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
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].