A new analysis by researchers at the US Department of Energy’s (DOE’s) Pacific Northwest National Laboratory (PNNL) finds that the US’ land and water resources could likely support the growth of enough algae to produce up to 25 billion gallons of algae-based fuel per year—one-twelfth of the country’s yearly needs. The partial techno−economic assessment was based on the availability of freshwater, saline groundwater, and seawater for use in open pond algae cultivation systems.
Achieving larger production volumes would require the utilization of less water-efficient sites and relatively expensive saline waters, they suggested. Freshwater availability and saline water delivery costs are most favorable for the coast of the Gulf of Mexico and Florida peninsula, where evaporation relative to precipitation is moderate. The results are published in the ACS journal Environmental Science & Technology.
The team in an earlier study had estimated US algal production potential assuming that all suitable sites have unlimited freshwater—i.e., alternative saline water sources were not considered. In this expanded study, they constrained freshwater supplies and estimated the delivery costs of saline alternatives for sites lacking freshwater.
|“While there are many details still to be worked out, we don’t see water issues as a deal-breaker for the development of an algae biofuels industry in many areas of the country.”|
—Erik Venteris, first author
In the earlier study, water consumption was limited to evaporation; with the addition of saline resources, they we modeled the added water demand required to maintain salt concentrations in the pond (“blowdown”). For the new study, they economically constrained the use of saline groundwater and seawater with GIS-based cost−distance models of operating and capital costs.
The model uses water consumption and biofuel production rates in conjunction with the water availability model to assess the range of viable water options and the most cost-effective source for each site. Freshwater was modeled as the primary water source, as it is usually the least expensive due to proximity and minimal blowdown and concentrate disposal expenses.
For its analysis, the team limited the amount of freshwater that could be drawn in any one area, assuming that no more than 5% of a given watershed’s mean annual water flow could be used in algae production. That number is a starting point, says Venteris, who notes that it’s the same percentage that the US Environmental Protection Agency allows power plants to use for cooling.
Geographic climate contrasts result in marked differences in water use efficiency among sites, varying by 3 orders of magnitude across the US. Freshwater consumption rates (evaporation minus precipitation) are largest in the southwestern US and lowest in the eastern US. Saline water is a potential alternative, as algae can also be grown in waters ranging from brackish to hyper saline. However, the use of saline alternatives amplifies the geographic patterns in water consumption (and attendant costs) because additional water must be discharged to maintain a constant operating salinity (blowdown), which increases with net evaporative loss.
For example, the national average annual freshwater loss is 2,950 m3 ha−1 for sites with available freshwater and 10,740 m3 ha−1for sites without. Assuming a seawater source with a salinity of (35 g kg−1) and a pond operating salinity of 60 g kg−1, the average annual water use for sites without freshwater more than doubles to 25,780 m3 ha−1. An additional issue not considered in this study is the disposal costs for saline concentrates which also increase with blowdown.—Venteris et al.
Venteris and colleagues weighed the pluses and minuses of the various water sources. They note that freshwater is cheap but in very limited supply in many areas. Saline groundwater is attractive because it’s widely available but usually at a much deeper depth, requiring more equipment and technology to pump it to the surface and make it suitable for algae production. Seawater is plentiful, but would require much more infrastructure, most notably the creation of pipelines to move the water from the coast to processing plants.
Our expanded analysis does not change the general geographic conclusions of Wigmosta et al. From a water resources perspective, the largest portion of the production potential occurs in the Texas Coast, South, Florida Peninsula, and the South Atlantic Coast, where the majority is based on inexpensive freshwater. However, as this is not a full techno−economic analysis, no judgment is made as to the economic viability of any region and there are many issues requiring further exploration before definitive conclusions can be reached.
A source of uncertainty in this and similar analyses results from lack of knowledge of commercial scale algae biofuel production rates and the potential for temporal instability in fuel prices...Several aspects of the current modeling approach also introduce uncertainty in our geographic conclusions. The assessment of freshwater resources is approximate at best. We assume a steady supply based on long-term means. However, the availability of this water source is subject to interannual weather variations as well as long-term climate trends.
...Likewise, national-scale information on saline groundwater occurrence, chemistry, and availability is limited in both completeness and quality. The results point to the need for high-quality geologic assessment of saline groundwater in the western U.S. to better understand the strengths and limitations of this resource...Finally, total costs to utilize saline resources are likely underestimated, as the model does not account for concentrate disposal, a serious issue for inland sites,28 but less likely so for those proximal to the ocean.—Venteris et al.
The work was funded by the DOE’s Office of Energy Efficiency and Renewable Energy.
Erik R. Venteris, Richard L. Skaggs, Andre M. Coleman, and Mark S. Wigmosta (2013) A GIS Cost Model to Assess the Availability of Freshwater, Seawater, and Saline Groundwater for Algal Biofuel Production in the United States, Environmental Science and Technology doi: 10.1021/es304135b