Steam Turbine Electricity Generation Plants

Conventional Energy Generation

The first practical electricity generating system using a steam turbine was designed and made by Charles Parsons in 1885 and used for lighting an exhibition in Newcastle. Since then,
apart from getting bigger, turbine design has hardly changed and
Parson’s original design would not look out of place today. Despite the
introduction of many alternative technologies in the intervening 120 years, over 80 percent of the world’s
electricity is still generated by steam turbines driving rotary
generators.

The Energy Conversion Processes

Electrical energy generation using steam
turbines involves three energy conversions, extracting thermal energy
from the fuel and using it to raise steam, converting the thermal energy
of the steam into kinetic energy in the turbine and using a rotary
generator to convert the turbine’s mechanical energy into electrical
energy.

• Raising steam (Thermal Sources)

Steam is mostly raised from fossil fuel sources, three of
which are shown in the above diagram but any convenient source of heat
can be used.

• Chemical Transformation

In fossil fuelled plants steam is raised by burning fuel,
mostly coal but also oil and gas, in a combustion chamber. Recently
these fuels have been supplemented by limited amounts of renewable
biofuels and agricultural waste.

The chemical process of burning the fuel releases heat by
the chemical transformation (oxidation) of the fuel. This can never be
perfect. There will be losses due to impurities in the fuel, incomplete
combustion and heat and pressure losses in the combustion chamber and
boiler. Typically these losses would amount to about 10% of the
available energy in the fuel.

• Nuclear Power

Steam for driving the turbine can also be raised by
capturing the heat generated by controlled nuclear fission. This is
discussed more fully in the section on Nuclear Power.

• Solar Power

Similarly solar thermal energy can be used to raise steam, though this is less common.

• Geothermal Energy

Steam emissions from naturally occurring aquifers are also used to power steam turbine power plants.

 

• The Steam Turbine (Prime Mover)

 

• Working Principles

High pressure steam is fed to the turbine
and passes along the machine axis through multiple rows of alternately
fixed and moving blades. From the steam inlet port of the turbine
towards the exhaust point, the blades and the turbine cavity are
progressively larger to allow for the expansion of the steam.

The stationary blades act as nozzles in which the steam expands and emerges at an increased speed but lower pressure. (Bernoulli’s
conservation of energy principle – Kinetic energy increases as pressure
energy falls). As the steam impacts on the moving blades it imparts
some of its kinetic energy to the moving blades.

 

There are two basic steam turbine types,
impulse turbines and reaction turbines, whose blades are designed
control the speed, direction and pressure of the steam as is passes
through the turbine.

 

 

• Impulse Turbines

The steam jets are directed at the
turbine’s bucket shaped rotor blades where the pressure exerted by the
jets causes the rotor to rotate and the velocity of the steam to reduce
as it imparts its kinetic energy to the blades. The blades in turn
change change the direction of flow of the steam however its pressure
remains constant as it passes through the rotor blades since the cross
section of the chamber between the blades is constant. Impulse turbines
are therefore also known as constant pressure turbines.

The next series of fixed blades reverses the direction of the steam before it passes to the second row of moving blades.

 

• Reaction Turbines

The rotor blades of the reaction
turbine are shaped more like aerofoils, arranged such that the cross
section of the chambers formed between the fixed blades diminishes from
the inlet side towards the exhaust side of the blades. The chambers
between the rotor blades essentially form nozzles so that as the steam
progresses through the chambers its velocity increases while at the same
time its pressure decreases, just as in the nozzles formed by the fixed
blades. Thus the pressure decreases in both the fixed and moving
blades. As the steam emerges in a jet from between the rotor blades, it
creates a reactive force on the blades which in turn creates the
turning moment on the turbine rotor, just as in Hero’s steam engine.
(Newton’s Third Law – For every action there is an equal and opposite
reaction)

• The Condenser

The exhaust steam from the low pressure
turbine is condensed to water in the condenser which extracts the latent
heat of vaporization from the steam. This causes the volume of the
steam to go to zero, reducing the pressure dramatically to near vacuum
conditions thus increasing the pressure drop across the turbine enabling
the maximum amount of energy to be extracted from the steam. The
condensate is then pumped back into the boiler as feed-water to be used
again.

It goes without saying that condenser
systems need a constant, ample supply of cooling water and this is
supplied in a separate circuit from the cooling tower which cools the
condenser cooling water by direct contact with the air and evaporation
of a portion of the cooling water in an open tower.

Water vapour seen billowing from power plants is evaporating cooling water, not the working fluid.

 

Back-Pressure Turbines, often used for electricity generation in process industries, do not use condensers. Also called Atmospheric or Non- Condensing Turbines, they
do not waste the energy in the steam emerging from the turbine exhaust
however, instead it is diverted for use in applications requiring large
amounts of heat such as refineries, pulp and paper plants, desalination
plants and district heating units. These industries may also use the
available steam to power mechanical drives for pumps, fans and materials
handling. The boiler and turbine must of course be oversized for the
electrical load in order to compensate for the power diverted for other
uses.

 

• Practical Machines

Steam turbines come in many configurations. Large machines are
usually built with multiple stages to maximise the energy transfer from
the steam.

 

 

To reduce axial forces on the turbine
rotor bearings the steam may be fed into the turbine at the mid point
along the shaft so that it flows in opposite directions towards each end
of the shaft thus balancing the axial load.

The output steam is fed through a cooling tower through which cooling water is passed to condense the steam back to water.

 

 

Turbine power outputs of 1000MW or more are typical for electricity generating plants.

 

• The Steam Turbine as a Heat Engine

Steam turbine systems are essentially
heat engines for converting heat energy into mechanical energy by
alternately vaporising and condensing a working fluid in a process in a
closed system known as the Rankine cycle.
This is a reversible thermodynamic cycle in which heat is applied to a
working fluid in an evaporator, first to vaporise it, then to increase
its temperature and pressure. The high temperature vapour is then fed
through a heat engine, in this case a turbine, where it imparts its
energy to the rotor blades causing the rotor to turn due to the
expansion of the vapour as its pressure and temperature drops. The
vapour leaving the turbine is then condensed and pumped back in liquid
form as feed to the evaporator.
In this case the working fluid is water
and the vapour is steam but the principle applies to other working
fluids such as ammonia which may be used in low temperature applications
such as geothermal systems. The working fluid in a Rankine cycle thus follows a closed loop and is re-used constantly.
The efficiency of a heat engine is determined
only by the temperature difference of the working fluid between the
input and output of the engine (Carnot’s Law).

 

Carnot 

showed that the maximum efficiency available = 1 – T_c / T_h where T_h is the temperature in degrees Kelvin of the working fluid in its hottest state (after heat has been applied) and T_c is its temperature in its coldest state (after the heat has been removed).

 

To maximise efficiencies, the temperature
of the steam fed to the turbine can be as high as 900°C, while a
condenser is used at the output of the turbine to reduce the temperature
and pressure of the steam to as low a value as possible by converting
it back to water. The condenser is an essential component necessary for
maximising the efficiency of the steam engine by maximising the
temperature difference of the working fluid in the machine.

Using Carnot’s law, for a typical steam turbine system with an
input steam temperature of 543°C (816K) and a temperature of the
condensed water of 23°C (296K), the maximum theoretical efficiency can
be calculated as follows:

Carnot efficiency = (816 – 296)/816 = 64%

But this does not take
account of heat, friction and pressure losses in the system. A more
realistic value for the efficiency of the steam turbine would be about
50%

Thus the heat engine is responsible for most of the system energy conversion losses.

 

See also Gas Turbines and Heat Engines

 

• Electromechanical Energy Transfer (Generator)
  The steam turbine drives a generator, to convert the mechanical energy into electrical energy. Typically this will be a rotating field synchronous machine. These machines are described more fully in the section on Generators.
  The energy conversion efficiency of these high capacity generators can be as high as 98% or 99% for a very large machine.

Note: This means that a 1000MW generator must dissipate 20 MW of waste heat and such generators require special cooling techniques.

 

Ancillary Systems

Apart from the basic steam raising and
electricity generating plant, there are several essential automatic
control and ancillary systems which are necessary to keep the plant
operating safely at its optimum capacity. These include:

• Matching the power output to the demand. Current controls
• Maintaining the system voltage and frequency
• Keeping the plant components within their operating pressure, temperature and speed limits
• Lubrication systems
• Feeding the fuel to the combustion chamber and removing the ash
• Pumps and fans for water and air flow
• Pollution. control – Separating harmful products from the combustion exhaust emissions
• Cooling the generator
• Electricity transmission equipment. Transformers and high voltage switching
• Overload protection, emergency shut down and load shedding