Advantages and limitations of exergy indicators to assess sustainability of bioenergy and biobased materials

Abstract

Innovative bioenergy projects show a growing diversity in biomass pathways, transformation technologies and end-products, leading to complex new processes. Existing energy-based indicators are not designed to include multiple impacts and are too constrained to assess the sustainability of these processes. Alternatively, indicators based on exergy, a measure of "qualitative energy", could allow a more holistic view. Exergy is increasingly applied in analyses of both technical and biological processes. But sustainability assessments including exergy calculations, are not very common and are not generally applicable to all types of impact. Hence it is important to frame the use of exergy for inclusion in a sustainability assessment. This paper reviews the potentials and the limitations of exergy calculations, and presents solutions for coherent aggregation with other metrics. The resulting approach is illustrated in a case study. Within the context of sustainability assessment of bioenergy, exergy is a suitable metric for the impacts that require an ecocentric interpretation, and it allows aggregation on a physical basis. The use of exergy is limited to a measurement of material and energy exchanges with the sun, biosphere and lithosphere. Exchanges involving services or human choices are to be measured in different metrics. This combination provides a more inclusive and objective sustainability assessment, especially compared to standard energy-or carbon-based indicators. Future applications of this approach in different situations are required to clarify the potential of exergy-based indicators in a sustainability context.

The use of exergetic indicators in the food industry – A review

“…ExA is continuously extended to include economic and environmental aspects. Maes and Van Passel (2014) give an overview of such methodologies such as the cumulative exergy content developed by Szargut et al (1988), the extended exergy accounting developed by Sciubba (2001), the ecological cumulative exergy consumption developed by Hau and Bakshi (2004), and the cumulative exergy extraction from the natural environment developed by Dewulf et al (2007). Tsatsaronis and Morosuk (2012) discuss other exergy-based methods such as exergoeconomics, exergo-environmental analysis, and the advanced exergy analysis.…”

“…However, there is still no clear agreement on how to treat the exergetic content of such waste streams. Maes and Van Passel (2014) argue that ExA cannot capture the immaterial aspects of emissions and waste streams (e.g. land degradation and biodiversity loss), which should be considered by additional metrics in a sustainability assessment.…”

“…The use of renewable and non-renewable energy sources can be of relevance in a thermodynamic analysis because both of these energy sources might have the same exergy content but different overall thermodynamic impact, as suggested by Stougie and Van der Kooi (2011). Recently, Maes and Van Passel (2014) introduced the renewability fraction that is useful in identifying the actual exergetic value of renewable sources, and considers the sunlight required to produce the renewable resource, the forest abatement costs for carbon dioxide sequestration and oxygen production, and the actual sunlight captured by the process studied. A similar indicator has been used in assessing the performance of strawberry cultivation in greenhouses by different heating methods (Hepbasli, 2011).…”

“…According to Wall (1988), energy should be considered as an indestructible quantity that is conserved in every closed process. Upon transformation of one energy form to another, part of its initial quality is destroyed (irreversibly lost), leading to a lower, degraded quality (Maes and Van Passel, 2014). The concept of energy quality has been described by Van Gool (1980) as the possibility of energy exchange between a donating and an accepting stream.…”

“…This possibility was defined by Cornelissen (1997) as the "maximum work potential of a material or of a form of energy in relation to its environment," and it is known as available work or exergy, a term which was originally given by Rant (1956) after the Greek words "eξ" (out of) and "έrgοn" (work). Now, consensus among many authors from different scientific fields develops in using exergy analysis (ExA) as an objective methodology for assessing the efficiency and hence sustainability of processes and systems, because it is based on the first and the second law of thermodynamics, considering both the quantity and the quality of material and energy streams simultaneously without having to resort to subjective weighing factors (Berg, 1980;Hevert and Hevert, 1980;Dincer, 2002b;Szargut, 2005;Zvolinschi et al, 2007;Rosen et al, 2008;Sciubba, 2009;Wall, 2009;Dincer and Rosen, 2013a;Maes and Van Passel, 2014). The advantages of using ExA over other assessment methods have been discussed in detail by various authors, e.g.…”