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Comment by thaumasiotes | Hacker News Reader

> half the size

This made me think. I would kind of expect the energy consumption of a fridge to be related to the surface area of the cooled volume rather than the volume itself. It's not necessary to spend any energy cooling air that is already surrounded by equally cold air.

Assuming all fridges are cubes, doubling the volume of the fridge will increase the surface area by a factor of 2^{2/3} = 59%. This will also be true if the fridge has any rectangular dimensions and you double the volume by scaling it up evenly in all three dimensions.

Things are more complicated if you scale the dimensions of the fridge unevenly.

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> Assuming all fridges are cubes, doubling the volume of the fridge will increase the surface area by a factor of 2^{2/3} = 59%.

You don’t have to assume any shape for this, as long as the shape remains the same, and that it is not fractal. You could have a spherical fridge and it would still be true.

Mathematics: you don’t have to assume a shape Mathematician: you could have a spherical fridge

That would look really cool, though!

> You don’t have to assume any shape for this, as long as the shape remains the same, and that it is not fractal. You could have a spherical fridge and it would still be true.

Why assume that the old fridge will be geometrically proportional to the new fridge? I think that's particularly unlikely to be true in the case of a person replacing their existing refrigerator.

> I would kind of expect the energy consumption of a fridge to be related to the surface area of the cooled volume rather than the volume itself.

I'd expect that for a fridge that's permanently closed, but what if you open it regularly? I'd expect a lot of the cold air inside (so volume) to mix with the room-temperature air once you open it..?

> Things are more complicated if you scale the dimensions of the fridge unevenly.

They are. Unless you reach the scale of the walk-in fridge, you don’t want a fridge to be deeper than an arm’s length, and even small fridges have that depth because they’re designed to fit a kitchen top, which also is scaled for an arm’s length.

Vertical scaling is a bit more flexible, but will stop at about 2 meters (until you reach sizes that you can drive a forklift in)

On the plus side, if you scale things evenly, wall thickness also goes up, so heat losses per m² of area will go down.

The energy consumption of a fridge depends on what it takes to cool the entire contents within a fridge, because heat from center of the food will travel to the surface when the surface cools down. Till the temperature of the entire food item matches the set temperature of the fridge, the fridge will be running. When you open the fridge, warm air enters the fridge, and the contents of the fridge gain heat.

Then there's variability depending on thermal conductivity and specific heat capacity of various foods. So what you're trying to calculate is heavily dependent on the actual contents, not just surface area.

Of course, fridges have cycling behavior similar to an AC that turns on as soon as temp dips below a set temp, but has a window of sorts. Making the above math even more complicated.

Biggest factor for many people is that the fridge is wedged in an alcove with insufficient gaps for air flow, so it drowns in it's own waste heat and efficiency plummets.

If you're putting the fridge in an alcove you need a built in friendly model which vents to the front.

I always laugh when hotels have a tiny ice-machine room that’s sweltering. Swing and a miss by the designers.

> The energy consumption of a fridge depends on what it takes to cool the entire contents within a fridge, because heat from center of the food will travel to the surface when the surface cools down.

This assumes that the dominant energy expenditure of the fridge is bringing new food down to fridge temperature.

But almost all of the contents of a fridge have been in there for a while and are already at fridge temperature. Most of what the fridge is doing is maintaining the internal temperature against the outside environmental temperature.

That's a constant value, not a variable.

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