TURBINES: TYPES OF TURBINES & THEIR APPLICATIONS
What is turbine?
A turbine is a rotating part which converts kinetic energy of a working fluid into useful mechanical energy and/or electrical energy. Simple, right? Well, nothing is simple when you go deeper.
There are set of blades mounted on a rotor which helps in extracting energy from the moving fluid. The efficiency of turbines depend on the design of the blades.
Different applications need different designs & designing them isn’t a layman’s job.
The 4 types of turbines are-
1. Water turbines
2. Steam turbines
3. Gas turbines &
4. Wind turbines
1. WATER TURBINES
The turbines used in hydroelectric powerplants are water turbines which have water as their working fluid.
First of all, millions of litres of water is collected in the dam. More the height of dam, more the pressure. The highly pressurized water is then made to flow via large pipe called as penstock.
The turbine is located at the end of penstock from where the pressurized water strike the blades of turbine at high velocity making it to rotate. This turbine is connected to a generator which generates electricity.
The shape of turbine blades depend upon the pressure & velocity of water. Water turbines are classified into 2 types-
1. Impulse type
2. Reaction type
1. Impulse type turbines
Impulse turbines basically work on Newton’s 2nd law. In impulse turbines, number of elliptical half sized buckets are fitted instead of blades on the rotor hub. When water strike the buckets at high speed, the rotor starts rotating. In short, the kinetic energy of water gets converted into rotational mechanical energy!
Thus, electricity is generated when one end of turbine shaft is connected to generator!
Example – Pelton turbine
2. Reaction turbines
The turbine blades or the impeller blades are designed in such a way that a force is generated on one side when water flows through it just like an airfoil. The force produced by airfoil is responsible for lift of aeroplane. Similarly here, that force makes the blades rotate.
Example – Kaplan turbine
Different types of turbines have their own ideal operating conditions. For eg.
→ Pelton turbines are preferred where low discharge rate can be obtained & high head(80-1600m) is available.
→ Kaplan turbines require high discharge rate along with low or medium head(2-70m).
→ Francis turbine work on medium flow rate & medium head. Francis turbine is a combination of impulse & reaction turbine. Francis turbines are most widely used turbines because they offer the highest efficiency & could also work in wide range of operating conditions.
1m head of water = 9810 Pa (100m of head is almost 7 times of atmospheric pressure)
Also called as steam turbines, they are used in nuclear & thermal powerplants where water is heated to form steam & then flowed through turbines to produce electricity. Alike water turbines, steam turbines are also classified into impulse & reaction types but the arrangement & design is different. All the modern steam turbines are a combination of impulse & reaction type.
Steam turbines consist not only rotating blades called as rotor but also static blades called as stator. Rotors & stators are placed alternately in order to extract most energy out of it. This method is called as compounding.
Also, if you observe, the moving buckets in impulse turbine are designed to get pushed by the steam. While the rotor blades in reaction turbine are aerofoiled shape, which lets itself generate reaction & also let steam maintain its velocity !
The steam first flows through high pressure(H.P) turbine followed by intermediate pressure(I.P) turbine. Then again after reheating the steam, it is made to flow through low pressure(L.P) turbines(huge set of blades).
The reason behind increase in blade sizes from inner side to outer side is because steam expands while losing its pressure & kinetic energy & giving it to turbines.
3. GAS TURBINES :-
Gas turbines in other words are internal combustion engines, which are not only used in powerplants for generating electricity but also for propelling airplanes & helicopters. Gas turbines as a whole system has a axial compressor at the inlet. These are sets of rotating blades which suck huge amount of air & compress it which also increases the temperature. This air is then supplied to the combustion chamber. Fuel is added into the combustion chamber & ignitor ignites the fuel. Thus large amount of exhaust gases are produced which are made to flow through turbines.
The different types of gas turbines/jet engines are –
The above mentioned are open cycle gas turbines where the exhaust gases are let directly into atmosphere. The other type i.e. closed cycle gas turbines where the exhausted are reused again for reheating are used in powerplants.
4. WIND TURBINES:-
Wind turbines are a boon to mankind- affordable, clean & sustainable ! Some windfarms are so big that they could produce 50MW of power. Well, coming to working of wind turbines, the story remains same as other turbines. The rotor has 3 blades & are designed in such a way that when wind flows straight through them, they start rotating. The only problem here is wind turbines rotate at a very low of RPM. The low RPM doesn’t produce electricity of required frequency & that is why we require a gearbox which increases the speed of shaft. The output shaft is then connected to the generator.
The 3 primary types of wind turbines are –
1. Horizontal-axis wind turbines (HAWT)
2. Savonius vertical-axis wind turbine (Savonius VAWT)
3. Darrieus vertical-axis wind turbine (Darrieus VAWT)
HAWTs are much older & common while VAWTs produce less power, are less efficient & hence are not used commonly.
Well, what’s interesting is why do wind turbines have 3 blades ?
Adding more number of blades increases torque which is not the need here. Increase in torque decreases the RPM which is undesirable. Also cost increases significantly.
Also lowering the number of blades to 2 leads to gyroscopic instability & periodic stresses which makes it unsafe.
Hence, 3 blades makes wind turbines the most efficient in all terms