Assessment of Physical and Ecological Limits of Biofuels


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Sugar cane is used to produce biofuels. Phot by José Reinaldo da Fonseca.

We can produce biofuel from sugar cane- but should we? Photo by José Reinaldo da Fonseca.

Biofuel Yield Productivity

Using this model, Professor de Castro and his coworkers studied several biofuel-related issues along the whole production process. Regarding the cultivation of the crops, for instance, they looked at the yield productivity of the lands for biofuels. They considered as an example the production of bioethanol from sugar cane in Brazil because ethanol is the most common biofuel, and Brazil is one of the biggest producers.

Researchers analyzed two parameters: The land and the biofuel yield, that is, the tonnes of sugar cane grown and the liters of biofuel produced per hectare of cultivated land, respectively. For both indicators, the values calculated in this study were lower than those previously reported in literature; this may indicate previous overestimations for the biofuel yield.

Energy Return and Power Density

The Energy Returned on Energy Invested (EROEI) serves as a very important parameter to assess the economic viability of a fuel, representing the ratio of usable energy to the energy necessary to obtain this fuel.

According to Professor de Castro: “Considering as an example the Brazilian ethanol, the EROEI we calculated is lower than some other estimates previously reported. The Brazilian cultivation and production are generally more advanced and efficient than those of other countries, such as India; therefore, we expect the overall/average energy return for biofuels to be even lower.”

The values of the biofuel power density (power generated per area of cultivated land) demonstrate a similar situation; once more, the estimates of professor de Castro and coworkers are lower than the published versions.

The ecological footprint of biofuels is higher than for standard fossil fuels. Photo by Institut Escola Les Vinyes.

The ecological footprint of biofuels is higher than for standard fossil fuels. Photo by Institut Escola Les Vinyes.

Ecological Footprint (EF) of Biofuels

Evaluating the ecological footprint (EF) is essential for every fuel; it is even more important, however, for biofuels, to verify if they really are more “green” and “friendly” to the environment.

For fossil fuels, we calculate EF taking into account the associated carbon dioxide (CO2) emissions, subtracting a percentage for the absorption by the oceans, and hence estimating the woodland surface area necessary to absorb this CO2 quantity from the atmosphere (this is the usual methodology in EF calculations). The EF has, therefore, units of hectares per capita.

In order to compare both sources of energy, the EF for fossil fuels and for global biofuels are estimated per net energy delivered to society.

Professor de Castro explains: “In the case of biofuels, we do not considered directly the emissions associated with the fossil fuels used during the production process but only and those due to the change in the land use (from agriculture to biocrops) and its erosion.”

With all these contributions, researchers estimated the biofuel EF to be twice or more the value of that of fossil fuels: >16.3 global square meters per watt (m2/W) delivered for biofuels against 8.2 m2/W for fossil fuels.

Are Biofuels a Good Energy Source?

Professor de Castro commented with these results:

“From our study it seems clear that biofuels are not such a good source of renewable energy; the high value of the EF is particularly worrying.

Until now the debate on the biofuel production was mainly linked to socioeconomic implications; we think it is time to consider also other aspects, such as the actual sustainability of this energy source, compared to fossil fuels and other renewable sources. These are all features which should not be ignored in policy making.”



de Castro, C. et al. A top-down approach to assess physical and ecological limits of biofuels. (2013). Energy. Accessed November 2013.

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