Articoli di riviste: "Carnot efficiency"

1

Lucia, Umberto. "Carnot efficiency: Why?" Physica A: Statistical Mechanics and its Applications 392, n. 17 (settembre 2013): 3513–17. http://dx.doi.org/10.1016/j.physa.2013.04.020.

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Polettini, Matteo, e Massimiliano Esposito. "Carnot efficiency at divergent power output". EPL (Europhysics Letters) 118, n. 4 (1 maggio 2017): 40003. http://dx.doi.org/10.1209/0295-5075/118/40003.

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Jennings, R. C., S. Santabarbara, E. Belgio e G. Zucchelli. "The Carnot efficiency and plant photosystems". Biophysics 59, n. 2 (marzo 2014): 230–35. http://dx.doi.org/10.1134/s0006350914020080.

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Su, Shanhe, Yanchao Zhang, Guozhen Su e Jincan Chen. "The Carnot efficiency enabled by complete degeneracies". Physics Letters A 382, n. 32 (agosto 2018): 2108–12. http://dx.doi.org/10.1016/j.physleta.2018.05.042.

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Jacob, K. T. "Fuel Cell Efficiency Redefined: Carnot Limit Reassessed". ECS Proceedings Volumes 2005-07, n. 1 (gennaio 2005): 629–39. http://dx.doi.org/10.1149/200507.0629pv.

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Chen, Lingen, Zewei Meng, Yanlin Ge e Feng Wu. "Performance Analysis and Optimization for Irreversible Combined Carnot Heat Engine Working with Ideal Quantum Gases". Entropy 23, n. 5 (27 aprile 2021): 536. http://dx.doi.org/10.3390/e23050536.

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An irreversible combined Carnot cycle model using ideal quantum gases as a working medium was studied by using finite-time thermodynamics. The combined cycle consisted of two Carnot sub-cycles in a cascade mode. Considering thermal resistance, internal irreversibility, and heat leakage losses, the power output and thermal efficiency of the irreversible combined Carnot cycle were derived by utilizing the quantum gas state equation. The temperature effect of the working medium on power output and thermal efficiency is analyzed by numerical method, the optimal relationship between power output and thermal efficiency is solved by the Euler-Lagrange equation, and the effects of different working mediums on the optimal power and thermal efficiency performance are also focused. The results show that there is a set of working medium temperatures that makes the power output of the combined cycle be maximum. When there is no heat leakage loss in the combined cycle, all the characteristic curves of optimal power versus thermal efficiency are parabolic-like ones, and the internal irreversibility makes both power output and efficiency decrease. When there is heat leakage loss in the combined cycle, all the characteristic curves of optimal power versus thermal efficiency are loop-shaped ones, and the heat leakage loss only affects the thermal efficiency of the combined Carnot cycle. Comparing the power output of combined heat engines with four types of working mediums, the two-stage combined Carnot cycle using ideal Fermi-Bose gas as working medium obtains the highest power output.

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Haseli, Y. "Substance Independence of Efficiency of a Class of Heat Engines Undergoing Two Isothermal Processes". Journal of Thermodynamics 2011 (25 maggio 2011): 1–5. http://dx.doi.org/10.1155/2011/647937.

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Three power producing cycles have been so far known that include two isothermal processes, namely, Carnot, Stirling, and Ericsson. It is well known that the efficiency of the Carnot cycle represented by is independent of its working fluid. Using fundamental relationships between thermodynamic properties including Maxwell's relationships, this paper shows in a closed form that the Ericsson and the Stirling cycles also possess the Carnot efficiency irrespective of the nature of the working gas.

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Ying Ng, Nelly Huei, Mischa Prebin Woods e Stephanie Wehner. "Surpassing the Carnot efficiency by extracting imperfect work". New Journal of Physics 19, n. 11 (7 novembre 2017): 113005. http://dx.doi.org/10.1088/1367-2630/aa8ced.

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Moreno, Daniel, e Marta C. Hatzell. "Efficiency of Carnot and Conventional Capacitive Deionization Cycles". Journal of Physical Chemistry C 122, n. 39 (7 settembre 2018): 22480–86. http://dx.doi.org/10.1021/acs.jpcc.8b05940.

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Purwanto, A., H. Sukamto e B. A. Subagyo. "Quantum Carnot Heat Engine Efficiency with Minimal Length". Journal of Modern Physics 06, n. 15 (2015): 2297–302. http://dx.doi.org/10.4236/jmp.2015.615234.

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Ferreiro Garcia, Ramon, e Dr Jose Carbia Carril. "Analysis of a thermal cycle that surpass Carnot efficiency undergoing closed polytropic transformations". JOURNAL OF ADVANCES IN PHYSICS 15 (19 febbraio 2019): 6165–82. http://dx.doi.org/10.24297/jap.v15i0.8029.

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This research work deals with a feasible non-regenerative thermal cycle, composed by two pairs of closed polytropic-isochoric transformations implemented by means of a double acting reciprocating cylinder which differs basically from the conventional Carnot based thermal cycles in that: -it consists of a non condensing mode thermal cycle -all cycle involves only closed transformations, instead of the conventional open processes of the Carnot based thermal cycles, -in the active processes (polytropic path functions), as heat is being absorbed, mechanical work is simultaneously performed, avoiding the conventional quasi-adiabatic expansion or compression processes inherent to the Carnot based cycles and, -during the closed polytropic processes, mechanical work is also performed by means of the working fluid contraction due to heat releasing. An analysis of the proposed cycle is carried out for helium as working fluid and results are compared with those of a Carnot engine operating under the same ratio of temperatures. As a result of the cycle analysis, it follows that the ratio of top to the bottom cycle temperatures has very low dependence on the ideal thermal efficiency, but the specific work, and, furthermore, within the range of relative low operating temperatures, high thermal efficiency is achieved, surpassing the Carnot factor.

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Aneja, Preety. "Optimization and Efficiency Studies of Heat Engines: A Review". Journal of Advanced Research in Mechanical Engineering and Technology 07, n. 03 (7 ottobre 2020): 37–58. http://dx.doi.org/10.24321/2454.8650.202006.

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This review aims to study the various theoretical and numerical investigations in the optimization of heat engines. The main focus is to discuss the procedures to derive the efficiency of heat engines under different operating regimes (or optimization criteria) for different models of heat engines such as endreversible models, stochastic models, low-dissipation models, quantum models etc. Both maximum power and maximum efficiency operational regimes are desirable but not economical, so to meet the thermo-ecological considerations, some other compromise-based criteria have been proposed such as Ω criterion (ecological criterion) and efficient power criterion. Thus, heat engines can be optimized to work at an efficiency which may not be the maximum (Carnot) efficiency. The optimization efficiency obtained under each criterion shows a striking universal behaviour in the near-equilibrium regime. We also discussed a multi-parameter combined objective function of heat engines. The optimization efficiency derived from the multi-parameter combined objective function includes a variety of optimization efficiencies, such as the efficiency at the maximum power, efficiency at the maximum efficiency-power state, efficiency at the maximum criterion, and Carnot efficiency. Thus, a comparison of optimization of heat engines under different criteria enables to choose the suitable one for the best performance of heat engine under different conditions.

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Shaw, John E. "Comparing Carnot, Stirling, Otto, Brayton and Diesel Cycles". Transactions of the Missouri Academy of Science 42, n. 2008 (1 gennaio 2008): 1–6. http://dx.doi.org/10.30956/0544-540x-42.2008.1.

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Comparing the efficiencies of the Carnot, Stirling, Otto, Brayton and Diesel cycles can be a frustrating experience for the student. The efficiency of Carnot and Stirling cycles depends only on the ratio of the temperature extremes whereas the efficiency of Otto and Brayton cycles depends only on the compression ratio. The efficiency of a Diesel cycle is generally expressed in terms of the temperatures at the four turning points of the cycle or the volumes at these turning points. How does one actually compare the efficiencies of these thermodynamic cycles? To compare the cycles, an expression for the efficiency of the Diesel cycle will be obtained in terms of the compression ratio and the ratio of the temperature extremes of the cycle. It is found that for a fixed temperature ratio that the efficiency increases with compression ratio for the Otto, Brayton and Diesel cycles until their efficiency is the same as that of the corresponding Carnot cycle. This occurs at the point where the heat input to the cycles is zero. For a fixed compression ratio the efficiency increases with temperature ratio for the Carnot and Stirling cycles but decreases for the Diesel cycle. This is an important factor in understanding how a Diesel cycle can be made to be more efficient than an Otto cycle.

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NAGATA, Masaru. "Carnot Cycle and Energy Efficiency. Improved Theory of Energy Conversion and Energy Efficiency." Transactions of the Japan Society of Mechanical Engineers Series B 62, n. 603 (1996): 3976–81. http://dx.doi.org/10.1299/kikaib.62.3976.

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Yerra, Pavan Kumar, e Chandrasekhar Bhamidipati. "Heat engines at criticality for nonlinearly charged black holes". Modern Physics Letters A 34, n. 27 (6 settembre 2019): 1950216. http://dx.doi.org/10.1142/s021773231950216x.

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Within the extended phase–space thermodynamics, we study heat engines in power Yang–Mills and power Maxwell black holes at criticality, as the corresponding nonlinearity power parameters [Formula: see text] and [Formula: see text] are varied. For the computation of efficiency of such engines, starting from power Maxwell black holes, a map is proposed for carrying out the computations in power Yang–Mills theories. On comparison, the approach of efficiency of heat engines to Carnot limit in both the systems is shown to coincide when [Formula: see text], but, for [Formula: see text], Maxwell (Yang–Mills) system dominates over Yang–Mills (Maxwell). Higher values of [Formula: see text] aid in improving the approach of efficiency to Carnot limit for Maxwell heat engines. On the contrary, efficiency in Yang–Mills heat engines approaches Carnot limit faster for lower values of the power [Formula: see text].

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16

Chang, T. B. "Exergetic Efficiency Optimization for an Irreversible Carnot Heat Engine". Journal of Mechanics 23, n. 2 (giugno 2007): 181–86. http://dx.doi.org/10.1017/s1727719100001209.

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AbstractIn this paper, an exergetic efficiency optimization method that combines the concept of exergy and finite-time thermodynamic theory is developed to analyze an irreversible heat engine. With the total thermal conductance constraint, the analytical solutions of optimal allocation of thermal conductance and the corresponding maximum exergetic efficiency, thermal efficiency, as well as operating temperatures of hot and cold sides are obtained under a fixed overall heat supply rate. The results show that the exergetic efficiency optimization method can effectively analyze an irreversible heat engine.

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Hernández, A. Calvo, J. M. M. Roco, S. Velasco e A. Medina. "Irreversible Carnot cycle under per-unit-time efficiency optimization". Applied Physics Letters 73, n. 6 (10 agosto 1998): 853–55. http://dx.doi.org/10.1063/1.122023.

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Hondou, Tsuyoshi, e Ken Sekimoto. "Unattainability of Carnot efficiency in the Brownian heat engine". Physical Review E 62, n. 5 (1 novembre 2000): 6021–25. http://dx.doi.org/10.1103/physreve.62.6021.

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Pednekar, Abhijit. "The Blue System That Can Exceed the Carnot Efficiency". Proceedings of the National Academy of Sciences, India Section A: Physical Sciences 83, n. 1 (12 febbraio 2013): 59–61. http://dx.doi.org/10.1007/s40010-013-0064-x.

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20

Panarella, Emilio. "Energy saving and climate change mitigation through improved thermodynamic efficiency". Physics Essays 33, n. 3 (28 settembre 2020): 283–88. http://dx.doi.org/10.4006/0836-1398-33.3.283.

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The second Law of Thermodynamics is fundamental in the analysis of thermodynamic cycles. It dictates that the conversion of heat to work is limited. It reaches an upper limit in a classical thermodynamic cycle, and such a limit is provided by the Carnot cycle, which is the most efficient. Motivated by a recent allowance of a patent to this author (U.S. Patent 10,079,075), the present study tutorially attempts to expand on the subject and shows that the efficiency can go above the Carnot efficiency, provided a novel cycle is used, and heat, rather than being discarded, is recirculated in the same engine used to generate work. The significant energy saving consequential to this finding and climate change mitigation are reported.

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Herrera Alcantar, Hiram Kalid, José Carlos Carvajal García, Osvaldo Rosales Pérez, Rubén Cesar Villarreal-Sánchez e Priscilla Elizabeth Iglesias-Vázquez. "Dimensionality and geometry effects on a quantum carnot engine efficiency". Revista de Ciencias Tecnológicas 2, n. 1 (27 febbraio 2019): 45–48. http://dx.doi.org/10.37636/recit.v214548.

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Calculamos la eficiencia de un ciclo de Carnot cuántico para una partícula confinada en dos pozos de potencial infinitos diferentes, un pozo de potencial cilíndrico de radio variable y un pozo de potencial bidimensional cuadrado con periodicidad en uno de sus lados. Encontramos que la eficiencia depende directamente de la dimensionalidad y la geometría del pozo que confina a la partícula.

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Bonança, Marcus V. S. "Approaching Carnot efficiency at maximum power in linear response regime". Journal of Statistical Mechanics: Theory and Experiment 2019, n. 12 (3 dicembre 2019): 123203. http://dx.doi.org/10.1088/1742-5468/ab4e92.

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23

Rebhan, E. "Efficiency of nonideal Carnot engines with friction and heat losses". American Journal of Physics 70, n. 11 (novembre 2002): 1143–49. http://dx.doi.org/10.1119/1.1501116.

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Askin, M., M. Salti e O. Aydogdu. "Polytropic Carnot heat engine". Modern Physics Letters A 34, n. 24 (8 agosto 2019): 1950197. http://dx.doi.org/10.1142/s0217732319501979.

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Recent astrophysical datasets have implied that the universe has entered a speedy expansion phase. The Polytropic gas model, which describes a unified formulation of dark contents (matter plus energy), is one of the most reasonable definitions of this mysterious phenomenon. This interesting formulation allows to simulate the dark contents in the cosmic form of the perfect fluid and gives an interesting point of view in the discussion of fundamental theories of physics. In the first step of our investigation, we discuss the thermal equation-of-state (EoS henceforth) and obtain the EoS and deceleration parameters as explicit functions of temperature. Subsequently, we obtain a relation for the thermal efficiency of the Carnot heat engine which depends on free parameters given in the cosmological Polytropic gas description and the limits of maximal and minimal temperatures imposed on the Carnot cycle.

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Khalatov, A. A., S. D. Severin, O. S. Stupak e O. V. Shihabutinova. "EFFICIENCY OF THE REGENERATIVE CYCLE OF BRIGHTON WITH VARIABLE THERMOPHYSICAL PROPERTIES OF THE WORKING FLUID (Part 2)". Thermophysics and Thermal Power Engineering 41, n. 3 (18 dicembre 2018): 5–13. http://dx.doi.org/10.31472/ttpe.3.2019.1.

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The data about thermodynamic efficiency of the ideal Brighton cycle with heat regeneration with constant thermophysical properties of the working fluid, as well as the Brighton cycle with heat recovery and the wetting of the working fluid at the inlet to the turbine (with variable thermophysical properties of the working fluid). The inapplicability of comparison of the thermal efficiency of the Brighton cycle with heat recovery and the wetting of the working fluid at the inlet to the turbine with the thermal efficiency of the equivalent ideal Carnot cycle is shown. The analysis of the thermodynamic efficiency of an ideal regenerative Brighton cycle with a decrease in the working body at the entrance to the turbine allows us to make the following conclusions: With the growth of the mass moisture content of the working fluid when entering the turbine, the thermal efficiency of the regenerative cycle increases, but decreases with an increase in the degree of increase in the pressure level in the cycle. High values of the thermal efficiency of the cycle () can be achieved with relatively small values of the degree of increase in the pressure in the cycle () and high (up to d = 0,5) values of the mass moisture content of the working body when entering the turbine. It is shown that under certain conditions the thermal efficiency of the regenerative cycle with the decrease of the working body when entering the turbine may be greater than the thermal efficiency of a similar ideal Carnot cycle, which does not contradict the second law of thermodynamics, since the condition for the implementation of the Carnot cycle is the immutability of the thermophysical properties of the working body in a loop In this regard, the use of the expression for the thermal efficiency of the ideal Carnot cycle is not used as a criterion for assessing the efficiency of cycles of power plants with highly variable thermophysical properties of the working fluid. It is also shown that the thermal efficiency of the regenerative cycle with the decrease of the working body when entering the turbine is always lower than the thermal efficiency of the equivalent non-equilibrium Carnot cycle with a change in the specific heat of the working fluid, which corresponds to the second law of thermodynamics. It is shown that the Brighton regenerative cycle with a decrease in the working body before the turbine can be represented as a conditional cycle with a higher maximum temperature of the cycle, which, depending on the mass content of the moisture content of the working body, can in 1,2 ... 2,5 times exceed the actual maximum temperature cycle, which determines the high values of its thermal efficiency.

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Benenti, Giuliano, e Giulio Casati. "Increasing thermoelectric efficiency: dynamical models unveil microscopic mechanisms". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, n. 1935 (28 gennaio 2011): 466–81. http://dx.doi.org/10.1098/rsta.2010.0266.

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Dynamical nonlinear systems provide a new approach to the old problem of increasing the efficiency of thermoelectric machines. In this review, we discuss stylized models of classical dynamics, including non-interacting complex molecules in an ergodic billiard, a disordered hard-point gas and an abstract thermoelectric machine. The main focus will be on the physical mechanisms, unveiled by these dynamical models, which lead to high thermoelectric efficiency approaching the Carnot limit.

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Ghanavati, Mehdi, e Hossein Movahhedian. "Self-contained n-qubit quantum refrigerator". International Journal of Quantum Information 12, n. 03 (aprile 2014): 1450018. http://dx.doi.org/10.1142/s021974991450018x.

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Brunner et al. [Phys. Rev. E 85 (2012) 05111] have claimed that, "essentially only the smallest machines can approach Carnot efficiency". We have verified this claim by raising self-contained four-qubit quantum refrigerator, and we have shown that according to concepts of virtual qubit, it can reach the maximum efficiency in other words Carnot efficiency. But its efficiency, such as self-contained three-qubit quantum refrigerator is not universal. We also investigated a special case of self-contained four-qubit quantum refrigerator, in other words self-contained four-qubit quantum refrigerator with two hot baths in the same temperature. We demonstrated that its efficiency has the form as efficiency of a self-contained three-qubit quantum refrigerator. In other words, from the perspective of efficiency, this particular model is equivalent to self-contained three-qubit quantum refrigerator. We also demonstrated the efficiency of this particular model in the Carnot limit that is independent from details of system model, but only depends on the environmental temperatures. Also, we raised a system that consists of n-qubit which acts as a refrigerator. According to self-contained four-qubit quantum refrigerator, we also investigated a special case of self-contained n-qubit quantum refrigerator — a self-contained n-qubit quantum refrigerator with (n - 2) baths in the same temperature. We considered the three different special situations of the n-qubit refrigerator and demonstrated their efficiency in three different situations which has the form as efficiency of self-contained three-qubit quantum refrigerator. In this special situations, (n - 2) qubits are in thermal contact with isothermal heat baths.

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Ibrahim, O. M., S. A. Klein e J. W. Mitchell. "Optimum Heat Power Cycles for Specified Boundary Conditions". Journal of Engineering for Gas Turbines and Power 113, n. 4 (1 ottobre 1991): 514–21. http://dx.doi.org/10.1115/1.2906271.

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Optimization of the power output of Carnot and closed Brayton cycles is considered for both finite and infinite thermal capacitance rates of the external fluid streams. The method of Lagrange multipliers is used to solve for working fluid temperatures that yield maximum power. Analytical expressions for the maximum power and the cycle efficiency at maximum power are obtained. A comparison of the maximum power from the two cycles for the same boundary conditions, i.e., the same heat source/sink inlet temperatures, thermal capacitance rates, and heat exchanger conductances, shows that the Brayton cycle can produce more power than the Carnot cycle. This comparison illustrates that cycles exist that can produce more power than the Carnot cycle. The optimum heat power cycle, which will provide the upper limit of power obtained from any thermodynamic cycle for specified boundary conditions and heat exchanger conductances is considered. The optimum heat power cycle is identified by optimizing the sum of the power output from a sequence of Carnot cycles. The shape of the optimum heat power cycle, the power output, and corresponding efficiency are presented. The efficiency at maximum power of all cycles investigated in this study is found to be equal to (or well approximated by) η=1−TL,in/φTH,in where φ is a factor relating the entropy changes during heat rejection and heat addition.

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Sadia, Yatir, Dana Ben-Ayoun e Yaniv Gelbstein. "Evaporation–condensation effects on the thermoelectric performance of PbTe-based couples". Physical Chemistry Chemical Physics 19, n. 29 (2017): 19326–33. http://dx.doi.org/10.1039/c7cp03159a.

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Opatrný, Tomáš, e Marlan O. Scully. "Enhancing Otto-mobile Efficiency via Addition of a Quantum Carnot Cycle". Fortschritte der Physik 50, n. 5-7 (maggio 2002): 657–63. http://dx.doi.org/10.1002/1521-3978(200205)50:5/7<657::aid-prop657>3.0.co;2-#.

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de Boer, P. C. T. "Maximum Attainable Performance of Stirling Engines and Refrigerators". Journal of Heat Transfer 125, n. 5 (23 settembre 2003): 911–15. http://dx.doi.org/10.1115/1.1597618.

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The flow through the regenerator of a Stirling engine is driven by differences of pressure in the compression and expansion spaces. These differences lead to power dissipation in the regenerator. Using linearized theory, it is shown that this dissipation severely limits the maximum attainable thermal efficiency and nondimensional power output. The maximum attainable values are independent of the value of the regenerator conductance. For optimized nondimensional power output, the thermal efficiency equals only half the Carnot value. The power dissipated in the regenerator is removed as part of the heat withdrawn at the regenerator’s cold side. Analogous results are presented for the Stirling refrigerator. At optimized nondimensional rate of refrigeration, its coefficient of performance is less than half the Carnot value.

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Johansson, Jonas. "Pedagogical Visualization of a Nonideal Carnot Engine". Journal of Thermodynamics 2014 (21 luglio 2014): 1–7. http://dx.doi.org/10.1155/2014/217187.

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We have implemented a visualization tool for the demonstration of a nonideal Carnot engine, operating at finite time. The cycle time can be varied using a slide bar and the pressure-volume, temperature-entropy, power-time, and efficiency-time diagrams change interactively and are shown on one screen. We have evaluated the visualization tool among engineering students at university level during an introductory course on thermodynamics and we review and discuss the outcome of the evaluation.

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Karim, M., Owen Arthur, Prasad Yarlagadda, Majedul Islam e Md Mahiuddin. "Performance Investigation of High Temperature Application of Molten Solar Salt Nanofluid in a Direct Absorption Solar Collector". Molecules 24, n. 2 (14 gennaio 2019): 285. http://dx.doi.org/10.3390/molecules24020285.

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Nanofluids have great potential in a wide range of fields including solar thermal applications, where molten salt nanofluids have shown great potential as a heat transfer fluid (HTF) for use in high temperature solar applications. However, no study has investigated the use of molten salt nanofluids as the HTF in direct absorption solar collector systems (DAC). In this study, a two dimensional CFD model of a direct absorption high temperature molten salt nanofluid concentrating solar receiver has been developed to investigate the effects design and operating variables on receiver performance. It has been found that the Carnot efficiency increases with increasing receiver length, solar concentration, increasing height and decreasing inlet velocity. When coupled to a power generation cycle, it is predicted that total system efficiency can exceed 40% when solar concentrations are greater than 100×. To impart more emphasis on the temperature rise of the receiver, an adjusted Carnot efficiency has been used in conjunction with the upper temperature limit of the nanofluid. The adjusted total efficiency also resulted in a peak efficiency for solar concentration, which decreased with decreasing volume fraction, implying that each receiver configuration has an optimal solar concentration.

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Lefebvre, Lucie, Ward De Paepe, Mario L. Ferrari e Alberto Traverso. "Carnot cycle in practice: compensating inefficiencies of ORC expanders through thermal regeneration". E3S Web of Conferences 238 (2021): 10005. http://dx.doi.org/10.1051/e3sconf/202123810005.

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The Organic Rankine Cycle (ORC) is a thermodynamic cycle that can operate with a hot source over a wide range of temperatures, especially with low-grade heat (below 200°C). One of the main limitations for the success of small-scale ORC cycles (few to 100 kWe) is the relatively low isentropic efficiency of the typically used turbomachinery. Low turbine efficiency leads to low ORC cycle performance. To increase the performance of the cycle, the turbine efficiency must be increase, however, this significantly increases the cost of the machinery, making the cycle less profitable. In this work, the performance evaluation of low-temperature ORC cycles (100-150°C) with heat extraction along the expansion process is investigated, in an attempt to overcome this limitation. The studied cycle works in the same way as a conventional ORC, except that during the expansion process, heat is extracted. This heat is re-used later in the cycle, just before the hot source, allowing to reduce its load. The different cycles presented in this paper, using pentane as working fluid, are compared based on their exergetic and energetic efficiencies. The influence of three parameters on the cycle performance is studied: the regeneration ratio, the maximum temperature of the cycle and the turbine isentropic efficiency. In the case of a cycle using pentane with a maximum temperature of 150 °C and an turbine isentropic efficiency of 65%, the energy efficiency increases from 6.2% to 16.3% when going from no regeneration to full regeneration, and the exergy efficiency increases from 21.1 to 45.8%.. Secondly, the influence of the maximum temperature of the cycle is studied. Using pentane as the working fluid, the higher the maximum temperature is, the larger the benefits of heat extraction. However, this temperature cannot exceed the critical temperature of the organic fluid to stay in the case of a subcritical cycle. Finally, considering the turbine isentropic efficiency, it is possible to demonstrate that using a less efficient turbine, for example in small ORC systems, the performance of a cycle with an ideal turbine isentropic efficiency (100%) can be achieved compensating at cycle level the turbine losses with the heat extraction along the expansion process.

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Ma, Yu-Han. "Effect of Finite-Size Heat Source’s Heat Capacity on the Efficiency of Heat Engine". Entropy 22, n. 9 (8 settembre 2020): 1002. http://dx.doi.org/10.3390/e22091002.

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Heat engines used to output useful work have important practical significance, which, in general, operate between heat baths of infinite size and constant temperature. In this paper, we study the efficiency of a heat engine operating between two finite-size heat sources with initial temperature difference. The total output work of such heat engine is limited due to the finite heat capacity of the sources. We firstly investigate the effects of different heat capacity characteristics of the sources on the heat engine’s efficiency at maximum work (EMW) in the quasi-static limit. Moreover, it is found that the efficiency of the engine operating in finite-time with maximum power of each cycle is achieved follows a simple universality as η=ηC/4+OηC2, where ηC is the Carnot efficiency determined by the initial temperature of the sources. Remarkably, when the heat capacity of the heat source is negative, such as the black holes, we show that the heat engine efficiency during the operation can surpass the Carnot efficiency determined by the initial temperature of the heat sources. It is further argued that the heat engine between two black holes with vanishing initial temperature difference can be driven by the energy fluctuation. The corresponding EMW is proved to be ηMW=2−2.

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Chmielniak, Tadeusz, e Henryk Łukowicz. "Condensing power plant cycle — assessing possibilities of improving its efficiency". Archives of Thermodynamics 31, n. 3 (1 settembre 2010): 105–13. http://dx.doi.org/10.2478/v10173-010-0017-6.

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Condensing power plant cycle — assessing possibilities of improving its efficiency This paper presents a method for assessing the degree of approaching the paper output of the Clausius-Rankine cycle to the Carnot cycle. The computations to illustrate its use were performed for parameters characteristic of the current state of development of condensing power plants as well as in accordance with predicted trends for their further enhancing. Moreover there are presented computations of energy dissipation in the machines and devices working in such a cycle.

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Hassanzadeh, H., e S. H. Mansouri. "Efficiency of ideal fuel cell and Carnot cycle from a fundamental perspective". Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 219, n. 4 (1 giugno 2005): 245–54. http://dx.doi.org/10.1243/095765005x28571.

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In this paper, we accept the fact that fuel cell and heat engine efficiencies are both constrained by the second law of thermodynamics and neither one is able to break this law. However, we have shown that this statement does not mean the two systems should have the same maximum thermal efficiency when being fed by the same amounts of chemical reactants. The intrinsic difference between fuel cells (electrochemical systems) and heat engines (combustion engines) efficiencies is a fundamental one with regard to the conversion of chemical energy of reactions into electrical work. The sole reason has been shown to be due to the combustion irreversibility of the latter. This has led to the statement that fuel cell efficiency is not limited by the Carnot cycle. Clarity is achieved by theoretical derivations and several numerical examples.

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Tjiang, Paulus C., e Sylvia H. Sutanto. "The efficiency of the Carnot cycle with arbitrary gas equations of state". European Journal of Physics 27, n. 4 (2 maggio 2006): 719–26. http://dx.doi.org/10.1088/0143-0807/27/4/004.

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Badescu, Viorel. "Is Carnot efficiency the upper bound for work extraction from thermal reservoirs?" EPL (Europhysics Letters) 106, n. 1 (1 aprile 2014): 18006. http://dx.doi.org/10.1209/0295-5075/106/18006.

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Reed, B. Cameron. "A note on the overall efficiency of back-to-back Carnot cycles". Physics Education 56, n. 4 (21 aprile 2021): 043004. http://dx.doi.org/10.1088/1361-6552/abf5b1.

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Ren, Xuefeng, Yiran Wang, Anmin Liu, Zhihong Zhang, Qianyuan Lv e Bihe Liu. "Current progress and performance improvement of Pt/C catalysts for fuel cells". Journal of Materials Chemistry A 8, n. 46 (2020): 24284–306. http://dx.doi.org/10.1039/d0ta08312g.

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Fuel cell is an electrochemical device, which can directly convert the chemical energy of fuel into electric energy, without heat process, not limited by Carnot cycle, high energy conversion efficiency, no noise and pollution.

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42

Bannon, Peter R. "Entropy Production and Climate Efficiency". Journal of the Atmospheric Sciences 72, n. 8 (1 agosto 2015): 3268–80. http://dx.doi.org/10.1175/jas-d-14-0361.1.

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Abstract Earth’s climate system is a heat engine, absorbing solar radiation at a mean input temperature Tin and emitting terrestrial radiation at a lower, mean output temperature Tout < Tin. These mean temperatures, defined as the ratio of the energy to entropy input or output, determine the Carnot efficiency of the system. The climate system, however, does no external work, and hence its work efficiency is zero. The system does produce entropy and exports it to space. The efficiency associated with this entropy production is defined for two distinct representations of the climate system. The first defines the system as the sum of the various material subsystems, with the solar and terrestrial radiation fields constituting the surroundings. The second defines the system as a control volume that includes the material and radiation systems below the top of the atmosphere. These two complementary representations are contrasted using a radiative–convective equilibrium model of the climate system. The efficiency of Earth’s climate system based on its material entropy production is estimated using the two representations.

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Pal, P. S., Arnab Saha e A. M. Jayannavar. "Operational characteristics of single-particle heat engines and refrigerators with time-asymmetric protocol". International Journal of Modern Physics B 30, n. 31 (5 dicembre 2016): 1650219. http://dx.doi.org/10.1142/s0217979216502192.

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We have studied the single-particle heat engine and refrigerator driven by time-asymmetric protocol of finite duration. Our system consists of a particle in a harmonic trap with time-periodic strength that drives the particle cyclically between two baths. Each cycle consists of two isothermal steps at different temperatures and two adiabatic steps connecting them. The system works in irreversible mode of operation even in the quasistatic regime. This is indicated by finite entropy production even in the large cycle time limit. Consequently, Carnot efficiency for heat engine or Carnot coefficient of performance (COP) for refrigerators is not achievable. We further analyzed the phase diagram of heat engines and refrigerators. They are sensitive to time-asymmetry of the protocol. Phase diagram shows several interesting features, often counterintuitive. The distribution of stochastic efficiency and COP is broad and exhibits power-law tails.

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Smith, I. K. "Matching and Work Ratio in Elementary Thermal Power Plant Theory". Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 206, n. 4 (novembre 1992): 257–62. http://dx.doi.org/10.1243/pime_proc_1992_206_042_02.

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For most thermal power plant, the Carnot cycle efficiency is not the true ideal. Matching the cycle to the source leads to alternative limits and improved perceptions of how practical power plant can be improved. A method of including the work ratio into an ideal cycle analysis is presented which simplifies the estimation of practical power plant efficiencies and highlights the historic course of thermal efficiency improvement.

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Woods, Mischa P., Nelly Huei Ying Ng e Stephanie Wehner. "The maximum efficiency of nano heat engines depends on more than temperature". Quantum 3 (19 agosto 2019): 177. http://dx.doi.org/10.22331/q-2019-08-19-177.

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Sadi Carnot's theorem regarding the maximum efficiency of heat engines is considered to be of fundamental importance in thermodynamics. This theorem famously states that the maximum efficiency depends only on the temperature of the heat baths used by the engine, but not on the specific structure of baths. Here, we show that when the heat baths are finite in size, and when the engine operates in the quantum nanoregime, a revision to this statement is required. We show that one may still achieve the Carnot efficiency, when certain conditions on the bath structure are satisfied; however if that is not the case, then the maximum achievable efficiency can reduce to a value which is strictly less than Carnot. We derive the maximum efficiency for the case when one of the baths is composed of qubits. Furthermore, we show that the maximum efficiency is determined by either the standard second law of thermodynamics, analogously to the macroscopic case, or by the non increase of the max relative entropy, which is a quantity previously associated with the single shot regime in many quantum protocols. This relative entropic quantity emerges as a consequence of additional constraints, called generalized free energies, that govern thermodynamical transitions in the nanoregime. Our findings imply that in order to maximize efficiency, further considerations in choosing bath Hamiltonians should be made, when explicitly constructing quantum heat engines in the future. This understanding of thermodynamics has implications for nanoscale engineering aiming to construct small thermal machines.

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Ye, Wenlian, Zhe Yang e Yingwen Liu. "Exergy loss analysis of the regenerator in a solar Stirling engine". Thermal Science 22, Suppl. 2 (2018): 729–37. http://dx.doi.org/10.2298/tsci170911058y.

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In order to evaluate the irreversibility and exergy losses of the regenerators in a solar beta-type free piston Stirling engine due to flow friction, 1-D thermodynamic model to quantify exergy loss in the regenerators are built. The effects of important parameters, such as oscillating flow pressure drop, the exergy loss to flow friction, the exergy losses to conduction heat transfer at the hot and cold side of the regenerator and the percentage of Carnot efficiency of Stirling engine are presented and studied in detail. Results show that exergy loss decreases with the increase of the porosity and matrix diameter. As for the regenerator length, there is an optimum value that is equal to 0.035 m where the exergy loss is minimal and the percentage of Carnot efficiency is maximal. Therefore, some parameters should be selected reasonably to meet the overall design requirements of a solar Stirling engine.

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Smith, Zackary, Priyo S. Pal e Sebastian Deffner. "Endoreversible Otto Engines at Maximal Power". Journal of Non-Equilibrium Thermodynamics 45, n. 3 (26 luglio 2020): 305–10. http://dx.doi.org/10.1515/jnet-2020-0039.

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AbstractDespite its idealizations, thermodynamics has proven its power as a predictive theory for practical applications. In particular, the Curzon–Ahlborn efficiency provides a benchmark for any real engine operating at maximal power. Here we further develop the analysis of endoreversible Otto engines. For a generic class of working mediums, whose internal energy is proportional to some power of the temperature, we find that no engine can achieve the Carnot efficiency at finite power. However, we also find that for the specific example of photonic engines the efficiency at maximal power is higher than the Curzon–Ahlborn efficiency.

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Chen, Jincan. "The maximum power output and maximum efficiency of an irreversible Carnot heat engine". Journal of Physics D: Applied Physics 27, n. 6 (14 giugno 1994): 1144–49. http://dx.doi.org/10.1088/0022-3727/27/6/011.

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Parker, Michael C., e Stuart D. Walker. "A Unified Carnot Thermodynamic and Shannon Channel Capacity Information-Theoretic Energy Efficiency Analysis". IEEE Transactions on Communications 62, n. 10 (ottobre 2014): 3552–59. http://dx.doi.org/10.1109/tcomm.2014.2351412.

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Abe, Sumiyoshi. "General Formula for the Efficiency of Quantum-Mechanical Analog of the Carnot Engine". Entropy 15, n. 12 (17 aprile 2013): 1408–15. http://dx.doi.org/10.3390/e15041408.

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