Winter fridge assist - using jugs of ice

[Daox]

So, I was reading my latest issue of HomePower Magazine and stumbled across a hopefully good tip. In winter, use jugs of water/ice to assist with cooling your fridge. Basically, a manual heat pump!

So, I think I will be testing this idea. From previous logging, I know my fridge uses ~1.4 kWh per day. I will test this again with 1 week of logging without jugs followed by one week of using the ice jugs followed again by one week without jugs. I currently envision using 2 milk jugs filled with water. I'll have one in the fridge and the other outside cooling down again. I go to work about 5am and get home about 5pm, I will swap these out at those times. If I find that this is insufficient time for the water to freeze outside, I will add another jug to the mix so they have 24 hrs vs 12 hrs to freeze. All the while, I'll be logging the refrigerator power use with a kill a watt.

So, anyone think they have any ideas about this? Think the milk jugs will crack eventually and leak all over? Think that the thermal mass is too small and won't assist cooling much? I'm basically open to any and all discussion.

[davranyou]

I have an idea of trying this as well! Thanks.

[Phantom]

Milk jugs should work for testing only fill them about 3/4 the way and you should be fine.

[Piwoslaw]

Ha! This has crossed my mind, but I decided it's not worth it. Why? Because, even though it will reduce the amount of energy your refrigerator uses, it will increase your global (whole house) energy usage. By putting the water jugs outside to cool down you are removing heat from inside the fridge. But, you are removing that heat from you house too, so your heating system will have to compensate.

Until I realised this, I would sometimes put food outdoors to cool down before putting it in the fridge. Now I do that only when the house doesn't need to be heated.

So, I think you may see a reduction in refrigerator energy usage, and yes, I think the jugs may start to leak after a while, but 5-10 freeze cycles should be OK. But your heating system will also need to work more, though that will be much harder to measure.

[Clev]

I use several 2-liter soda bottles full of water in my chest freezer. They increase the thermal mass (supposedly so the freezer kicks on less often), while providing drinking water in case of emergency. (They also seal better and are stronger than milk jugs.)

[Daox]

Excellent point Piwoslaw. Taking it a bit further though, wouldn't my ~92% efficient furnace be more efficient than my fridge which is at most running off ~40% efficient grid power (which is supposedly renewable energy)?

[RobertSmalls]

The fridge is a 100% efficient electric resistive heater, and so are HeatBalls (with the curtains drawn), so you could leave both running while nobody's home.

Piwoslaw, when you pull water out of your fridge at 5°C and set it outside at -10°C, it's not the 15K that matters (much), it's the phase change that counts. Water can hold 4.2kJ per kg*K (so 63kJ/kg for the 15K), but it takes 334kJ/kg to freeze/melt water. This phase change doesn't count against you when it comes to setting "warm" jugs of water outside to freeze.

Which leads me to a simple calculation for Daox to estimate the power savings per jug. Use the above temperature figures, and 3kg of water in a gallon jug (about 80% full). Each time you swap the indoor jug with the outdoor jug, you transport 1.2MJ (.33kWh) of heat from the fridge.

Assuming a CoP of 2 for your fridge (SWAG), each jug will save you about 0.16kWh of electricity, and I'd start with three jugs inside / three jugs outside, and make sure the arrangement doesn't cause melting in the freezer portion of the fridge.

[Piwoslaw]

Originally Posted by Daox

Excellent point Piwoslaw. Taking it a bit further though, wouldn't my ~92% efficient furnace be more efficient than my fridge which is at most running off ~40% efficient grid power (which is supposedly renewable energy)?

Your fridge is a heat pump with CoP ~2-3. If you heated your house with a heat pump with similar CoP, then you wouldn't be losing too much efficiency, but with a furnace (with CoP < 1) you need to use more energy to make up for the heat that you literally threw out the window. As I mentioned earlier, it will be almost impossible to measure an increase in fuel for heating, since that depends on so many parameters and the amount of heat being tranferred outside in the jugs will be small compared to how much is needed to heat the whole house. Now, a controlled experiment, with only a refrigerator in a closed off room... blahblahblah. Factoring in grid efficiency also changes the equation slightly.

If your refrigerator's coils went through the wall and always pumped heat to the great outdoors, then doing this experiment wouldn't effect your house's heating.

In any case, I haven't taken into account waste heat from the refrigerator's compressor helping to heat the house, nor heat lost while opening windows/doors to swap jugs.

[strider3700]

completely ignoring the benefit/cost of placing frozen water in the fridge there is another big advantage to having jugs of water in the fridge. When you open the door the cold air pours out the bottom and is replaced by warm air in the room which your fridge now needs to cool down. Replace that cold air with enclosed cold water and when you open the door there is less air to fall out and need to be cooled.

I've noticed the more I've added to my cold cellar the better it is at reducing short duration temperature swings. The fridge should act the same.

[Daox]

Woo, lots of good discussion here.

Piwoslaw is right, the results will be hard to see, especially with respect to the increased heating load. But, I think I will still give the experiment a try as I have stated, but perhaps with more jugs as RobertSmalls has suggested. I'd like to get as drastic an effect as I can to see how much this can effect things. Perhaps I'll also monitor my kitchen temperature. Its somewhat isolated from the rest of the house heating wise and is always the coldest room. A substantial reduction in the fridge putting off heat should show a little.