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 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.
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.
Source: Government of Australia
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 - Tc / Th where Th is the temperature in degrees Kelvin of the working fluid in its hottest state (after heat has been applied) and Tc 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.
Note: This only includes the conversion of the heat energy in the steam to mechanical energy on the turbine shaft. It does not include the efficiency loss in the combustion chamber and boiler inconverting the chemical energy of the fuel to heat energy in the steam nor does it include the efficiency losses incurred in the generator if the turbine is used to generate electricity. Taking these losses into account, the overall efficiency of converting the fuel's chemical energy in coal and oil fired plants to electrical energy is typically around 33%.
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.