Thermodynamic Metrics for Aggregation of Natural Resources in Life Cycle Analysis: Insight via Application to Some Transportation Fuels

Abstract

While methods for aggregating emissions are widely used and standardized in life cycle assessment (LCA), there is little agreement about methods for aggregating natural resources for obtaining interpretable metrics. Thermodynamic methods have been suggested including energy, exergy, and emergy analyses. This work provides insight into the nature of thermodynamic aggregation, including assumptions about substitutability between resources and loss of detailed information about the data being combined. Methods considered include calorific value or energy, industrial cumulative exergy consumption (ICEC) and its variations, and ecological cumulative exergy consumption (ECEC) or emergy. A hierarchy of metrics is proposed that spans the range from detailed data to aggregate metrics. At the fine scale, detailed data can help identify resources to whose depletion the selected product is most vulnerable. At the coarse scale, new insight is provided about thermodynamic aggregation methods. Among these, energy analysis is appropriate only for products that rely primarily on fossil fuels, and it cannot provide a useful indication of renewability. Exergy-based methods can provide results similar to energy analysis by including only nonrenewable fuels but can also account for materials use and provide a renewability index. However, ICEC and its variations do not address substitutability between resources, causing its results to be dominated by dilute and low-quality resources such as sunlight. The use of monetary values to account for substitutability does not consider many ecological resources and may not be appropriate for the analysis of emerging products. ECEC or emergy explicitly considers substitutability and resource quality and provides more intuitive results but is plagued by data gaps and uncertainties. This insight is illustrated via application to the life cycles of gasoline, diesel, corn ethanol, and soybean biodiesel. Here, aggregate metrics reveal the dilemma facing the choice of fuels: high return on investment versus high renewability.

Life cycle assessment of fresh pineapple from Costa Rica

“…This method is traditionally called emergy (Odum, 1996) and has been implemented in LCA with that name, but has also been implemented as cumulative ecological exergy consumption (Baral and Bakshi, 2009;Zhang et al, 2010) and solar energy demand (SED) (Rugani et al, 2011). Because SED was a complete implementation of emergy for the Ecoinvent database, SED is used here, but the characterizations factors are modified in accordance with a newly recalculated global baseline for emergy of 15.2E24 sej/yr (Brown and Ulgiati, 2010), and a characterization value for organic matter in eroded sediment of 1.06E5 sej/J is added for including the emergy of lost organic matter.…”

Environmental Comparison of Straw Applications Based on a Life Cycle Assessment Model and Emergy Evaluation

“…The conversion factor called transformity or unit energy value (UEV) is defined as the emergy required to make one unit of a given product or service (Odum 1996). Indices such as the renewability indicator (RI) (Hau and Bakshi 2004), potential greenhouse gas emissions (GHGemitted) (Baral and Bakshi 2010), and emergy per unit money (EmPM) (Brown and Ulgiati 2004), can be used to assess the best use of straw for society and the environment.…”

Life Cycle Impact Assessment

“…According to the authors, it overcomes some weaknesses of emergy, and if identical system boundaries, allocation and quantification methods are used, emergy and ECEC should produce equivalent results. It accounts for several ecosystem services as well and is commonly used in complementation to the industrial ICEC Urban and Bakshi 2009;Baral and Bakshi 2010;Baral et al 2012). It accounts for several ecosystem services as well and is commonly used in complementation to the industrial ICEC Urban and Bakshi 2009;Baral and Bakshi 2010;Baral et al 2012).…”