Andres Clarens and Lisa Colosi

Abstract Algae-derived bioenergy is being widely discussed as a promising alternative to bioenergy produced from terrestrial crops. Several life cycle assessment (LCA) studies have been published recently in an effort to anticipate the environmental impacts of large-scale algae-to-energy systems. LCA is a useful tool for understand­ing the environmental implications of technology, but it is very sensitive to model­ing assumptions and techniques. In this chapter, the methodological issues surrounding LCA of algae-to-energy systems are reviewed in the context of several of the recent papers with a particular focus on system boundaries, cultivation tech­niques, metrics, coproduct allocation, and uncertainty. The issues raised here are useful in two regards: (1) they enable an understanding of the differences between the published studies and allow LCA practitioners and others to more directly inter­pret the results and (2) they serve as a good starting point for future analysis of algae-to-energy technologies.

1 Introduction

The promise of using algae as a bountiful and renewable source of bioenergy has been attracting increasing attention over the last few decades [26] . This is because algae have a number of characteristics that make them appealing relative to other bioenergy sources. They are generally fast growing and produce more biomass per area of land than most terrestrial crops [19]. Certain species generate high concen­trations of lipids so they can be used to produce liquid fuels, such as biodiesel, using existing conversion technologies [18]. And since they are grown in water, they could

A. Clarens (*) • L. Colosi

Civil and Environmental Engineering, University of Virginia, Charlottesville, VA 22904, USA e-mail: aclarens@virginia. edu

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_32, 759

© Springer Science+Business Media New York 2013 also be cultivated in man-made ponds, which suggests their cultivation can be scaled up and operated in steady-state mode, greatly enhancing their potential for large — scale energy production. Over the past few years, interest in algae-to-energy tech­nologies has surged for a variety of reasons. Among them is the idea that algae could be used to sequester CO2 from fossil fuel burning sources, thereby reducing a major contributor to climate change [3] . Increasing petroleum prices, concerns about our dwindling fossil fuel reserves, and the perceived competition between food and fuel uses for crops that can be consumed as food have also contributed to interest in algae as a fuel source [23].

The heightened attention on algae-to-energy systems has resulted in a prolifera­tion of academic and industrial publications describing these technologies. A num­ber of these studies focus on quantifying the environmental impacts of algae-to-energy systems using life cycle assessment (LCA) techniques [8, 14, 24, 31]. LCA is a framework for assessing the environmental and energy implications of a process or product over its entire life cycle (LC), from resource extraction to final disposal. Over the past 10 years, LCA has emerged as a valuable tool for understanding the full environmental costs of complex engineering systems. It allows designers and engineers to avoid media shifting, whereby one environmental impact is avoided at the cost of some other, often hidden and worse, environmental burden [13]. LCA can also serve as a useful design tool that allows for a priori evaluation of different engineering decisions. By applying LCA in this way, it is possible that many tradi­tional sources of pollution can be avoided upstream rather than remediated after they are generated. Even though LCA has been widely practiced for over a decade, only recently have the techniques been applied to algae-to-energy processes.

The algae-related LCA studies appearing in the academic literature to date offer multiple perspectives on how large-scale algae-to-energy systems might be deployed. These studies are largely speculative because there is a lack of empirical data for long-term operation of full-scale commercial algae cultivation systems. In general, the results of algae LCA studies published to date are difficult to compare because of key modeling differences. The differences originate from several stages of the analyses. To begin with, the scope. e. g., system boundaries and functional unit of the studies, is different. Second, the data sources used in the studies, the way in which the studies report their results (i. e., metrics), and the manner in which they allocate burdens to different processes (e. g., coproducts) also vary quite a bit. This variability is to be expected given that there are, as yet, no norms for the industry that would suggest the most reasonable set of assumptions. Finally, the unsatisfac­tory way in which the studies handle uncertainty speaks to the lack of data in this fi eld. Table 1 highlights the array of different modeling assumptions that have been used in some of the LCA studies of algae-to-energy systems that have been pub­lished to date. It should be pointed out that each of these studies utilized a different functional unit and many use different modeling assumptions. Thus, it is no surprise that the results are difficult to compare.

In general, it cannot be said that one particular study is more or less “correct” than any of the others. LCA challenges exist even for processes and products that

are well characterized and widely practiced. One widely cited, and related, example is the case of petroleum-based liquid fuels. Since the early 1990s, a substantial number of studies have been conducted describing the process of extracting the crude oil from the ground, transporting it, refining it, distributing and selling it, then burning it in cars and trucks [25]. Different studies resulted in very different esti­mates for the burdens of similar processes that are practiced in more or less the same manner around the world. To address these challenges, Argonne National Laboratory in the United States created the Greenhouse Gases and Regulated Emissions, and Energy Use in Transportation (GREET) model for estimating LC burdens associ­ated with petroleum-based transportation fuels in 1996 [33]. By synthesizing the results from various published LC models, and normalizing the system boundaries and allocation assumptions among analyzed cases, the creators of GREET produced a meta-model that is more representative of petroleum fuel production than any one given analysis. This occurs because the meta-model effectively neutralizes (i. e., washes out) some assumptions that can make any one particular study either over — or underestimate the true impacts of a given process. Since the algae-to-energy industry is currently undergoing such rapid development, it seems timely to con­sider standardization of LC methodology to improve the accuracy of LCA for algae — derived fuels.

This chapter is written for two primary audiences. The first is the algae-to-energy researchers wishing to model LC impacts of specific products or processes. For these uses, the material presented here should serve as a useful primer into the lan­guage of LCA as it relates to algae-to-energy processes. The second audience is the broader scientific and journalistic community. This community has occasionally misinterpreted the results of several recent algae LCAs. The material presented here should help educate the science-literate reader who has no background in LCA so that they can better understand the implications and conclusions of algae LCA studies. It is expected that successful engagement of both audiences should improve the quality of future algae LCA studies and contribute to discourse about the merits of algae-to-energy technologies.