How Steam Turbines Work

Steam turbines like water turbines are used to generate electricity. Steam turbines require an outside fuel sources such as nuclear power, coal power, and burning natural gas, and they are much more common than water turbines. Currently in the U.S., steam turbines account for 88 percent of the electricity generation. Steam turbines use water that is heated to extremely high temperatures and converted into steam to rotate the blades of the turbine.


Courtesy of http://turbinegenerator.org/steam/steam-turbine-works

These steam turbines rely heavily on the second law of thermodynamics that states: energy has quality and quantity, and actual process occur in the direction of decreasing quality of energy. We cant forget about the first law of thermodynamics either that states: energy can be neither created nor destroyed during a process.

1)     A heat source is needed which is usually provided by the combustion of fossil fuels.

2)     This part of the figure is the boiler which uses the heat energy provided by the heat source to convert water into high pressure steam.

3)     An exhaust pipe is needed to exhaust the pollutants from the combustion of the fossil fuel. This is not needed if solar energy is used for heat conversion.

4)     The steam from the boiler is now pumped into the turbine. The steam energy is used to rotate the turbine blades.

5)     Just like the water turbines, the steam turbine is attached by a shaft to a generator to create electricity.

6)     The used steam now passes through and is cooled using a cooling tower.

7)   The cooling tower releases the already used, lower energy, and cooled steam into the atmosphere.  The remaining water that is cooled back into a liquid state is pumped back into the boiler and repeats the process again, creating more energy from steam. The is part of the figure is crucial for the cycle. Some people wonder why you have to waste that steam into the atmosphere or other thermal reservoir. Steam power plant cannot run unless the cycle is completed and heat rejection is part of this cycle.This reason can be explained simply by the second law of thermodynamics. This now lower quality steam cannot travel back and be used again without violating the second law of thermodynamics[1].



Diagram courtesy of http://geothermal.marin.org/geopresentation/sld038.html

Two Main Types of Steam Turbines


Impulse Turbines

Like the name states impulse turbines run by the impulse of steam.
They have fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. The velocity of the steam is then reduced once it pass over the blades[4]

      "As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss"[4]". Image above courtesy of http://powerplantstechnology.blogspot.com/2010/04/steam-turbine-use-in-power-plant.html.

Disadvantages of Impulse Turbines-

The velocity of rotor is to high for practical purposes

The velocity of the steam is very when it leaves the turbine. So there is a lot of energy wasted

These problems though can be fixed with different expansion processes.


Reaction Turbines

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor[4].