ENERGEX - Switched ON

Calculating heat energy

Calculations

Before we can produce electricity, the energy in coal needs to be converted to heat energy.

So how much energy do we get from the coal?

We know that the temperature of the steam before it enters the turbine reaches 540 ^o C.

For the purpose of this exercise let us suppose we want to raise the temperature of 1 kg of water from room temperature 25 ^o C to 540 ^o C.

To calculate the heat required to raise the temperature of the water to 540 ^o C we use the formula:

Heat energy = heat required to raise the temperature to 100 ^o C + heat required to change state + heat required to raise the temperature to 540 ^o C.

i.e.

Q =	heat energy to raise the temperature of water to 540 ^o C
m _w =	mass of water
c _w =	specific heat of water (heat required to raise the temperature of a 1kg of water 1 ^o C)
m _s =	mass of steam
c _s =	specific heat of steam (heat required to raise the temperature of a 1kg of steam 1 ^o C)
H _f =	latent heat of vaporisation of water (heat required to change liquid water to a gas)
T =	change in temperature.

In this case:

m _w = m _s =1kg
c _w = 4.19 kJ/kg ^o C
c _s = 2.01 kJ/kg ^o C
T _w = (100 ^o C - 25 ^o C)
T _s = (540 ^o C - 100 ^o C)
H _f = 334 kJ/kg

Therefore:

Q = [1 x 4.19 x (100 - 25)] + [1 x 334] + [1 x 2.01 x (540 - 100)] kJ
= 1532.65 kJ

So to raise the temperature of 1kg water to 540 ^o C the coal must release at least 1532.65 kJ of energy. Remember that some of the stored energy in coal is either not released or is lost.