We propose a unified optimization criterion for energy converters. It represents the best compromise between energy benefits and losses for a specific job and neither an explicit evaluation of entropies nor the consideration of environmental parameters are required. For all considered systems the criterion predicts a performance regime laying between those of maximum efficiency and maximum useful energy. Such regime has been invoked as optimum not only in macroscopic heat engines but also in some molecular motors.

A thermodynamic model is developed for predicting the performance records of a solar hybrid gas turbine power plant with variable irradiance and ambient temperature conditions. The model considers a serial solar hybridization in those periods

An upper bound for the coefficient of performance (COP) of endoreversible refrigerators which depends only on the ratio t T c ͞T h between the cold and hot reservoir temperatures has been elusive to date. We address here this long standing problem by analyzing an endoreversible Carnot refrigerator that operates in conditions of maximum per-unit-time COP. Two novel results are obtained: (1) A long sought t-dependent upper bound for the COP of refrigerators. (2) A t-dependent optimum distribution of the heat conductances associated with the coupling between the refrigerant and the heat reservoirs. Moreover, when the method is applied to heat engines, the resulting optimum efficiency is even closer to real efficiencies than the well-known Curzon-Ahlborn result. [S0031-9007(97)03029-9] PACS numbers: 05.70.-a, 07.20.M, 44.60.+k, 44.90.+c According to classical equilibrium thermodynamics, a Carnot cycle provides upper bounds for the efficiency of heat devices working between two heat reservoirs at fixed temperatures. However, Carnot's results are of very limited practical interest since they correspond to the reversible limit, i.e., to infinite-time processes. Real applications involve finite-time processes; hence the interest in deriving new upper bounds for the performance of heat devices operating with finite-time cycles. Finite-time thermodynamics (FTT) has attracted much attention over the past two decades because of its realistic predictions for a large number of real heat devices, especially heat engines [1][2][3][4]. The main goal of FTT is to ascertain the best operating mode of heat devices with finite-time cycles. Basically, finite-rate constraints arising from several sources of irreversibility are modeled and then a suitable functional (efficiency, power output, cooling power, entropy generation, etc.) is optimized with respect to the involved parameters. Endoreversible cycle models [5-7] (internally reversible with all irreversibilities coming from the coupling between the system and its environment) have played a central role in the birth and development of FTT. The most simple and popular model that has been proposed is an endoreversible Carnot cycle with linear (Newtonian) finite-time heat transfer in the two isothermal processes. In a seminal paper, Curzon and Ahlborn (CA) [5] showed that the optimum efficiency (recently called the Chambadal-Novikov-Curzon-Ahlborn efficiency [4]) at maximum power output for this endoreversible heat engine is given by h CA 1 2 p t, where t T c ͞T h is the ratio between the cold and hot reservoir temperatures. This remarkable result provides a very simple alternative expression to Carnot efficiency ͑h C 1 2 t͒, giving a much better agreement with observed values in real power plants [8].Concerning the analysis of refrigerators, the results of FTT are considerably less satisfactory than for heat engines. In spite of continued and extensive work in this field [9][10][11][12][13][14][15][16][17][18][19], it is known that, using endoreversible refrigeration cycle models,...

We present new results obtained from the Carnot-like low-dissipation model of heat devices when size-and time-constraints are taken into account, in particular those obtained from the total cycle time and the contact times of the working system with the external heat reservoirs. The influence of these constraints and of the characteristic time scale of the model on the entropy generation allows for a clear and unified interpretation of different energetic properties for both heat engines and refrigerators (REs). Some conceptual subtleties with regard to different optimization criteria, especially for REs, are discussed. So, the different status of power input, cooling power, and the unified figure of merit χ are analyzed on the basis of their absolute or local role as optimization criteria.

The optimal performance of the Feynman ratchet-and-pawl engine is analysed by taking the power and the efficiency of the engine as objective functions. The power-efficiency curves are also obtained. These curves show a loop shape similar to those characteristic of some real heat engines. Explicit analytical expressions are reported in the so-called linear regime.

We show in this work that a finite-time-thermodynamics model of an irreversible Otto cycle is suitable to reproduce performance results of a real spark ignition heat engine. In order to test our model we have developed a computer simulation including a two-zone combustion model and compared the evolution of the performance parameters of the simulated engine as functions of the rotational speed ͑͒ with those obtained from a simple theoretical scheme including chemical reactions. A theoretical Otto cycle with irreversibilities arising from friction, heat transfer through the cylinder walls, and internal losses properly reproduces simulation results by considering extreme temperatures and mass inside the cylinder as functions of. Furthermore we obtain realistic values for the parameters characterizing global irreversibilities, their evolution with , and a clearer understanding of their physical origin not always well established in theoretical models.

This paper presents a general thermodynamic model for hybrid Brayton central tower thermosolar plants. These plants have been proved to be technically feasible but R+D efforts need to be done in order to improve its commercial interest. From the thermodynamic viewpoint it is necessary to increase its performance to get larger power production with reduced fuel consumption, and so reduced emissions. We develop a model for multi-step compression and expansion stages with that aim. The model is flexible and allows to simulate recuperative or non-recuperative plants, with an arbitrary number of stages and working with different subcritical fluids. The results for multi-step configurations are compared with those obtained for a plant with one turbine and one compressor. Different working fluids are analyzed, including air, nitrogen, carbon dioxide, and helium. Several plant layouts and the corresponding optimal pressure ratios are analyzed. It is concluded that configurations with two-stages compression with intercooling combined with one or two expansion stages can significantly improve overall plant efficiency and lower fuel consumption. Power block efficiencies can reach 0.50 and overall plant efficiency can attain values about 0.40 working with air or CO 2. For instance, comparing with a single stage plant running with air, a plant with subcritical CO 2 , two compression stages with intercooling and single step expansion can reach an overall efficiency about 19% larger and a fuel conversion rate around 23% larger. For such configuration, the specific fuel consumption is predicted to be about 108 kg/(MW h) at design point conditions.

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