Chih Wu

Finite Time Thermodynamic Optimization or Entropy Generation Minimization of Energy Systems

Abstract The historical background, research development, and the state-of-the-art of finite time thermodynamic theory and applications are reviewed from the point of view of both physics and engineering. The emphasis is on the performance optimization of thermodynamic processes and devices with finite-time and/or finite-size constraints, including heat engines, refrigerators, heat pumps, chemical reactions and some other processes, with respect to the following aspects: the study of Newtons law systems, an analysis of the effect of heat resistance and other irreversible loss models on the performance, an analysis of the effect of heat reservoir models on the performance, as well as the application for real thermodynamic processes and devices. It is pointed out that the generalized thermodynamic optimization theory is the development direction of finite thermodynamics in the future.

Effect of heat transfer law on the performance of a generalized irreversible Carnot engine

In a classical endoreversible Carnot engine model, irreversibility in the form of heat resistance between the reversible Carnot cycle and its heat reservoirs is taken into account. This paper presents a generalized irreversible Carnot engine model that incorporates several internal irreversibilities, such as heat leak, friction, turbulence etc. These added irreversibilities are characterized by a constant parameter and a constant coefficient. The relation between optimal power output and efficiency is derived based on a generalized heat transfer law . The effect of heat leakage, internal irreversibility and heat transfer law on the optimal performance of the generalized irreversible heat engine is investigated.

Analysis of waste-heat thermoelectric power generators

A real thermoelectric power generator utilizing waste heat is proposed. The generator is treated as an external and internal irreversible heat engine. The specific power output of the generator is analyzed and compared with that of the Carnot, endoreversible and external reversible thermoelectric heat engines.

Heat transfer effects on the net work output and efficiency characteristics for an air-standard Otto cycle

Finite-time thermodynamic analysis of an air-standard Otto cycle is performed in this paper. The relation between net work output and efficiency of the cycle is derived. The maximum net work output and the corresponding efficiency bound of the cycle with heat transfer considerations are also found. Detailed numerical examples are given. The result obtained herein provides a guide to the performance evaluation and improvement for practical Otto engines.

Theoretical analysis of the performance of a regenerative closed Brayton cycle with internal irreversibilities

Abstract Using the technique of finite-time thermodynamic analysis, the performance of a closed Brayton cycle with regeneration is assessed. Specifically, analytical expressions for the power output and the thermal efficiency, as functions of the pressure ratio and the reservoir temperatures, are derived. The analysis also takes into account all the irreversibilities associated with finite-time heat transfer processes. The paper also shows that the power output can be maximized with judicious selection of parameters such as the heat exchanger surface areas and the heat conductances. Maximum power is obtained when the effectiveness of the hot- and cold-side heat exchangers are related in a specific manner, and this power output is a strong function of the regenerator effectiveness.

Effect of heat transfer on the performance of thermoelectric generators

The power output and efficiency expressions for thermoelectric (semiconductor) generators which is composed of multi-elements are derived with considerations of heat transfer irreversibility in the heat exchangers between the generator and the heat reservoirs. Numerical examples are provided. The effects of heat transfer and the number of elements on the performance are analyzed.

Ecological optimization for generalized irreversible Carnot engines

The optimal ecological performance of a Newtons law generalized irreversible Carnot engine with losses due to heat-resistance, heat leak and internal irreversibility is derived by taking an ecological optimization criterion as the objective. This consists of maximizing a function representing the best compromise between the power and entropy production rate of the heat engine. A numerical example is given to show the effects of heat leakage and internal irreversibility on the optimal performance of the generalized irreversible heat-engine.

Efficiency of an Atkinson engine at maximum power density

In studies of finite-time thermodynamics, most performance analyses concern the maximum power output and the corresponding efficiency for heat engines. In this paper, instead of just maximizing power for a cycle, the power density (the ratio of the power to the maximum specific volume in the cycle) is maximized for an Atkinson engine. The results showed that the efficiency at maximum power density is always greater than that at maximum power, and the design parameters at maximum power density lead to smaller and more efficient Atkinson engines with larger pressure ratios.

Optimum performance of irreversible stirling engine with imperfect regeneration

Abstract An optimal performance analysis is performed for a Stirling engine with heat transfer and imperfect regeneration irreversibilities. The relationship between the net power output and thermal efficiency of the engine is derived. Detailed numerical examples are given. The results obtained inthis paper provide guidance to performance evaluation and design improvement for Stirling engines.

Performance analysis and optimization of a supercharged Miller cycle Otto engine

One of the major alternatives of the Otto cycle has been examined to determine its potential for increased efficiency and net work power in the spark ignited internal combustion engine is to shorten the compression process relative to the expansion process by early close or late of intake valve. The modified Otto cycle is called Miller cycle. This paper deals with the analysis of a supercharged Otto engine adopted for Miller cycle operation. The Miller cycle shows no efficiency advantage and suffers a penalty in power output in the normally aspirated version. In the supercharged Otto engine adopted for Miller cycle version, it has no efficiency advantage but does provide increased net work output with reduced propensity to engine knock problem. Sensitivity analysis of cycle efficiency versus early close of intake valve and that of cycle net work versus early close of intake valve are performed. Optimization on the cycle efficiency is obtained.