Study Finds Water Footprint for Bioenergy Larger Than Other Forms of Energy; Bioelectricity the Smallest, Biodiesel the Largest
Researchers at the University of Twente, Netherlands have calculated the water footprints (WFs) of bioenergy from 12 crops that currently contribute the most to global agricultural production: barley, cassava, maize, potato, rapeseed, rice, rye, sorghum, soybean, sugar beet, sugar cane, and wheat. In addition, their study includes jatropha, an energy crop.
In general they found that bioelectricity is more water-efficient than first-generation biofuels (due largely to the ability to use the entire biomass to produce energy, rather than just the starch or oil fraction of the yield for liquid fuel production). They also found that the WF of bioethanol on a m3 of water per GJ of fuel basis appears to be smaller than that of biodiesel. Their results appeared 2 June in an open access paper in the journal Proceedings of the National Academy of Sciences (PNAS).
The WF of a product is defined as the volume of freshwater used for production at the place where it was actually produced. In general, the researchers note, the actual water content of products is negligible compared with their WF, and water use in product life cycles are dominated by the agricultural production stage. The water footprint comprises three components:
- Green WF: rainwater that evaporated during production, mainly during crop growth.
- Blue WF: surface and groundwater for irrigation evaporated during crop growth.
- Gray WF: volume of water that becomes polluted during production.
The study did not include energy inputs in the production chain, such as energy requirements in the agricultural system (e.g., energy use for the production of fertilizers and pesticides) or energy use during the industrial production of the biofuel. As a result, they point out, the study underestimates the WF of bioenergy, most particularly so in cases where agricultural systems have a relatively large energy input.
The WF of bioenergy shows large variation, they found, depending on 3 factors: (i) the crop used, (ii) the climate at the location of production, and (iii) the agricultural practice.
For electricity generation, sugar beet, maize, and sugar cane with WFs of~50m3/GJ are the most favorable crops, followed by barley, rye, and rice with WFs of ~70–80m3/GJ. Rapeseed and jatropha, typical energy crops showing WFs of ~400 m3/GJ, are the least water-efficient. For the production of ethanol, 2 crops grown in a temperate climate (sugar beet and potato) with WFs of ~60 and 100 m3/GJ, respectively, are most efficient, followed by a crop typical for a warm climate, sugar cane, showing a WF just below 110 m3/GJ. Values for maize [corn] and cassava are larger than for sugar beet, sugar cane, and potato at 110 and 125 m3/GJ, respectively. With a WF of ~400m3/GJ, sorghum is by far the most disadvantageous crop. For biodiesel production, soybean and rapeseed, crops mainly grown for food, show the best WF at~400m3/GJ; jatropha has the least favorable WF of ~600 m3/GJ.—Gerbens-Leenes et al.
Using a different metric, on average it takes 14,000 liters of water to produce one liter of biodiesel from rapeseed or soya. However, the water footprint for rapeseed in Western Europe is significantly smaller than in Asia. For soya, India has a large water footprint, while the figures for countries such as Italy and Paraguay are more favorable. Jatropha, which is increasingly used for biomass production, has an even less favorable water footprint of 20,000 litres of water on average for one litre of biodiesel.
For the production of bioethanol, sugar beet is the best performer with one liter of bioethanol made from sugar beet requiring on average 1,400 litres of water, as against 2,500 liters for sugarcane. Corn requires 2,570 liters of water per liter of ethanol; sorghum requires 9,812 liters of water per liter bioethanol.
The researchers note that the advent of cellulosic biofuels, in which the entire biomass can be used for fuel production, will offer a water footprint more comparable to that of bioelectricity. They assume, though, that the WF of next-generation biofuels will never be lower than the WF of the total crop biomass (m3/ton) divided by the energy content (GJ/ton).
The scientific and the international political communities promote a shift toward renewable energy sources, such as biomass, to limit the emission of greenhouse gases. This study has shown that biomass production goes hand in hand with large water requirements. There are already reasons for profound concern in several regions and countries with limited water resources about whether the food and fiber needs of future generations can be met. If a shift toward a larger contribution from bioenergy to total energy supply takes place, results of this study can be used to select the crops and countries that (under current production circumstances) produce bioenergy in the most water-efficient way.—Gerbens-Leenes et al.
Winnie Gerbens-Leenes, Arjen Y. Hoekstra, and Theo H. van der Meer (2009) The water footprint of bioenergy. PNAS doi: 10.1073/pnas.0812619106