Crops have been proposed as future sources of renewable fuel, and as future sources of energy storage, such as hydrogen. There are certainly many challenges to address with the use of crops for fuel and energy storage.
These include land-use considerations, possible hinderance of pest control, and whether or not biofuels are actually environmentally-friendly at all depending on the type of biofuel and how it is produced. However, the utilization of biofuels is inarguably necessary, at least for producing chemicals in a sustainable manner.
An important consideration is the water required for crop production. If biofuels are going to be produced on a large scale, the crops used to produce the biofuels must use as little water as possible; freshwater is already limited, and will only be more so in the future.
Winnie Gerbens-Leenes (University of Twente, The Netherlands) and coworkers have made an important contribution towards addressing this challenge. They have determined the most efficient crops to utilize for electricity, ethanol, and diesel production, with respect to their water requirements per unit of energy produced from the crops, on an average global level.
Crops studied.
The scientists' intent was for their research to be of use on a global level. They therefore focused their study on the 12 crops that account for 80 percent of worldwide crop production (as well as one additional tree species), and averaged results for each crop on a global level.
These 13 species were sugar beet, wheat, jatropha, rye, soybean, cassava, barley, potato, maize, sorghum, rice, sugar cane, and rapeseed. They assume that electricity generation comes from total biomass yields, not simply crop yields.
Region-to-region variability obviously comes into play here; for example, conditions in the United States may be more favorable for a particular crop than in Brazil. This variation should be taken into consideration when decisions on which crop to plant are made.
Water requirements: Bioelectricity vs bioethanol and biodiesel.
The scientists found that, for most crops, utilizing crops for bioelectricity requires twice as much water than for bioethanol or biodiesel. This is because bioelectricity can use the entire plant, but bioethanol and biodiesel can only use the starch and oil components.
They further found that the crop water requirements varied widely, depending on three factors. These factors are agricultural techniques, climate of crop production, and the crop utilized.
Agricultural techniques..
Water usage is high when crop yields are low. Conversely, water usage is low when crop yields are high.
Consequently, yields of potato, wheat, and barley are low in Kazakhstan. However, wheat yields are high in Denmark, and water requirements are low.
Climate of crop production.
Differences in climate drastically impact the water requirements of crop production. For example, sugar beets require twice as much water to grow in Iran as, on average, the rest of the world.
The crop utilized.
The scientists found maize, sugar cane, and sugar beet to be the most efficient for electricity production, in terms of water utilization per unit of energy produced, at roughly 50 cubic meters of water per gigajoule (m3/GJ) of energy. Rapeseed and jatropha were the least efficient, at roughly 8 times that amount.
Sugar beet (temperate climate, at roughly 60 m3/GJ) and sugar cane (warm climate, at roughly 110 m3/GJ) were the most efficient for ethanol production. Sorghum was by far the least efficient, at greater than 400 m3/GJ.
Rapeseed and soybeans were the most efficient for biodiesel production, at roughly 400 m3/GJ. Jatropha was the least efficient, at roughly 600 m3/GJ.
Implications.
It seems clear that, on an average global level, sugar beet, sugar cane, and maize grown for electricity require the least amount of water per unit of energy produced, followed closely by sugar beet in a temperate climate for ethanol production. Rapeseed and soybeans for biodiesel generation fair poorly in comparison.
A choice must be made as to how much crop acreage is to be used for food relative to energy production. If biofuels are to be more widely utilized, studies such as these are clearly needed in order to minimize the amount of water required, depending on regional conditions.
for more information:
Gerbens-Leenes, W., Hoekstra, A. Y., & van der Meer, T. H. (2009). The water footprint of bioenergy Proceedings of the National Academy of Sciences, 106 (25), 10219-10223 DOI: 10.1073/pnas.0812619106