture Control" and had mistakenly left it in the ON position. There are strip heaters around the door openings that are intended to drive off condensation on the cold door edges. I did not mean to have this on for last nights recording. This would definitely have used more power. I am not sure when the heaters come on but likely they are turned on and off the same as the compressor. But I still see the same 150 watts? Puzzling.

So THREE variables!

Thanks for your interest
George Plhak
Lion's Head, Ontario, Canada

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Unknown said...

Great story, I just added insulation to the exterior of my mini fridge and wondered if anyone else had experimented.
Jon in Phoenix

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