On exergy analysis of industrial plants and significance of ambient temperature

• Main Author: Rian, Anne Berit
• Format: Doctoral Thesis
• Language: English
• Published: Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk 2011
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-13964; http://nbn-resolving.de/urn:isbn:978-82-471-2690-5 (printed ver.)

The exergy analysis has been a relatively mature theory for more than 30 years. However, it is not that developed in terms of procedures for optimizing systems, which partly explains why it is not that common. Misconceptions and prejudices, even among scientists, are also partly to blame. The main objective of this work was to contribute to the development of an understanding and methodology of the exergy analysis. The thesis was mainly based on three papers, two of which provided very different examples from existing industrial systems in Norway, thus showing the societal perspective in terms of resource utilization and thermodynamics. The last paper and the following investigation were limited to certain aspects of ambient conditions. Two Norwegian operational plants have been studied, one operative for close to 30 years (Kårstø steam production and distribution system), while the other has just started its expected 30 years of production (Snøhvit LNG plant). In addition to mapping the current operational status of these plants, the study of the Kårstø steam production and distribution system concluded that the potential for increasing the thermodynamic performance by rather cautious actions was significant, whereas the study of the Snøhvit LNG plant showed the considerable profit which the Arctic location provided in terms of reduced fuel consumption. The significance of the ambient temperature led to the study of systems with two ambient bodies (i.e. ambient water and ambient air) of different temperatures, here three different systems were investigated: A regenerative steam injection gas turbine (RSTIG), a simple Linde air liquefaction plant (Air Liq) and an air-source heat pump water heater (HPWH). In particular, the effect of the chosen environment on exergy analysis was negligible for RSTIG, modest for Air Liq and critical for HPWH. It was found that the amount of exergy received from the alternative ambient body, compared to the main exergy flow of the system gave an indication of whether the choice of environment would affect the exergy results or not. Furthermore, the additional study, where the effect on exergy results due to a fixed environmental state versus a natural environment was investigated, also suggested that neither calculations nor software tools should uncritically be based on the fixed environment when it does not correspond with the natural environment. The findings in these studies can be useful at different levels, such as for further studies and optimization of similar plants, for the authorities to encourage or demand even better performances from future plants and for developing methodical engineering tools for exergy.