J. M. M. Roco

Optimum performance of a regenerative Brayton thermal cycle

The optimum performance of a regenerative Brayton cycle was analyzed. The model includes external and internal irreversibilities coming from four main sources: coupling to external heat reservoirs, turbine and compressor nonisentropic processes, pressure losses in the heater and the cooler, and the regenerator. In terms of the parameters accounting for each type of irreversibility, explicit numerical results are presented for the maximum efficiency, maximum power output, efficiency at maximum power output, power output at maximum efficiency, as well as for the pressure ratios required for maximum efficiency and maximum power. This analysis could provide a general theoretical tool for the optimal design and operation of real regenerative gas turbine power plants.

Regenerative gas turbines at maximum power density conditions

A new kind of power analysis has recently been presented which is based on the maximization of the power density and predicts smaller and more efficient non-regenerative Joule - Brayton engines than those designed at maximum power. In this paper we apply the power density maximization method to regenerative gas turbines using a theoretical framework where the optimal operating conditions of the heat engine are expressed in terms of the isentropic efficiencies of the compressor and turbine and of the heat exchanger efficiency. It is shown that, unlike non-regenerative results, real regenerative gas turbines are less efficient at maximum power density conditions than at maximum power conditions.

Feynman's ratchet optimization: maximum power and maximum efficiency regimes

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.

Optimization of heat engines including the saving of natural resources and the reduction of thermal pollution

The use of the new concept of a saving function as a measure of possible reductions of undesired side effects in heat engine operation is proposed. Two saving functions are introduced, one associated with fuel consumption and another associated with thermal pollution. Two optimization paths including the maximization of power output and these saving functions are presented. The first is based on a linear formalism and the second is based on a power-law formalism. When these optimization criteria are applied to a Curzon-Ahlborn heat engine, both criteria lead to a very similar optimum efficiency, opt = 1- 3/4 , where is the ratio between the temperatures of the cold and the hot external reservoirs. A numerical comparison with the efficiency of some modern nuclear power plants is reported.

Power and efficiency in a regenerative gas turbine

The effect of a regenerative heat exchanger in a gas turbine is analysed using a regenerative Brayton cycle model, where all fluid friction losses in the compressor and turbine are quantified by an isentropic efficiency term and all global irreversibilities in the heat exchanger are taken into account by means of an effective efficiency. This analysis, which generalizes that reported by Gordon and Huleihil for a simple, nonregenerative Brayton cycle, provides a theoretical tool for the selection of optimal operating conditions in a regenerative gas turbine. For a fixed ratio of the compressor inlet temperature to the turbine inlet temperature and in terms of the isentropic and regenerator efficiencies we present explicit results for the behaviour of the maximum efficiency, efficiency at maximum power, maximum power and power at maximum efficiency as well as for the behaviour of the pressure ratios required for maximum efficiency and maximum power.

Vibration-rotation spectra of HCl in rare-gas liquid mixtures: Molecular dynamics simulations of Q-branch absorption

New experimental results are presented on the fundamental IR band shape of HCl dissolved in neat liquid Ar and Ar doped with Kr and Xe. A strong enhancement of the absorption in the range of a central Q-branch is observed in the spectra of doped solutions. Semiclassical molecular dynamics simulations of the spectral band profile are carried out using (12-6) Lennard-Jones site–site interaction potentials. The parameters of these model potentials were deduced by fitting the available anisotropic interaction surfaces, accurately describing the structure of binary rare-gas-HCl van der Waals complexes. Simulations realistically reproduce the observed triplet band structure and its evolution with changing thermodynamic conditions. The analysis of the influence of anisotropic interactions on the orientational dynamics of solutes and orientation-dependent radial distribution functions reveals the mechanisms that contribute to appearance of the Q-branches. It is shown that long-living solute-solvent spatial correl...

Power and efficiency in a regenerative gas-turbine cycle with multiple reheating and intercooling stages

Using an improved Brayton cycle as a model, a general analysis accounting for the efficiency and net power output of a gas-turbine power plant with multiple reheating and intercooling stages is presented. This analysis provides a general theoretical tool for the selection of the optimal operating conditions of the heat engine in terms of the compressor and turbine isentropic efficiencies and of the heat exchanger efficiency. Explicit results for the efficiency, net power output, optimized pressure ratios, maximum efficiency, maximum power, efficiency at maximum power, and power at maximum efficiency are given. Among others, the familiar results of the Brayton cycle (one compressor and one turbine) and of the corresponding Ericsson cycle (infinite compressors and infinite turbines) are obtained as particular cases.

Coefficient of performance under optimized figure of merit in minimally nonlinear irreversible refrigerator

We apply the model of minimally nonlinear irreversible heat engines developed by Izumida and Okuda (EPL, 97 (2012) 10004) to refrigerators. The model assumes extended Onsager relations including a new nonlinear term accounting for dissipation effects. The bounds for the optimized regime under an appropriate figure of merit and the tight-coupling condition are analyzed and successfully compared with those obtained previously for low-dissipation Carnot refrigerators in the finite-time thermodynamics framework. Besides, we study the bounds for the nontight-coupling case numerically. We also introduce a leaky low-dissipation Carnot refrigerator and show that it serves as an example of the minimally nonlinear irreversible refrigerator, by calculating its Onsager coefficients explicitly.

Irreversible refrigerators under per-unit-time coefficient of performance optimization

A finite-time thermodynamics analysis is used to investigate the optimal coefficient of performance (COP) of an irreversible Carnot refrigerator using the per-unit-time COP as an objective function for optimization. The model includes finite-rate heat transfers between the refrigerant and the external heat reservoirs, heat leak between heat reservoirs, and internal dissipations of the refrigerant. Heat conductances associated with heat transfers are optimized by maximizing the cooling power per unit of capital invested in the refrigerator. The obtained results are consistent with performance data for real low-temperature refrigerators.

On an irreversible air standard Otto-cycle model

We present a simplified model for an irreversible Otto cycle which accounts for the characteristic power-versus-efficiency curve of real heat engines. This model avoids the usual hypotheses of endoreversible heat engines and it considers an air Otto engine with internally dissipative friction and a pure sinusoidal law for the piston velocity on the adiabatic branches.