Energy-efficient refrigerator-freezer usage

[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.uk/shopcomet/product/465232/INDESIT-BAN12NF/tab/specification#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 September 15, 2010 by dougal (log)

[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.

[Edward J]

Yup, remote compressors. Most, if not all supermarkets have them, a lot of the larger restaurants and hotels do this, and a lot of the air conditioning units have remote compressors too. Been that way for quite some time now, but it's about time someone started to make a residential unit like this.

My Dad worked for a large Western Cnaadian supermarket chain, he was in charge of fixtures and equipment. He came up with a scheme on paper that would utilize the heat from the compressors to heat the stores in winter. However, this was in the early 80's and the cost of natural gas was ridiculously low. The cost of HVAC upgrades would only be paid for in 5 years from the savings of natural gas. The store owners never went for it.

Another method for cooling down the gasses and the compressors is to use water, and N.America has quite a few water cooled compressors. Since water is not metered in many municiplaities, it was thought of as "energy efficient" back inthe 70's and popular with bakeries and other industries where dust and flour meant much more maintainence to conventional refrigeration. In an "open loop" system this means that the heated water is dumped down the drain. The water is not potable, and many municipalities are banning the use of this system. Some of the more "Clever" hotels and institutions (prisons) use this water as pre-heated water entering water boilers exclusively for laundry.

My mind boggles with the thought of utilizing the heat from, say a dairy plant or meat packing plant's compressors to heat green houses in the winter, or water cooled systems combined with geo-thermal technology to heat homes in the general area. This will only happen when the cost of natural gas and electricty become more and more expensive.

But it's nice to see a residentail version of remote comperssors for home fridges.

[paulraphael]

But it's nice to see a residentail version of remote comperssors for home fridges.

I have no idea if that design will ever make it to market. This was years ago, and the researchers had doubts back then, because the appliance market is so conservative. Their fridge was also horizontal and mounted at eye level, above the counter ...

Their stuff worked, though. The institute is in Aspen, and they use all their own technology. They said their winter energy bill went from something like $15 a month to $30 when they got a xerox machine.

When I visited, their work in progress was a solar powered hot tub.

Edited September 16, 2010 by paulraphael (log)