Regenerative Rankine Cycle

10 Key excerpts on "Regenerative Rankine Cycle"

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  Principles of Engineering Thermodynamics, SI Edition

  • John Reisel(Author)
  • 2021(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  This is left as an exercise. 7.3.5 Rankine Cycle with Regeneration With thermal efficiencies in the range of 30–40% for the forms of the Rankine cycle discussed previously, it should be obvious that much of the input heat is ultimately rejected to the environment (60–70%). If some of this heat could be captured and used as heat input to the cycle, the heat input load could be reduced and the thermal efficiency could be increased. This is the concept behind the various forms of the Rankine cycle with regeneration, with “regeneration” being the internal transfer of heat within the cycle. A fundamental problem with directly using the heat removed through the condenser to heat the water elsewhere in the cycle is that the working fluid passing through the condenser is at the lowest temperature in the cycle. From the Second Law of Thermodynamics, heat is transferred from a hotter substance to a colder substance, and so although the heat can be sent to the environment from the condensing working fluid, the heat transfer only occurs because the environment is at an even lower temperature. It is not possible to use this energy to heat the working fluid elsewhere in the cycle, because the temperatures of the fluid elsewhere in the cycle are hotter than in the condenser. Therefore, to achieve regeneration, it is necessary to extract energy from the working fluid before the working fluid reaches the condenser. If we look at a T-s diagram of a Rankine cycle, the most logical use for the regeneration is to preheat the working fluid heading toward the steam generator. If the compressed liquid enters the steam generator at a hotter temperature, less energy is needed in the steam generator to heat the compressed liquid to the saturated liquid state; this results in an overall reduction in heat input. This heat transfer process can be achieved through relatively simple heat exchangers. One such regeneration scheme is shown in Figure 7.19.

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  Waste Heat Recovery in Process Industries

  • Hussam Jouhara(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  Figure 1.20 ), and this steam is then reheated and expanded in the second turbine from MP to LP. The main advantage of the reheat is to increase the steam quality to ensure a steadier thermodynamic efficiency.

  Rankine cycles with reheat also ensure a higher overall mechanical efficiency by using two turbines for the cycle compared to only using one. In this configuration, the overall efficiency of the cycle is given by:

  (1.24)

  where, q

  inB

  corresponds to the heat (kJ/kg) provided by the boiler to the fluid and q

  inR

  corresponds to the heat (kJ/kg) provided by the reheater to the fluid (In real-life power plant, the boiler and the superheater use the same heat source.)

  Figure 1.19

  Schematic of the Rankine cycle with reheat.

  Figure 1.20

  T–s diagram of a Rankine cycle with reheat.

  1.2.4.5 Regenerative Rankine Cycles

  To improve energetic and exergetic efficiency of the thermodynamic cycle, the Regenerative Rankine Cycles extracts a portion of the steam from the turbine to heat the fluid before it is sent to the boiler. This results in a small reduction in the work provided to the turbine but a substantial reduction in the heat input, because the heating phase creates entropy and thus irreversibility if this is all provided by the boiler. The portion of steam used therefore helps the boiler heat the fluid, reducing the fuel required by the boiler and hence both energetic and exergetic efficiencies improve. Furthermore, the average temperature of the heat input by the boiler is increased and this also results in a higher energetic efficiency [8] .

  Regenerative cycles require a new component. A feed water heater (FWH ) also called a regenerator is required, its role being to heat the feed water before it enters the boiler. There are two different kinds of regenerative cycle: the open FWH cycle and the closed FWH cycle.

  Open FWH cycles use an Open FWH. As seen in Figure 1.21 , this component mixes the liquid delivered from pump 1 with the extracted steam arriving from turbine. It corresponds to point 6 on the T–s diagram shown in Figure 1.22

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  Advanced Thermodynamics

  Fundamentals, Mathematics, Applications

  • Mehrzad Tabatabaian, R. K. Rajput(Authors)
  • 2017(Publication Date)
  • Mercury Learning and Information

    (Publisher)

  . ., Work done – 1– i e h h m h = + ( ) ( ) ( ) 2 1 2 2 3 – 1– – – h m m h h         +   Advantages of the Regenerative cycle over the Simple Rankine Cycle: • The heating process in the boiler tends to become reversible. • The thermal stresses set up in the boiler are minimized. This is due to the fact that temperature ranges in the boiler are reduced. • The thermal efficiency is improved because the average temperature of heat addition to the cycle is increased. • Heat rate is reduced. • The blade height is less due to the reduced amount of steam passed through the low-pressure stages. • Due to many extractions there is an improvement in the turbine drainage and it reduces erosion due to moisture. • A small size condenser is required. BASIC STEAM POWER CYCLES • 289 Disadvantages of Regenerative cycle over Simple Rankine cycle: • The plant becomes more complicated. • Because of addition of heaters greater maintenance is required. • For given power a large capacity boiler is required. • The heaters are costly and the gain in thermal efficiency is not much in comparison to the heavier costs. Note that in the absence of precise information (regarding actual tem- perature of the feed water entering and leaving the heaters and of the con- densate temperatures) the following assumptions should always be made while doing calculations: • Each heater is ideal and bled steam just condenses. • The feed water is heated to saturation temperature at the pressure of bled steam. • Unless otherwise stated the work done by the pumps in the system is considered negligible. • There is an equal temperature rise in all the heaters (usually 10 °C to 15 °C). Example 12 A steam turbine is fed with steam having an enthalpy of 3100 kJ/kg. It moves out of the turbine with an enthalpy of 2100 kJ/kg. Feed heating is done at a pressure of 3.2 bar with steam enthalpy of 2500 kJ/kg. The con- densate from a condenser with an enthalpy of 125 kJ/kg enters into the feed heater.

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  Thermodynamics and Heat Power

  • 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.

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  Elements of Energy Conversion

  • Charles R. Russell(Author)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  This can be avoided and, at the same time, the efficiency of the cycle can be increased by superheating the steam. The amount of superheat is limited by the allowable operating temperatures of the super-heater and turbine. However, these limits have been raised by the development of high-temperature alloys. The Rankine cycle with (pressure volume) (temperature-entropy) FIG. 3-2 Rankine cycle with superheat. superheat (Fig. 3-2) illustrates the expansion extending into the wet steam area until the maximum allowable liquid content of about 10 percent is reached. The efficiency of the cycle can be increased further by reheating the vapor after partial expansion (Fig. 3-3). In addition, the reheat cycle permits a greater pressure ratio between the boiler and the condenser. The condenser temperature and corresponding pressure are established by the cooling water or other heat rejection system. As mentioned previously, the process of mixing cold conden-sate with the high-temperature liquid in the boiler is thermo-dynamically inefficient. Heating of the boiler feed can be made more nearly reversible by the use of regenerative stages-that is, by withdrawing vapor from the turbine at several intermediate temperatures to heat the boiler feed (Figs. 3-4 and 3-5). As many HEAT ENGINES 83 as eight stages of regenerative heating may be justified in a large steam power plant to increase efficiency. As the number of regenerative stages is increased, the efficiency of the cycle ap-proaches that of the Carnot cycle. Λ4 J 4 Φ r ^. * ^. -*_ ♦ FIG. 3-3 Rankine cycle with reheat. FIG. 3-4 Rankine cycle with superheat and regenerative heaters. boiler Regenerative heaters FIG. 3-5 Turbine with feedwater heaters. Nuclear power stations have been built without superheat in order to avoid complexities in the reactor.

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  Nuclear Reactor Thermal Hydraulics

  An Introduction to Nuclear Heat Transfer and Fluid Flow

  • Robert E. Masterson(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  https://ecourses.ou.edu/ .)(b) The temperatures and pressures at each step in the regeneration process for a coal-fired power plant (on the left) and for a nuclear power plant (on the right). Notice that coal-fired power plants operate at higher boiler temperatures and pressures than nuclear power plants do and are therefore more efficient. However, they also produce considerably more emissions.

  The thermodynamic cycle is completed by adding heat to the water directly from the core or through the steam generators so that the steam enters the high-pressure turbine at a pressure and an enthalpy, which corresponds to state 5. In other words, the feedwater “climbs” the left-hand side of the vapor dome shown in Figure 9.12a , and because its energy content is higher, the area under the thermal cycle curve is higher as well. This implies that the thermal efficiency is also greater . The efficiency of the Rankine cycle can be increased even further by adding additional feedwater heaters to the flow. Figure 9.12b shows the pressures at each step in this process for a coal-fired power plant on the left and for a nuclear power plant on the right. In the case of a coal-fired power plant, the temperatures and pressures at state 5 are higher because the boiler operates at a much higher temperature. Thus, the steam at this point has more internal energy. For example, in a BWR, the system pressure is about 7 MPa and the saturation temperature is about 286°C, while in the secondary loop of a PWR, the system pressure is about 7 MPa and the superheat temperature is about 292°C because the steam generators are designed to superheat the steam. The pressure at point 5 in a coal-fired power plant is about 15 MPa, while the temperature is about 600°C. Now let us compare the enthalpies at each step in this process. For a coal-fired power plant, the enthalpies at points 1 through 7 are

  The Enthalpies for a Coal-Fired Power Plant with an Ideal Regenerative Rankine Cycle

  h 1

  = 191.8   kJ/kg

  h 2

  = 193.0   kJ/kg

  h 3

  = 798.3   kJ/kg

  h 4

  = 814.0   kJ/kg

  h 5

  = 3,583.3   kJ/kg

  h 6

  = 2,860.2   kJ/kg

  h 7

  = 2,115.3   kJ/kg

  (9.27)

  These enthalpies assume the use of a single open feedwater heater. If a BWR or PWR is designed with the same feedwater heater, the corresponding enthalpies would be

  The Enthalpies for a PWR with an Ideal Regenerative Rankine Cycle

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  Thermodynamics

  A Smart Approach

  • Ibrahim Dinçer(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  6.8 Rankine Cycle In the previous Brayton cycle, the working gas was air, leading to the name the air-standard Brayton cycle. It is sometimes called a gas turbine cycle. In this section, the focus is on another very significant heat engine cycle, which was discovered by in 1859 by a Scottish engineer, William J.M. Rankine, the so-called Rankine cycle. This cycle is also recognized as a steam engine cycle, steam turbine cycle, steam Rankine cycle, etc. In the cycle, the work-ing fluid is water, which goes through phase changes to be steam at high temperature and high pressure with high work potential (and energy content) and generate work in a steam turbine. This is a heat-driven cycle and what makes it the heat engine cycle. It has been the most widely used cycle since the industrial revolution. The Rankine cycle consists of four key components, namely a pump, a boiler (or a heat exchanger depending on the heat source), a steam turbine, and a condenser (Figure 6.25a). In addition, a list of processes of the ideal and actual steam Rankine cycles is shown in Table 6.6; these processes are also clearly seen in Figure 6.25b on a T-s diagram. Steam turbine Boiler Pump (a) (b) 3 4 T 2 s 2 1 4 s s 4 3 2 1 Condenser Q in W in W out W in W out Q out Q in Q out Figure 6.25 (a) A schematic diagram of a simple Rankine cycle and (b) its T-s diagram showing both ideal and actual behaviors for the pump and steam turbine. Table 6.6 A list of processes of the ideal and actual steam Rankine cycles. Process Ideal cycle Actual cycle Adiabatic pumping (compression) Isentropic (1-2 s ) Nonisentropic (1-2) Isobaric Heat addition (2 s -3) Heat addition (2-3) Expansion Isentropic (3-4 s ) Nonisentropic (3-4) Isobaric Heat rejection (4 s -1) Heat rejection (4-1) 6.8 Rankine Cycle 369 Table 6.7 The mass, energy, entropy, and exergy balance equations for each component of a simple Rankine cycle.

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  Thermal Cycles of Heat Recovery Power Plants

  • Tangellapalli Srinivas(Author)
  • 2021(Publication Date)
  • Bentham Science Publishers

    (Publisher)

  Steam Rankine Cycle Tangellapalli Srinivas

  Abstract

  Steam Rankine cycle (SRC) is a widely used thermal power cycle due to its ideal properties of the fluid. Compared to organic Rankine cycle (ORC) and organic flash cycle (OFC), the working fluid and equipment are not expensive. A water treatment plant is associated with a steam thermal power plant. SRC is a power plant cycle in thermal power plant, bottoming cycle in a combined cycle power plant or power generation cycle for a waste heat recovery plant. In this chapter, the performance characteristics of SRC have been developed and analyzed. A single pressure heat recovery steam generator (HRSG) with a deaerator is considered to draw the heat from the industrial waste heat. The conditions for maximum power and maximum efficiency have been developed. Correlations are presented to find the HRSG pressure at the available hot gas supply temperature. The work is extended to a case study on SRC operating with the heat recovery of a cement factory. A power plant layout suitable to the identified waste heat sources from a cement factory has been developed. From the evaluation, it has been concluded that approximately 15 MW of electricity can be generated from its waste heat to meet the electrical load of 15 MW. It indicates that at the optimized conditions, it is possible to self-generate the electrical demand of the cement factory from its own waste heat recovery. The analysis recommended a low pressure in HRSG for maximum power generation.

  Keywords: Heat recovery, Rankine cycle, Steam power cycle, Thermal efficiency, Thermal power plant.

  INTRODUCTION

  In majority of countries, more than 50% of electricity is from thermal power plant with steam as a working fluid. The power cycle is a steam Rankine cycle (SRC). Water is the ideal fluid for power. To operate SRC, a water treatment plant is required. The thermal properties of water/steam are superior than the others. Large size power plants can be suitably built with water as the working fluid. The equipment and systems are well developed and available around the world. The main challenges of thermal power plants are the handling of dust, ash and environmental pollution caused by the burning of coal. The ash and emission issues can be addressed by shifting from coal firing to heat recovery option or biomass firing. SRC is not suitable for a power plant with low temperature heat source. Organic Rankine cycle (ORC) is similar to SRC layout.

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  14th International Conference on Turbochargers and Turbocharging

  Proceedings of the International Conference on Turbochargers and Turbocharging (London, UK, 2021)

  • Institution of Mechanical Engineers, Institution of Mechanical Engineers, IMECHE, IMECHE(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  113 14th International Conference on Turbochargers and Turbocharging Institution of Mechanical Engineers, ISBN: 978-0-367-67645-2 A system-level study of an organic Rankine cycle applied to waste heat recovery in light-duty hybrid powertrains A. Pessanha¹, C.D. Copeland², Z. Chen³ 1,2 University of Bath, Bath, UK 3 Jaguar Land Rover Powertrain Research, Coventry, UK ABSTRACT The global transportation sector produces approximately 20% of human-induced greenhouse gas and pollutant emissions. Road transport accounts for almost 75% of that amount. As a result, the automotive industry has invested heavily in powertrain electrification as a long-term strategy for reducing the environmental impact of urban mobility. However, in-vehicle energy storage and access to charging infrastructure are still large bottlenecks to widespread uptake of battery electric vehicles (BEV). There- fore, hybrid electric vehicle (HEV) architectures are a short-to-medium-term solution that will enable a gradual transition to a cleaner transportation system. Current HEV platforms continue to rely on the internal combustion engine (ICE) for a significant proportion of tractive effort. Therefore, optimising the efficiency of the thermal power plant remains a key challenge. In fact, modern ICE’s only convert up to 40% of the energy released in the combustion process into useful mechanical work. The remainder is lost in the form of high-enthalpy exhaust gases and engine cooling. Waste heat recovery (WHR) aims to reclaim a proportion of this energy to increase the overall efficiency of the vehicle and has been identified as a key enabler of real-world emissions reductions by the UK Advanced Propulsion Centre (APC). The organic Rankine cycle (ORC) has been identified as a promising technology for WHR in HEV powertrains. Basic thermodynamic principles were used to identify three key parameters that determine ORC power output: condenser pressure, evaporation pressure and fluid superheating.

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  Thermal Design of Nuclear Reactors

  • R. H. S. Winterton(Author)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  h cannot be increased because of temperature limitations in the core. Water in particular is not a high-temperature reactor coolant. Since the critical temperature and pressure of water are 374ºC and 221 bar respectively, there is little scope for increasing core outlet temperatures much above the present levels of 310º to 320º (PWR). If the water at the core outlet were brought up to the critical conditions the quite modest improvement in the temperature would be accompanied by one-fifth the density and nearly double the pressure. To achieve the same moderating effect with the reduced density the flow area between the fuel rods would have to be increased by a factor of 5. The increased capital cost of the larger, higher pressure, core would outweigh any improvement in thermal efficiency. In fact, a single pressure vessel to contain the core would be difficult to construct with present techniques.

  The low temperature in the cycle is limited by the availability of cooling water. If cooling water at 15ºC is available from the sea or a river and is allowed to rise in temperature to 25ºC before discharge, and the pinch-point temperature difference in the condenser is 5ºC, then the steam will condense at 30ºC (saturation pressure 0.042 bar). Clearly it will not be possible to do much better than this, so T c is largely fixed by the location of the power station and the time of year.

  FEED WATER HEATING (REGENERATION)

  Heat rejection in the Rankine cycle occurs entirely at the lowest temperature in the cycle, but not all of the heat in the boiler is supplied at the highest temperature; some heat is required to bring the feed water up to the boiling-point.

  If the heat required between points 1 and 2 (Figs. 9.3 and 9.4 ) could be supplied internally within the cycle then all the heat supplied by the reactor would be at the highest temperature, the boiling-point, and for an ideal reversible cycle the efficiency would equal that of the Carnot cycle.

  The way in which this is done is to extract some of the steam at intervals in the turbine, and use this steam to warm the feed water in a number of feed heaters, as shown in Fig. 9.6 . The two feed heaters shown are of the closed type, that is, they are shell and tube heat exchangers with the steam and water flows kept separate. The condensed steam from the higher temperature heater is throttled to the pressure of the steam in the next, lower temperature, feed heater, and mixed with it. The other type of feed heater is the open or direct contact feed heater where the steam bled off from the turbine and the feed water are simply mixed together.

  Fig. 9.6

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