Physics
Rankine Cycle
The Rankine cycle is a thermodynamic cycle used in power plants to convert heat into mechanical work. It involves four main processes: compression, heating, expansion, and cooling. Water or steam is typically used as the working fluid, and the cycle is commonly used in steam turbine power plants to generate electricity.
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11 Key excerpts on "Rankine Cycle"
eBook - PDFSustainable Utility Systems
Modelling and Optimisation
- Petar Sabev Varbanov, Jiří Škorpík, Jiří Pospíšil, Jiří Jaromír Klemeš(Authors)
- 2020(Publication Date)
- De Gruyter(Publisher)
6.2 Rankine – Clausius cycle (R – C cycle, steam cycle) The steam cycle is the oldest heat cycle which is used in technical practise. For transformation of heat to work through the steam cycle, initially steam piston en-gines were used (Schobert, 2014). The current state of the art in power generation uses steam turbines (US EPA, 2015). The use of a particular mechanism or machine does not change principle of the cycle, even though the steam turbine is a turbo-machine and the piston steam engine is a reciprocating engine. Unlike the internal combustion engine cycle, the steam cycle connects several devices. Figure 6.1 shows the steam cycle for power generation only. The description of each part of the flow chart follows: State 1 – state 2: feed water pressure increase. This is performed by using a boiler feed water pump. The power input to the pump is lower compared with the work of the steam expansion in the steam turbine, because water is almost an incompressible fluid. State 2 – state 3: steam generation . Raising steam (steam generation) in the boiler (Chapter 5) can be divided into three steps. In the first step 2 – 3 ʹ , the water is heated up to the liquid saturation point. This is referred to as boiler feed water pre-heating. The second part of the process is the evaporation of water to the state of steam saturation (3 ʹ – 3 ʹʹ ). The steam is then superheated to state 3 in a super-heater section of the boiler. Obtaining superheated steam is essential for power generation using this cycle, as steam turbines use exactly the superheated part of the steam energy content and steam at the exit of the turbine has to be essentially dry (Varbanov, 2004). Note, states 3 ʹ and 3 ʹʹ are internal to the steam boiler. 3 1 2 4 3'' CT ~ FP SB ST G C Fig. 6.1: Flow chart of Rankine – Clausius cycle.
eBook - ePubThermal Power Plant
Design and Operation
- Dipak Sarkar(Author)
- 2015(Publication Date)
- Elsevier(Publisher)
Thermodynamically, all three cycles, i.e., Carnot, Stirling, and Ericsson cycles are identical in nature, since all of them work between constant temperature at heat source and constant temperature at heat sink. All are theoretically capable of attaining identical efficiency, working between similar hot and cold end temperatures if there are no losses. However, none of these cycles finds practical application in commercial engines.1.6 Rankine Cycle
In a steam power plant supply and rejection of heat is more easily realized at constant pressure than at constant temperature. It was William John Macquorn Rankine , after whom the Rankine Cycle is named, who first calculated the maximum possible work that could be developed by an engine using dry saturated steam between the pressure limits of the boiler and condenser. The simplest steam cycle using dry saturated steam as the working fluid has the following basic components (Figure 1.8 ):a. Steam Generator/Boiler b. Steam Turbine c. Condenser d. Boiler Feed PumpThe Rankine Cycle is an ideal thermodynamic cycle involving the following processes:Figure 1.8 Simplest steam cycle.i. Steam generation in boiler at constant pressure ii. Isentropic expansion in steam turbine iii. Condensation in condenser at constant pressure iv. Pressurizing condensate to boiler pressure by isentropic compressionFigure 1.9
- John Reisel(Author)
- 2021(Publication Date)
- Cengage Learning EMEA(Publisher)
Later, we will discuss some options for increasing the thermal efficiency of the Rankine Cycle. In general, these options require additional capital costs in constructing the cycle, and so they are not universally used. Smaller Rankine Cycle facilities will often rely on the less expensive option of a basic Rankine Cycle. Operators of systems that use co-generation, during which both process steam and electricity from a Rankine Cycle are produced, may not be particularly concerned with increasing the cycle’s thermal efficiency significantly because they may view any electricity produced as a bonus. Furthermore, one large-scale power plant design also relies on the basic Rankine Cycle—a boiling water reactor version of a nuclear power plant. A boiling water reactor nuclear power plant uses saturated steam to enter the T s 1 4 3 2 FIGURE 7.11 A comparison between the T-s diagrams of the vapor Carnot power cycle (shown in a dashed box) and the nonideal, basic Rankine Cycle. Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Chapter 7 Power Cycles 266 turbine, following the basic Rankine Cycle design. This is done because in the reactor only a portion of the water in the nuclear reactor is allowed to vaporize, whereas most remains a liquid. The resulting saturated steam (both liquid and vapor are present, so it is by necessity a saturated mixture) is extracted and sent directly toward the turbine.
No longer available |Learn more- Irving Granet, Maurice Bluestein(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
373 8 Vapor Power Cycles LEARNING GOALS After reading and studying the material in this chapter, you should be able to 1. Understand the definition of the term cycle , and differentiate between gas and vapor cycles 2. Recall that our conclusions regarding the Carnot cycle were independent of the working fluid used in the cycle 3. Sketch and analyze the elements of the simple Rankine Cycle 4. Conclude that for the same pressures, the efficiency of the Rankine Cycle is less than that of a Carnot cycle 5. Define the type efficiency as the ratio of the ideal thermal efficiency of a given cycle divided by the efficiency of a Carnot cycle operating between the same maxi-mum and minimum temperature limits 6. Sketch the simple reheat cycle elements and T–s and h–s diagrams for this cycle 7. Conclude that reheat does not greatly increase the efficiency of the Rankine Cycle but does decrease the moisture content of the steam in the later stages of the turbine 8. Understand that regeneration is a method of heating the feedwater with steam that has already done some work and that in the limit, regenerative cycle approaches the efficiency of the Carnot cycle operating between the same temperature limits 9. Apply regeneration to the Rankine Cycle, sketch the T–s diagram, and show how regeneration improves the Rankine Cycle efficiency 10. Qualitatively understand the construction and operation of the major pieces of equipment used in commercial steam-generating plants 11. Understand other methods that have been proposed and/or constructed for the production of power, including direct energy devices 12. Understand the use of cogeneration as a combination of power production and process or space use 374 Thermodynamics and Heat Power 8.1 Introduction A cycle has been defined as a series of thermodynamic processes during which the work-ing fluid can be made to undergo changes involving energy transitions and subsequently is returned to its original state.
eBook - ePub- R. H. S. Winterton(Author)
- 2014(Publication Date)
- Pergamon(Publisher)
Fig. 9.3 The basic Rankine Cycle.Fig. 9.4 The points of the Rankine Cycle superimposed on the layout of the plant.Since liquids are almost incompressible the increase in temperature on passing through the pump is in fact much smaller than indicated in the figure. Drawn to scale it would not appear at all, and process 1-2 would follow the saturation line. In the ideal cycle the expansion through the turbine is assumed to be isentropic (the prime indicates that 4′ represents the end point of the ideal expansion, not the actual condition of the steam at point 4).Changes of potential and kinetic energy of the water/steam around the circuit are small and normally neglected, so the steady-flow energy equation reduces toand since in the turbine no heat is exchanged and in the boiler no work is done we have the simple relations: work out of the turbine equals the decrease in enthalpy of the steam; and, as before, heat supplied in the boiler equals the increase in enthalpy of the water/steam. The ratio of these two quantities is the thermal efficiency of the steam cycle. A high thermal efficiency is important because it means that a given electrical power output can be met with a smaller and cheaper nuclear plant.In practice, of course, there is some increase in entropy as the steam expands through the turbine, and the simplest way to allow for this is to use an experimentally determined isentropic efficiency :Values of the isentropic efficiency are typically around 0.85.In view of the importance of enthalpy values in steam cycle calculations it is useful to have a chart that gives enthalpy values directly. The enthalpy-entropy diagram (or Mollier chart) for unit mass of water/steam is shown in Fig. 9.5 . The shape of this chart can be roughly explained as a distorted version of the temperature-entropy graph. In the region where there is a mixture of water and steam the constant pressure lines rise steeply, since both enthalpy and entropy increase as the mixture changes from saturated water to saturated steam. The enthalpy increase in going from saturated water to a mixture with a dryness fraction x is xh lv , and the entropy increase is xh lv /T sat , since entropy = . Consequently the constant pressure lines in the wet steam region can be divided into equal intervals of dryness fraction x
eBook - PDFThermodynamics
Fundamentals and Engineering Applications
- William C. Reynolds, Piero Colonna(Authors)
- 2018(Publication Date)
- Cambridge University Press(Publisher)
Small Rankine Cycle power plants are adopted to generate electricity from the combustion of biomass, i.e., residues from forestry maintenance and agriculture, and material waste from the wood industry. The optimal capacity of such plants is small because the cost of gathering biomass is proportional to the dis-tance from the plant. Solar concentrators of various types are increasingly used to transfer the sun’s energy to the working fluid of a Rankine Cycle plant, 154 Chapter 7 Energy Conversion Systems F F F F F F F Figure 7.11 Process flow diagram of an actual steam power station ( ≈ 600 MW e ) displaying typical enhancements with respect to the simple Rankine Cycle configuration, i.e., superheating, reheating, and regeneration. A deaerator is also present and one of the feedwater pumps is actuated by an auxiliary turbine deriving steam from the low-pressure stages of the main turbine. thus achieving comparatively high energy conversion efficiency (Figure 7.13 ). Finally, the conversion of the energy content of geothermal reservoirs is another example of the use of the Rankine Cycle principle to generate electricity from a sustainable energy source. In some cases geothermal steam can be expanded directly into the turbine, while, if the geothermal fluid is pressurized water or if it contains too many impurities, other more complex configurations are needed. Example: efficiency of a Rankine Cycle power plant. Consider the superheated Rankine Cycle whose representation in the T–s thermodynamic diagram is in Figure 7.5 a. A regenerated cycle configuration is derived from it by extracting steam from the turbine at 0.7 MPa = 7 bar. Such steam is mixed with compressed liquid delivered by the first feedwater pump, thus heating this liquid up to the saturation temperature at the extraction pressure (Figure 7.14 ).
No longer available |Learn moreAdvanced Thermodynamics
Fundamentals, Mathematics, Applications
- Mehrzad Tabatabaian, R. K. Rajput(Authors)
- 2017(Publication Date)
- Mercury Learning and Information(Publisher)
264 • ADVANCED THERMODYNAMICS This can be achieved by making suitable changes in the conditions of steam generation or condensation, as discussed in the following: Increasing boiler pressure. It has been observed that by increasing the boiler pressure (other factors remaining the same), the cycle tends to rise and reaches a maximum value at a boiler pressure of about 166 bar (see Figure 8.5[a]). Superheating. All other factors remaining the same, if the steam is superheated before allowing it to expand in the turbine, then the Rankine Cycle efficiency may be increased (see Figure 8.5[b]). The use of superheated steam also ensures longer turbine blade life because of the absence of erosion from high velocity water particles that are suspended in wet vapor. FIGURE 8.5. Effect of operating conditions on the thermal efficiency of the Rankine Cycle. BASIC STEAM POWER CYCLES • 265 Reducing condenser pressure. The thermal efficiency of the cycle can be amply improved by reducing the condenser pressure (see Figure 8.5[c]), hence reducing the temperature at which heat is rejected, especially in high vacuums. But the increase in efficiency is obtained at the increased cost of the condensation apparatus. The thermal efficienty of the Rankine Cycle is also improved by modify- ing its system components and layout as listed by the following methods: • By reheating of steam. • By regenerative feed heating. • By water extraction. • By using binary vapor. In further sections, we will discuss in detail these methods along with numerical examples. We use the following numerical examples to demonstrate applications of the Carnot and Rankine Cycles. Example 1 The following data refer to a simple steam power plant: State No. Location Pressure Quality/Tem. Velocity 1. 2. 3. 4. 5. Inlet to turbine Exit from turbine inlet to condenser Exit from condenser and inlet to pump Exit from pump and inlet to boiler Exit from boiler Rate of steam flow = 10000 kg/h.
eBook - PDFThermodynamics
A Smart Approach
- Ibrahim Dinçer(Author)
- 2020(Publication Date)
- Wiley(Publisher)
In many industries, such as paper, oil production, refining, and others, process heat is required to operate. Process heat is delivered usually from a source with a temperature that varies from 150 to 200 C , although it can also be at lower temperatures than that range depending on the type of industry and specific application. Such industries and plants also consumes power in the form of electricity, which means that with specific parameters and operating conditions we can use the usual Rankine Cycle as a power generation unit as well as a heat production system, where the process heat comes from the condenser that might be operating at a higher pressure or receive a very hot steam, as shown in Figure 6.33a along with its T-s diagram in Figure 6.33b. In Figure 6.33, a basic Rankine Cycle is used in a cogeneration plant, where the heat being rejected by the condenser is used as process heat for various purposes: Q out = Q p = m a h b -h a 6 25 where the subscript p refers to process heat. In regards to the performance of a cogeneration Rankine Cycle (as shown in Figure 6.33a), we will define both energy and exergy efficiencies of the cogeneration Rankine Cycle as follows: η en = W net + m × Δ h ab Q in where Δ h ab is the amount of flow energy gained from the condenser. Steam turbine 3 –4 Boiler Condenser 2-1 Pump Cold water Hot water T 2 2 s 4 s s 4 3 1 a b W in W out (a) (b) Q in Q out Q out W in Q in W out Figure 6.33 A basic Rankine Cycle with a useful heat output to a cogeneration plant. 394 6 Power Cycles It may also be defined in the following form if the heat is directly transferred: η en = W net + Q p Q in and η ex = W net + Ex ab Ex Q in = W net + m × Δ ex ab 1 -T 0 T s Q in based on the closed type arrangement given in Figure 6.33a, where Δ ex ab is the amount of flow exergy gained from the condenser.
eBook - PDFEnergy Conversion Engineering
Towards Low CO2 Power and Fuels
- Ahmed F. Ghoniem(Author)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
Some pack- ages allow the user to add data and functionality to extend the capability. 6.13 Summary Converting low- and intermediate-temperature heat sources, such as geothermal, solar, and nuclear energy sources, to work is efficiently done using a two-phase power cycle. In the absolute majority of applications, the working fluid is water/steam. With even lower- temperature sources, such as waste heat (from industrial plants) and low-quality geo- thermal sources (to be discussed in detail in Chapter 9), it is possible to use an organic fluid (with lower critical pressure) instead of water as a working fluid. Rankine Cycles are closed cycles in which the working fluid is externally heated and cooled in a cyclical fashion, and a pump (instead of a compressor) is used to raise the pressure of the liquid (instead of a gas) following condensation, which contributes to improving the cycle efficiency even with relatively lower maximum temperature (compared to the case of gas turbine cycles). Because the working fluid is isolated, it is possible to use lower- quality fuels (such as coal of different grades [see Chapter 12] or biomass [see Chapter 14]) as the heat source. On the other hand, maintaining the efficiency requires lowering the lower (condenser) pressure by using low-temperature coolant, as well as raising the maximum pressure of the cycle. Similar to Brayton cycles, reheat and regener- ation (internal transfer of heat between the expansion side and the liquid heating side) are used to further improve the efficiency. 6.13 Summary 361 Cycles whose maximum pressure is higher than the critical pressure of the working fluid are called supercritical cycles, and gain efficiency improvements by reducing the temperature difference between the heat source and the working fluid (and the corresponding irreversi- bility).
- Allan D. Kraus, James R. Welty, Abdul Aziz(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
The pressure on the steam side of the condenser is set by the temperature of the cooling water and this temperature is consistent with the temperature difference maintained between the steam and the cooling water. Vapor Power and Refrigeration Cycles 263 29.26 M Superheater Air Heater Air In Air Out Economizer To Stack Pulverizer 7.42 M 8.38 M FIGURE 9.2 A typical central station steam generator. 9.3 The Ideal Rankine Cycle 9.3.1 The Ideal Rankine Cycle The T -s diagram for the ideal Rankine Cycle is indicated in Figure 9.3 and we observe that the cycle is composed of four ideal processes: • Process 1-2: An isentropic compression in the pump from the condenser pressure to the boiler pressure. 264 Introduction to Thermal and Fluid Engineering s T 2 3 4 1 FIGURE 9.3 The T -s diagram for the Rankine Cycle. In some cases, a steam engine may take the place of the steam turbine between points 3 and 4 because of the adverse effect of moist or saturated steam on the turbine blades. • Process 2-3: A constant pressure heat addition in the steam generator (and its as-sociated components such as economizers and feedwater heaters) until saturated steam at the boiler pressure is obtained. • Process 3-4: An isentropic expansion in the turbine from the pump outlet pressure to the condenser pressure. • Process 4-1: A constant pressure heat rejection in the condenser. We note in Figure 9.3 that, in the basic ideal Rankine Cycle, steam leaves the steam genera-tor and enters the turbine as saturated steam. If all work and heat interactions are considered as positive quantities, then the energy rate balance for a control volume representing just the turbine operating between state points 3 and 4 will be ˙ W turb = ˙ m ( h 3 − h 4 ) (9.1) where ˙ m is the mass flow rate. Equation 9.1 presumes that there is no heat transfer to the surroundings through the casing of the turbine and that there are no kinetic and potential energy changes.
eBook - PDF- S. C. McBirnie(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
In dealing with practical cycles, the first point which must be appreciated is that the operations are not ideal as they are in theory—there are always losses and practical limitations. For example, expansion is not adiabatic, isentropic and reversible, nor can heat be received or rejected reversibly since practically, the condition of equality of temperature cannot be realised. There is also always some pressure drop in steam pipes. Confining our attention to the engine and condenser aspects, let us consider first a straight Rankine Cycle having an upper limit of 14.0 bar, dry saturated, i.e. an upper tem-perature limit of 468.2 K. In thermodynamic theory, the minimum lower cycle temperature limit is the temperature of the environment, i.e. of the earth, the sea, the atmosphere, since it is to that temperature that all heat rejected—and indeed all work done—is ultimately degraded. For a marine engine cycle we may regard the sea as the immediate heat sink, and thermo-dynamically therefore, the minimum lower cycle temperature limit is the sea temperature. Suppose the sea temperature to be 2 1 ° C (294.2 K ) . This corresponds to a saturation pressure of 0.025 bar, and an ideal Rankine condenser would therefore give a minimum lower cycle pressure of 0.025 bar, resulting in the cycle illustrated in Figure 12.2(a) and (b). To achieve the Rankine ideal of reversible condensation however, would mean that the sea water would have to pass through the condenser at constant temperature 21°C—clearly impossible in practice, since cold water cannot receive heat without its temperature rising, nor can heat flow from the exhaust steam to the circulating water unless there is a tem-perature difference between the steam and the water at all points in the condenser (see Chapter 6). Suppose the temperature difference at the cold water end of the condenser to be 1 8 ° C .
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