The economics of defrosting

 

I’ve seen frozen-food packaging that suggests defrosting in the fridge. Intuitively, this makes perfect sense.

The frozen item defrosting in your fridge will reduce the time the fridge’s thermostat runs the compressor, allowing you to recover the energy that the manufacturer used to freeze the item in the first place.

Essentially, the supermarket has sold you food plus a chunk of energy that your fridge can recover.

Alternatively, you could defrost the item at room temperature or in a microwave oven.

• Room temperature. Not only will you loose the freezing energy, but defrosting in the winter will make the kitchen marginally cooler resulting in the thermostat for your heating turning the boiler (furnace) on for more of the time.
• Microwave. This will be fast, but require you to explicitly inject the microwave-energy-induced heat into the food.

Getting Quantitative

Let’s do some very rough sums to work out the implications of this energy use on our bills.

The specific heat capacity of water is approximately 4kJ/kg/degree C. That means to raise 1kg of water 1 degree C in temperature requires the addition of about 4kJ (kilo Jules) of energy.

Ice has a different specific heat capacity from water. It’s more like 2kJ/kg/degree C.

To keep the sums easy, lets make some assumptions:

• our frozen goods arrive home at -20 degrees C
• our fridge is set to 5 degrees C
• room temperature is 20C
• We have a 1kg (2.2 pounds) bag of frozen veg with a specific heat capacity the same as water and ice.
• We will ignore latent heat of fusion (the energy involved in changing from solid to liquid.
• Everything works at 100% efficiency
• Domestic energy costs £0.30/kWh
• a single AA cell holds 4Wh of energy

If we defrost the 1kg bag in the fridge up to 5 degrees C, we will recover energy of:

-20C to 0C: 20C * 2 * 1kg = 40 kJ

0C to 5C: 5C * 4 * 1kg = 20 kJ

Total: 60 kJ reduction in the energy our fridge needs to use to keep the fridge at 5C.

If we were to defrost it on the side in the kitchen, the energy our heating would have to provide to defrost it to 5C would be the same, at 60 kJ as would the microwave way of defrosting.

How much is 60 kJ

This sounds very impressive, but just how much energy is 60 kJ, which can also be expressed as 16.7 Wh (Watts for an hour)?

Energy Source	Cost
4 x AA battery (4Wh)	£ 5.00	Yes, but the fridge doesn’t run on batteries
Domestic electricity	£ 0.005	What the fridge costs

So, defrosting in the fridge gains us 0.5p which we could double to a 1p saving if we were to defrost by microwaving or leaving at room temperature.

This is only really an order of magnitude calculation, but, as you can see, it’s not going to save us a lot of money – or the planet. Unless that is, your fridge runs on AA batteries.