Entransy Theory for the Analysis and Optimization of Thermal Systems

DOI: 10.1615/IHTC15.kn.000014
pages 271-290

Zeng-Yuan Guo
Education Ministry Key Lab of Enhanced Heat Transfer and Energy Conservation, Tsinghua University

Qun Chen
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Engineering Mechanics, Tsinghua University

Xin-Gang Liang
Tsinghua University,Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Engineering Mechanics

Abstract

New optimization methods are needed to increase the energy efficiencies of various kinds of thermal systems as entropy theory is not always suitable for optimizing heat transfer problems, such as the derivation of Fourier's law of heat conduction, volume to point heat conduction, and counter-flow heat exchangers, which lead to the development of the new quantities of entransy, entransy dissipation and the corresponding optimization principle. Entransy is of the nature of "potential energy" because it is in fact the simplified expression of potential energy of thermomass, which describes the heat release capability during a time period. Unlike the traditional definition of thermal resistance, the entransy dissipation thermal resistance (EDTR) is defined, which may describe the irreversibility and optimization of various heat transfer process not related to heat-work conversion. A temperature-heat transfer rate (T-Q) diagram is also introduced, which can graphically calculate the entransy dissipation rate of heat transfer processes and be used to conveniently optimize thermal systems. Then a brief review is made on applications of the minimum EDTR principle and T-Q diagram for optimization of different thermal systems, including a heat exchanger couple, a single stream heat exchanger network and chemical processes with recuperation technologies. Comparisons of the optimized results for different thermal systems indicate that the minimum EDTR principle, rather than the minimum entropy generation principle, should be used to optimize the thermal systems without heat to work conversion. Finally, a global optimization method is then given for thermal systems with the EDTR as an intermediate parameter with applications to a building central chilled water system, a district heating network and a spacecraft heat exchanger network.