[digitalprimate]

Recently I moved from living with my parents to having my own place in Antwerp.
And now I also own a rather large fridge/freezer (two doors).

A former roommate of mine used to put blocks of styrofoam in her freezer, to make the available space smaller, thus needing less energy to freeze the food that was in there. As I was rearranging the blocks of styrofoam today, I wondered wether this was actually true. I haven't found any website that states this use. So I thought I'd pose the question here:
Does it really make a difference if you put styrofoam into your freezer, to use up less energy?

PS: I've read somewhere that freezing clear plastic containers of water to achieve the same goal is not doing much.

[Dorian Gray]

Absolutely true! However, you can get the same effect by simply keeping your fridge or freezer nearly full with food. The reason it works is that every time you open the door, the cold dense air inside the fridge falls out, being replaced with air at room temperature. When you close the door the fridge has to use energy to extract the heat from that air, so by reducing the air space in the fridge, you reduce the amount of air at room temperature that must be cooled each time you open the fridge.

This won't have any effect on how much energy is required to cool (or freeze) the warm food that you put in the fridge, but that is a tiny proportion of the overall energy used. Most energy is spent keeping the food cool for days and weeks, despite less-than-perfect insulation. And of course, opening the door is the biggest threat to insulation, so reducing the energy used to re-cool once the door is closed is a very sensible approach to reducing the overall energy consumption of the fridge or freezer. It is for this reason that chest freezers have the door on the top, so that the cold air is less likely to fall out (and be replaced by warm air) when the door is opened.

No need to use styrofoam though. Anything that displaces a large proportion of the empty (air) space in the fridge will have the same effect. The more frequently you open your fridge door, the more energy you would save by eliminating empty space. So another way to reduce energy consumption is to use the fridge in short bursts of activity. For example, replacing everything after a meal in two minutes is better than opening the fridge at five-minute intervals to replace items one-by-one (warming the air each time), while doing other things like washing the dishes.

I gave this stuff considerable thought when I lived in a tropical climate in Brazil. Nice to see someone reducing their energy footprint in simple ways that don't impinge on their freedom. It grates on my nerves to see people leave their fridge door open for minutes on end while getting bits and pieces out of it, or opening it just to peer in and ultimately take nothing out!

[leachboy]

Originally Posted by digitalprimate PS: I've read somewhere that freezing clear plastic containers of water to achieve the same goal is not doing much.

I would thnk that the water would do a much better job than styrofoam. I haven't studied thermodynamics in a long time, but I would think that water has a much higher specific heat and thus would do more to regulate the temperature in the freezer. The other side of that would be that it would take a lot longer (and thus more energy) for the water to achieve the same temp as the rest of the freezer, but that's a one-time thing.

[Banana]

leachboy's explanation is something that I'd have thought of myself, but one thing bother me is that if you allow a fridge to ice up, you're going to see higher energy bill because more work must be spent in keep the extra water frozen.

Now, if we're talking about block of styrofoam, I'd see nothing wrong with it since styrofoam, for lack of better word, doesn't really freeze.. it just get colder. I could be wrong, though.

[billybobsky]

The amount of energy saved is negligible. With water, you are wasting the initial freezing energy. Air doesn't have a high heat content.

meh.

[GrayDiggity]

Originally Posted by digitalprimate PS: I've read somewhere that freezing clear plastic containers of water to achieve the same goal is not doing much.

That is because they are using clear containers. If they are colored, then it will work better.



I have actually heard the opposite. Honestly, though, concentrating on thermal systems in college (no longer in that field), I have never really studied this or even given it much thought. I would think tubs of water help.

It does take a great deal of energy to freeze the water, so maybe that is where the problem is. Once you get it frozen, it seems like it would increase the efficiency.

Originally Posted by Banana Now, if we're talking about block of styrofoam, I'd see nothing wrong with it since styrofoam, for lack of better word, doesn't really freeze.. it just get colder. I could be wrong, though.

I see what you are saying, but in the same way, water just gets colder too. It is not the freezing, or solidification, that takes so much energy, it is the getting colder part.

[Dorian Gray]

Originally Posted by leachboy I would thnk that the water would do a much better job than styrofoam. I haven't studied thermodynamics in a long time, but I would think that water has a much higher specific heat and thus would do more to regulate the temperature in the freezer.

The specific heat capacity of the material used to displace the air is irrelevant, because its temperature doesn't vary much (and to the extent that it does, it merely acts as a buffer against temperature fluctuations in the fridge, with no long-term effect). Temperature regulation is not a problem in fridges or freezers, because the cold foodstuffs have vastly higher specific heat capacity than the warm air that's introduced when the door is opened. Styrofoam, with its very low specific heat capacity, is good if you frequently want to remove it to reclaim the fridge space, of course.

Originally Posted by billybobsky The amount of energy saved is negligible. With water, you are wasting the initial freezing energy. Air doesn't have a high heat content.

I calculated this years ago for our family fridge, billybobsky, and it wasn't negligible at all for our usage. In fact, the majority of the energy used by our fridge was used to re-cool air introduced from outside (at 25-30 degrees C in our case). I made educated assumptions about the insulation of the fridge and the amount of air displaced by opening the door (50% of free fridge capacity). Air has a very low specific heat capacity, but huge volumes of air are displaced every time the fridge is opened, that air must be cooled by fully 20 degrees C, and the fridge may be opened fifty times a day in a family home.

Fridges are generally pretty efficient, because they are a heat pump (a heat engine in reverse). Therefore lowering the temperature of a litre of water by 1 degree C in a fridge takes far less energy than heating it by 1 degree C on a stove. But fridges still use a lot of watt-hours in a month, so reducing that consumption yields a solid return.

[digitalprimate]

Originally Posted by Dorian Gray Nice to see someone reducing their energy footprint in simple ways that don't impinge on their freedom. It grates on my nerves to see people leave their fridge door open for minutes on end while getting bits and pieces out of it, or opening it just to peer in and ultimately take nothing out!

And to make it even greener - I actually shopped around to get an A+ fridge/freezer. Unfortunately, I couldn't find an A++ freezer (unless it was 500 euros, which was a tiny bit out of budget). So I bought this one.

They should make fridgeTRUNKS too, instead of only freezertrunks, because those babies would help towards saving money AND energy like crazy!