Editorial

2 pages, 148 KiB

Open AccessEditorial

Carnot Cycle and Heat Engine: Fundamentals and Applications

by Michel Feidt

Entropy 2020, 22(3), 348; https://doi.org/10.3390/e22030348 - 18 Mar 2020

Cited by 10 | Viewed by 3190

After two years of exchange, this specific issue dedicated to the Carnot cycle and thermomechanical engines has been completed with ten papers including this editorial [...] Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

Research

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15 pages, 6749 KiB

Open AccessArticle

Second Law Analysis for the Experimental Performances of a Cold Heat Exchanger of a Stirling Refrigeration Machine

by Steve Djetel-Gothe, François Lanzetta and Sylvie Bégot

Entropy 2020, 22(2), 215; https://doi.org/10.3390/e22020215 - 14 Feb 2020

Cited by 7 | Viewed by 3159

Abstract

The second law of thermodynamics is applied to evaluate the influence of entropy generation on the performances of a cold heat exchanger of an experimental Stirling refrigeration machine by means of three factors: the entropy generation rate

N_{S}

, the irreversibility distribution [...] Read more.

The second law of thermodynamics is applied to evaluate the influence of entropy generation on the performances of a cold heat exchanger of an experimental Stirling refrigeration machine by means of three factors: the entropy generation rate

N_{S}

, the irreversibility distribution ratio ϕ and the Bejan number

{B e |}_{N_{S}}

based on a dimensionless entropy ratio that we introduced. These factors are investigated as functions of characteristic dimensions of the heat exchanger (hydraulic diameter and length), coolant mass flow and cold gas temperature. We have demonstrated the role of these factors on the thermal and fluid friction irreversibilities. The conclusions are derived from the behavior of the entropy generation factors concerning the heat transfer and fluid friction characteristics of a double-pipe type heat exchanger crossed by a coolant liquid (55/45 by mass ethylene glycol/water mixture) in the temperature range 240 K < T_C < 300 K. The mathematical model of entropy generation includes experimental measurements of pressures, temperatures and coolant mass flow, and the characteristic dimensions of the heat exchanger. A large characteristic length and small hydraulic diameter generate large entropy production, especially at a low mean temperature, because the high value of the coolant liquid viscosity increases the fluid frictions. The model and experiments showed the dominance of heat transfer over viscous friction in the cold heat exchanger and

{B e |}_{N_{S}} \to 1

and ϕ → 0 for mass flow rates

\dot{m} \to 0.1

kg.s⁻¹. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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19 pages, 4838 KiB

Open AccessEditor’s ChoiceArticle

Energy and Exergy Evaluation of a Two-Stage Axial Vapour Compressor on the LNG Carrier

by Igor Poljak, Josip Orović, Vedran Mrzljak and Dean Bernečić

Entropy 2020, 22(1), 115; https://doi.org/10.3390/e22010115 - 17 Jan 2020

Cited by 7 | Viewed by 4186

Abstract

Data from a two-stage axial vapor cryogenic compressor on the dual-fuel diesel–electric (DFDE) liquefied natural gas (LNG) carrier were measured and analyzed to investigate compressor energy and exergy efficiency in real exploitation conditions. The running parameters of the two-stage compressor were collected while [...] Read more.

Data from a two-stage axial vapor cryogenic compressor on the dual-fuel diesel–electric (DFDE) liquefied natural gas (LNG) carrier were measured and analyzed to investigate compressor energy and exergy efficiency in real exploitation conditions. The running parameters of the two-stage compressor were collected while changing the main propeller shafts rpm. As the compressor supply of vaporized gas to the main engines increases, so does the load and rpm in propulsion electric motors, and vice versa. The results show that when the main engine load varied from 46 to 56 rpm at main propulsion shafts increased mass flow rate of vaporized LNG at a two-stage compressor has an influence on compressor performance. Compressor average energy efficiency is around 50%, while the exergy efficiency of the compressor is significantly lower in all measured ranges and on average is around 34%. The change in the ambient temperature from 0 to 50 °C also influences the compressor’s exergy efficiency. Higher exergy efficiency is achieved at lower ambient temperatures. As temperature increases, overall compressor exergy efficiency decreases by about 7% on average over the whole analyzed range. The proposed new concept of energy-saving and increasing the compressor efficiency based on pre-cooling of the compressor second stage is also analyzed. The temperature at the second stage was varied in the range from 0 to −50 °C, which results in power savings up to 26 kW for optimal running regimes. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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13 pages, 979 KiB

Open AccessArticle

Progress in Carnot and Chambadal Modeling of Thermomechanical Engine by Considering Entropy Production and Heat Transfer Entropy

by Michel Feidt and Monica Costea

Entropy 2019, 21(12), 1232; https://doi.org/10.3390/e21121232 - 16 Dec 2019

Cited by 32 | Viewed by 3126

Abstract

Nowadays the importance of thermomechanical engines is recognized worldwide. Since the industrial revolution, physicists and engineers have sought to maximize the efficiency of these machines, but also the mechanical energy or the power output of the engine, as we have recently found. The [...] Read more.

Nowadays the importance of thermomechanical engines is recognized worldwide. Since the industrial revolution, physicists and engineers have sought to maximize the efficiency of these machines, but also the mechanical energy or the power output of the engine, as we have recently found. The optimization procedure applied in many works in the literature focuses on considering new objective functions including economic and environmental criteria (i.e., ECOP ecological coefficient of performance). The debate here is oriented more towards fundamental aspects. It is known that the maximum of the power output is not obtained under the same conditions as the maximum of efficiency. This is shown, among other things, by the so-called nice radical that accounts for efficiency at maximum power, most often for the endoreversible configuration. We propose here to enrich the model and the debate by emphasizing the fundamental role of the heat transfer entropy together with the production of entropy, accounting for the external or internal irreversibilities of the converter. This original modeling to our knowledge, leads to new and more general results that are reported here. The main consequences of the approach are emphasized, and new limits of the efficiency at maximum energy or power output are obtained. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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20 pages, 6051 KiB

Open AccessEditor’s ChoiceArticle

OTEC Maximum Net Power Output Using Carnot Cycle and Application to Simplify Heat Exchanger Selection

by Kevin Fontaine, Takeshi Yasunaga and Yasuyuki Ikegami

Entropy 2019, 21(12), 1143; https://doi.org/10.3390/e21121143 - 22 Nov 2019

Cited by 26 | Viewed by 6621

Abstract

Ocean thermal energy conversion (OTEC) uses the natural thermal gradient in the sea. It has been investigated to make it competitive with conventional power plants, as it has huge potential and can produce energy steadily throughout the year. This has been done mostly [...] Read more.

Ocean thermal energy conversion (OTEC) uses the natural thermal gradient in the sea. It has been investigated to make it competitive with conventional power plants, as it has huge potential and can produce energy steadily throughout the year. This has been done mostly by focusing on improving cycle performances or central elements of OTEC, such as heat exchangers. It is difficult to choose a suitable heat exchanger for OTEC with the separate evaluations of the heat transfer coefficient and pressure drop that are usually found in the literature. Accordingly, this paper presents a method to evaluate heat exchangers for OTEC. On the basis of finite-time thermodynamics, the maximum net power output for different heat exchangers using both heat transfer performance and pressure drop was assessed and compared. This method was successfully applied to three heat exchangers. The most suitable heat exchanger was found to lead to a maximum net power output 158% higher than the output of the least suitable heat exchanger. For a difference of 3.7% in the net power output, a difference of 22% in the Reynolds numbers was found. Therefore, those numbers also play a significant role in the choice of heat exchangers as they affect the pumping power required for seawater flowing. A sensitivity analysis showed that seawater temperature does not affect the choice of heat exchangers, even though the net power output was found to decrease by up to 10% with every temperature difference drop of 1 °C. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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13 pages, 14973 KiB

Open AccessArticle

Performance of a Simple Energetic-Converting Reaction Model Using Linear Irreversible Thermodynamics

by J. C. Chimal-Eguia, R. Paez-Hernandez, Delfino Ladino-Luna and Juan Manuel Velázquez-Arcos

Entropy 2019, 21(11), 1030; https://doi.org/10.3390/e21111030 - 24 Oct 2019

Cited by 6 | Viewed by 1819

Abstract

In this paper, the methodology of the so-called Linear Irreversible Thermodynamics (LIT) is applied to analyze the properties of an energetic-converting biological process using simple model for an enzymatic reaction that couples one exothermic and one endothermic reaction in the same fashion as [...] Read more.

In this paper, the methodology of the so-called Linear Irreversible Thermodynamics (LIT) is applied to analyze the properties of an energetic-converting biological process using simple model for an enzymatic reaction that couples one exothermic and one endothermic reaction in the same fashion as Diaz-Hernandez et al. (Physica A, 2010, 389, 3476–3483). We extend the former analysis to consider three different operating regimes; namely, Maximum Power Output (MPO), Maximum Ecological Function (MEF) and Maximum Efficient Power Function (MEPF), respectively. Based on the later, it is possible to generalize the obtained results. Additionally, results show analogies in the optimal performance between the different optimization criteria where all thermodynamic features are determined by three parameters (the chemical potential gap

Δ = \frac{μ_{1} - μ_{4}}{R T}

, the degree of coupling q and the efficiency

η

). This depends on the election that leads to more or less efficient energy exchange. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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9 pages, 852 KiB

Open AccessArticle

What Is the Real Clausius Statement of the Second Law of Thermodynamics?

by Ti-Wei Xue and Zeng-Yuan Guo

Entropy 2019, 21(10), 926; https://doi.org/10.3390/e21100926 - 24 Sep 2019

Cited by 10 | Viewed by 4141

Abstract

In this paper, we first analyze the difference between the second law of thermodynamics and the laws in other disciplines. There are some phenomena in other disciplines similar to the Clausius Statement of the second law, but none of them has been accepted [...] Read more.

In this paper, we first analyze the difference between the second law of thermodynamics and the laws in other disciplines. There are some phenomena in other disciplines similar to the Clausius Statement of the second law, but none of them has been accepted as the statement of a certain law. Clausius’ mechanical theory of heat, published in the nineteenth century, is then introduced and discussed in detail, from which it is found that Clausius himself regarded “Theorem of the equivalence of the transformation of heat to work, and the transformation of heat at a higher temperature to a lower temperature”, rather than “Heat can never pass from a colder to a warmer body without some other change”, as the statement of the second law of thermodynamics. The latter is only laid down as the fundamental principle for deriving the theorem of the equivalence of transformations. Finally, based on the theorem of the equivalence of transformations and the average temperature method, a general quantitative relation among the heat, the work, and the temperatures is obtained for arbitrary cycles, which is thus recommended as an alternative mathematic expression of the second law. Hence, the theorem of the equivalence of transformations is the real Clausius Statement of the second law of thermodynamics. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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763 KiB

Open AccessArticle

Global Efficiency of Heat Engines and Heat Pumps with Non-Linear Boundary Conditions

by Per Lundqvist and Henrik Öhman

Entropy 2017, 19(8), 394; https://doi.org/10.3390/e19080394 - 31 Jul 2017

Cited by 2 | Viewed by 4893

Abstract

Analysis of global energy efficiency of thermal systems is of practical importance for a number of reasons. Cycles and processes used in thermal systems exist in very different configurations, making comparison difficult if specific models are required to analyze specific thermal systems. Thermal [...] Read more.

Analysis of global energy efficiency of thermal systems is of practical importance for a number of reasons. Cycles and processes used in thermal systems exist in very different configurations, making comparison difficult if specific models are required to analyze specific thermal systems. Thermal systems with small temperature differences between a hot side and a cold side also suffer from difficulties due to heat transfer pinch point effects. Such pinch points are consequences of thermal systems design and must therefore be integrated in the global evaluation. In optimizing thermal systems, detailed entropy generation analysis is suitable to identify performance losses caused by cycle components. In plant analysis, a similar logic applies with the difference that the thermal system is then only a component, often industrially standardized. This article presents how a thermodynamic “black box” method for defining and comparing thermal efficiency of different size and types of heat engines can be extended to also compare heat pumps of different apparent magnitude and type. Impact of a non-linear boundary condition on reversible thermal efficiency is exemplified and a correlation of average real heat engine efficiencies is discussed in the light of linear and non-linear boundary conditions. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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1189 KiB

Open AccessArticle

Carnot-Like Heat Engines Versus Low-Dissipation Models

by Julian Gonzalez-Ayala, José Miguel M. Roco, Alejandro Medina and Antonio Calvo Hernández

Entropy 2017, 19(4), 182; https://doi.org/10.3390/e19040182 - 23 Apr 2017

Cited by 20 | Viewed by 5809

Abstract

In this paper, a comparison between two well-known finite time heat engine models is presented: the Carnot-like heat engine based on specific heat transfer laws between the cyclic system and the external heat baths and the Low-Dissipation model where irreversibilities are taken into [...] Read more.

In this paper, a comparison between two well-known finite time heat engine models is presented: the Carnot-like heat engine based on specific heat transfer laws between the cyclic system and the external heat baths and the Low-Dissipation model where irreversibilities are taken into account by explicit entropy generation laws. We analyze the mathematical relation between the natural variables of both models and from this the resulting thermodynamic implications. Among them, particular emphasis has been placed on the physical consistency between the heat leak and time evolution on the one side, and between parabolic and loop-like behaviors of the parametric power-efficiency plots. A detailed analysis for different heat transfer laws in the Carnot-like model in terms of the maximum power efficiencies given by the Low-Dissipation model is also presented. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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Other

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1312 KiB

Open AccessPerspective

The History and Perspectives of Efficiency at Maximum Power of the Carnot Engine

by Michel Feidt

Entropy 2017, 19(7), 369; https://doi.org/10.3390/e19070369 - 19 Jul 2017

Cited by 40 | Viewed by 5124

Abstract

Finite Time Thermodynamics is generally associated with the Curzon–Ahlborn approach to the Carnot cycle. Recently, previous publications on the subject were discovered, which prove that the history of Finite Time Thermodynamics started more than sixty years before even the work of Chambadal and [...] Read more.

Finite Time Thermodynamics is generally associated with the Curzon–Ahlborn approach to the Carnot cycle. Recently, previous publications on the subject were discovered, which prove that the history of Finite Time Thermodynamics started more than sixty years before even the work of Chambadal and Novikov (1957). The paper proposes a careful examination of the similarities and differences between these pioneering works and the consequences they had on the works that followed. The modelling of the Carnot engine was carried out in three steps, namely (1) modelling with time durations of the isothermal processes, as done by Curzon and Ahlborn; (2) modelling at a steady-state operation regime for which the time does not appear explicitly; and (3) modelling of transient conditions which requires the time to appear explicitly. Whatever the method of modelling used, the subsequent optimization appears to be related to specific physical dimensions. The main goal of the methodology is to choose the objective function, which here is the power, and to define the associated constraints. We propose a specific approach, focusing on the main functions that respond to engineering requirements. The study of the Carnot engine illustrates the synthesis carried out and proves that the primary interest for an engineer is mainly connected to what we called Finite (physical) Dimensions Optimal Thermodynamics, including time in the case of transient modelling. Full article

(This article belongs to the Special Issue Carnot Cycle and Heat Engine Fundamentals and Applications)

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