Abstract:
The thermodynamics of the Curzon-Ahlborn engine, which is a prototype of endoreversible engines, is elucidated. In particular, their criterion for adiabatic equilibration is revised. The so-called irreversibility of endoreversible engines arises from the selection of the coldest reservoir for heat rejection. Rather, if the reservoirs are allowed to come into thermal and mechanical contact, a mean value results which optimizes the work output and heat uptake, and is entirely reversible. The Carnot efficiency cannot be beaten because nothing is as cold as the coldest reservoir.

Abstract:
In recent decades, the approach known as Finite-Time Thermodynamics has provided a fruitful theoretical framework for the optimization of heat engines operating between a heat source (at temperature ) and a heat sink (at temperature ) . The aim of this paper is to propose a more complete approach based on the association of Finite-Time Thermodynamics and the Bond-Graph approach for modeling endoreversible heat engines. This approach makes it possible for example to find in a simple way the characteristics of the optimal operating point at which the maximum mechanical power of the endoreversible heat engine is obtained with entropy flow rate as control variable. Furthermore it provides the analytical expressions of the optimal operating point of an irreversible heat engine where the energy conversion is accompanied by irreversibilities related to internal heat transfer and heat dissipation phenomena. This original approach, applied to an analysis of the performance of a thermoelectric generator, will be the object of a future publication.

Abstract:
En este artículo se hace uso de la termodinámica endorreversible para evaluar el comportamiento de un sistema de refrigeración por compresión mecánica de vapor. Se describe la obtención del Coeficiente de Operación del ciclo de Carnot de refrigeración endorreversible (COP CE), en el que se consideran las irreversibilidades externas asociadas a la transferencia de calor. Con los valores de presiones y temperaturas obtenidos experimentalmente en un sistema de refrigeración por compresión mecánica de vapor, se calcularon, a) el Coeficiente de Operación real (COP R), b) el Coeficiente de Operación de Carnot reversible (COP C) y c) el Coeficiente de Carnot endorreversible (COP CE). Se hizo la comparación de estos resultados y se llegó a la conclusión que es el COP CE, el parámetro que debe usarse para establecer los límites teóricos del comportamiento de la operación de los sistemas de refrigeración, y no el COP C, como tradicionalmente se ha hecho. This paper describes the use of endoreversible thermodynamics to evaluate the behavior of a mechanical compression refrigeration system. The coefficients of performance of the endoreversible Carnot Cycle, COP CE, were obtained by including the external irreversibility associated with heat transfer. The real coefficient of performance (COP R), the reversible Carnot coefficient of performance (COP C) and the endorreversible Carnot coefficient of performance (COP CE) were determined using experimentally obtained pressures and temperatures. The results were compared with experimental data from a laboratory refrigeration system, and the conclusion was that the COP CE was the parameter of choice for setting the theoretical limits for refrigeration system operation instead of the COP C as has been conventionally done.

Abstract:
The derivation of general performance benchmarks is important in the design of highly optimized heat engines and refrigerators. To obtain them, one may model phenomenologically the leading sources of irreversibility ending up with results which are model-independent, but limited in scope. Alternatively, one can take a simple physical system realizing a thermodynamic cycle and assess its optimal operation from a complete microscopic description. We follow this approach in order to derive the coefficient of performance at maximum cooling rate for \textit{any} endoreversible quantum refrigerator. At striking variance with the \textit{universality} of the optimal efficiency of heat engines, we find that the cooling performance at maximum power is crucially determined by the details of the specific system-bath interaction mechanism. A closed analytical benchmark is found for endoreversible refrigerators weakly coupled to unstructured bosonic heat baths: an ubiquitous case study in quantum thermodynamics.

Abstract:
In this lecture we briefly review the definition, consequences and applications of an entropy, $S_q$, which generalizes the usual Boltzmann-Gibbs entropy $S_{BG}$ ($S_1=S_{BG}$), basis of the usual statistical mechanics, well known to be applicable whenever ergodicity is satisfied at the microscopic dynamical level. Such entropy $S_q$ is based on the notion of $q$-exponential and presents properties not shared by other available alternative generalizations of $S_{BG}$. The thermodynamics proposed in this way is generically {\it nonextensive} in a sense that will be qualified. The present framework seems to describe quite well a vast class of natural and artificial systems which are not ergodic nor close to it. The a priori calculation of $q$ is necessary to complete the theory and we present some models where this has already been achieved.

Abstract:
Performance of an endoreversible Carnot heat pump cycle with finite speed of the piston is investigated by using finite time thermodynamics. The analytical formulae between the optimal heating load and the coefficient of performance (COP), as well as between the optimal heating load and speed ratio of the piston are derived. It is found that the heating load versus COP characteristics are parabolic-like, and there exist a maximum heating load and the corresponding COP. These are different from the monotonically decreasing characteristic of the endoreversible Carnot heat pump without consideration of the finite speed of the piston. At the same time, the effects of reservoir temperature ratio on the optimal relations are analyzed by numerical examples. In the analysis and optimization, two cases with and without limit of cycle period are included.

Abstract:
In this work, we focused mainly in the analysis of stability of a non-endoreversible Curzon-Ahlborn engine working in an ecological regime. For comparison purposes we also include the Maximum Efficient Power (MEP) regime taking into account the engine time delays. When the system’s dynamic stability is compared with its thermodynamics properties (efficiency and power output), we find that the temperature ratio τ = T1/T2 represents a trade-off between stability and energetic properties. When we take the non-endoreversible case, τ can increases to values greater than R (where R is the non-endoreversible parameter) but not greater than one. We reformulate an important difference between this case and the other two, Maximum Power (MP) and MEP regime, in which τ = R. Finally, we demonstrated that the total time delay does not destabilize the steady state of system. It does not seem to play a role in the dynamic thermodynamic property trade-off.

Abstract:
An endoreversible Meletis-Georgiou (MG) cycle model with constant specific heat of working fluid is established and the analytical formulae of performance parameters including working fluid temperatures, work output and efficiency are derived using the finite time thermodynamic theory. The performance of the endoreversible MG cycle is analyzed and optimized. The characteristics of the work output versus compression ratio, efficiency versus compression ratio, and work output versus efficiency are obtained, respectively, by detailed numerical examples, and the effects of changeover ratio, over-expansion ratio, heating value of fuel, heat transfer loss coefficient, initial temperature of working fluid, and the transferred volume ratio on the relationship mentioned above are also discussed. The maximum work output and the corresponding optimal compression ratio, changeover ratio, over-expansion ratio as well as the maximum efficiency and the corresponding optimal changeover ratio and over-expansion ratio are obtained by taking the cycle work output and efficiency as the optimization objectives, respectively. Moreover, the effects of the parameters such as the heating value of fuel, heat transfer loss coefficient, initial temperature of working fluid, and the transferred volume ratio on the maximum work output, the maximum efficiency and the corresponding optimal ratios are analyzed. The results may provide guidelines for the optimal design of practical MG engine.

Abstract:
Optimal ecological performances of endoreversible chemical engine cycles with both linear and diffusive mass transfer laws are derived by taking an ecological optimization criterion as the objective, which consists of maximizing a function representing the best compromise between the power output and entropy production rate of the chemical engines. Numerical examples are given to show the effects of mass-reservoir chemical potential ratio and mass-transfer coefficient ratio on the ecological function versus the efficiency characteristic of the cycles. The results can provide some theoretical guidelines for the design of practical chemical engines.