There’s one last wrinkle that is often added to the design
of gas turbine power plants. Instead of delivering the heat to a factory to
be used in their manufacturing process, we can use the steam that’s recovered
to create even more electricity. This is called ‘combined cycle’.
Check out the diagram below to see how it’s done.
| Again, we start with our gas turbine: |
And again we stick a boiler on the end to
collect waste heat, but with a twist: |
1. The
fuel goes in (in this case, it’s natural gas).
2. Fresh, combustion air is added.
3. The fresh air is compressed.
4. The fuel is burned in the combustion chamber.
5. Burning the fuel creates a lot of expanding gases, which flow through
the turbine, causing it to spin.
6. The spinning turbine is attached to a generator, which also spins.
7. The generator creates electricity. |
8. The hot air from the
gas turbine is collected.
9. In some cases, duct firing is added to increase the amount of heat
available to make steam.
10. The hot air flows over the first set of tubes – the High Temperature/Pressure
Tubes. The temperature of the exhaust gases makes high temperature (~800
°F) and high pressure (~600psig) steam.
11. The second set of tubes recovers slightly less energy, creating steam
with less pressure at a lower temperature.
12. The exhaust gases are vented to the atmosphere (additional scrubbers,
electrostatic precipitators and other environmental controls can be added
here).
14. The hot steam is fed into a steam turbine, causing it to spin.
15. The spinning steam turbine turns a generator, which creates more electricity.
16. After the steam turbine has used some of the energy in the hot steam,
the cooler steam is returned to be reheated. |
Now, when we put 100 units of fuel in, we get about 50 units
of electricity for sale. Now that we’ve seen the most common ways that large
gas turbines are installed in power plants, let’s take a moment to look at
an interesting development in gas turbine technology – micro turbines. |
