The Rankine cycle is a thermodynamic cycle that illustrates the conversion of heat into mechanical energy, which is ultimately converted into electrical energy. The Rankine cycle is the essential operating cycle for all power plants, and most solar power plants operate on this cycle.
Rankine cycle is named after Scottish engineer William Rankine, who analyzed an ideal heat engine with a condenser in 1859.
Rankine Cycle Process
Working Fluid
Rankine cycle operates on a working fluid that is continuously evaporated and condensed. Water is the most commonly used fluid for several reasons. First, it is non-toxic, cheap, and widely available. Second, it has high specific heat and heat capacity. Water absorbs a large amount of heat, which is necessary to perform valuable mechanical work.
Schematics
The schematic of a Rankine cycle is shown below. The Rankine cycle system comprises a pump, boiler, turbine, and condenser. The pump delivers liquid water to the boiler (1), which converts water to superheated steam. The steam drives the turbine that powers an electric generator (not shown) (2). It then leaves the turbine, cools down, and condenses to a liquid state in the condenser (3). Finally, the condensed liquid is pressurized by the pump (4) and sent back to the boiler (1).
Work Done and Efficiency
The different stages of the Rankine cycle are described below. The amount of heat exchange and the work done during each stage are shown.
Step 1: Isentropic Compression
The pump pressurizes liquid water from the condenser before it enters the boiler. The work done by the pump to compress water (Wpump) can be estimated from the enthalpy (H) change of water before entering (H1) and after leaving (H2) the pump.
Wpump = H2 – H1
We assume an ideal scenario where no heat is lost to the surroundings during this step. Also, the pump requires little input energy, which is negligible for thermodynamic calculations.
Pressurized liquid water enters the boiler and boils until it forms superheated steam. The amount of heat (Qin) entering the boiler is equal to the enthalpy difference between superheated steam (H3) and liquid water (H2).
Qin = H3 – H2
The heat, Qin, is supplied to the boiler by fuel. The pressure inside the boiler does not change during this step, so no work is done.
Step 3: Isentropic Expansion
At a high temperature and pressure, dry superheated steam from the boiler expands through the turbine, resulting in work. Pressure and temperature are reduced, and the steam is discharged to the condenser. The magnitude of the work done (Wturbine) is simply the negative change in enthalpy.
Wturbine = -(H4 – H3) = H3 – H4
Again, we assume that there is no heat exchange with the surroundings.
Step 4: Isobaric Heat Rejection
It is the last step in the Rankine cycle. Wet steam from the turbine condenses into liquid water. Heat is given out during this process, which is equal to
Qout = H4 – H1
Let us not calculate the heat absorbed (Qtotal) and the net work done (Wout) during one cycle.
Qtotal = Qin – Qout
Wout = Wturbine – Wpump
From the first law of thermodynamics
Qin – Qout = Wturbine – Wpump
=> Wout = Qin – Qout
The thermal efficiency (η) of the Rankine cycle is
The above formula for efficiency is derived under the assumption that there is no heat loss in the system. However, in a real Rankine cycle, each stage is associated with irreversible processes like friction, resulting in heat loss. Therefore, real Rankine cycle efficiency is far lower than the ideal.
Temperature-Enthalpy (TS) Diagram
Below is the temperature-entropy (TS) diagram of the Rankine cycle. The different stages are represented by the letters A to F. In AB, water is compressed by the pump, and in EF, the steam expands and drives the turbine. These two steps occur isentropically, ΔS = 0, and maximize the work output.
In a real Rankine cycle, this scenario does not happen. The compression by the pump and the expansion in the turbine are not isentropic, which means that these processes are not reversible, resulting in an entropy increase. Ultimately, the power generated by the turbine is reduced. The critical issue here is the formation of water droplets in the turbine, which leads to the deterioration of the turbine and a decrease in the engine’s overall efficiency.
The practical way of overcoming this problem is by superheating the steam, represented by the DE portion of the diagram. Point D is at the border of the dual-phase region of steam and water. So, after expansion, the steam will be very wet. Superheating moves point D to the right and up to point E in the diagram. Therefore, the expansion will result in drier steam. This process is also called reheating the Rankine cycle.
FAQs
Q.1. What is an Organic Rankine Cycle?
Ans. Organic Rankine Cycle or ORC is a Rankine cycle where an organic compound such as toluene, isobutene, or isopentane is used as the flowing medium instead of water.
The Rankine cycle or Rankine Vapor Cycle is the process widely used by power plants such as coal-fired power plants or nuclear reactors. In this mechanism, a fuel is used to produce heat within a boiler, converting water into steam which then expands through a turbine producing useful work.
Rankine cycle is the main cycle on which modern-day thermal power plants work. In a good steam power plant, the Rankine cycle efficiency varies from 35 to 45 %.
2[5]The cycle consists of four processes: (1-2) Isentropic compression on pump; (2-3) Constant pressure heat addition in a boiler; (3-4) Isentropic expansion in a turbine; (4-1) Constant pressure heat rejection in a condenser.
The Rankine cycle is an ideal thermodynamic cycle involving a constant pressure heat engine which converts heat into mechanical work. The heat is supplied externally in this cycle in a closed loop, which uses either water or any other organic fluids (Pentane or Toluene) as a working fluid.
A PV diagram represents the relationship between pressure and volume during a thermodynamic process. In the context of the Rankine Cycle, this diagram visually portrays the changes in state that occur as steam goes through the various stages of the cycle.
That is, to increase the efficiency one should increase the average temperature at which heat is transferred to the working fluid in the boiler, and/or decrease the average temperature at which heat is rejected from the working fluid in the condenser.
The Rankine cycle is less efficient than the Carnot cycle for given maximum and minimum temperatures, but, as said earlier, it is more effective as a practical power production device.
The four main components of the Rankine cycle are the water pump, boiler, turbine, and condenser. There are four main stages to the Rankine cycle. Compression, heating, expansion, and condensation of the working fluid. The Rankine cycle can be represented by a pressure-volume diagram for each stage.
The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat source and heat sink.
The Brayton cycle uses a gas turbine, whereas the Rankine cycle uses a steam turbine. The working fluid in a Brayton cycle is always a gas, whereas the working fluid in a Rankine cycle can be either liquid or vapor.
The Rankine cycle is a thermodynamic power cycle which uses a working fluid to convert heat into mechanical work (or power) following a specific set of processes [1].
A real-life refrigerator follows a cycle that is essentially a backwards Rankine cycle. A PV diagram for this cycle is shown in Fig. 1. The path 1 → 2 is an adiabatic compression of the refrigerant fluid (a common fluid is the hydrofluorocarbon HFC-134a) which is a gas along this path.
The Rankine scale (/ˈræŋkɪn/) is an absolute scale of thermodynamic temperature named after the University of Glasgow engineer and physicist Macquorn Rankine, who proposed it in 1859.
Rankine cycle is an ideal cycle of heat engine which uses water and steam as a working fluid to generate power with the help of a steam turbine. Process 1-2: Isentropic compression in Pump.Process 2-3: Constant pressure heat addition in the Boiler.Process 3-4: Isentropic expansion in Turbine.
A simple Rankine cycle is based on pressurizing water with a pump, heating the pressurized water beyond its boiling point to produce superheated steam in the boiler, expanding the steam through a steam turbine, cooling the residual steam to regenerate water, and returning the water to the pump to start through the ...
The Rankine cycle, also called the Rankine vapor cycle, is a thermodynamic cycle that converts heat into mechanical energy. The Rankine cycle is name after William Johnson Macquorn Rankine, a 19th century Scottish engineer and physicist known for his research in the thermodynamic properties of steam.
The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid. Unless the pressure and temperature reach supercritical levels in the boiler, the temperature range over which the cycle can operate is quite small.
In thermodynamics, the Carnot efficiency is the maximum possible efficiency of a heat engine operating between two reservoirs at different temperatures.
For any heat engine operating on this cycle, the thermal efficiency is the ratio of work done to the input heat QH at the working temperature. Mathematically, ηth=WQH. Now, the energy of the system must be equal according to the first law of thermodynamics.
Introduction: My name is Gregorio Kreiger, I am a tender, brainy, enthusiastic, combative, agreeable, gentle, gentle person who loves writing and wants to share my knowledge and understanding with you.
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