Can you explain more about this?
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.
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
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.
"If you wish to make an apple pie from scratch ... you must first invent the universe." - Carl Sagan