(525b) Hybrid Thermodynamic Life Cycle Assessment of Six Alternatives for Generating Electricity

Life Cycle Assessment (LCA) has fast emerged as one of the most popular techniques for sustainable engineering. It promises to consider environmental implications of all the processes including the extraction of raw materials, fabrication and packaging of products and use and final disposal. As a result, LCA can provide the broad systems perspective required for economically and ecologically conscious industrial systems. However, LCA faces numerous practical challenges. Traditional process-based LCAs consider only a few selected components of the supply chain due to computational limitations, and ignore vast fractions of complementary economic networks. This introduces an arbitrary system boundary and yields results with large truncation errors. Economic Input-Output LCA (eioLCA) considers the entire economic network and employs a consistent system boundary that corresponds with the national economy. However, its results are at the level of industry sectors, and are too aggregate to analyze individual processes and products. Hybrid LCA techniques have also been developed that marry the specificity of process LCA with the comprehensiveness of eioLCA. Hybrid techniques are a major improvement over traditional LCA techniques and represent the state-of-the-art today.

However, all the LCA techniques suffer from two fundamental shortcomings. Firstly, they yield results in disparate units - such as tons of SO2 and tons of CO2 - that cannot be combined without human valuation. Secondly, they ignore the contribution of ecosystem goods and services (natural capital) to the industrial production network. Examples of natural capital include various natural functions that supply natural resources to economic system and dissipate emissions from it. Thermodynamic methods can address both shortcomings by allowing scientifically rigorous conversion of mass and energy streams into consistent units of exergy (available energy), and by joint analysis of ecological and industrial systems as networks of exergy flow [1,2]. This does not replace human valuation, but provides more scientifically rigorous data to assist valuation. Furthermore, joint thermodynamic analysis of industrial and ecological systems provides unique insight into the quality and renewability of ecological resources. Our previous work on thermodynamic input-output analysis combines these benefits with LCA based on economic input-output data [2]. This approach allows consideration of diverse ecological and economic resources, emissions and their impact on human and ecosystem health, capital and labor on a theoretically sound basis addressing the problem of arbitrary human valuation in LCA. Use of a common currency also helps construction of hierarchical metrics of sustainability that balance the accuracy of detailed multidimensional data with the ease of making decision via aggregated metrics. Such metrics are easy-to-calculate, robust, stackable and communicable to diverse stakeholders. In addition, reliance on thermodynamic principles also imparts Thermodynamic Input-Output Analysis a sound theoretical basis.

This presentation will demonstrate the advantages of Thermodynamic Input-Output Analysis and hierarchical thermodynamic metrics by applying it to six alternative electricity generation systems, including oil-, coal-, and natural gas-based thermoelectric systems and wind-, geothermal-, and hydroelectric systems. The results will include exergetic or second law efficiencies of the six systems based on machinery and fuel inputs during operation and construction phases of the six systems and net annual electricity yields from them. This will be followed by proposal of an algorithm that would enable systematic expansion of the scope of exergy analysis beyond the scale of the process to include economic and ecological stages of the production chain. Efficiencies at the process-scale indicate that the thermoelectric alternatives are more efficient than wind and geothermal alternatives because coal, natural gas and oil are more concentrated forms of energy sources than wind or geothermal heat. However, at the ecosystem scale the trend is reversed. Wind and geothermal alternatives have higher efficiencies than the thermoelectric alternatives because coal, natural gas and oil are non-renewable resources that are produced much more inefficiently in ecological systems than wind and geothermal heat.

This presentation will also compare the six alternatives using performance indicators such as environmental loading ratio, and impact per value added. The former measures the relative reliance of the six systems on non-renewable resources whereas the second measures the human health impact of emissions from the six systems per unit electricity production. Finally, based on the results, a plausible correlation between second law efficiency at the ecosystem scale and impact per value added would be hypothesized. This correlation, if true, would be very significant as it would provide a proxy-indicator of the impact based on input-side numbers. Such a proxy-indicator would be especially useful at early stages of product and process development and for evaluating emerging technologies, when knowledge about emissions and their environmental impacts is often unavailable.

References

1. Hau, J. L., and Bakshi, B. R., 2004, Expanding Exergy Analysis to Account for Ecosystem Products and Services, Env. Sci. Tech. 38 (13): 3768-3777.

2. Ukidwe, N. U., and Bakshi, B. R., 2004, Thermodynamic Accounting of Ecosystem Contribution to Economic Sectors with Application to 1992 US Economy, Env. Sci. Tech. 38 (18): 4810-4827.