Workings of a generator
Energy sources
Renewable and non-renewable
Energy density
If a magnet is
moved inside a
coil an electric
current is
induced
(produced)
A electric current is induced because the
magnetic field around the coil is changing.
A generator
works in this
way by rotating
a coil in a
magnetic field
(or rotating a
magnet in a coil)
“This is our most
successful energy
project. When we
dug the
foundation for the
wind turbine, we
struck oil.”



Finite (being depleted – will run out)
In general from a form of potential energy
released by human action
Coal, oil, gas (fossil fuels), Uranium.


Mostly directly or indirectly linked with the
sun
The exception is tidal energy
Fuel
Oil
Natural gas
Coal
Nuclear
Hydroelectric
Others
% total energy
production
40
23
23
7
7
<1
CO2 emission
g.MJ-1
70
50
90
-



The energy that can be obtained from a unit
mass of the fuel
J.kg-1
If the fuel is burnt the energy density is
simply the heat of combustion




Coal - 30 MJ.kg-1
Wood - 16 MJ.kg-1
Gasoline – 47 MJ.kg-1
Uranium – 7 x 104 GJ.kg-1 (70000000
MJ.kg-1)



Imagine 1 kg falling 100m.
Energy loss = mgh = 1x10x100 = 103 J
If all of this is turned into electrical energy
it gives an “energy density” of the “fuel” of
103 J.kg-1
Generally (except for solar cells) a turbine is
turned, which turns a generator, which
makes electricity.
In electricity production they are burned,
the heat is used to heat water to make
steam, the moving steam turns a turbine
etc.




Relatively cheap
High energy density
Variety of engines and devices use them
directly and easily
Extensive distribution network in place




Will run out
Pollute the environment (during mining
sulphur and heavy metal content can be
washed by rain into the environment)
Oil spillages etc.
Contribute to the greenhouse effect by
releasing greenhouse gases

Freeing Energy from the Grid

A coal powered power plant has a power
output of 400 MW and operates with an
overall efficiency of 35%

1.
2.
3.
A coal powered power plant has a power
output of 400 MW and operates with an
overall efficiency of 35%
Calculate the rate at which thermal energy is
provided by the coal
Calculate the rate at which coal is burned (Coal
energy density = 30 MJ.kg-1)
The thermal energy produced by the power plant
is removed by water. The temperature of the
water must not increase by more than 5 °C.
Calculate the rate of flow of water.

Calculate the rate at which thermal energy
is provided by the coal

Calculate the rate at which thermal energy
is provided by the coal
Efficiency = useful power output/power input
Power input = output/efficiency
Power input = 400/0.35 = 1.1 x 103 MW

Calculate the rate at which coal is burned
(Coal energy density = 30 MJ.kg-1)

Calculate the rate at which coal is burned
(Coal energy density = 30 MJ.kg-1)
1 kg of coal burned per second would
produce 30 MJ. The power station needs
1.1 x 103 MJ per second.
Mass burned per second = 1.1 x 103/30
= 37 kg.s-1
Mass per year = 37x60x60x24x365
= 1.2 x 109 kg.yr-1

The thermal energy produced by the power
plant is removed by water. The temperature
of the water must not increase by more than
5 °C. Calculate the rate of flow of water.
The thermal energy produced by the power
plant is removed by water. The temperature
of the water must not increase by more than
5 °C. Calculate the rate of flow of water.
Rate of heat loss = 1.1 x 103 – 0.400 x 103
= 740 MW
In one second, Q = mcΔT
740 x 106 = m x 4200 x 5
m = 35 x 103 kg
So flow needs to be 35 x 103 kg.s-1
