Original in Spanish: Termodinámica de nevera
Letting the tap run to put colder water in the fridge vs. putting the warmer water in the fridge without waiting.
Data and assumptions:
- We’re going to have to open the fridge door either way.
- I’m not in a hurry, but I want to drink the water at fridge temperature (say, 4 degrees Celsius).
- The fridge is of the old A+++ class (what are we otherwise? animals?) and has a COP=5 (refrigerator coefficient of performance).
- We’re in standard conditions (25 Celsius), while the tap water, once it runs cold, is at 19 degrees (seems optimistic to me, but fine, right now at my place it comes out ready for brewing tea XD).
- At my place it took 4 litres to drop 2 degrees, so I’ll assume that to get it down to 19 degrees I’ll throw away 12 litres.
- Water costs 1.19 eur/m3 while electricity is 0.28374 eur/kWh (just to say something).
- The glass is 250 ml.
- The heat capacity of water is 1 kcal/degree.kg (that is, 4.186 joules/kg.degree) and its density 1 kg/litre.
o Case: put the glass in after “throwing away” 12 litres:
12 litres H2O = 0.014 eur
o Case: put the glass in with room-temperature water without waiting
0.25 litres x 4.186 J/degree.kg x 6 degrees of difference = 6.279 joules of energy needed to reach tap-water temperature
6.279 joules = 1.74E-6 kWh x 0.28374 eur/kWh = 4.95E-7 eur
So it looks like it’s better to put the warm water in without waiting, by a difference of 5 orders of magnitude in euros :)
(Facebook, 2022-07-19)