31 replies to this topic

[Fat Guy]

In the process of doing a little research on the relative energy efficiency of different refrigerator-freezer designs, I came across some surprising numbers regarding energy efficiency. For example, according to thisweb page:

Keep your refrigerator's coils clean. Brushing or vacuuming the coils can improve efficiency by as much as 30 percent.

and

Check the temperature - a fridge that is 10 degrees colder than necessary can use 25 percent more energy. Refrigerators should be kept between 35 and 38 degrees - freezers at 0 degrees Fahrenheit.

Those numbers, if true, are pretty shocking. A 30 percent efficiency boost from keeping the coils clean? I have never in my life cleaned a refrigerator's coils. I guess I'll be starting.

There are a number of other tips on that web page and others. Assuming most of the population doesn't know this stuff, I imagine there is a lot of energy to be saved out there.

[Edward J]

A full refrigerator will consume less energy than a empty one or partially full one.

I have no idea as to the % of energy saved, but it is true.

It is much harder to refrigerate empty space than it is to refrigerate solid objects.

Think about it...

Say you crammed a fridge full of soda pop. It would take the fridge considerble energy to get the pop cold, but once cold, it is very easy to maintain that temperature.

The refrigeration guys I talk to tell me if you want a fridge to run, but don't have anything to put into it, stuff in hunks of styrofoam--anything to soak up the space.

[Peter the eater]

Earlier this summer I did a set of design drawings for a 40' houseboat. I learned that when you choose to live on a boat, every inch and every watt counts. This particular barge/boat relies heavily on photovoltaic cells and storage batteries so the appliances are all 12V, including the 2' x 2' x 5' fridge/freezer. My point is, there are innovative and efficient appliances out there for the RV and marine people.

[Darienne]

We live on a dirt road and have two large dirty dogs and often other doggy visitors. We clean our fridge coils from time to time and I am always stunned by the amount of dust on them.

Hmmm...would probably be better if the cleaning were noted on the calendar for the first day of each month, along with change or furnace filters, septic treatment and DH's change of blades (he does change them more often...I don't know his schedule )

Good note about the styrofoam although I can't recall the last time our fridge was half empty.

[dougal]

In the process of doing a little research on the relative energy efficiency of different refrigerator-freezer designs, I came across some surprising numbers regarding energy efficiency. For example, according to thisweb page:

Keep your refrigerator's coils clean. Brushing or vacuuming the coils can improve efficiency by as much as 30 percent.

...

Those numbers, if true, are pretty shocking. A 30 percent efficiency boost from keeping the coils clean? I have never in my life cleaned a refrigerator's coils. I guess I'll be starting.
...

I think something may have got lost in translation.

You aren't going to lose any significant efficiency because of normal household dust on the EXTERNAL radiator (most of whose surface area is in any case vertical or downward-facing, and so not going to get covered with dust).
But it is usually forgotten that the radiator on the back of your fridge or freezer is there to dump heat into the room air. (So on top of the back of the fridge is usually not a cool, but actually a warm place, for things like bread proofing or beer brewing.)
However, permitting good airflow over that radiator DOES matter to efficiency - so don't box it in - ventilate it!

The thing is that you will lose a LOT of efficiency if the INTERNAL (cooling) coils are covered with thick ice. The ice acts as an insulator ...
And it's even worse when the thermostat disappears inside a lump of 'frost'
This is one reason for the popularity with modern officialdom of 'frost-free' freezers (even if they do make your ice cream go icy).

A useful energy-efficiency cleaning tip is to make sure that the door seals can ... seal! Air leakage there wastes energy, wastes money.

A full refrigerator will consume less energy than a empty one or partially full one.

I have no idea as to the % of energy saved, but it is true.

It is much harder to refrigerate empty space than it is to refrigerate solid objects.

Think about it...

Say you crammed a fridge full of soda pop. It would take the fridge considerble energy to get the pop cold, but once cold, it is very easy to maintain that temperature.

The refrigeration guys I talk to tell me if you want a fridge to run, but don't have anything to put into it, stuff in hunks of styrofoam--anything to soak up the space.

Sorry, but I don't think this is true either.

I think it comes from a misunderstanding of comments about the cost or energy cost of keeping the stuff frozen.
It probably started as something along the lines of "per pound of content, it costs more to have a freezer only half full."
Which is perfectly true.
As long as you include the "per pound" bit.



"Think about it".
The freezer motor and compressor have to run to remove the heat that leaks IN, through the insulation, or when the door is open. (We aren't dealing with 'warm' stuff being added to the freezer and needing cooling - just steady-state running.)
Heat leakage through the insulation (and door) has (almost) nothing whatsoever to do with what is in the freezer.
And it is actually easier to cool air ("empty space") than solid objects, because the air has less thermal capacity. It has less heat to remove.


Almost nothing to do with it?
Well, if you have an upright freezer, with no 'shelf-doors' (or solid drawer fronts), you will probably lose more cold air when you open the door of a pretty empty freezer than you'll lose from a 'full' one.
The answer is to get a more modern, more energy-efficient freezer - with drawer-fronts or per-shelf doors. Or even a chest freezer, which "loses less cold" on opening the door than any upright.


As to being harder to refrigerate "empty space", bear in mind that the thermostat (which controls the motor) is measuring the AIR temperature inside the freezer. Again, having 'plenty stuff' in the freezer makes little difference. Except this - you NEED to allow air circulation, so that the thermostat is exposed to a temperature typical of the whole freezer, not just its own corner. You NEED air circulation (its not "empty space" really) to spread and 'even out' the cold - otherwise the motor will be running too much or too little. So DON'T cram it so full as to impede the air circulation.


Sure, the more thermal mass there is in the freezer (not styrofoam), the less the temperature will be affected by each opening of the door.
However here we have to understand that temperature and heat are different things.
The amount of heat getting into the freezer on opening the door will be pretty similar, whatever is in there. (Yes very slightly more will get in for an 'empty' freezer.) The point here is that even though a larger thermal mass in the freezer will show a smaller temperature change - its essentially the same amount of heat getting in, and thus the same amount of heat for the compressor to remove.




In Europe (the EU), by law all new fridges and freezers must display their energy efficiency results, with a standardised easy-to-understand grading. http://www.energycho...cy-ratings.html

Good insulation, good door seals, an effective thermostat and an efficient compressor and refrigerant make for low energy use.

Placing the unit in a cool location (so less temperature difference between inside and out, plus cooler air flowing over the external radiator) will reduce energy consumption.
Conversely putting it in a hotspot (like next to the oven) will increase the energy demand.
A word of warning however - many (but not all) modern ozone-friendly refrigerants are NOT suited to being used in a COLD environment. If you plan to put the thing in the cool of the garage (or a shed or outhouse) -- check the technical detail on this specific point before you buy. The risk is that you can destroy the compressor - an expensive repair.

Edited by dougal, 04 September 2010 - 07:50 AM.

[slkinsey]

In the process of doing a little research on the relative energy efficiency of different refrigerator-freezer designs, I came across some surprising numbers regarding energy efficiency. For example, according to thisweb page:

Keep your refrigerator's coils clean. Brushing or vacuuming the coils can improve efficiency by as much as 30 percent.

Those numbers, if true, are pretty shocking. A 30 percent efficiency boost from keeping the coils clean? I have never in my life cleaned a refrigerator's coils. I guess I'll be starting.

How would you even do this? Modern refrigerators almost always have the coils sealed behind something rather than open like they used to be.

[Edward J]

Dougal,

Don't take my word for it.
Please, Please, please consult a Hvac guy or a refrigeration mechanic.

Who me, a refrigeration expert?

Just a lowly cook who can hear when a compressor kicks in and kicks out, and knows a compressor under load consumes more power than just the case fan. A loaded fridge's compressor doesn't kick in and out as much as a empty one or a half full one. That much I know.

Please consult a refrigeration expert.

[Chris Hennes]

Dougal is coming at this from a scientific, thermodynamics, immutable-laws-of-the-universe perspective. Absent any actual experimental measurements of power draw, I'd tend to trust the thermodynamics before the anecdotal evidence (or evidence in an HVAC mechanic's handbook) anytime. A compressor kicking on more frequently is not the same as a compressor running longer.

[andiesenji]

One of the easiest ways to use your refrigerator and/or freezer efficiently is to minimize the amount of time you have the door open.

Rather than spend several minutes looking for something in one of the many little containers usually loose on a shelf, utilize a tray or basket like the one in the photo so you only have to pull the tray out and set it on the counter to look for the item.

This also makes cleaning the shelves much easier and much faster.
This one is for one of the shallow shelves at the top of the fridge. I have deeper ones for taller bottles and jars that fit onto lower shelves that are taller.
They also allow me to store jars or bottles upside down, if the contents can be affected by exposure to the air.

I've been using these and similar items for many years and can't imagine being without them.

[Edward J]

A compressor kicking on more frequently is not the same as a compressor running longer.

No, it isn't. I've been working with commercial refrigerators in my job now for close to 27 years now. I do know that when a typical N. American compressor starts up it draws max amperage, and when it runs, the amperage goes down a bit. I also know that if a compressor runs continously for more than an hour, the thermal overload kicks in and it shuts down.

I also know that dust and lint have quite an insulation value and can choke up inbetween the fins, the compressor has to work harder to compress. In most N.American municipalities water cooled refrigeration systems are banned, and the N. Americans are very slow to make closed loop water cooled systems available. The Euros and the Asians have got some very intelligent commercial eqpt that put the N. American commercial sytems to shame. If I had the extra cash and could reverse the decison I made 4 years ago, I would put my 5' and 3' display cases as well as my 2 door reach in coolers on one remote, closed loop system.

For some reason, no wants to think that the heat a refrigerator produces is NOT waste, but rather a by product that can and should be utilized....

One of the worst things about leaving the fridge/freezer doors open or lousy seals on the doors is not the compressor working harder to compensate, but the warm humid air sticking to the coil and subsequently icing up the coil. Temp in the fridge goes up and up, then the compressor shuts down. One of the most frequent tasks I've done is de-icing the coil manually rather than unplug the unit and let it defrost gradually.

In the last 10 or so years I have bought close to $80,000.00 worth of commercial refrigeration and repairs. I am by no means any expert on refigeration, but know enough to diagnose a problem and know when to call in a repair guy--at $75/ hr AND additional truck/travel fee, and when to "do it myself".

[weinoo]

How would you even do this? Modern refrigerators almost always have the coils sealed behind something rather than open like they used to be.

Exactly. Our refrigerator has no visible coils.

But, there are coils on the bottom side of the unit. Which the manual recommends to clean thusly:

Condenser Coil Remove base grille to access.
Use a vacuum cleaner hose nozzle.

Also, I think it's most efficient to have a nearly full freezer and a less full refrigerator.

[Toliver]

And it is actually easier to cool air ("empty space") than solid objects, because the air has less thermal capacity. It has less heat to remove.

Can you explain more about this?
I would think if I have multiple objects in my refrigerator they would act as "cold sinks" (the opposite of "heat sinks" in a BBQ). They would help stabilize the cold temp inside the fridge and this would mean the refrigerator wouldn't have to work so hard to maintain a certain temp inside. So the more objects I have inside my refrigerator (while still allowing proper air circulation), the easier it would be to maintain the constant cold temp.
edited to add: If I have a near empty refrigerator, I would put large bottles/jugs of water inside (to act as "cold sinks") thinking this would improve the efficiency of keeping the inside cold.

Edited by Toliver, 09 September 2010 - 09:35 AM.

[Edward J]

This is the point I was making in the above post, and fwiw, the very same advice I was given and have given.

Like you, I'm waiting for an answer.........

[dougal]

And it is actually easier to cool air ("empty space") than solid objects, because the air has less thermal capacity. It has less heat to remove.

Can you explain more about this?

I can try.

My attempt was to deal with the fallacy that "It is much harder to refrigerate empty space than it is to refrigerate solid objects."
That statement shows an unfamiliarity with the significance of the terms Specific Heat, and Heat (or Thermal) Capacity, which I deal with below.
Note that in your quote, I was explaining about cooling - changing the temperature - rather than either the steady-state condition or the transient effects of opening the door (which is what you refer to).
We can best deal with each of those three situations individually, rather than muddling them together.


The first thing to be clear about is the difference between 'temperature' and 'heat'. They are different in the sort of way that speed and momentum (or perhaps more accurately, kinetic energy) are different.
'Heat' refers to a quantity of energy.
Such as the quantity of (heat) energy you would transfer to a pan of water by turning on a particular electric heater element for exactly one minute.
That is a set amount of heat - regardless of what is in the pan.
But a different power heater or different duration, or both, changes the quantity of heat.
Now lets apply an identical amount of heat (energy) to both a pint and to a gallon of water, when we will see that the temperature of the pint will change by eight times more than the gallon. Same heat energy input, different mass, thus different temperature change.
Turning it round, to change the temperature of both by the same amount, the gallon requires 8x the heat energy that the pint needs.
And that's the same if the 'heat' in question is negative. (Like with a fridge) It needs 8x more energy ("more heat") to be taken out to chill a gallon than to chill a pint (by the same temperature change).

OK, now what about different materials?
The same volume of different materials require different amounts of heat energy to change their temperature by the same amount. Physics fact - not up for discussion.
This is the property of the material called its "Specific Heat" (sometimes Heat/Thermal Capacity.)
The volumetric Specific Heat of air is only about 1/3000 th of that of water. So, compared to a pint of water, it only takes 1/3000th of the Heat (Energy) to make the same change in the temperature of a pint of air.

So, the amount of energy transfer required to cool (to drop the temperature of) the air in a fridge is TINY, compared to the energy transfer required to cool anything "solid" - like meat, which is actually about 70% water - or to change the temperature of the actual structure of the fridge itself.
Which is what I was saying in the section quoted above.
Its not "harder" -- its about 3000 times easier!



The energy transfer of a fridge is achieved with a heat pump (usually a compressor system), which (as a first approximation) uses energy roughly in proportion to the amount of energy it as asked to shift. The more heat energy it has to shift, the longer it has to run and the more it costs.
The more heat energy getting into the fridge, the more the electricity bill. Pretty much in direct proportion.



Now lets consider the steady state.
The fridge and contents are fully down to temperature, the door is kept closed.
In this situation, it will make no real difference whether the fridge is empty or well filled.
The only energy that the heat pump needs to shift is the heat energy that leaks in through the insulation (including the door seals).
In the steady state, it makes no difference what is in the fridge - the heat energy leakage does not vary with the content.



OK, now the door opening.
For a first approximation, when you have the door open for a set time, you would let in (roughly) the same amount of heat energy (or 'let cold out' - same thing), whether the fridge is full or empty. (Though if its full, it may take you longer to find what you want, so the door will be open longer, allowing MORE heat in!)
Now, yes indeed, with a smaller thermal mass inside the fridge, that amount of heat WILL produce a greater temperature change. Likely even enough of a change to trip the (temperature sensitive) thermostat and start the compressor promptly.
However, because it was the same amount of heat getting in (regardless of the contents), it is the same amount of heat energy that has to be removed by the compressor - and THAT means that the compressor will run for the same number of total extra minutes because of the door opening. It just may not happen instantly (because we have only a small temperature change when 'full', and it is temperature that the thermostat sees, but that would be a small temperature change applied to a large mass), however that same amount of heat energy that got in, still HAS to be removed eventually, and that means the compressor doing it.
If the same amount of heat gets in, then the compressor has the same amount of heat energy to remove. Regardless of whether that energy has gone into a large or a small thermal mass, and consequently regardless of whether the fridge air temperature swing (after the door has been closed for a minute or so) is large or small -- its the same amount of energy - and it will take the same amount of work by the compressor to remove it!

However, it IS indeed an approximation to say that the 'cold loss' (heat entry) on opening the door is independent of the content. Its quite a good approximation though, because air doesn't carry much heat (or cold) - see the heat capacity discussion above, AND modern high-efficiency freezers have drawer fronts (walk-ins have plastic strip curtains), to minimise the air-change on door opening.

Nevertheless, as Andiesenji said above, it is always very good practice to minimise the time that the door is open ... (and thus the heat entering, or cold escaping, the freezer.)


Its also worth noting that the thermostat controlling the compressor, is triggered by air temperature.
Because the air cools faster (because it cools more easily) than the food, the thermostat will usually switch off the compressor LONG before the food is properly chilled down to temperature. The food then slightly warms the air, which turns on the thermostat again. It normally takes MANY MANY such cycles to chill down fresh food. And its the total extra running-time (beyond normal "steady state" running) that represents the extra cost of chilling extra food. Its very hard to subjectively add-up that extra time.


Chris Hennes explained very well that its nothing to with the soon-ness with which the compressor might turn on, the energy cost of opening the door or adding extra (relatively warm) food is seen in the total extra compressor running time - which might be made up as extra seconds on each compressor cycle over the following hours.


Despite 'real world' concerns (like the compressor having a minimum 'off time' between runs, and being less efficient in very short bursts -- both of which should be addressed by the controller design) the total running costs of a fridge or freezer doesn't really depend on how much material is stored (long term) in it.
The running cost does depend on the heat pump efficiency (which will be affected by how much insulation (ice, dust) you have blanketing the coils (inside and maybe outside)), the inside/outside temperature difference, the quality of the fridge insulation, the size of the unit (with the same insulation, a bigger wall area allows more heat to leak in), the state of the seals, how long you have the door open, and how much stuff you are asking the fridge to bring down in temperature (how much new stuff goes in).
But it depends barely at all on the amount of stuff kept in there.
Its also important to remember that it is only air circulation inside the fridge that allows the temperature sensor of the thermostat to promptly experience a temperature 'typical' of the whole space.
If the fridge is so full that air circulation is poor, that is going to 'fool' the thermostat - with the result that the compressor could either run too long (expensive) or too little (bad for the food).
Which means that its a bad idea to block air circulation by filling up your freezer with styrofoam - as some 'internet wisdom' would have us do.



In each of the three different conditions involved, there is no real energy-saving advantage in keeping the freezer full.
Which means that its only when expressed as cost per pound of stuff stored in a specific fridge or freezer, that its less costly if its kept well filled.
And it makes no sense to add pounds of junk, because that doesn't change the cost per pound of food refrigerated.
Much more sensible would be to get a smaller freezer that is sized appropriately to your actual needs.
And with similar insulation quality, etc, that WILL cost less to run.

[ChickenStu]

I've found the best way to keep my fridge and freezer cold is to close the doors whenever my roommate leaves them open.

[Edward J]

Thank you for your reply Dougal.

Throughout my career I've been told by countless HVAC and refrigeration guys that a full fridge operates the best. Perhaps what they meant is that due to poor insulation and lousy seals and most of all frequent opening, it might be best if the fridge were full. In any commercial setting, it is a "given" that the fridge will be be open alot, and most fridges come with the defrost timer preset to defrost at 16:00, midnight, and 08:00.

I do know that, with the exception of home-style chest freezers and gravity coil type "sushi" fridges, all refrigeration has a cabinet fan that circulates air. In my work abroad in Europe, S.E Asia, and here in N.America I have noticed that, without fail, the thermostat probe is located near or directly into the airstream of the cabinet fan. It is my understanding that there is always airflow in the cabinet and that shelving and shelving standards are designed to enourage airflow.

[Fat Guy]

Dougal, thanks for the amazing and comprehensive explanation. While I was reading I thought of a couple of minor points: Not everything that goes into a freezer is at the same starting temperature. When I buy a 5-pound bag of frozen strawberries at the supermarket, somebody else has already paid to freeze it. It may come up a few degrees on the way home but the input required to get it to my freezer's temperature is a lot less than with an equivalent quantity of room-temperature strawberries. At the other end of the spectrum, freezing something that's hot takes a lot of energy and something hot can also warm the things around it. So putting hot stock in the freezer is probably not a very efficient thing to do. Also, while the physics of the energy transfer may not be in dispute, I definitely find in real-world experience that full freezers (and refrigerators) hold temperature better. This echoes the experience I have with large volumes of pasta water. Even though the same amount of energy needs to be replaced once you add the pasta, and even though it takes more energy to re-boil a large volume of water, you still get better cooking performance and quicker rebound with large volumes of water because the initial temperature drop is so much less.

[paulraphael]

The empty vs. full fridge theory is simple; it's not about how hard it is to cool air vs. food.

It's about real world use and what happens when you open the door. Cooled air is denser than the air in the room. When you open the door to the fridge, most of the air inside literally falls out. Imagine a fridge filled with some other dense, fluid substance, like water. Every time you open the door, most of the cooled air leaves ... it spills out and flows away along the floor. The fridge then needs to work to cool the warm air that's replaced it.

Unless you have packing issues, food and drink containers do not spill onto the floor. Having a full fridge simply means a smaller volume of air that needs to be cooled every time you open the door. The bigger the fridge, the bigger the issue. And this is not an issue at all for things like top-loading chest freezers. Walk-in fridges and freezers are more efficient by design in this regard. Still, they often have plastic curtains to help slow air loss when the door is open.

[dougal]

The empty vs. full fridge theory is simple; it's not about how hard it is to cool air vs. food.

It's about real world use and what happens when you open the door. Cooled air is denser than the air in the room. When you open the door to the fridge, most of the air inside literally falls out. ...

... Having a full fridge simply means a smaller volume of air that needs to be cooled every time you open the door. The bigger the fridge, the bigger the issue. And this is not an issue at all for things like top-loading chest freezers.

But Paul, I've already pointed out that that air change represents comparatively little heat energy.



My domestic freezer is about 100 litres capacity.

If I lost 100 litres of cold air (a whole freezerful) and replaced it all with room temp air, (and modern freezers have drawer fronts to minimise the air change anyway) the heat content of the warm air going in would be the same as putting in 100/3000 litres of room temperature water. See the discussion of specific heats and the data reference above.

Now 1/30 of a litre (or 33 ml) is just over 2 tablespoonfuls of water.

So why did you open the fridge door? How significant is that compared to to giving it an extra couple of tablespoonfuls of water to chill?

It seems I must repeat - the heat (or cold) content of air is TINY compared to water. (And food is largely water.) Yes, I haven't even included the Latent Heat of Freezing (it'll compensate for the Specific Heat of ice being less than that of liquid water.) Basically - air is something like 3000 times easier (it takes 3000x less energy) to heat or cool than water.


If you leave the freezer door open long enough to lose cold from (or rather gain heat into) your precious food (rather than just the air), because you have MUCH more energy 'invested' in the cooling of your precious food than is invested in the mere air in the freezer -- you actually have more to lose when the freezer is full and you leave the door open longer than needed.

And, as was pointed out previously, the fuller the freezer the longer it is going to take to locate and extract whatever you are wanting, so the longer the door will be open and therefore the greater the energy wastage!

I'm afraid its just an 'internet myth' that filling your freezer will reduce your energy consumption.



>> Paul - since you state that your point does not apply to Chest Freezers then how would this make any sense at all?

To help your fridge or freezer run more efficiently:
...
• Keep your fridge and freezer at least three quarters full. If you have a large chest freezer and cannot fill it with food, place cardboard boxes or rolled up newspaper to help fill it up.

http://www.thegreenp.../energy-saving/ Just the first example I happened to find of the myth.

Surely you must agree that is nonsense?
Well-intentioned nonsense certainly - but its still nonsense!

[paulraphael]

Dougal, I'd like to see a real world test. I suspect you'd be surprised.

I know you're correct about the relative heat capacities of air and water, but the difference is that when you open the door, you lose air. Most of it. Perhaps drawers are effective in stemming some of convective loss; I don't know because I throw those things out. While it takes less energy to cool a liter of air than a liter of ... anything else, that liter of air is getting replaced constantly with warm air in a fridge that's getting used a lot.

This difference only applies when you're dealing with a fridge that's being opened repeatedly. Overnight, or in a storage freezer, it makes no difference.

Also, if the stuff filling the fridge is getting cycled in and out (like, you're putting room temp 2 liter bottles of soda in it, cooling them, and then removing them) then of course you'll use tons of energy.

I'm not saying it's a big difference. It's dwarfed by simply having a newer, efficient fridge (or by not setting it colder than necessary. But I'm betting it's more than measureable.

[dougal]

Paul, I ask again, what do you believe to be the basis of any benefit to keeping a CHEST freezer "at least three quarters full" - and specifically HOW do you believe that filling it up with cardboard boxes and/or newspapers could reduce the chest freezer's energy consumption?

Would you agree that that much is nonsense?



I'd like to get that dealt with before progressing to anything more complex and confusing.

[paulraphael]

[quote name='dougal' date='12 September 2010 - 04:50 PM' timestamp='1284324643' post='1760270']
Paul, I ask again, what do you believe to be the basis of any benefit to keeping a CHEST freezer "at least three quarters full"

I said, "And this is not an issue at all for things like top-loading chest freezers."

We agree on this!

[Edward J]

Y'know, reading through this thread, I think a seat-of-the-pants test shouldn't be that hard to do.

What we need is two identical refirgerators, preferably both new, and two power meters--these are becoming more and more common and cheaper now, I can get a "Black and Decker" device at my supermarket.

Plug both friges into the meters, run one empty and one full of, say 2 lt pop bottles or milk jugs filled with water. Both fridges get opened twice a day, with the door wide open for 20 seconds. Test should go on for 7 consecutive days.

I'd be very interested to know what kind of results would be given.

[pastameshugana]

Y'know, reading through this thread, I think a seat-of-the-pants test shouldn't be that hard to do.

What we need is two identical refirgerators, preferably both new, and two power meters--these are becoming more and more common and cheaper now, I can get a "Black and Decker" device at my supermarket.

Plug both friges into the meters, run one empty and one full of, say 2 lt pop bottles or milk jugs filled with water. Both fridges get opened twice a day, with the door wide open for 20 seconds. Test should go on for 7 consecutive days.

I'd be very interested to know what kind of results would be given.

Great idea - surely someone here works for/has family at/etc - an appliance store that would be willing to run a test.

Anyone?

[naguere]

Years ago in England out fridge freezer had two compressors . replacements now only have one for the whole thing.

A friend said it was more efficient to buy a separate fridge and freezer to get round this.

It seems sensible to me.

[dougal]

...
Plug both friges into the meters, run one empty and one full of, say 2 lt pop bottles or milk jugs filled with water. Both fridges get opened twice a day, with the door wide open for 20 seconds. Test should go on for 7 consecutive days.
...

The test would of course have to measure the extra energy consumption while chilling down that load ...


There are two different things that have been confused at this point.
1 - Filling up the airspace with low thermal mass material - for example styrofoam - in order to reduce the airchange on door opening. The concept being that the air is 'hard to refrigerate'.
2 - Filling up the freezer with high thermal mass material - for example water - in order to minimise the temperature change produced by whatever amount of heat energy gets in when the door is opened. The rationale being that as well as reducing the airchange, the high thermal mass makes the compressor run longer, but less frequent cycles - which ought to be more efficient.

Now, while I could believe that with a high-powered commercial system, designed to chill down large quantities of material quickly, it might well run short and inefficient compressor cycles if it was given much less than its designed cooling load, I really don't see this as being of any significance with typically low powered domestic equipment (with a freezing capacity of only 13 lb/24 hours (6kg/day) not being atypical. http://www.comet.co....cification#spec ) ADDED - we can also see that the experiment of filling it with 60 litres of water would therefore give it about 10 days of flat-out work to freeze it all ... which is a pretty considerable energy cost to recover from supposed extra efficiency.

Note that short-cycling would be caused by the air in the freezer being chilled too quickly, too easily, by the cooling power of the compressor. The thermostat shuts off (too quickly for 'compressor efficiency') because it sees that the air is down to temperature. Its not because its too hard to do - its too easy! Filling the space with styrofoam (as reportedly advocated by "refrigeration guys" in post number 2) is NOT going to address the problem of a compressor that is too powerful for the job it has been given to do!
ADDED -- I'm not convinced that giving a powerful compressor extra work to do (like chilling bottles of water) is going to reduce its total energy consumption. It may be working "more efficiently", but its doing LOTS of extra work! (that is unless you actually want all that water chilled ...)



One of the aspects that has not been touched on is the humidity of the air change. This could be more important than the air's thermal capacity.
This humidity gives rise to frost on the cooling tubes, in turn either reducing efficiency or needing frost-removal.
I'm not saying that door-opening has NO energy cost, I'm saying that I don't think that the energy cost of door-opening for a domestic freezer (particularly a chest freezer) is going to be very different depending on how much food is stored in there. As long as door opening is infrequent, then its a matter of a small change to a small proportion of the total energy consumption.
There are many factors that are more important. IMHO the principal factors are right-sizing and buying a high-efficiency unit. Get those wrong and it doesn't matter how you use it, you are going to be on a loser.
Even well-run (defrosted regularly, etc) an inefficient appliance (as many old ones typically are) would be expected to use about double the electricity of a new one. The payback time for a replacement might only be about five years. And remembering that domestic energy is paid for out of after-tax income, that makes buying a new, efficient (and right-size) appliance a pretty good financial investment for the future, if your present one is not very efficient.



In general, the experts at getting any particular fridge or freezer to run at maximum efficiency are the manufacturers.
In the UK, the product labelling has to include the annual energy usage under standard test conditions.
But just how standard? One dodge has been to 'recommend' in the instructions that the unit be placed well away from the wall - even though its stops allow it to be positioned quite close to the wall!
The greater airspace permits the heat to be better dumped from the radiator.
So one valid energy saving tip seems to be "pull your fridge or freezer further away from the wall".
The UK test spec is being changed to 'back-stops against the wall' rather than using 'recommended clearances mentioned in the small print".
The test conditions discussion paper makes quite interesting reading.

Edited by dougal, 15 September 2010 - 04:46 AM.

[Alex]

Dougal mentioned the door seals a couple of times. Let me second (or third) that recommendation. Here's a quick guide to checking the seals.

And yes, definitely clean those horizontal coils. We have a special bristle brush just for that purpose, and vacuum out the loose dust using a radiator attachment.

We have a 12-year-old, 18 cu. ft., top-freezer Whirlpool that we keep at exactly the temperatures Fat Guy mentioned in his lead post. It's never needed a repair. (Can I get a group eGullet wood-knocking and finger-crossing now, please?)

[Edward J]

The test would of course have to measure the extra energy consumption while chilling down that load ...


Well, yes, of course.

There are two different things that have been confused at this point.
1 - Filling up the airspace with low thermal mass material - for example styrofoam - in order to reduce the airchange on door opening. The concept being that the air is 'hard to refrigerate'.
2 - Filling up the freezer with high thermal mass material - for example water - in order to minimise the temperature change produced by whatever amount of heat energy gets in when the door is opened. The rationale being that as well as reducing the airchange, the high thermal mass makes the compressor run longer, but less frequent cycles - which ought to be more efficient.




No, a refrigerator


Now, while I could believe that with a high-powered commercial system, designed to chill down large quantities of material quickly, it might well run short and inefficient compressor cycles if it was given much less than its designed cooling load, I really don't see this as being of any significance with typically low powered domestic equipment (with a freezing capacity of only 13 lb/24 hours (6kg/day) not being atypical.

This sounds like a "Shock" or a "Blast freezer". Very different (and expensive) animal with quite a different air circualtion system, and technology to deal with humidity issues. Besides, we're talking about refrigerators here, not freezers

One of the aspects that has not been touched on is the humidity of the air change. This could be more important than the air's thermal capacity.
This humidity gives rise to frost on the cooling tubes, in turn either reducing efficiency or needing frost-removal.

Yes, I DID mention this in my second or third post


I'm not saying that door-opening has NO energy cost, I'm saying that I don't think that the energy cost of door-opening for a domestic freezer (particularly a chest freezer) is going to be very different depending on how much food is stored in there. As long as door opening is infrequent, then its a matter of a small change to a small proportion of the total energy consumption.

No, we're talking about refrigerators, NOT freezers

[paulraphael]

One of the easiest ways to use your refrigerator and/or freezer efficiently is to minimize the amount of time you have the door open.

With most fridge designs (vertical boxes with doors on the front), this actually makes less difference than most people assume. You loose most of the cold air in the first couple of seconds the fridge is open; it basically dumps out onto the floor. Additional seconds lead to comparatively little heat loss. Opening the door less frequently makes a bigger difference than shortening the time it's open.

[paulraphael]

For some reason, no wants to think that the heat a refrigerator produces is NOT waste, but rather a by product that can and should be utilized....

I saw a prototype / concept fridge designed by the energy researchers at the Rocky Mountain Institute that took advantage of this. The compressor was mounted in a separate box from the fridge freezer, in an outside wall. In the summer, the heat blew outside. In the winter it blew inside. And it was kept far away from the container that you're trying to keep cold. Brilliant.