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Author Topic: Does keeping water bottles in the fridge save on energy costs?  (Read 10301 times)

Offline thedoc

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Rik Lapham asked the Naked Scientists:
   
I read all the Q & A's about it and I was wondering how much money monthly a person with a standard 18 cu ft fridge (maybe 3 or 4 cu ft for the freezer) would save filling it with water bottles?

My friend insists he saves $5 or $10 a month but I think he's exagerating.I know you're in the UK, so if you could give me a percentage of the total operating cost if dollard amounts are unfamiliar to you, I would love that.

I enjoy going to visit and cooking him dinner but those darn frozen water bottles fallingall over the place when I try to use the freezer are a pain! (I'm trying to convince him to eitherreplace with larger bottles of water or boxes filled with insulation.)

What do you think?
« Last Edit: 29/05/2013 14:30:00 by _system »


 

Offline confusious says

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They used to tell us to fill empty spaces in the freezer with loaves of bread, this takes up empty space and in turn saves electricty, I suppose water bottles act in the same way, there are of course many other items which can be frozen, cheese, milk are two others I know of.
 

Offline peppercorn

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Water has a high thermal mass, as well as being easily available.
However it does expand when frozen so watch that your bottles don't rupture.

The lager the thermal mass one has where a thermal reservoir is concerned, the more stable the it will remain over time (no such thing as 100& insulation, of course).
It takes a large proportion of your energy demand to take all that water down to freezing, which is useful if you are running a freezer that remains only partially full. If on the other hand, your freezer's contents are varied from one week to the next then you may find you are taking frozen bottles out to make room, which is dumping much of the work done in the first place.
So, the simple answer is, it depends.... useful huh!
[NB. if ou put the frozen bottles in the fridge to defrost, you ought to get quite a lot of the 'work' back :) ]
 

Offline CliffordK

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There were a few pertinent questions asked earlier.

Is a full fridge/freezer better than an empty one?
http://www.thenakedscientists.com/forum/index.php?topic=23913.0
http://www.thenakedscientists.com/forum/index.php?topic=43522.0

And, why is frost bad for a freezer.
http://www.thenakedscientists.com/forum/index.php?topic=35365

I presume that space filling with something that is a good thermal heat sink is not a bad idea, as well as displacing cold air that is lost when you open the door.

However, depending on the air circulation in the fridge, covering the "coils" may lead to inefficiencies. 

Another option, of course, is to buy a smaller fridge.  I suppose it might depend a bit on insulation and design, but a smaller fridge would also have less surface area on the outside of the fridge for heat exchange.
 

Offline cheryl j

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I don't know if it saves energy, but in one clinic I worked in we kept water bottles in there to keep the temperature more constant when the door was opened and closed. The vaccines had to be kept in a narrow temperature range.
 

Offline peppercorn

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I don't know if it saves energy, but in one clinic I worked in we kept water bottles in there to keep the temperature more constant when the door was opened and closed. The vaccines had to be kept in a narrow temperature range.
That's the same principle as mentioned above. That the bottles of water in a fridge (or bottles of ice in the freezer) greatly increase the thermal mass that is being cooled; creating a far more stable temperature, and usually an increase in efficiency as well.

Clifford - if the coils in the freezer are 'covered' (in contact with bottles) then heat transfer should be at least as effective as with convection I would have thought.  However, iced up coils can have a reduced capacity to transfer heat effectively (due to all the tiny bubbles of trapped air I assume), and it may be the case that having a bottle (or any other item) pushed right up against the coils may encourage ice to 'bridge' to the walls of the enclosure, so increasing conductive losses through to the outside.
 

Offline CliffordK

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I mentioned Mini-Fridges earlier.
Apparently they vary significantly.  While their overall energy consumption may be less than a full-sized fridge, they typically have a greater energy consumption per cubic foot. 

New refrigerators may also be significantly better than 20 or 30 year old refrigerators.

Another option that I'm seeing is refrigerators without freezers have somewhat lower energy consumption than those with freezers. 

There are a couple of refrigerators designed with extra insulation for the solar industry that are significantly more efficient than others on the market, for example the Sun Frost RF12 is a bit small, but it is rated at about 172kw/year which is half to a third the energy consumption of most other refrigerators. 

--------

As far as frost covered coils vs placing objects directly on the coils. 

Air is a poor heat conductor (insulator), but it does experience convection, and is easily moved with fans.
Water & Ice are also relatively poor heat conductors compared to many metals.
Table of thermal conductivity

Water and ice have a relatively high specific heat capacity compared to many other substances.  However, the specific heat capacity is listed in grams.  If one factors in the density, then copper might be similar to water.

Anyway, if you place an insulator like foam over the coils, then the freezer will have to work longer to cool down the entire freezer.  Likewise embedding the coils in ice (porous or not) would be bad.  I would think that covering every inch of the coils would similarly be bad. 

Now, if one has a large upright freezer, then it may have coils in the middle of some of the shelves.  Heat goes up, Cold goes down.  So, covering the tops of the coil shelves with food, or something with generally high thermal conductivity, and allowing the bottoms of the shelves to be open may in fact not be bad.

One might consider putting copper blocks into the freezer, but water and plastic milk jugs are cheap.
 

Offline quantemleep

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The answer depends on how you use your freezer. The basics of thermodynamics states that the rate of heat transfer when the freezer is closed, from the cold freezer to the ambient room temperature is directly proportional to the difference in temperature. So as long as the freezer is closed, what is in the freezer should have little or no effect on energy consumption.

The difference in energy loss, then, is when you open the freezer, and the energy loss is proportional to the quantity of cold air that leaves the freezer when open, which would be proportional to the amount of open air and how frequently and how long the freezer is open. You can minimize this by knowing what you are retrieving and do your business quickly. If a water container is in your way and you remove it, you have wiped out any benefit by the warm air that must replace the water container you removed AND you are keeping the door open longer, INCREASING your energy loss.

Also, you must consider the energy required to cool the water to freezing temperature and the significant phase transition energy required to convert water from liquid to the frozen state. This energy might not make much difference if the water bottles remain permanently in the freezer and is never removed when retrieving or adding food.

But if the contents of the freezer change (and naturally it will in the process of consumption and resupplying the freezer) then you must consider the loss of energy when you remove water containers and then place them back after you remove a frozen pizza.

If you are repeatedly removing water containers to resupply and then putting in room temperature water when you take something out, the energy to recool and refreeze the water containers probably more than offsets any savings. Due to the trouble an inconvenience of the water containers being in the way (and hiding food that gets forgotten and probably tossed out because it goes bad), putting water containers in the open space is probably a very bad idea.
 

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