A Virtual-system Concept for Exergy Analysis of Flow Network Plant; Part I: Principles

Abstract

A new type of virtual system, named a flow constraint system (FCS), is proposed to facilitate, clarify, and simplify exergy analyses of plant that involve material flow networks. The need for the virtual system is outlined and the concept is demonstrated by applying it to a CHP steam cycle. The FCS concept allows the physical constraints on the exergy interactions associated with flow streams to be taken into account fully. It also simplifies the treatment of bifurcations in material flows and considerably reduces the need for absolute exergy evaluations. The new concept follows from the work already published by the authors on conceptual devices for exergy analysis and builds on this and the work of other authors relating to exergy and exergoeconomic analysis, especially using matrix methods. A bond graph type of diagram is described as an alternative to the usual Grassmann diagram. A numerical illustration is given in a separate paper — Part II. NOMENCLATURE Abbreviations CHP Combined heating and power FCS Flow constraint system LFCS Linked flow constraint system RN Reversible node Symbols b Specific flow exergy function ) ( s T h b o   h Specific enthalpy s Specific entropy o T Temperature of the environment 13 Ξ Exergy interaction rate with subscript identifier ) 6 5 4 (   Ξ Exergy interaction rate that is the algebraic sum of the exergy interactions identified by the signed subscripts ) ( 6 5 4 ) 6 5 4 ( Ξ Ξ Ξ Ξ         A Virtual-System Concept—Part I McGovern & O’Toole 1992 2 INTRODUCTION In this paper a new, simple, and powerful concept is presented to deal with real constraints, which have generally been neglected, on the net exergy interactions due to the transport of exergy by material flows between subsystems and external systems of plant. A DISCUSSION OF CONVENTIONAL SYSTEMS, AS BACKGROUND A system can be conventionally defined as a region in space surrounded by a boundary, which can be either real or imaginary. It is closed if no transfer of matter occurs across the boundary; and otherwise it is open. Beretta and Gyftopoulos (1990) have given a very general definition of the state of a system: a vast amount of data may be required to describe it. However, if a system is in equilibrium its state can be described by means of a relatively small number of parameters. An example would be a simple closed system that contains a gas in equilibrium: it could be described by specifying the gas and stating two independent properties such as pressure and specific volume: the state would be the same throughout the system; i.e., all subsystems would have the same state. It is also possible that the state might vary throughout a system Fig. 1 (a) An equilibrium (reversible) flow system with two entry and two exit streams, all of the same fluid. (b) The reversible process paths for the equilibrium flow system in (a) shown on a s T diagram. 1