Does keeping ice packs in fridge save energy?

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lthenderson
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Re: Does keeping ice packs in fridge save energy?

Post by lthenderson »

mediahound wrote: Tue Apr 02, 2024 9:24 am So what is the bottom line? If I'm going to dispose of some ice packs that were delivered to me already frozen. Should I thaw them out at room temperature first or thaw them in the fridge?
The ice packs that often come with food deliveries are just water and plastic. Dispose of in the trash. The gel packs are made from sodium polyacrylate which is safe to put in the trash though is not recommended to flush down your sink as it can cause skin irrigation. I certainly wouldn't thaw either of them in the fridge only to dispose of them.
Valuethinker
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Re: Does keeping ice packs in fridge save energy?

Post by Valuethinker »

Riprap wrote: Tue Apr 02, 2024 8:32 am Does keeping ice packs in fridge save energy? (Please explain, show all work)

Sounds like a question from a thermodynamics course final exam.
High school physics, on the face of it.

Unless the buried question in the University Level exam is "Under what circumstances would keeping ice packs in the fridge save energy? Illustrate your answer with calculations".

Actually that sounds rather like the entrance exam for Engineering at Oxford or Cambridge.
Last edited by Valuethinker on Wed Apr 03, 2024 3:55 am, edited 1 time in total.
Scotttheking
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Re: Does keeping ice packs in fridge save energy?

Post by Scotttheking »

rkhusky wrote: Mon Apr 01, 2024 10:07 pm I’ve wondered if in winter I should put some water jugs outside, let them freeze, and then put them in the fridge or freezer.
So an ice box. https://en.wikipedia.org/wiki/Icebox
eigenperson
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Re: Does keeping ice packs in fridge save energy?

Post by eigenperson »

rkhusky wrote: Tue Apr 02, 2024 9:36 am
eigenperson wrote: Tue Apr 02, 2024 9:34 am
rkhusky wrote: Mon Apr 01, 2024 10:07 pm I’ve wondered if in winter I should put some water jugs outside, let them freeze, and then put them in the fridge or freezer.
No, don't bring cold stuff into your house in winter. Your fridge will run less, but your heater will run more to compensate.
But in our area electricity is more expensive than natural gas.
In that case this is roughly the same question as "heat pump or furnace."

Say you bring roughly 100 g of ice from your backyard into your fridge. It will take 30 kJ to melt. The heat to accomplish this leaks into your fridge from the house. At the same time, the heat pump in your fridge takes some unpaid time off, saving -- let's say -- 10 kJ of electricity. Both the 30 kJ of heat and 10 kJ of electricity (which turns into heat) must be replaced by your gas furnace, which produces an extra 40 kJ to compensate. You save 10 kJ of electricity at the cost of 40 kJ of gas.

Most gas and electric bills are quoted in different units, but if you translate them to equal units of energy, you can figure out whether it is more cost-effective to bring in ice from outside during the winter. But in general, the impact on your bills will be too small to measure either way.
rkhusky
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Re: Does keeping ice packs in fridge save energy?

Post by rkhusky »

eigenperson wrote: Tue Apr 02, 2024 7:54 pm
rkhusky wrote: Tue Apr 02, 2024 9:36 am
eigenperson wrote: Tue Apr 02, 2024 9:34 am
rkhusky wrote: Mon Apr 01, 2024 10:07 pm I’ve wondered if in winter I should put some water jugs outside, let them freeze, and then put them in the fridge or freezer.
No, don't bring cold stuff into your house in winter. Your fridge will run less, but your heater will run more to compensate.
But in our area electricity is more expensive than natural gas.
In that case this is roughly the same question as "heat pump or furnace."

Say you bring roughly 100 g of ice from your backyard into your fridge. It will take 30 kJ to melt. The heat to accomplish this leaks into your fridge from the house. At the same time, the heat pump in your fridge takes some unpaid time off, saving -- let's say -- 10 kJ of electricity. Both the 30 kJ of heat and 10 kJ of electricity (which turns into heat) must be replaced by your gas furnace, which produces an extra 40 kJ to compensate. You save 10 kJ of electricity at the cost of 40 kJ of gas.

Most gas and electric bills are quoted in different units, but if you translate them to equal units of energy, you can figure out whether it is more cost-effective to bring in ice from outside during the winter. But in general, the impact on your bills will be too small to measure either way.
Any heat that is not produced by my fridge is replaced by the furnace to maintain room temperature. Therefore, saving 40 KJ of electricity at the expense of 40 KJ of gas. And, even though I would probably be using 5000 g of ice, I suspect that is still not much of a cost savings.
eigenperson
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Re: Does keeping ice packs in fridge save energy?

Post by eigenperson »

rkhusky wrote: Tue Apr 02, 2024 8:11 pmAny heat that is not produced by my fridge is replaced by the furnace to maintain room temperature. Therefore, saving 40 KJ of electricity at the expense of 40 KJ of gas. And, even though I would probably be using 5000 g of ice, I suspect that is still not much of a cost savings.
If you have a 1-for-1 tradeoff in this situation, I'm afraid your fridge has a COP of zero.

By bringing the ice into your constant-temperature house, whether it's inside or outside of the fridge, you're committing your furnace to producing enough heat to melt the ice. (If the ice is inside of the fridge, the heat has to leak through the fridge's insulation, but that makes no difference.)

For any fridge with a COP greater than 1.0, the energy cost of producing this heat is more than the fridge saves. For a more typical COP near 3.0, it's significantly more.

And you're right -- even 5 kg of ice is not very significant. Even a 1-ton heat pump (or 12 kBTU/hr furnace) can nominally make (or melt) 1 ton of ice per day, and is not particularly impressed by 5 kg.
Valuethinker
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Re: Does keeping ice packs in fridge save energy?

Post by Valuethinker »

eigenperson wrote: Tue Apr 02, 2024 8:37 pm
rkhusky wrote: Tue Apr 02, 2024 8:11 pmAny heat that is not produced by my fridge is replaced by the furnace to maintain room temperature. Therefore, saving 40 KJ of electricity at the expense of 40 KJ of gas. And, even though I would probably be using 5000 g of ice, I suspect that is still not much of a cost savings.
If you have a 1-for-1 tradeoff in this situation, I'm afraid your fridge has a COP of zero.
Would this not be COP = 1.0 ?

i.e. 40 KJ cooling from 40 KJ of electricity?

COP of a gas furnace around 0.9 (90% efficiency) although it's not measuring quite the same thing because that's a measure of the conversion of heat energy from a combustible fuel into usable heat?
By bringing the ice into your constant-temperature house, whether it's inside or outside of the fridge, you're committing your furnace to producing enough heat to melt the ice. (If the ice is inside of the fridge, the heat has to leak through the fridge's insulation, but that makes no difference.)

For any fridge with a COP greater than 1.0, the energy cost of producing this heat is more than the fridge saves. For a more typical COP near 3.0, it's significantly more.

And you're right -- even 5 kg of ice is not very significant. Even a 1-ton heat pump (or 12 kBTU/hr furnace) can nominally make (or melt) 1 ton of ice per day, and is not particularly impressed by 5 kg.
Mr. Rumples
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Re: Does keeping ice packs in fridge save energy?

Post by Mr. Rumples »

It's just me in the house and so I keep empty milk containers in the refrigerator thinking that there would be less cold air escaping when I open and close the doors. I also store sodas in there, more than I ever use but have on hand. I keep the frozen ice packs in the freezer for the same reason. They can come in handy for putting on my face after dental surgery (implants).
"History is the memory of time, the life of the dead and the happiness of the living." Captain John Smith 1580-1631
rkhusky
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Re: Does keeping ice packs in fridge save energy?

Post by rkhusky »

eigenperson wrote: Tue Apr 02, 2024 8:37 pm
rkhusky wrote: Tue Apr 02, 2024 8:11 pmAny heat that is not produced by my fridge is replaced by the furnace to maintain room temperature. Therefore, saving 40 KJ of electricity at the expense of 40 KJ of gas. And, even though I would probably be using 5000 g of ice, I suspect that is still not much of a cost savings.
If you have a 1-for-1 tradeoff in this situation, I'm afraid your fridge has a COP of zero.

By bringing the ice into your constant-temperature house, whether it's inside or outside of the fridge, you're committing your furnace to producing enough heat to melt the ice. (If the ice is inside of the fridge, the heat has to leak through the fridge's insulation, but that makes no difference.)

For any fridge with a COP greater than 1.0, the energy cost of producing this heat is more than the fridge saves. For a more typical COP near 3.0, it's significantly more.

And you're right -- even 5 kg of ice is not very significant. Even a 1-ton heat pump (or 12 kBTU/hr furnace) can nominally make (or melt) 1 ton of ice per day, and is not particularly impressed by 5 kg.
You’re right. I was only considering the heat generated by the motor and forgetting the heat pumped out of the refrigerator.

For a COP of 3, you do get a factor of 4. But looking at my gas and electric bills and doing some conversions, it appears that electricity costs 6x more than gas per KJ (0.006 cents vs 0.001 cents).

5 Kg of ice (1.3 gal) -> 1700 KJ of electricity (10 cents) -> 6800 KJ of heat (7 cents). So, I am saving 10 cents of electricity by adding the ice to the refrigerator, but I have to spend 7 cents in gas to replace the missing heat from the refrigerator, for a net of 3 cents per 5 Kg of ice.

Probably not worth the effort.
Valuethinker
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Re: Does keeping ice packs in fridge save energy?

Post by Valuethinker »

Mr. Rumples wrote: Wed Apr 03, 2024 5:22 am It's just me in the house and so I keep empty milk containers in the refrigerator thinking that there would be less cold air escaping when I open and close the doors. I also store sodas in there, more than I ever use but have on hand. I keep the frozen ice packs in the freezer for the same reason. They can come in handy for putting on my face after dental surgery (implants).
It's worth knowing that the heat capacity of air is vastly smaller than water (I am thinking it is 1/100th, but someone more knowledgeable will pitch in with the right number). Therefore it's not a big saving.

Depending on the efficient operating range of the fridge or freezer, there is benefit from the greater thermal mass. But I am not sure which way that goes - either net benefit or penalty from higher thermal mass?

I think the main reason is probably if one has power cuts - then the freezer packs can come in handy in delaying disaster.
billaster
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Re: Does keeping ice packs in fridge save energy?

Post by billaster »

Mr. Rumples wrote: Wed Apr 03, 2024 5:22 am It's just me in the house and so I keep empty milk containers in the refrigerator thinking that there would be less cold air escaping when I open and close the doors. I also store sodas in there, more than I ever use but have on hand. I keep the frozen ice packs in the freezer for the same reason. They can come in handy for putting on my face after dental surgery (implants).
You hear a lot about keeping your refrigerator full but it isn't really that significant. This is because the heat capacity of air is very small. There just isn't that much heat in the air. Even if you had a completely empty refrigerator and opened the door for a while to fill it completely with 70 degree air, the heat in that air amounts to only the heat to melt one or two ice cubes. And that's for a completely empty refrigerator.

The idea that having a full refrigerator allows less warm air to enter, while true in theory, the practical significance is very small. It's sort of like the theory that driving with half a tank of gas gives better mileage because your car is lighter. While theoretically true, the difference is almost immeasurable.
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