Ideal Rankine Cycle
Principles of thermodynamics are useful for power cycle for of electrical power generation (i.e net power output) and to study refrigeration & heat pump which requires input of net power.
Classification of thermodynamics power cycles can be done into two types:
- Vapor cycle working fluid exists in liquid phase during one part of the cycle (i.e from condenser outlet to Boiler) and mixed phase wit in the steam boiler and in vapor phase at the Boiler outlet.
- Gas cycle working fluid during the cycle remains in gas phase.
Steam power generation units run on vapor power cycle using water as the working fluid.
Under this section attempt is made to familiarize the readers with the concepts of ideal vapor cycle called Rankine cycle.
Typical Power Plant Cycle
Electrical power is generated by using vapor cycle power plants by using Coal, Lignite, Diesel, Heavy furnace oil as fuel depending upon the availability and cost. The flow scheme of the vapor power cycle is given below:
The entire power plant can be broken down into following sub-systems.
- Sub-system A: Classified as main-components of power plant (Turbine, Condenser, Pump, Boiler) for power generation.
- Sub-system B: Classified as stack/chimney, from where the waste gases are exhausted to atmosphere.
- Sub-system C: Classified as electric generator for converting the mechanical energy to electrical energy.
- Sub-system D: Classified as Cooling water system for absorbing the heat of the rejected steam in the condenser and helping in changing the phase of the steam to liquid (condensate).
The focus is to study sub-system a which deals Rankine cycle. Many of the practical limitations related with the Carnot cycle can be conveniently overcome in Rankine cycle.
Typical Ideal Rankine Cycle
In a vapor cycle if the working fluid in a vapor cycle passes through various components of the power plant without irreversibility and frictional pressure drop, then the cycle is called as Ideal Rankine Cycle.
The Rankine cycle is the basic operating cycle for all power plants where an working fluid is continuously changing its phase from liquid to vapour and vice-versa.
The (p-h) and (T-s) diagrams are useful in understanding the working of Rankine cycle along with the description given below:
1-2-3 Isobaric Heat Transfer or Constant pressure heat addition in a boiler
Boiler is a large heat exchanger where heat liberating fuel like coal, lignite or oil transfers the heat indirectly to water at constant pressure. Water enters the steam boiler from boiler feed pump as a compressed liquid at state-1 and is heated to the saturation temperature as shown in the T-s diagram as state-3.
The energy balance in the boiler is or energy added in steam generator,
3-4 Isentropic Expansion or Isentropic expansion in a turbine
Vapor from the boiler outlet enters the turbine at state 3, where it expands isentropically over the turbine fixed and moving blade to produce work done in the form of mechanical rotation of the turbine shaft which in turn is connected to the electrical generator.
Work delivered by turbine, (Neglecting heat transfer with the surroundings)
Wturbine out= h3-h4
4-5 Isobaric Heat Rejection or Constant pressure heat rejection in a condenser
At state-4 vapor enters the condenser and the change of phase occurs as vapor is condensed to liquid at constant-pressure in the condenser by transferring the heat of the steam to the circulating water flow through the tubes of the condenser. Change of phase occurs in condenser and the working fluid leaving the condenser is in liquid state and marked as point 5.
Energy rejected in the condenser, qout= h4-h5
5-1 Isentropic Compression or Isentropic compression in a pump
Water exits the condenser at state 5 and enters the pump. This pump raises the pressure of the water by imparting work during the processes. In units of smaller size and because of low specific volume this small work can be neglected when compared to work-output of steam turbine.
Work done on pump, per kg of water, W51= h5-h1
The thermal efficiency of the Rankine cycle is given by,