Exergetic analysis of systems for expansion processes within a gas grid

This paper deals with configurations used to expand natural gas in a grid from a pressure of above 40 bar to a final pressure of approximately 12 bar. The thermodynamic performance of two system configurations is studied: (1) A conventional system in which all natural gas passes through a throttling valve, (2) a system that uses an expander as the expansion device. A conventional system is from the exergetic viewpoint dissipative because no useful result is obtained during expansion. The overall energetic efficiency for the second system is around 89%, whereas the corresponding exergetic efficiency is around 30%. The sensitivity analysis demonstrates how the mass flow rate of natural gas and the seasonal fluctuation of the ambient temperature affect the thermodynamic efficiency of these systems. The data given in this paper demonstrate that the average throughout the year energetic and exergetic characteristics of the systems are almost the same, and the more complex analysis taking into account the seasonal fluctuations of the ambient temperature.