Measuring the sustainability of energy systems

A couple of years ago, during the peak of the global bioenergy mania, a series of short articles appeared in the journal Biofuels, Bioproducts & Biorefining, which summarized the debate about the pros and cons of net energy analysis as a methodology for measuring the sustainability of biomass-based energy production systems. On one hand, the renowned biofuel guru, Prof. Bruce Dale, argued that the net energy concept was flawed and misleading. Dr. Franzi Poldy, on the other hand, made the counterargument that net energy analysis was useful and relevant if used judiciously. It is interesting to note that their debate, which took place in early 2008, echoed similar controversies regarding the thermodynamic analysis of bioenergy systems during the global energy crisis of the 1970s. Two interesting articles were published in the journal Science in 1979, the first of which was entitled “Gasohol: does it or doesn’t it produce positive net energy?” by Chambers and co-workers, and the other which was entitled “High-grade biofuels from biomass farming: Potentials and constraints” by Weisz and Marshall. These two research groups raised points that were largely similar to those debated by Dale and Poldy.   

Firstly, let’s define precisely what we mean by net energy. Net energy may be thought of in simple terms as the “energy profit” of a fuel life cycle. Suppose we have a system for producing a biofuel (e.g., bioethanol or biodiesel) on a commercial scale. Net energy is simply the sum of all useful energy outputs, minus the sum of all energy expenditures or inputs within the entire fuel supply chain. The outputs that are counted normally include the final fuel product itself, plus byproducts with energy value, such as cogenerated electricity; in some cases, the equivalent energy content of non-fuel byproducts (e.g, livestock feed or fertilizers derived from biofuel production residues) is also included. In most cases, the energy inputs used for calculations include only non-renewable or fossil energy inputs, since solar energy absorbed by plants during photosynthesis is regarded as a free, natural input. In the case of biofuel systems, such energy inputs are needed for irrigation, farm operations, transportation, storage, fertilizer production, and biomass conversion. The difference between total useful output and total non-renewable input provides a numerical measure of whether the production system as a whole makes thermodynamic sense. A positive net energy value indicates that the fuel supply chain produces more useful energy than it consumes. On the other hand, negative net energy indicates that the system operates at a loss, and one would be better off using the non-renewable energy inputs directly, instead of using them to make biofuels. Of course, zero net energy corresponds to a thermodynamic break-even. In some countries, an alternative metric called “energy ratio” is used instead. It is found by dividing the total useful energy output by the non-renewable energy input. Clearly, energy ratios greater than one correspond to positive net energy, and values less than one correspond to negative net energy.

The policy implications of net energy (or energy ratio) analysis should be evident: if a country tries to gain some degree of energy independence by establishing local biofuel production systems, proper quantitative analysis of the fuel life cycle reveals whether such a system actually reduces or increases energy imports. The literature contains numerous examples of net energy analysis of various biofuel systems. For example, typical analysis of sugarcane-based bioethanol production systems in Brazil show very high net energy values, which results from highly efficient production systems that reflect the accumulation of nearly four decades of incremental technological improvements. On the other hand, similar analysis of microalgal biofuel systems have shown mixed results (my colleague at La Salle, Prof. Luis Razon, and I have written a paper showing significant energy losses for selected microalgal biodiesel systems; we submitted the manuscript to Applied Energy and it is currently under review).

It should be stated here that net energy measures only the thermodynamic aspect of energy systems. Thus, it provides an important perspective, especially for comparing alternative energy sources; however, by itself it does not give a comprehensive description of the sustainability of fuel supply chains. In the case of biofuels, in particular, recent research has led to greater appreciation of other important environmental (e.g, land use, water footprint, carbon footprint) and socio-economic (e.g., social acceptability, job creation) dimensions that are relevant to decision-makers. As a final note, I should emphasize that it is difficult to say in general whether or not any given biofuel system is sustainable. The answer depends too much on operational details, much in the same way the financial profitability of a business enterprise depends on how it is run. What is important is to have the systematic, rigorous techniques necessary to assess biofuels within very specific contexts.

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Raymond R. Tan is a full professor of chemical engineering and university fellow at De La Salle University. His main research interests are process systems engineering (PSE), life cycle assessment (LCA) and pinch analysis. He received his BS and MS in chemical engineering and Ph.D. in mechanical engineering from De La Salle University. He is the author of more than 50 articles in ISI-indexed journals in the fields of chemical, environmental and energy engineering. He is a member of the editorial board of the journal “Clean Technologies and Environmental Policy,” and co-editor of the forthcoming book “Recent Advances in Sustainable Process Design and Optimization.” He is also the recipient of multiple awards from the Philippine National Academy of Science and Technology and the National Research Council of the Philippines. E-mail at [email protected].