TABLE 4-2 Life-Cycle Water Requirements to Produce Biofuel from Corn and Soybean


Crop Study Water Consumption (L/L) Notes

Corn

Chiu et al., 2009

513-1,402

Average water use when irrigated. Range comes from different U.S. states.

Dominguez-Faus et al., 2009

5-2,138

Average irrigated and nonirrigated. Range comes from state variability.

Chiu et al., 2009

142

National average of irrigated and nonirrigated.

Soybean

Dominguez-Faus et al., 2009

1,400-2,900

Average water use when irrigated. Range comes from different U.S. states.

Harto et al., 2010

133

National average irrigated and nonirrigated.


NOTE: L/L = water consumption per liter of ethanol or biodiesel produced. Ethanol has about two-thirds of the energy content of gasoline. Biodiesel has about the same energy content as gasoline.

cultivated in regions in which rainfall makes up for evaporative losses. Allocation of water use to coproducts in addition to fuel can significantly reduce water use associated with the biofuel product.

Wigmosta et al. (2011) developed a geographically resolved model of variability in water and land requirements in different areas in the United States. They estimated water requirements that range from 22 to 3,600 liters of water per liter of oil depending on location of cultivation. Their assumptions for productivity of algae are much lower than those of Harto et al. (2010) and Yang et al. (2011), highlighting the uncertainty associated with critical factors driving material requirements. Wigmosta et al. (2011) also constructed scenarios that build out the geographical distribution of algal biofuel production, starting with areas with lower evaporation and more rainfall. They found steep increases in water requirements as production moves to more water-intensive areas. Yang et al. (2011) explored water recycling, use of saline water instead of fresh water, performance of different algal strains, and geographic variability. They find that recycling harvest water is critical in managing water requirements.

The committee reviewed what is known about water requirements for other algal biofuel pathways. Sapphire Energy estimated that its proposed biorefinery in Columbus, New Mexico, would require 3,500 acre feet (4.32 billion liters) of fresh water to produce 30,000 barrels (4.77 million liters) of green crude each year, or 906 liters of water per liter of green crude. The green crude can be upgraded to drop-in fuels (USDA-RD, 2009). Therefore, Sapphire Energy’s production pathway is comparable to either the reference pathway in Chapter 3 or the alternative pathway #1 depending on whether coproducts are included.

The estimates of life-cycle water use of algal biofuels (Table 4-1) were compared to those of other biofuels to explore whether algal biofuels are more or less water intensive than other biofuels. Table 4-2 shows results of studies of life-cycle water requirements of corn-grain ethanol and soybean biodiesel. For biofuels produced from corn grain, soybean, and algae cultivated in open ponds, water use depends more on the climate (rainfall in particular) where the biomass is grown rather than the type of biomass.

4.1.2.2 Life-Cycle Water Use in Closed Systems—Photobioreactors

Cultivating algae in photobioreactors has the potential to eliminate water consumption from evaporation, which could significantly reduce overall water demand. However, data for closed systems are even scarcer than for open systems. Harto et al. (2010) estimated the



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