Research Publications

Electricity is a key energy carrier for residential, commercial and industrial activities, particularly in developed countries. In the United States, electricity generation accounts for 39% of total primary energy consumption with renewable sources providing only 9% of the total [1]. This reliance on fossil and nuclear fuels is not sustainable on a long term basis and is a major contributor to environmental challenges such as global climate change, acidification, smog formation, and radioactive waste disposal. Transition to renewable technologies provides an opportunity for enhancing the sustainability of electricity generation. The objectives of this paper are to compare the life cycle energy performance of competing renewable technologies through the use of net energy ratio (NER) and to examine alternative life cycle assessment (LCA) methods for evaluating the NER. The net energy ratio as defined here is the total life cycle electrical energy output of a system relative to the total life cycle primary energy input from nonrenewable sources and is used to discuss the non-renewable energy leveraging capacity of electricity generation. NER values can be calculated using either a process-based LCA approach or an input-output based LCA approach. Examination of NER values for alternative technologies is extremely complex. Materials of construction, processing systems, operating parameters, durability, and other factors all affect technology performance. In the case of many renewable energy technologies, location factors such as solar radiation levels, wind speeds, and crop yields directly impact NER values. Examples that demonstrate these factors for select renewable technologies are shown in Table 1. Not surprisingly, the identification and examination of key technology parameters is an important component of understanding energy performance.This paper highlights critical factors included in energy analyses, and provides NER values for the major electricity generation technologies available in the US.

Process-based LCA studies require detailed information regarding a well-defined system and the associated processes. EIOLCA requires information regarding economic transactions and environmental impacts associated with broad industrial sectors within a specified economy (e.g. the US national economy). The resulting analyses provide a detailed understanding of individual system components in the case of process-based LCA or a comprehensive view based on a more extensive system boundary in the case of EIOLCA. Figure 1 compares the NER of a wind farm using the two LCA methods studied. The electrical output of the facility at any point on the figure is identical for both methods, however, differences in the amount of life cycle energy input captured by the two methods results in different NER values. EIOLCA tends to be more comprehensive and inclusive than process based approaches, which yields a larger denominator for the computation of the NER. Consequently, EIOLCA based NER values are smaller than process based LCA results. Two key system parameters, the lifetime of the facility and the capacity factor, affect the NER regardless of the LCA method used. Overall results from EIOLCA and process-based LCA methods indicate that the greatest leveraging of non-renewable energy occurs with wind turbines and
hydroelectric plants followed by biomass and photovoltaics.

[1] U.S. Department of Energy, Annual Energy Review 2003, DOE/EIA-0384, Energy Information Administration (2004).
[2] D. V. Spitzley, G. A. Keoleian, Life Cycle Environmental and Economic Assessment of Willow Biomass Electricity: A Comparison with Other Renewable and Non-Renewable Sources, CSS04-05, Center for Sustainable Systems, University of Michigan, (2004).