Thailand fuels up with cassava
Net energy value calculations favor ethanol production from cassava.
While most of the world is scrambling after corn and sugar for answers to its renewable energy needs, many developing countries are focusing on a lesser known plant—cassava. New research published today on ES&T's Research ASAP website (DOI: 10.1021/es0620641) calculates the net energy value (NEV) of cassava and suggests that the tuber would be an efficient source of renewable energy in Thailand.
The findings provide a "framework" for policy makers to evaluate whether "ethanol from cassava is feasible and practical," says study leader Shabbir Gheewala of King Mongkut's University of Technology Thonburi (Thailand).
Although molasses is the current source of ethanol in Thailand, the country already has a cassava-derived ethanol pilot plant, and the government is aiming to build 12 full-scale ones by 2008.
Using "scaled-up" data from the existing pilot plant, the authors calculate the NEV of cassava-based ethanol as 10.22 megajoules per liter (MJ/L), an overall positive yield. NEV is a measure of the energy content of ethanol minus the net energy used in the production process. The usefulness of NEV in evaluating an ethanol source is debatable, but with well-defined boundaries and clearly stated assumptions, it can provide a measure of the energy consumption and yield of the ethanol production from a one particular source.
NEV "may not be the best instrument to evaluate biofuels' contribution to energy security," the authors admit in the paper. The amount of fossil fuels used in the ethanol-manufacturing process is an important factor that determines "renewability".
Consequently, they used yet another tool, the net renewable energy value, defined as the energy content of ethanol minus the total fossil-energy inputs. When fossil-fuel inputs (97.35% of the energy inputs) were taken into consideration, the NREV of cassava fell to 9.15 MJ/L. This value, however, is still greater than that reported for energy costs of cassava-derived ethanol in China.
Several features make cassava more advantageous than sugar cane or cane molasses. The tuber is already used as a starch source and for chip/pellet products. It can be grown in any season and can be a year-round source of ethanol. "In Thailand, cassava is the third most important cash crop after rice and sugar cane," says Gheewala. "Over the past 20–30 years, cassava agronomy research has contributed significantly to the development of improved agricultural practices, such as time and method of planting, intercropping, soil erosion control, and especially, weed control and fertilizing." The result: Thailand uses less fertilizers and herbicides than China to grow cassava, yet it provides a comparable yield.
The study "addresses a question that's of policy interest right now," says agricultural economist Satish Joshi of Michigan State University. Uwe Fritsche at the Institute for Applied Ecology (Germany) agrees. "Cassava is a feedstock that is not in the major debate so far," and yet it is a crop of "relevance for a lot of developing countries," he says. The findings also explain the "life-cycle implications of this kind of biofuel," he adds.
However, the authors' calculations will remain meaningless without an understanding of their economic relevance, says Fritsche. "We have a lot of solar energy, for example, but it's very expensive so it does not mean much." The authors agree. "A general idea about the market dynamics is useful to understand the overall situation," they report in the paper.
Fritsche also points out the absence of any reference to greenhouse-gas emissions. "Net energy balance is for scientists," he says. The real economic implications of any biofuel source remain unknown without an estimate of its greenhouse-gas emissions, because that is where "most of the current debate" lies, Fritsche adds.
