Economy 7 Trick

Cardew

podcake wrote: »

it is generally the case that it is better to keep the difference in temperature 'topped up' because you are just using a small extra bit of energy every so often.

Hi,
The first part of your post is correct, but IMO you contradict that statement with the quote above - and incidentally defy the laws of thermodynamics.

By keeping the temperature low in the freezer(by 'topping up') you are keeping the differential between the internal and external at a maximum and thus(by your own admission) the rate of heat transfer will be greater.

If you let the temperature in the freezer rise(by having it switched off longer) then the rate of heat transfer will be lower.

This is exactly the same false argument that holds it is cheaper to keep the CH in a house on permanantly, rather than timed.

We can use the old chessnut of asking if you were to turn off the power to the freezer and go away for say 2 years*, and switch it back on when you returned, you would no doubt conceed that you would use less power that keeping it 'topped up' for that 2 years.

So how about 1 year? 1 month? 1 week? 1 day? 1 hour? at which point would the law of thermodynamics not apply?

* If someone tells me the food in the freezer will be ruined in 2 years - I will scream;)

podcake

sorry, I didn't make myself clear in what I was trying to say, I'm somewhat stressed with work atm, and this is a welcome break! . My point is this:

the rate of change of temperature is proportional to the difference between the temperature of the air in the freezer and the ambient. That said, whilst the freezer might be capable of keeping the food cool enough for 11 hours for only (as a guess) 2 hours of that will the food be at a suitably low temperature for safe LONG TERM storage.

There is a valid point to what you say RE: leaving the freezer for 2 years. by that point the freezer will have evened out its temperature and have become ambient, any point from after the point at which that happens, your theory holds true, you need only the same amount of energy to re-freeze a freezer after one year, or after 10. So if you ( as you have done) divide the energy usage by the time you come up with a number of energy per time, which gives the apparent result of the use of less energy.

Yes, I agree with what you say, the longer you leave the freezer the less ENERGY is required to re-attain the desired temperature per time unit. But think as stated above, if it takes 1kwh to re-freeze from ambient, it takes that regardless of the time spent AT ambient.
as well, the freezer can only cool at a give rate, accepting this to be, maybe 1 degree per hour if your food goes 5 degrees over what is deemed a safe temperature, your food is held above that for FIVE HOURS..

The small amount of energy used regularly keeps the freezer from dropping above the set temperature this is where the CH analogy doesn't hold true, because you don't need your room to be at 20 degrees, you just like it there, a freezer is temperature critical.
This is specifically why freezers have a thermostat, and don't keep on 24/7.


to clarify:

you thought i meant better was better in terms of energy usage, what I mean is in terms of food hygiene and wear and tear on the freezer

Cardew

podcake wrote: »

There is a valid point to what you say RE: leaving the freezer for 2 years. by that point the freezer will have evened out its temperature and have become ambient, any point from after the point at which that happens, your theory holds true, you need only the same amount of energy to re-freeze a freezer after one year, or after 10.

I am sorry if I appear pedantic, but it is not 'my theory' but the laws of thermodynamics.

The whole point I was making is that this statement is incorrect:

"it is generally the case that it is better to keep the difference in temperature 'topped up' because you are just using a small extra bit of energy every so often."

My 'theory'(as you term it) doesn't just apply when the temperature of the freezer contents and ambient temperature reach parity as you suggest.

However warm you allow the freezer contents to go, it takes less energy to bring it back to the desired set temperature than it does to maintain the contents at the desired set temperature.

So if your desired set temperature is -20C and you allow it to rise to, say, -15C it will take less energy to bring it back to -20C than it would have taken to have maintained it at a constant -20C

podcake

Again, another point to clarify, it takes the same energy to change the temperature of the air in the freezer a given amount regardless of where we are in the temperature spectrum. However, the extra energy required to keep something at extremes is due to the losses. (the specific heat capacity of the fluid in the partially sealed container - in this case air) is a constant and:

Q = MC ΔT

where ΔT is the change in temperature, Q is the ENERGY, M is mass and C is the specific heat capacity of the fluid (air)

podcake

I think you are looking at this very simplistically:

Cardew wrote: »

I am sorry if I appear pedantic, but it is not 'my theory' but the laws of thermodynamics.

The whole point I was making is that this statement is incorrect:

"it is generally the case that it is better to keep the difference in temperature 'topped up' because you are just using a small extra bit of energy every so often."

My 'theory'(as you term it) doesn't just apply when the temperature of the freezer contents and ambient temperature reach parity as you suggest.

However warm you allow the freezer contents to go, it takes less energy to bring it back to the desired set temperature than it does to maintain the contents at the desired set temperature.

So if your desired set temperature is -20C and you allow it to rise to, say, -15C it will take less energy to bring it back to -20C than it would have taken to have maintained it at a constant -20C

An interesting point of view.

Firstly, if we ignore thermal losses for now:

It takes 0 Joules to 'maintain' something at a given level assuming that there are, as stated above, no losses

It would take a number of Joules of energy to drop something from -15 to -20 degrees

those two statements are true, the mass hasn't changed, the temperature hasn't changed and the specific heat capacity hasn't changed.. therefore the energy is 0 if the losses are 0

now, the losses come into play,

The freezer will have some losses (hence why you will lose more at the more extreme differences) but these will, in all likely-hood be minimal.

now, without specifics (and i'm not going to go in depth into Newton's laws of cooling, freezer SHCs etc..) we cannot say what the difference is. There will be a point in time where both of us are correct.

BUT you simply cannot categorically state that

So if your desired set temperature is -20C and you allow it to rise to, say, -15C it will take less energy to bring it back to -20C than it would have taken to have maintained it at a constant -20C

Because you simply do not know that..
not to mention that if people open the door, the losses will be horrendous..

The key point is that a freezer's main function is to keep stuff cold not to save money.. The ultimate extent of that could be that, we would all be better off not having a freezer, as it would take less energy to make our stuff cold again if we ever did get one!

(as zeupater points out)

A four star freezer maintains a temperature of -18C which will store most food for 12 months, but if, as a result of your energy saving measures, the temperature regularly rises to -12C your food will degrade by the same amount in around one month, and at -6C in around a week

So it is MORE important to keep the thing cold than it is to save on the energy powering it, because the functionality for a week is not particularly helpful..

Cardew

podcake wrote: »

Firstly, if we ignore thermal losses for now:

.

You know full well that this discussion is not about the condition of the food!!! It was raised to save on electricity costs.

If there were no thermal losses???

If there were none you could go away for 20 years and the freezer would still remain at the same temperature.

The whole point of this discussion is about thermal losses and the energy required to replace those losses.!!

I can absolutely and categorically say that this statement is true.

So if your desired set temperature is -20C and you allow it to rise to, say, -15C it will take less energy to bring it back to -20C than it would have taken to have maintained it at a constant -20C

I can absolutely and categorically say that this statement is not true.

it is generally the case that it is better to keep the difference in temperature 'topped up' because you are just using a small extra bit of energy every so often.

zeupater

Hi

From an energy only point, Cardew is correct ..... the energy (heat) gain within the freezer will be dependent on the average temperature differential over a period of time, therefore a system maintained at an average of -18C will gain more energy than one with an average temperature of say -12C ... thermodynamics does apply. It is only the energy (heat) gained over a period which needs to be pumped out of the freezer, which is achieved by consuming energy (electricity) .....

There is a valid point that there may not be enough hours of cooling to pull the temperature of the contents down to the same starting point on a daily basis without some form of creep.

Putting the energy debate aside and looking at this as a moneysaving idea, which is why it was raised by the OP, this is something which could cost far more in wasted food than it would ever save in monetary terms, it is also possibly a health-risk ......

The only way that I would consider doing this would be if I had a freezer which could be set to something like -30C (without using the manual superfreeze button) in order to ensure that the maximum temperature would remain below -18C, however, this would consume much more energy, therefore any moneysaving would depend entirely on the cost of the energy source - solar pv would therefore probably be preferential over E7 to achieve this due to there being, on average, more daylight generating hours than E7 hours, thus allowing a longer temperature recovery period.

Just my thoughts
Z

Cardew

zeupater wrote: »

Hi

From an energy only point, Cardew is correct ..... the energy (heat) gain within the freezer will be dependent on the average temperature differential over a period of time, therefore a system maintained at an average of -18C will gain more energy than one with an average temperature of say -12C ... thermodynamics does apply. It is only the energy (heat) gained over a period which needs to be pumped out of the freezer, which is achieved by consuming energy (electricity) .....

Z

I made it very clear that it was only the energy aspects that were being considered.

grahamc2003

podcake wrote: »

Again, another point to clarify, it takes the same energy to change the temperature of the air in the freezer a given amount regardless of where we are in the temperature spectrum. )

Sorry, but this bit isn't true aswell.

The specific heat of air varies widely enough with temperature to make it a variable, and not a constant, in calculations such as air con design. Moist air, as found in a fridge, is even more variable.

If I could figure out how to type out a partial differential curly delta, I'd write the relationship (Curly delta Cpair by curly delta T not equal zero.)

What the op originally posted had occured to me, except I though about switching my fridge/freezer off just for an hour before e7 rates kicked in. Since that would be about 00:30 and I've usually finished eating by then, the fridge door would remain shut and there'd by only a slight temp gain, not enough to damage anything. I didn't do it because the savings would be tiny. There'd be no detrimetntl effect on the compressor as far as I can see - in fact, it would have to work very slightly less.

st999

How does a freezer work?

In simple terms.

The motor, or whatever it is called, cools the freezer to -20C and the thermostat then switches the motor off.

Then because the temperature outside the freezer is warmer than the temperature inside the freezer the inside of the freezer warms up. When it reaches -15C or some temperature ordained by the thermostat, the thermostat switches the motor, or whatever it is called, on, and the temperature falls to -20C. This repeats throughout the day and night, whenever the temperature rises to -15C or some temperature ordained by the thermostat, or even higher because the door is opened the motor switches on.

So someone thinks up a brilliant new idea to control a freezer.

He puts a thermometer in the freezer and notes the temperature. If the temperature is -20C or lower he switches the motor off. He goes back 1 hour later and checks the temperature. If the temperature is -20C or lower he does nothing, but if the temperature is higher than -15C he switches on the motor and goes back 1 hour later.

He repeats this forever.

Then he thinks, this is crazy, when will I ever get any sleep or work done.

So he comes up with a brilliant idea. Why don't I put a thing in the freezer that keeps the temperature no lower than -20C and no higher than -15C then I can get some sleep.

So he puts in a thermostat that automatically keeps the temperature between 2 temperatures. The temperature is not kept at a constant temperature, it varies between 2 temperatures. It is called hysteresis.

That is why freezers work the way that they do, so you can sleep.

Also that is why they do not have a time switch so that the time they can be on for can be altered.

They are designed to be switched on continuously and the thermostat controls the on off period