Physics
Reversed Heat Engines
Reversed heat engines are devices that operate in the opposite direction of traditional heat engines, converting work into heat rather than heat into work. They are based on the principle of thermodynamic reversibility and are used in theoretical studies and practical applications such as refrigeration and heat pumps. Reversed heat engines play a crucial role in understanding the fundamental principles of thermodynamics.
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12 Key excerpts on "Reversed Heat Engines"
eBook - PDF- Donald Olander(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
109 4 Heat Engines, Power Cycles, and the Thermodynamics of Open Systems 4.1 HEAT ENGINES In Section 1.9, it was noted that the first law regards heat and work as completely interchangeable; if a certain number of Joules of heat added to a system increases the internal energy of a body by, say, Δ U , the same number of Joules of work performed on the body would produce the same Δ U . In addition, work can be completely converted to heat, as everyday experience with friction attests. However, the reverse is not true; heat cannot be completely transformed into work. This limitation, which is a consequence of the second law, is best demonstrated by studying the properties of heat engines . A heat engine is a system operating in a cycle that receives heat from a high-temperature source (called a thermal reservoir) and produces useful work. However, since the efficiency of conversion must be less than 100%, some of the input heat is rejected to a cold reservoir. Figure 4.1 shows a schematic of a heat engine/heat pump and their associated thermal reservoirs. The reservoirs supply or receive heat without alteration of their temperatures. Heat flows in the reservoirs are reversible whether or not the engine is. FIGURE 4.1 A schematic of a heat engine or heat pump. The heat pump is a heat engine running in reverse. Hot Reservoir T H Cold Reservoir T L Heat Pump Heat Engine Cold Reservoir T L Hot Reservoir T H W W Q H Q L Q L Q H 110 General Thermodynamics The circle with the arrows in Figure 4.1 is a shorthand representation of the heat engine. It is intended to signify that the working substance (a fluid such as an ideal gas or water) moves through many thermodynamic states in a never-ending cyclic process. The detailed structure of the heat engine can vary greatly, but the simplest version contains the following four steps: 1. One in which heat is absorbed isothermally from the high-temperature reservoir. 2. The next, in which work is produced adiabatically.
eBook - PDF- George C. King(Author)
- 2023(Publication Date)
- Wiley(Publisher)
A series of turbine blades is used as the more gradual expansion of the steam in the turbine results in a closer approach to an ideal reversible process. After passing through the turbine, the steam is condensed to water, which is then returned to the boiler, and the process repeats. 226 Physics of Matter or ε 1 Q C Q H (7.3) We see that the efficiency of a heat engine can never be greater than 1, and could only be 1 if Q C = 0. No heat engine is able to convert the heat absorbed from a hot reservoir into work with 100% efficiency, i.e. Q C can never be equal to zero. Some of the absorbed heat must be discarded to a reservoir at a lower temperature. This is a consequence of the second law of thermodynamics, as we will see in Section 7.7. In practice, the best heat engines operate with close to 40% efficiency. One version of the second law of thermodynamics, called the Kelvin statement, follows from the empir- ical observation that Q C can never be equal to zero: It is impossible for an engine that operating in a cycle will produce no other effect than the extraction of heat from a reservoir and the performance of an equivalent amount of work. If this were not true, it would be possible, for example, to extract heat from the ocean to power a ship. There is a huge amount of thermal energy in the oceans of the world, but without the availability of a heat reservoir at a lower temperature, it cannot be converted into useful work. 7.2.2 Refrigerators and heat pumps We can think of a refrigerator as a heat engine working in reverse. In the operation of a heat engine, heat is taken from a hot reservoir and heat is discarded to a cold reservoir. In the case of a refrigerator, heat is removed from a cold reservoir and heat is discarded to a hot reservoir. To achieve this, there must be an input of work to drive the engine. Hot reservoir, T H Cold reservoir, T C W Q H Q C Heat engine Figure 7.4 A schematic representation of a heat engine.
eBook - PDF- Harold Morowitz(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
If we are careful to define 33 34 4. The Second Law of Thermodynamics our concepts, this difference between real and idealized engines and refrig-erators will not be a cause of misunderstanding. The entities that we wish to deal with are then: 1. Heat Engine. A heat engine is a device which takes heat from a hot source and performs mechanical work (definition). The hot source can be combusting gasoline, the steam from a boiler, the sun, or any other high-temperature generator. In order to continuously produce work, it is necessary for an engine to operate in a cycle; that is, it must take in heat, produce work, and return to its original state so that it can repeat the process and continuously develop output. It is found that if an engine does not operate in a cycle, its internal state changes until it reaches a point where it can no longer function (empirical generalization). Our concern will then be with heat engines which operate in a cyclic fashion. 2. Refrigerator. A refrigerator is a device which transfers heat from a cold body to a hotter body (definition). Refrigerators which operate continuously also operate in cycles; that is, they must transfer heat and return to their original state so as to repeat the process. Work (generally mechanical) must be performed in order to accomplish this kind of heat transfer (empirical generalization). B. STA TEMENTS OF THE SECOND LA W The second law of thermodynamics was originally formulated in two different ways, one about heat engines and the second about refrigerators. In subsequent considerations it is necessary to use both statements inter-changeably, so that we must prove their equivalence. These inductions are the foundation upon which all of classical thermodynamics is based.
eBook - PDF- Lokesh Pandey(Author)
- 2020(Publication Date)
- Arcler Press(Publisher)
Carnot pictured about the heat engines as deriving the mechanical effect with the help of passing the heat from a place which has higher temperature to the place having comparatively low temperature. Carnot was able to depict that the greatest mechanical effect for any two these types of places was generated with the help of the heat engine that could be put into use with respect to the reverse and that all such reversible engines which can performed equally well, as well as independently of their construction and whether their working material was air, steam or anything else.The original analysis of the Carnot was direct within the caloric theory of heat under the postulation that heat is conserved through the course of its passage from hot to cold. There is another subsequent analysis that is done by Clausius (1851) and Thomson (1852) altered the analysis done by the Carnot with the help of requiring that the work produced with the help of a heat engine, which as an outcome, derived from the conversion of some of the heat into the work. The Concept of Reversibility and Irreversibility in Thermodynamics 115 The results of the Carnot are stated with respect to the internally contradictory notion of a reversible heat engine that could transfer heat without the variation in the temperature. A heat engine is a series of a thermodynamically reversible processes that draws and releases the heat from and to a hot place and cold place. On the other hand, through the course of the conversion of the heat energy into the work. Integrating them, this engine is just a single lengthy thermodynamically reversible process. The efficiency η of a heat engine is defined in the common way η = W/Q, where W is the work delivered with the help of the engine and Q the heat drawn from the hot place. A reversible heat engine is consisting of a set of irreversible processes that produces a reversible process.
eBook - PDF- Claus Borgnakke, Richard E. Sonntag(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
.................................................................................................................................................................... ............. HEAT ENGINES AND REFRIGERATORS 145 during the cycle. From the first law we conclude that the net heat transfer is positive and equals the work done during the cycle. A heat engine may be defined as a device that operates in a thermodynamic cycle and produces a certain amount of net positive work through the transfer of heat from a high-temperature body to a low-temperature body. Often the term heat engine is used in a broader sense to include all devices that produce work, either through heat transfer or through combustion, even though the device does not operate in a thermodynamic cycle. The internal combustion engine and the gas turbine are examples of such devices, and call- ing them heat engines is an acceptable use of the term. In this chapter, however, we are concerned with the more restricted form of heat engine, as just defined, one that operates on a thermodynamic cycle. A simple steam power plant is an example of a heat engine in this restricted sense. Each component in this plant may be analyzed individually as a steady-state, steady-flow process, but as a whole it may be considered a heat engine (Fig. 5.4) in which water (steam) is the working fluid. An amount of heat, Q H , is transferred from a high-temperature body, which may be the products of combustion in a furnace, a reactor, or a secondary fluid that in turn has been heated in a reactor. In Fig. 5.4 the turbine is shown schematically as driving the pump. What is significant, however, is the net work that is delivered during the cycle. The quantity of heat Q L is rejected to a low-temperature body, which is usually the cooling water in a condenser.
No longer available |Learn more- Irving Granet, Maurice Bluestein(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
The original transitional energy as work has been converted into heat, and this heat can be expressed as molecular work. However, this form of energy will not be available to return the body to its original state. From this simple example, we note that work can be converted into heat, but that the conversion of heat into useful work may not always be possible. Even though the first law states that energy is conserved, it does not furnish the necessary information to enable us to determine whether energy has become unavailable. It is now necessary to define certain terms. The first of these is the concept of a heat engine. As defined by Keenan (1941, p. 58): A heat engine may be defined as a continuously operating system across whose boundar-ies flow only heat and work. It may be used to deliver work to external devices, or it may receive work from external devices and cause heat to flow from a low level of temperature to a high level of temperature. This latter type of heat engine is known as a refrigerator. In essence, this definition of a heat engine can be taken to be the definition of a thermody-namic cycle , which we shall understand to be a series of thermodynamic processes during which the working fluid can be made to undergo changes involving only heat and work interchanges and is then returned to its original state. The purpose of the conventional engineering thermodynamic cycle is, of course, to con-vert heat into work. In an air-conditioning or refrigeration cycle, work is used to remove heat from an area in which it is undesirable. Other special cycles exist but are not treated in this text. Associated with the concept of a cycle is the term efficiency.
eBook - ePub- Nihal E Wijeysundera(Author)
- 2016(Publication Date)
- WSPC(Publisher)
heat engine cycle .6.1.1Efficiency of a heat engine cycle
The efficiency η of the heat engine cycle is defined as the net work output per unit heat input. This can be expressed as:Applying the first law to the cycle we haveFrom Eqs. (6.1) and (6.2) we obtainIt is seen that the efficiency of the cycle has an economic significance because in a typical power plant the heat source is maintained at the high temperature by burning fuel, which constitutes the main operating energy cost of the plant. The net work delivered to the surroundings, on the other hand, is the desired output of the plant. Therefore the maximization of the efficiency of the cycle should be the primary objective of power plant design.6.2The Reversed Heat Engine Cycle
In dealing with topics related to the second law we also encounter an important class of energy conversion systems called reversed heat engine cycles. In practical terms these are refrigerators or heat pumps as they are sometimes called. Whereas heat engines deliver work by absorbing heat from high temperature sources, Reversed Heat Engines transfer heat from bodies at low temperatures to bodies at high temperatures. The latter process does not occur without the aid of an external energy input.Illustrated in Fig. 6.4 is a typical reversed heat engine cycle which operates using a working fluid that undergoes phase change. It should be noted that this is an idealized cycle that differs somewhat from the ideal vapor-compression refrigeration cycle due mainly to practical reasons.Let us now follow the passage of a ‘packet’ of fluid through the various sub-components of the plant shown in Fig. 6.4
eBook - PDFThermodynamics
From Concepts to Applications, Second Edition
- Arthur Shavit, Chaim Gutfinger(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
Heat Engines and Second Law of Thermodynamics 153 The second principle, stating that all reversible heat engines operating between two given reservoirs have the same efficiency, follows directly from the first principle. Indeed, if there are two reversible engines A and B, it follows from principle 1 that the efficiency of engine A cannot exceed that of B, as B is reversible, and at the same time the efficiency of B cannot exceed that of A, as A is reversible. Hence, both engines must have the same efficiency. Principle 2 is instrumental in defining a thermodynamic temperature scale discussed in Section 7.8. Some real engineering cycles are presented in Chapter 11, and their thermal efficien-cies come out lower than those of reversible engines operating between the same limit temperatures. This is, perhaps, not surprising, because the efficiency of a reversible engine emerged as the maximum possible efficiency for any heat engine. A question may be asked: is the lower thermal efficiency of an irreversible heat engine a direct result of its irrevers-ibility? To answer this question we introduce a third theorem: The thermal efficiency of an irreversible heat engine is lower than that of a reversible engine operating between the same reservoirs. To prove this theorem we invoke the definition of reversibility, presented in Section 7.4, and conclude that an irreversible cycle is one for which no complementary cycle exists, such that the system and its environment return back to their respective original states. If such a complementary cycle could be found, the original cycle would be, by definition, reversible. Now consider again Figure 7.11 in which we take engine X as irreversible. Suppose it had the same thermal efficiency as the reversible RevR engine. We now reverse the reversible engine as shown in the figure and use the work delivered by X to run RevR.
eBook - PDF- William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
For example, a steam engine on an old-style train can produce the work needed for driving the train. Several questions emerge from the construction and application of heat engines. For example, what is the maximum percentage of the heat extracted that can be used to do work? This turns out to be a question that can only be answered through the second law of thermodynamics. The second law of thermodynamics can be formally stated in several ways. One statement presented so far is about the direction of spontaneous heat flow, known as the Clausius statement. A couple of other statements are based on heat engines. Whenever we consider heat engines and associated devices such as refrigerators and heat pumps, we do not use the normal sign convention for heat and work. For convenience, we assume that the symbols Q h , Q c , and W represent only the amounts of heat transferred and work delivered, regardless what the givers or receivers are. Whether heat is entering or leaving a system and work is done to or by a system are indicated by proper signs in front of the symbols and by the directions of arrows in diagrams. It turns out that we need more than one heat source/sink to construct a heat engine. We will come back to this point later in the chapter, when we compare different statements of the second law of thermodynamics. For the moment, we assume that a heat engine is constructed between a heat source (high-temperature reservoir or hot reservoir) and a heat sink (low- temperature reservoir or cold reservoir), represented schematically in Figure 4.4. The engine absorbs heat Q h from a heat source ( hot reservoir) of Kelvin temperature T h , uses some of that energy to produce useful work W, and then discards the remaining energy as heat Q c into a heat sink ( cold reservoir) of Kelvin temperature T c . Power plants and internal combustion engines are examples of heat engines.
eBook - PDF- Bernard Desmet(Author)
- 2022(Publication Date)
- Wiley-ISTE(Publisher)
As indicated in the previous section, heat transfers between the hot source, working fluid and the heat sink require finite temperature differences which constitute sources of irreversibility. Thus, the amount of heat Q h provided by the hot source of temperature T h is used by the Carnot engine at a temperature T h that is lower than T h , and the amount of heat Q c transferred by the working fluid at the temperature T c to the heat sink at the temperature T c is lower than T c (Figure 1.7). Figure 1.7. An endoreversible dithermal cycle According to the second law of thermodynamics: Q h T h + Q c T c + ΔS irr = 0 [1.15] 10 Thermodynamics of Heat Engines where ΔS irr is the increase in entropy due to temperature differences. By expressing Q c : − Q c = Q h T c T h + T c ΔS irr [1.16] and by transforming [1.16] into the expression of the first law of thermodynamics (equation [1.7]): W = T c ΔS irr − Q h 1 − T c T h [1.17] we deduce the thermodynamic efficiency of the endoreversible cycle to be: η th = |W | Q h = 1 − T c T h ⎛ ⎝ 1 − T c ΔS irr Q h 1 − T c T h ⎞ ⎠ [1.18] REMARK.– If ΔS irr = 0 (reversible cycle), then: η th = 1 − T c T h = η c [1.19] which is the endoreversible efficiency of the Carnot cycle. 1.2.3.5. Exergy of a closed system In the case of a closed system that evolves from an initial state i to a final state f , for example under the action of a piston as represented in Figure 1.8, by taking into account the mechanical equilibrium equation for the piston, the work exchanged with the volume V of the elastic fluid is given by: W e = (i→f ) − p dV = W m + (i→f ) − p a dV [1.20] The piston is assumed to be in equilibrium under the action of the fluid pressure p contained in the cylinder, the pressure p a of the environment and the force F of the mechanical actions applied to the piston. The displacement force F of the piston corresponds to the useful mechanical work W m that is exchanged with the exterior.
eBook - PDF- Petric? Vizureanu, Petrică Vizureanu, Petrică Vizureanu(Authors)
- 2020(Publication Date)
- IntechOpen(Publisher)
Similarly, by the First and Second Laws of Thermodynamics, for a standard endoreversible heat engine operating (without HTR) at Curzon-Ahlborn efficiency, the heat input Q H from its hot reservoir, the work output W , the waste heat Q C exhausted to its cold reservoir, the efficiency ϵ CA,std , the entropy change Δ S H ,CA,std of its hot reservoir, the entropy change Δ S C ,CA,std of its cold reservoir, and the total entropy change Δ S total,CA,std ¼ Δ S H ,CA,std þ Δ S C ,CA,std are related in accordance with W ¼ Q H � Q C ¼ Q H ϵ CA,std ¼ Q H 1 � R 1 = 2 T � � ) Q C ¼ R 1 = 2 T Q H : (7) and Δ S total,CA,std ¼ Δ S H ,CA,std þ Δ S C ,CA,std ¼ � Q H T H þ Q C T C ¼ � Q H T H þ R 1 = 2 T Q H R T T H ¼ Q H T H 1 R 1 = 2 T � 1 ! ¼ Q H T H R � 1 = 2 T � 1 � � > Δ S total,Carnot,std ¼ 0 : (8) Note that for any R T in general and as R T ! 0 in particular, Δ S total,Carnot,std ¼ 0; by contrast, for any R T in general Δ S total,CA,std > 0 ( Δ S total,CA,std ! 0 only in the limit R T ! 1), and as R T ! 0, Δ S total,CA,std ! ∞ . As we have already noted, heat engines ’ work outputs are, in almost all cases, totally frictionally dissipated as heat immediately or on short time scales [16, 17]. For example, an automobile heat engine ’ s work output in initially accelerating the automobile is typically frictionally dissipated only a short time later the next time the automobile decelerates; its work output while the automobile travels at constant speed is immediately and continually frictionally dissipated. [Rare exceptions include, for example, a noncyclic rocket heat engine ’ s work output being seques-tered essentially permanently as kinetic and gravitational potential energy in the launching of a spacecraft (but typically most of the kinetic energy accelerates the 114 Thermodynamics and Energy Engineering
- Anthony R. Philpotts, Jay J. Ague(Authors)
- 2022(Publication Date)
- Cambridge University Press(Publisher)
For example, will mineral A change into B during metamorphism, or will B change into A? This principle is embodied in the second law of thermo- dynamics, and it involves an obscure property of material known as entropy (S), for which the following discussion lays the ground. During the development of thermodynamics, the effi- ciency of heat engines received a great deal of attention. Heat engines use a working fluid, normally liquid or gas, to absorb or transmit energy. Examples include the steam engine and the internal combustion automobile engine. The fundamental principles governing heat engine efficiency were first examined by the engineer Nicolas Léonard Sadi Carnot. His brilliant study, published in 1824, laid the foun- dations for the second law and the Kelvin temperature scale. A geological example of a heat engine is the gener- ation of electricity by geothermal energy (Section 2.5; Figs. 2.9, 8.4). For example, consider the dry steam type of geothermal power plant. Heat is transferred upward from magma at depth to raise the temperature of circulating groundwater to produce steam. The high-temperature steam is brought rapidly to the surface where it expands to drive turbines and generate electricity. After passing through the turbines, the steam is condensed to water at lower tem- perature and exhausted from the system, where it may be pumped back underground, reheated, and reused. A defin- ing characteristic of this and all other heat engines is that they operate in a cycle between a high-temperature (HT ) heat 192 8 Introduction to Thermodynamics Figure 8.4 Geothermal electricity generation (schematic, not to scale). The H 2 O is pumped in a cycle between the high-temperature heat reservoir (hot rock heated by magma), and the low-temperature heat reservoir (condenser supplied by low-temperature water from the cooling towers). Electricity is produced by a generator (G) connected to the turbine (T).
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