Universal ecological performance for endo-reversible heat engine cycles

SYNOPSIS Exergy-based ecological optimisation of a universal endoreversible thermodynamic cycle model is performed using finite time thermodynamics or entropy generation minimisation. The cycle consists of two constant thermal-capacity heating branches, a constant thermal-capacity cooling branch and two adiabatic branches. An ecological optimisation criterion is taken as the objective, which consists of maximising a function representing the best compromise between the power output and exergy loss rate of the heat engine. The analytical formulae for power, efficiency and ecological function of the universal endoreversible thermodynamic cycle with heat resistance loss are derived. The comparative performance analyses for Diesel, Otto, Atkinson, Brayton and Dual-cycles are carried out by a detailed numerical example, which show that the ecological criterion is more available.

Power optimization of an endoreversible Brayton gas heat engine

Reciprocating heat-engine cycles

A new model for a class of irreversible absorption heat transformers is presented, in which not only the irreversibilities of heat conduction but also the irreversibilities resulting from the internal working fluid dissipation are considered. An ecological optimization criterion is proposed for the best mode of operation of such a class of heat transformers. It consists of maximizing a function representing the best compromise between the rate of heat pumping and the loss rate of heat pumping. It is shown that the model adopted here may be treated as a combined cycle of an irreversible Carnot heat engine and a reversible Carnot heat pump, and by using the combined cycle method, many problems of the heat transformer can be solved simply and conveniently. The ecological optimal performance of the heat transformer is investigated by using such a method and the significance of the ecological optimization criterion is discussed. The results obtained here are of importance in the optimal design and reasonable use of the energy of heat transformers.

The Department of Energy's (DOE) Solar Thermal Program has as one of its program elements the development and evaluation of heat engine technologies that are applicable to Distributed Receiver Systems. The primary research and development activities, for the past several years, have involved the dish--electric concept in which a heat engine, solar receiver, and generator are combined as a power conversion assembly (PCA) and mounted at the focus of a parabolic dish concentrator. PCAs based on Rankine, Brayton, and Stirling cycle heat engines have been designed and built at 10/endash/25 kWe power levels with varying degrees of success. The small Brayton cycle engines in particular have fallen short of expectations as a focal mounted PCA. This report analyzes larger Brayton engines (500 kWe and above) supplied with fossil fuel and solar energy from 15-m diameter parabolic dishes or small central receivers. In particular, a steam injected gas turbine (STIG) cycle is examined because of its ability to offer higher efficiencies as compared to the combined cycle, regenerative Brayton cycle, and water injected Brayton cycle. This report outlines quantitatively the cycle parameters that affect efficiency for a solar-augmented steam-injected gas turbine. 15 refs., 18 figs., 2 tabs.

The optimal ecological performance of a irreversible Carnot engine with the losses of heat-resistance, heat leak and internal irreversibility, in which the transfer between the working fluid and the heat reservoirs obeys a generalized heat transfer law Q ∝ ∆(Tn), is derived by taking an ecological optimization criterion as the objective, which consists of maximizing a function representing the best compromise between the power and entropy production rate of the heat engine. Some special examples are discusses. A numerical example is given to show the effects of heat transfer law, heat leakage and internal irreversibility on the optimal performance of the generalized irreversible heat engine. The results can provide some theoretical guidance for the designs of practical engine.

Performance optimization for an irreversible four-temperature-level absorption heat pump

Finite time thermodynamic evaluation of irreversible Ericsson and Stirling heat engines

This work establishes a quantum heat engine considering qubits as the external heat transfer mediums. The authors find that the efficiency and the power output of the quantum heat engine are closely dependent on the microproperties of qubits, and consequently relate the irreversible heat transfer with quantum effects. The equivalent conditions between the quantum heat engines and the classical endoreversible Carnot heat engines are revealed

SYNOPSIS On the basis of exergetic analysis, ecological performance analysis and optimisation of an endoreversible simple air heat pump cycle with constant-temperature heat reservoirs is carried out in this paper. An exergy-based ecological optimisation criterion, which consists of maximising a function representing the best compromise between the exergy output rate and exergy loss rate (entropy generation rate and environment temperature product) of the heat pump cycle, is taken as the objective function. The analytical formulae concerning the relationship between the exergy-based ecological function and pressure ratio is derived with heat resistance losses in the hot- and cold-side heat exchangers. The effects of the effectivenesses of the heat exchangers, the ratio of hot-side heat reservoir temperature to ambient temperature and the heat reservoir temperature ratio on the exergy-based ecological function are analysed. The cycle performance optimisations are performed by identifying the optimum pressure ratio and the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for a fixed total heat exchanger inventory, respectively. The expressions of the maximum ecological function are derived. The influences of some design parameters, including heat capacity rate of the working fluid and heat exchanger inventory on the optimal performance of the endoreversible air heat pump are provided by numerical examples. The results show that exergy-based ecological optimisation is an important and effective criterion for the evaluation of air heat pumps.

96/06754 Experimental study of turbulent flow in the near wakes of single and tandem prisms

Despite the remarkable success of Carnot’s heat engine cycle in founding the discipline of thermodynamics two centuries ago, false viewpoints of his use of the caloric theory in the cycle linger, limiting his legacy. An action revision of the Carnot cycle can correct this, showing that the heat flow powering external mechanical work is compensated internally with configurational changes in the thermodynamic or Gibbs potential of the working fluid, differing in each stage of the cycle quantified by Carnot as caloric. Action (@) is a property of state having the same physical dimensions as angular momentum (mrv = mr2ω). However, this property is scalar rather than vectorial, including a dimensionless phase angle (@ = mr2ωδφ). We have recently confirmed with atmospheric gases that their entropy is a logarithmic function of the relative vibrational, rotational, and translational action ratios with Planck’s quantum of action ħ. The Carnot principle shows that the maximum rate of work (puissance motrice) possible from the reversible cycle is controlled by the difference in temperature of the hot source and the cold sink: the colder the better. This temperature difference between the source and the sink also controls the isothermal variations of the Gibbs potential of the working fluid, which Carnot identified as reversible temperature-dependent but unequal caloric exchanges. Importantly, the engine’s inertia ensures that heat from work performed adiabatically in the expansion phase is all restored to the working fluid during the adiabatic recompression, less the net work performed. This allows both the energy and the thermodynamic potential to return to the same values at the beginning of each cycle, which is a point strongly emphasized by Carnot. Our action revision equates Carnot’s calorique, or the non-sensible heat later described by Clausius as ‘work-heat’, exclusively to negative Gibbs energy (−G) or quantum field energy. This action field complements the sensible energy or vis-viva heat as molecular kinetic motion, and its recognition should have significance for designing more efficient heat engines or better understanding of the heat engine powering the Earth’s climates.

Thermoelectric devices based on thermoelectric effect have special advantages in the conversion between heat and electric energy, and the performance optimization of thermoelectric device has the practical significance, Based on the finite-time thermodynamic theory, this paper studies the thermodynamic characteristics of thermoelectric generators and refrigerators by using the ecological criterion, On the basis of constructing the ecological criterion function,the expressions of efficiency and coefficient of performance (COP) of thermoelectric devices are derived, and the influence of ambient temperature and thermoelectric figure of merit on efficiency and COP is explained. It is found that when the ambient temperature is low (high).which is helpful for enhancing the efficiency (COP) of generators (refrigerators). In addition,the relationship between ecological criterion and trade-off optimization criterion is definitely clarified.

Ecological optimisation of irreversible harmonic oscillator Carnot refrigerator

Optimal configuration of a class of endoreversible heat engines with fixed duration, input energy and radiative heat transfer law (q ∝ Δ(T 4)) is determined. The optimal cycle that maximizes the efficiency of the heat engine is obtained by using optimal-control theory, and the differential equations are solved by the Taylor series expansion. It is shown that the optimal cycle has eight branches including two isothermal branches, four maximum-efficiency branches, and two adiabatic branches. The interval of each branch is obtained, as well as the solutions of the temperatures of the heat reservoirs and the working fluid. A numerical example is given. The obtained results are compared with those obtained with the Newton’s heat transfer law for the maximum efficiency objective, those with linear phenomenological heat transfer law for the maximum efficiency objective, and those with radiative heat transfer law for the maximum power output objective.