Abstract
The majority of liquid fuel used around the world is refined from oil. Despite ongoing advances in drilling and extraction technology, and the discovery of new reserves, the projected supply of oil is predicted to be substantively less than demand by 2030. 1 A number of initiatives exist to develop drop-in alternatives that can address this shortfall. Any effort to produce an alternative source of oil to compete in the global oil market needs to address scale and cost, as oil is traded globally in huge volumes and at relatively low costs.
Algae have long been viewed as having potential to be a viable renewable source of oil. The challenge with realizing this potential thus far has been finding a process that can be scaled in a cost competitive manner. The primary processes being used at present for the commercial production of algae are effective at producing niche food and chemical products targeting low volume markets that have high margins. Without substantial technological progress these processes will not be feasible for the cost competitive production of oil at scale. The two basic technology platforms for producing algae biomass are open and closed systems. Closed systems typically have higher productivity, are more stable, and are more expensive than open systems. 2 When considering the scale and costs needed to produce oil competitively, the only system that is currently feasible, despite its challenges, is the open system. 3
Civilizations across the globe have harvested and consumed algae for centuries. Yet research on how to mass culture microalgae successfully using open systems was not initiated until the late 1940s and early 1950s in the US, Germany, and Japan. 4 –6 Open pond systems present many challenges stemming primarily from the fact that they are exposed to unpredictable and uncontrollable meteorological conditions, are at risk of many forms of contamination, and are often poorly mixed. The most prominent limiting factor for reliable productivity is contamination. 7 Unwanted algal species, viruses, fungi, and grazers will inevitably enter into the culture and impact the target algae strain. 8 –11 A critical element in managing contamination in these open ponds is maintaining growth conditions that favor target species of algae and minimize possible contaminants. 12 Monocultures in open ponds are often achieved by maintaining an extreme culture environment, such as high salinity, alkalinity, or nutritional status, or by growing algae in batch systems. 12,13 However, for algae that grow in non-extreme environments, which include many strains of algae considered candidates for commercial biofuel production, stability has been more difficult to manage. 14
Despite the substantive challenges of open pond systems, they have been successfully used for the commercial production of algae for the last 50 or so years. These efforts were started in the 1960s with the growth of Chlorella in Japan, followed in the 1980s by the growth of Spirulina in the US, and then later in Thailand and China. 15 –18 In the 1980s, commercial production of Dunaliella started in Australia as well as in India, Israel, and the US. 15,19,20 These commercial achievements all indicate that at varying scales (up to 700 ha) algae have been successfully cultured outdoors in a limited range of culture conditions. However it is important to note that all of these applications support products that range in value from ∼$20-3,000/kg, whereas the projected maximum cost of biomass for the production of oil should be somewhere in the $0.20-0.30/kg range. 15 To begin commercial production of biofuels, costs and scale must roughly decrease and increase more than an order of magnitude, respectively.
In order for open systems to be viable options for scaling the production of algal biomass they have to become cheaper to build and operate while sustaining robust and productive growth. Most pond designs are similar to those of sanitary landfills, in which combinations of expensive, high density polyethylene (HDPE) and expensive clay liners are used to reduce or eliminate leakage, although there are some exceptions. 21 These liners make up the largest fraction of the overall cost of a pond and are economically impractical for large scale cultivation. 12 Unlined ponds have proven not to be suitable for the production of algal biomass for biofuels or any kind of high productivity algae culture, due to issues with flow, suspended material, and maintenance. 12 Some companies are developing unlined ponds that are able to support high productivity cultures.
Sapphire Energy is developing strategies to take advantage of unlined ponds. The company has continuously cultivated algae in a model unlined pond (∼2,000 m2 surface area, ∼500,000 L volume, ∼10 cm/s flow) at its Las Cruces test facility in New Mexico for more than a year with stable productivities and no issues with suspended materials. Additionally, it is exploring possible new commercial sites with low drainage soils that would substantially reduce the cost of constructing commercial ponds. In collaboration with Pacific Northwest National Labs, Sapphire has extensively screened 66,000 possible sites in the southern US for compatibility with unlined ponds and has found that many potentially good sites based on predicted productivity also have the undesirable characteristic of soils with high permeability. 22 Sapphire expects these types of screening approaches—modeling and deploying high productivity unlined ponds in R&D test sites, and focusing on sites that will allow for lower capital expenditure (capex) requirements for pond construction—will help to demonstrate the commercial viability of lower cost, scalable unlined ponds for the production of algae for biofuel.
Regarding open pond stability, a number of strategies have been deployed to crop protect strains of algae that do not grow in extreme environments, with varying levels of success. The primary approach has involved deploying a bulk chemical into the pond that alters the pond environment to create a differential advantage for the target algae strain, or a disadvantage for a particular pest. Examples of these strategies involve the use of hyperchlorine in Nannochloropsis cultures to control protozoa. 23 Ammonia has been used as a treatment for rotifers and cladocerans in open ponds, and pH adjustment (decrease to pH 3 for 1–2 hours, followed by adjustment to pH 7.5) has been used to target rotifers in Nannochloropsis cultures. 23,24 The use of pesticides to control zooplankton has been tested in the past but has not been used in large scale cultures. 25 A slightly different and innovative approach to managing pests in open ponds involves taking advantage of trophic interactions by actively deploying rotifers into a culture to consume contaminant green algae. This has been successfully used in cultures of Spirulina. 26 Sapphire Energy has made great strides in managing contamination in open ponds by developing a pest management system that tracks pests in real time using quantitative polymerase chain reaction (qPCR) and uses action thresholds to trigger deployment of pest management strategies. 27 qPCR gives real-time targeted feedback to pond operators about what containments are in their ponds and how quickly they are growing; the management strategies provide options to mitigate this growth immediately if needed. Using this strategy Sapphire has been able to culture green algae stably in non-extreme media for more than a year in outdoor open ponds. 28
Strategies for deploying low cost and reliable open ponds for the commercial production of biofuels continue to be honed as the field progresses toward commercial economics. Current data suggest that if improvements continue on expected trajectories, the industry will reap favorable energy return on investments for algae-based biofuel. 29 Data like these bode well for the potential for open pond algae technology to become a viable platform for producing oil to address the projected supply challenges facing the oil market in the coming decades.