This report gives a review of methods and approaches for assessment of greenhouse gas mitigation and cost-effectiveness in biomass-based energy systems, identifying methodological strengths and weaknesses. Beneficial methodological approaches should be accurate, transparent and efficient in the assessment of greenhouse gas mitigation potentials and costs.
The approaches most frequently used in project-level studies of greenhouse gas mitigation of biomass-based systems are lifecycle assessments (LCA), performed either with dedicated LCA-software or ‘manually’ in spreadsheets, and energy systems analysis (ESA) typically performed in spreadsheets. The general aspects which determines the quality of a methodology used for energy system assessments are; 1) accuracy, including the  comprehensiveness and consistency of the study, 2) transparency of assumptions and calculations and 3) efficiency, meaning that an appropriate level of detail must be balanced by ease-of-use. These aspects were used as the framework in a standard reporting format for a selection of reviewed published reports.
Some methodological aspects are discussed in greater detail. One aspect of fundamental importance is the baseline, or reference fossil system, against which the biomass-based systems are compared. For an accurate baseline the choice of reference technology should be made carefully, including e.g. future development, market influence and site-specific effects. The functional unit is also fundamental. The same unit should be used in the comparisons between fossil and biomass-based systems and have to be defined accurately. This requires specific considerations in studies of multifunctional systems like cogeneration plants. As in all system analysis the comprehensiveness of the assessment is dependent on how the system boundaries are chosen. The methodological treatment of different time-spans, and the use of different output parameters are discussed, as is the use of sensitivity analysis as a way to increase transparency of the uncertainty associated with the complex analyses.
In energy systems much focus has been on conversion technology, but with the life-cycle perspective most studies include the precluding production stages of the system. Still, the endues conversion is sometimes not included. Cost assessments often cover only conversion and/or production costs and not the complete life cycle. Costs, however, have also been estimated based on energy prices and from top-down models like input-output analysis. For calculations of investment costs parameters like the interest rate may have large influence on the results. External costs, covering the costs for impact on the ecosystem, have been used in energy system analysis, though the uncertainties in these estimates are relatively high.

By: P. Å. Vikman, L. Gustavsson, A. Klang

download this document:    270 kb