Title: Exergy calculations based on a fixed standard reference environment vs. the actual ambient conditions: gas turbine and fuel cell examples

Authors: Ivar S. Ertesvåg

Addresses: Department of Energy and Process Engineering, Norwegian University of Science and Technology, No. 7491 Trondheim, Norway

Abstract: Two approaches for calculating exergy values were compared: ne based on the actual local temperature, pressure and relative humidity, and the other one based on the fixed reference conditions of 25°C, 1 atm and 70% relative humidity. In the former, exergy values represent the actual potential of the system, while the latter is chosen by some commercial software. The objective was to investigate to which degree the choice of environment influence on the results of the exergy analysis. For power plants and chemical conversion, the effects were modest with respect to overall efficiency (which, however, can be assessed without exergy). Details of the exergy analysis were, however, influenced by the choice of environment. Irreversibilities (exergy destruction) and flow exergies were more affected, and the distribution of exergy losses between irreversibility and discharge was different. For elevations above the sea level, the flow exergies can get negative values.

Keywords: exergy analysis; ambient conditions; fixed environment; standard reference environment; dead state; gas turbines; fuel cells; loss distribution; discharged exergy; negative exergy; chemical exergy; pressure exergy component; temperature exergy component; relative humidity; environmental influence; power plants; chemical conversion; exergy efficiency; choice of environment; irreversibilities.

DOI: 10.1504/IJEX.2015.068209

International Journal of Exergy, 2015 Vol.16 No.2, pp.239 - 261

Received: 22 Aug 2013
Accepted: 14 Dec 2013

Published online: 24 Mar 2015 *

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