We prefer the term "energy life cycle analysis" over "energy balance" because the term "energy balance" could mislead people into thinking that the input and output energy should be balanced, or equal. Because EgLCA is usually concerned with the input of fossil fuel energy versus the energy in the fuel itself, usually the input of solar energy (captured during photosynthesis) is not accounted for.
The process is also called “energy life-cycle assessment.”
Energy life cycle analysis is different from Environmental Life Cycle Analysis (EvLCA), which measures the environmental impact or benefits of a product compared to other similar products. In other words, EgLCA is an energy accounting, whereas EvLCA is an accounting of greenhouse gas and other pollutants.
A Brief Review of Previous Energy Life-Cycle Analysis Studies
Several studies have been done on the energy life cycle of biodiesel. These studies often have widely differing results because the inputs were different, the assumptions were different, and the energy inputs were divided differently among the various products of the process (biodiesel, oilseed meal, and crude glycerin).
The “energy inputs” of biodiesel production include not only the energy used in the process of converting oil to biodiesel, but also could include the energy required to grow the soybeans, such as agricultural machinery use, and fertilizer and pesticide use; the energy required to transport the soybeans to the biodiesel production plant; and the energy required to construct the biodiesel plant. This is what makes EgLCA in general difficult and somewhat controversial. The energy contained in some of the consumables such as building material is hard to estimate. In addition, there is no specific rule on where to stop the accounting process: for example, should we include the energy used to construct the road to transport the soybeans to the crushing plant?
If the system boundaries (that is, what is included and what is excluded from accounting) were different for two comparable biofuel systems, comparing their performance would not only be meaningless, but also dangerously misleading. Therefore, before the results of an energy life-cycle analysis are interpreted, the assumptions and system boundaries should be carefully examined.
In addition, different studies have allocated the energy inputs differently to the oil and the meal resulting from extracting oil from soybeans. This can also produce widely differing results.
Because of the difficulties of comparing the various studies, and because some of the earlier studies used data that are no longer relevant to the biodiesel industry, the United States Department of Agriculture (USDA) publishes updated studies as new data becomes available. The latest study was published in 2011: Energy Life-Cycle Assessment of Soybean Biodiesel Revisited. This is an update of a 2009 study: Energy Life-Cycle Assessment of Soybean Biodiesel, which in turn updated a previous USDA study from 1998, Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus, which was the first comprehensive life-cycle assessment of biodiesel.
Latest Study: Soy Biodiesel Yields 5.54 Times the Energy of the Fossil Energy Inputs
The 1998 USDA study found that soy biodiesel yielded 3.2 times the fossil energy needed to produce the fuel. The 2011 study found that soy biodiesel yields 5.54 times the fossil energy needed to produce it (Sheehan et al., 1998, p. v; Pradhan et al., 2011, p. 1031).
Why the difference? One reason is that soybean production takes less energy now — farmers have largely adopted a no-till approach to soybean planting, which requires less fuel. Soybean yields have improved, and newer soybean crushing facilities are more energy efficient.
Soybean agriculture and processing are expected to become even more energy efficient in the future, and so the ratio of energy input to output of biodiesel ought to continue to improve.
Why is Energy Life-Cycle Analysis Important?
Much of the attention directed toward biofuels production focuses on the perception that biofuels are renewable and have superior environmental attributes compared to their petroleum counterparts. Based on these assumptions, the government and private businesses have spent a lot of resources to develop the infrastructure for biodiesel.
The renewability assumption would be unarguably true if there were no nonrenewable resources such as diesel and gasoline used in the biodiesel production process. However, that is not the case. Some amount of nonrenewable resources are consumed in agriculture, either directly or indirectly.
The use of nonrenewable fossil fuel obviously makes biodiesel less than 100% renewable. The amount of renewability could range from 0% renewable to 100% renewable. If we use as much or more nonrenewable energy to make biodiesel, then the biodiesel is 0% renewable. At the other extreme, if no nonrenewable fuel is used, then the biodiesel is 100% renewable. So we cannot tell for sure without doing some basic calculations how renewable biodiesel is.
Now let us take a step back and revisit our assumption that biodiesel is a renewable fuel. What if the assumptions were wrong and the fuel turned out to be nonrenewable? In that case, biodiesel would take in more fossil energy than it delivers, and it would therefore be an environmentally unfriendly fuel. It would not help the agricultural community in the long run, as the fuel would not be sustainable.
Energy life-cycle analysis provides a way to compare the relative benefits among alternative fuel sources, helps identify a subsystem that may need improvement, and helps make a go or no-go decision for a particular biofuel. A biofuel that is developed after a well-conducted EgLCA with positive results will ensure benefit to the society, country, and the world. Without EgLCA, money and resources may be wasted by investing in something that turns out not to be beneficial.
It should be noted that while assessing biodiesel for renewability, only the nonrenewable fuel that went into production is counted. This is different from the net energy return, which is the comparison of total energy produced by biodiesel compared to total energy (renewable + nonrenewable) that went into making it. If total energy input of a biofuel is greater than the energy produced from the biofuel, then it is called a net negative system.
Although a net negative system sounds not so good, this in itself may not be a bad thing because it may be desirable to convert low-value agricultural products such as corn stover and forest wastes into useful liquid fuel. In such cases, the benefit should be justified separately. Therefore, in order to have a correct assessment of the benefits from biodiesel, or biofuel in general, an energy life-cycle assessment must be conducted.
Methods of Energy Life-Cycle Analysis
Energy life-cycle analysis, like any other life-cycle analysis, should follow the general guidelines established by ISO (2006). ISO 14040 defines LCA as a four-step process:
- the goal and scope definition phase,
- the inventory analysis phase,
- the impact assessment phase, and
- the interpretation phase.
ISO (2006) further states that the scope, including the system boundary and level of detail, of an LCA depends on the subject and the intended use of the study. The depth and the breadth of LCA can differ considerably depending on the goal of a particular LCA. Therefore, even though there is a general guideline provided in ISO 14040, an individual study could vary significantly depending on how the system boundary was set.
The system boundary defines what is included and excluded in a model. It is almost impossible to track all the energy used over the life cycle of a product because each input has a life cycle of its own, and in turn the inputs required to produce an input each have a unique life cycle. Therefore, a researcher must limit the system boundary used in the analysis and still provide a meaningful EgLCA.
For example, considerable discrepancies regarding the system boundary definition were observed among four models of biodiesel energy life-cycle analysis. After careful consideration of these models, Pradhan et al. (2008) tried to streamline the system boundary and the definition of EgLCA so that the final results are comparable. Their study proposed a modified system boundary based on the merits and demerits of the earlier models to answer the biodiesel renewability question. The following observations were made after carefully examining each of the four system boundaries:
- Human food consumption should not be included as an energy input from labor because it does not aid in answering the renewability question, and it creates a circular reference within the system boundary. In any case, since the calorific value provided by human labor accounts for a negligible fraction of the total energy, this can be excluded without introducing much error.
- Life-cycle energy (not the calorific value) should be assigned as the equivalent energy for all the inputs. Life-cycle energy is the energy consumed in producing a specific input.
- Energy associated with inputs, including machinery, fertilizer, pesticides, lime, chemicals, liquid fuel, electricity, and other fuels used in production and transportation and processing should be included.
- Energy required for building and maintaining the biodiesel infrastructure, such as a biodiesel plant, should be included and amortized per unit of biofuel production.
- Each co-product should share a portion of the energy input according to either their proportion by weight of the total output (i.e., their mass fraction), or their economic value. The choice depends on the type of research question being answered. For renewability analysis, distributing the energy input based on the mass fraction of the co-product is helpful in determining the energy balance. On the other hand, distributing the energy input based on the economic value of the co-product is suitable for analyzing economic viability of the energy production system.
- FER (fossil energy ratio) as defined in the following equation can be used to quantify the renewability of the biodiesel.
