# Latest in Glastory: Glass tempering energy consumption: how not to be misled by false data by Riku Färm

In this second part of our blog on energy consumption in the glass tempering process, let me go deeper into details to explain some of the principles behind heating and cooling.

Knowing the derivation of different metrics will help you better understand the tempering process, as well as the costs linked to its energy requirements. Once learned, it’s handy to know, so that no one will ever be able to trick you with numbers by providing irrelevant or over-optimistic performance data.

### Laws of physics for energy consumption

Heating energy

The required energy to heat glass can be calculated with the formula:

E = ΔT * c * m, where

E = Energy required to heat the glass
ΔT = Temperature change
c = Specific heat capacity of the glass
m = Mass of the glass

Let’s take a 1 m2 sheet of 4 mm thick glass as an example. In this case, the values would be:

ΔT = 610 °C (from 20 °C to 630 °C)
c =  1.1 kJ / kg * °C
m = 1 m² * 2500 kg/m³ * 0,004 m = 10 kg

*Note that the specific heat capacity of glass changes as a function of temperature!  The value at room temperature is about 0.78 kJ / kG * °C, whereas the mean specific heat capacity between +20 °C and +630 °C is about 1.1 kJ / kG * °C

By placing these values into the formula above, we get:

E = 610 °C x 1.1  kJ / kg * °C * 10 kg = 6710 kJ = 1.9 kWh = 0.475 kWh/m²*mm

According to the calculated results, we see that it is impossible to heat a 4 mm glass sheet from +20 °C to +630 °C using less energy than 1.9 kWh. In more universal terms, this means that the heating requires at least 0.475 kWh/m²*mm. By multiplying this by the glass thickness, we get the minimum energy required to heat this piece of glass.

Keep in mind this is not all. To calculate the total energy consumption, you also need to add energy loss, convection blowers and the glass quenching process on top of the calculations above. Read more