Physics
Thermal Efficiency
Thermal efficiency is a measure of how well a device converts heat energy into useful work. It is calculated by dividing the useful output of the device by the input heat energy. A higher thermal efficiency indicates that a greater proportion of the input energy is being converted into useful work, making the device more efficient.
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8 Key excerpts on "Thermal Efficiency"
eBook - PDFEnergy and Society
An Introduction, Second Edition
- Harold H. Schobert(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
The heat engine efficiency is a measure of the energy that can be converted to work as a fraction of the total energy. This concept can be expressed mathemati-cally in an equation for calculating efficiency: η = - T T T h l h Using this equation, the numerical value of efficiency will be a fraction having a value between 0 and 1. Often it is more convenient or easier to express efficiency as FIGURE 11.8 Sir William Thomson, Lord Kelvin (1824–1907), one of the foremost physicists of the nineteenth century. Kelvin developed most of his theories while a professor at the University of Glasgow. The unit of the absolute temperature scale is named in his honor. (From http:// commons.wikimedia.org/wiki/FileSir_William_Thomson_Baron_Kelvin_1824_-_1907_ Scientist_resting_on_a_binnacle_and_holding_a_marine_azimuth_mirror.jpg.) 159 Heat and Thermal Efficiency a percentage, rather than as a fraction. This is done readily by using the comparable formula: η = - T T T % 100 h l h In this case, efficiency has a value between 0% and 100%. The equations are equiva-lent and provide the same information; the difference is whether the calculated effi-ciency is expressed as a fraction or as a percentage. However, there is one crucial fact in the use of efficiency calculations: these equations only provide the correct answer if the temperature is expressed in kelvins . The ratio of the available energy to the total energy represents the maximum efficiency of a heat engine. This efficiency value, sometimes called the Carnot effi-ciency , is reached only if the engine is mechanically perfect, for example, if there are no losses of energy through friction or losses of heat to the outside world. In any real heat engine, the efficiency is less—sometimes considerably so—than the predicted maximum (Carnot) efficiency, simply because of such problems as friction among the parts and losses of heat to the surroundings.
- Jonathan Wickert, Jonathan Wickert, Kemper Lewis(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
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. 300 Chapter 7 Thermal and Energy Systems The real efficiency is sometimes described as the ratio of “what you get” (the work produced by the engine) to “what you paid” (the heat input), and it is often expressed as a percentage. If an automobile’s engine has an overall efficiency of 20%, then for every 5 L of fuel consumed, only 1 L worth of energy is converted for the useful purpose of powering the vehicle. Such is the case even for a heat engine that has been optimized by millions of hours of research and development effort by skilled mechanical engineers. That seemingly low level of efficiency, however, is not as bad as you might think at first glance, given the limitations that physical laws place on our ability to convert heat into work. Table 7.5 lists real efficiencies for a variety of thermal and energy systems encountered in mechanical engineering.
- Lucien Borel, Daniel Favrat(Authors)
- 2010(Publication Date)
- PPUR(Publisher)
402 Thermodynamics and Energy Systems Analysis: from Energy to Exergy in which is the transformation energy received in the network n: (10.10) Particular cases 10.1.2 Effectiveness (or First Law efficiency) Definition Considering that the energy balance (10.7) expresses the conservation of energy, it is in principle inadequate to highlight the notion of thermodynamic loss. In general, the manipulation of a system always consists of receiving energy in one form or another as well as giving it in one form or another. Strictly speaking, the result of this is that the correct definition of “Thermal Efficiency” must approach a value of 1, i.e., 100 % – a value which in and of itself is, of course, of no interest. We know, however, that in practice it is opportune to define characteristic num- bers that can be used to express an interesting property of the considered system. In a general way, we denote by effectiveness the ratio between the useful energy and the used or dispensed energy. It is not possible to give a general definition for this ratio since the concept of usefulness and used vary from case to case. However, it is generally admitted that, considering the fact that it has no inter- est for the practitioner, the heat energy Q a transferred between the system and the atmosphere can neither be considered as an energy service received nor given by the system. This is why the term Q a will not be taken into account in the various formu- lations of the effectiveness. In other words, it has no value when we provide it and it The introduction of the concept of transformation energy is of great interest as it permits the inclusion in a single theory of closed systems, steady-state open sys- tems and non steady-state open systems.
eBook - ePubEnergy and Society
An Introduction, Second Edition
- Harold H. Schobert(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
low, respectively.The pool of water at the bottom of the waterfall still had some ENERGY . Analogously, the cold side of the steam engine still contains heat. Conceptually, more WORK could be extracted from the water in the pool until all the available gravitational potential energy of the water was used up—until we had reached the center of the Earth, which is the zero point of gravitational energy. Quite similarly, suppose that the condenser on a steam engine is at, say, 25°C. The water produced in the condenser could, in principle, be cooled further and further, until we reached a point at which no more work could be extracted from it. This point would be the temperature analog of the center of the Earth, the zero point of potential energy below which it is impossible to go. In a thermal system, this zero point of energy is the temperature called absolute zero. The total energy of the thermal system would be represented by the difference between the temperature of the hot side and absolute zero: Th − 0, or, simply Th .The heat engine efficiency is a measure of the ENERGY that can be converted to WORK as a fraction of the total ENERGY . This concept can be expressed mathematically in an equation for calculating efficiency:η =()T h−T 1T hUsing this equation, the numerical value of efficiency will be a fraction having a value between 0 and 1. Often it is more convenient or easier to express efficiency as a percentage, rather than as a fraction. This is done readily by using the comparable formula:% η = 100()T h−T 1T hIn this case, efficiency has a value between 0% and 100%. The equations are equivalent and provide the same information; the difference is whether the calculated efficiency is expressed as a fraction or as a percentage. However, there is one crucial fact in the use of efficiency calculations: these equations only provide the correct answer if the temperature is expressed in kelvins
- Jonathan Wickert, Kemper Lewis, Jonathan Wickert(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
With W being regarded as the heat engine’s useful output, all thermal and energy systems such as these have the characteristic that the supplied heat cannot be converted entirely into useful work. Referring to Equation (7.9), the energy balance for the heat engine is Q h 2 Q l 5 W (7.10) because there is no change in its internal energy. The real efficiency h (the lowercase Greek character eta) of the heat engine is defined as the ratio of the work output to the amount of heat supplied to it: 5 W Q h (7.11) In the light of Equation (7.10), we see that h 5 1 2 Q l /Q h for the heat engine. Since the amount of heat supplied to the engine is greater than the amount wasted (Q h . Q l ), the efficiency always lies between zero and one. Real efficiency Work, W Heat engine Q h Q l High-temperature energy source, T h Low-temperature reservoir, T l Figure 7.11 Conceptual view of a heat engine that operates between sources of energy that are maintained at high and low temperatures. Heat reservoir Copyright 2021 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. 300 Chapter 7 Thermal and Energy Systems The real efficiency is sometimes described as the ratio of “what you get” (the work produced by the engine) to “what you paid” (the heat input), and it is often expressed as a percentage. If an automobile’s engine has an overall efficiency of 20%, then for every 5 L of fuel consumed, only 1 L worth of energy is converted for the useful purpose of powering the vehicle.
eBook - PDFEngine Testing
Theory and Practice
- A. J. Martyr, M.A. PLINT(Authors)
- 2011(Publication Date)
- Butterworth-Heinemann(Publisher)
13 Thermal Efficiency, measurement of heat and mechanical losses Introduction This chapter deals with the measurements and calculations necessary to determine the energy balance, thermal performance and mechanical losses of an internal combustion engine. A brief account is given of the basic theory in order to provide a framework for an interpretation of these observations and as background reading for Chapter 14, where modern combustion analysis is discussed. A notation table is included at the end of the chapter. One ultimate measure of the performance of an internal combustion engine is the proportion of the heat of combustion of the fuel that is turned into useful work at the engine coupling. The Thermal Efficiency at full load of internal combustion engines ranges from about 20 per cent for small gasoline engines up to more than 50 per cent for large slow-running diesel engines, which are the most efficient means currently available of turning the heat of combustion of fuel into mechanical power. It is useful to have some idea of the theoretical maximum Thermal Efficiency that is possible, as this sets a target for the engine developer. Theoretical thermo-dynamics allows us, within certain limitations, to predict this maximum value. The proportion of the heat of combustion that is not converted into useful work appears elsewhere: in the exhaust gases, in the cooling medium and as convection and radi-ation from the hot surfaces of the engine. In addition, there may be appreciable losses in the form of unburned or late burning fuel. It is important to be able to evaluate these various losses. Of particular interest are losses from the hot gas in the cylinder to the containing surfaces, since these directly affect the indicated power of the engine. The so-called ‘adiabatic engine’ seeks to minimize these particular losses.
eBook - PDFEngineering Problem Solving
A Classical Perspective
- Milton C. Shaw(Author)
- 2001(Publication Date)
- William Andrew(Publisher)
Thermal Engineering 269 269 1.0 INTRODUCTION Important topics to be considered in this chapter are thermodynamics, thermal transformation systems, and heat transfer. Thermodynamics in-volves fundamental relationships between heat, work, and the properties of a system. It is concerned with the transformation of one form of energy into another and the basic laws that control such transformation. Of particular importance is the transformation of thermal energy into mechani-cal energy, which is the first step in the conversion of the energy associated with fossil fuels into electrical energy as discussed in Ch. 10. Thermal transformation systems are systems that transform thermal energy into mechanical energy. This includes steam power plants, steam engines, steam turbines, gas turbines, and internal combustion engines. Heat trans-fer is concerned with the transfer of thermal energy from one medium to another by: Radiation Conduction Convection 11 Thermal Engineering 270 Engineering Problem Solving: A Classical Perspective With radiation, heat is transferred by electromagnetic waves ema-nating from a hot body to a cold body where radiation waves are absorbed resulting in a temperature rise. Conductive heat transfer involves the passage of heat through a solid from a region of high temperature to one of lower temperature. Convective heat transfer involves the transport of thermal energy from a hot body to a fluid flowing across the hot body. 2.0 HISTORICAL BACKGROUND Before the 17 th century, little attention was given to thermal energy. The Phlogiston Theory of heat championed by Stahl (16601734) was the first generally accepted. This proposed that all combustible materials contain a massless material (phlogiston) that escapes on combustion. Some materials like sulfur were considered rich in phlogiston while others con-tained very little.
eBook - PDF- Raymond Serway, Chris Vuille(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Real processes proceed in an order governed by the second law of thermodynamics, which can be stated in two ways: 1. Energy will not flow spontaneously by heat from a cold object to a hot object. 2. No heat engine operating in a cycle can absorb energy from a reservoir and perform an equal amount of work. No real heat engine operating between the Kelvin tem- peratures T h h and h and h T c c can exceed the efficiency of an engine c can exceed the efficiency of an engine c operating between the same two temperatures in a Carnot cycle (Fig. 12.27), given by e C 5 1 2 T c c T h h [12.16] The Thermal Efficiency of a heat engine is defined as the y of a heat engine is defined as the y ratio of the work done by the engine to the energy trans- ferred into the engine per cycle: e ; W e e ng 0 Q h 0 5 1 2 0 Q c 0 0 Q h 0 [12.12] CONCEPTUAL QUESTIONS 1. Two identical containers each hold 1 mole of an ideal gas at 1 atm. Container A holds a monatomic gas and container B holds a diatomic gas. The gas in each container is compressed at constant pressure to half its original volume. (a) What is the ratio W A W A W /W B B of the work done on gas A to the work done on gas B? (b) What is the ratio ΔU A U A U /ΔU B B of the change in internal energy for gases A and B? (c) What is the ratio Q A /Q B of the energy transferred to gases A and B ? 2. Which one of the following statements is true? (a) The path on a PV diagram always goes from the smaller volume to the diagram always goes from the smaller volume to the larger volume. (b) The path on a PV diagram always goes diagram always goes Heat pumps (Fig. 12.26) are heat engines in reverse. In a refrigerator the heat pump removes thermal energy from inside the refrigerator. Heat pumps operating in cooling mode have coefficient of performance given by COP 1 cooling mode 2 5 0 Q c 0 W [12.13] Figure 12.26 Schematic diagram of a heat pump.
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