4.1.2 Life-Cycle Water Requirements
Quantification of life-cycle water requirements of algal biofuel production would support managing future impacts on water demand and enable comparison of water use for algal biofuel with other fuels. The estimation of life-cycle use of any requirement for algal biofuel production (for example, water, nutrients, and energy), however, is complicated by the developing nature of the technologies. In addition to uncertainty as to how algal biofuel will evolve on the path of commercialization, there is also a lack of data on material and energy requirements of the current technologies.
An additional complication in open-pond algae cultivation is that water use varies significantly with climatic differences in temperature, humidity, and rainfall. Other biofuels and agricultural crops show such variability in water use. For example, regional variability in irrigation results in estimates of life-cycle water requirements to make ethanol from corn varying from 5 to 2,140 liters of water per liter of fuel, depending on in which U.S. state the corn is grown (Chiu et al., 2009). If the national average of water demand for corn-grain production is used, Chiu et al. (2009) estimated the water use for corn-grain ethanol to be 142 liters of water per liter of fuel. However, the geographical distribution of additional corn grown to meet ethanol demand is uncertain so that whether their water demand matches the national average for all corn also is unclear. The water intensity of open-pond algae cultivation depends critically on the future geographic distribution of cultivation. This future distribution is difficult to forecast, however, being based on the conflux of uncertain future technological performance, policy, and industry response. In the absence of a reliable forecast, studies of water intensity can clarify relationships between location, climate, and water use.
22.214.171.124 Life-Cycle Water Use of Freshwater Open-Pond Systems
A number of studies have analyzed water requirements of biofuel produced from algae cultivated in open-pond systems and include different phases of the life cycle (Harto et al., 2010; Wigmosta et al., 2011; Yang et al., 2011). There are large differences in assumptions and results among studies, which is not surprising given the challenges mentioned above. Table 4-1 summarizes the assumptions and results of three studies on open-pond algae cultivation to highlight differences in results and the origins of these differences. The results span over two orders of magnitude, from 32 to 3,650 liters of water per liter of algal biofuel. As a comparison, 1.9-6.6 liters of water are consumed to produce 1 liter of petroleum-based gasoline from crude oil or oil sands (King and Webber, 2008; Wu et al., 2009; Harto et al., 2010). Resolving the variability and uncertainty in these results is beyond the scope of this report. Instead, the goal of this report is to identify and prioritize issues that could affect the long-term sustainability of algal biofuels. Prioritization of research and development (R&D) for issues of concern could contribute to developing algal biofuels as a sustainable part of the energy future.
Harto et al. (2010) analyze life-cycle water requirements of a number of alternative transportation fuels, including corn-grain and switchgrass ethanol, soybean biodiesel, solar and wind generated electricity, and algal biofuels with algae cultivation in open-pond systems and closed photobioreactors. The scope of processes analyzed includes embodied water in facilities and vehicles. In most scenarios, water use in evaporation and fuel production dominate the life cycles. Scenario analyses that combine pessimistic versus optimistic assumptions for productivities with evaporation yield variability from 32 to 656 liters of fresh water per liter of biodiesel. The low-end scenario of 32 liters per liter applies to algae