Do the Different Exergy Accounting Methodologies Provide Consistent or Contradictory Results? A Case Study with the Portuguese Agricultural, Forestry and Fisheries Sector

Abstract

Three exergy accounting approaches are used to evaluate exergy efficiency: the Energy Resources Exergy Accounting (EREA), the Natural Resources' Exergy Accounting (NREA) and the Extended Exergy Accounting (EEA). To test the consistency of the results provided by these methodologies, we apply them to evaluate the Portuguese agricultural, forestry and fisheries (AFF) sector, from 2000 to 2012. EREA shows an increase of 30% in the efficiency of the Portuguese AFF sector, while NREA and EEA methodologies increases of 27% and 43%, respectively. Although the results are consistent for the AFF sector, the same does not happen in the fisheries subsector, whose exergetic efficiency increases 14% with the EREA but decreases 42% with the NREA approach. The ratio of output to useful exergy reveals that a better thermodynamic efficiency is not translated into a higher energy service efficiency because fishing vessels have to travel more to get the same fish. Thus, results provided by the EREA and NREA approaches are complementary and both are needed to provide a realistic picture of exergy efficiency. On the other hand, results obtained by the EEA approach are dominated by capital and environmental impacts, revealing the disproportionality between material and immaterial inputs in this methodology.

The Way Forward in Quantifying Extended Exergy Efficiency

“…We compare the EEA methodologies that were collected in Table 1 to estimate the efficiency of the Portuguese Agriculture, Forestry, and Fisheries (AFF) sector from 2000 to 2012. The exergy flow diagram followed the one presented in Figure 4 [13], where all available fluxes in the respective databases were accounted for and converted to energetic and exergetic fluxes. Data sources and values gathered for this study are available in Appendixes A and B.…”

“…If wood is used for furniture or construction it maintains its carbon content; if used in combustion processes as an energy carrier or in the paper industry, the released carbon dioxide should be attributed to the related activity. Figure 5 presents the amount of intrinsic exergy in materials and energy entering the AFF sector [13]. These exergetic fluxes take into account the mass and energy magnitudes and the specific exergy values as explained in Appendix A.…”

“…Exergy flow Capital flow Intermediate consumption (IC) consists of the monetary value of services and products consumed as inputs and transformed or used up by a production process [46]. In the Natural Resources Exergy Accounting (NREA) methodology, this flow is quantified as an exergy flow, representing all matter and energy plus services that enter the system [13]. If the ratio between these flows was used to obtain the specific exergy of capital than = , where stands for all material and energy flows measured by their intrinsic exergy.…”

“…These inconsistencies led to EEA results where the equivalent exergies of capital and labour are the dominant flows by a large factor. This disparity between the exergy of different flows is shown in Manso et al [13], where the results obtained by the EEA approach, using as a case study the Agriculture, Forestry, and Fishing (AFF) economic sector of Portugal from 2000 to 2012, are completely dominated by the exergies of capital and environmental impacts.…”

“…In this paper, we review past studies to identify, synthesize, and discuss the different methods used to quantify exergy with a special focus on the three immaterial production factors of EEA (labour, capital, and environmental impact) and compare the several EEA methodologies using the Agriculture, Forestry and Fishing (AFF) economic sector of Portugal from 2000 to 2012 as a case study, with the aim of providing understanding and clarity for EEA practitioners. The case study and initial dataset is the same used in Manso et al [13], but here it is used to compare the multiple EEA methodologies while in Manso et al [13], it was used to compare the exergy efficiency obtained using the EREA, NREA, and one of the EEA approaches. Additionally, this paper addresses the challenge of improving the estimation of EEA externalities, labour, capital, and environmental impacts for sectorial studies.…”