Use of exergy as an indicator of environmental impact: influence of methodological choices
thesisposted on 23.02.2017, 03:09 by Brammer, Naomi Rachel
It has been suggested that exergy may provide a means of assessing and comparing the environmental impact caused by resource extractions and waste emissions associated with human activities. This work investigates the use of exergy as a measure of potential to cause environmental damage, and the influence of the choice of indicator and environmental temperature on the results and conclusions of exergy-based analyses of environmental impact, through a case study of five competing fossil fuel power plant technologies. To provide the necessary background for this investigation, the effects of environmental temperature on the chemical exergies of the elements are first examined based on Szargut’s reference environment model. The largest percentage variations in chemical exergy are observed for elements with gaseous reference species, where the exergy values depend only on the environmental temperature and concentration of the reference species. However, sensitivity analyses reveal that variations in the chemical exergies of elements with solid and aqueous reference species are primarily due to the thermochemical properties of the reference species and the exergies of other elements involved in its formation. Variations with environmental temperature in the chemical exergies of the fossil fuels used in the case study, and several related substances, are also evaluated and shown to depend on their compositions and thermochemical properties. A new classification of exergy-based indicators is introduced and used to identify the six fundamental exergy quantities underlying all such indicators, which are then determined for each of the power plants considered in the case study at environmental temperatures ranging from 273.15 K to 323.15 K (0°C to 50°C). Comparison of the calculated exergy quantities suggests that the choice of indicator has the greatest effect on the conclusions of exergy-based analyses of environmental impact for competing systems with similar resources but different technologies and wastes. Under these conditions, ranking of alternative systems in order of their potential to cause environmental damage is shown to differ depending on whether the indicator is based on the total exergy of the resource inputs or the total, chemical, or concentrational exergy of the waste outputs. Conclusions drawn from analyses of environmental impact that compare the total exergies of resource inputs or waste outputs with considerable physical exergy contents but temperatures close to the reference environment are particularly sensitive to the choice of environmental temperature. However, the choice of temperature can also influence the conclusions of analyses based on the chemical or concentrational exergies of waste outputs, especially when comparing gaseous emissions with differing chemical compositions. The findings of this work demonstrate the importance of carefully specifying the choice of indicator and reference environment conditions when reporting the results of an exergy-based analysis of environmental impact.