|Characteristics of the ideal microalga. Source: Wijffels and Barbosa. Click to enlarge.|
Although microalgae are not yet produced at large scale for bulk applications such as fuel production, recent advances—particularly in the methods of systems biology, genetic engineering, and biorefining—present opportunities to develop this process in a sustainable and economical way within the next 10 to 15 years, according to two researchers from the University of Wageningen, the Netherlands.
The perspective by René H. Wijffels and Maria J. Barbosa on the outlook for microalgal biofuels was published in the 13 August issue of the journal Science.
Algae store chemical energy in the form of oils such as neutral lipids or triglycerides; the algal oil can be extracted and converted into biodiesel by transesterification with short-chain alcohols or by hydrogenation of fatty acids into linear hydrocarbons for drop-in fuel components. Algae also synthesize other fuel products, such as hydrogen, ethanol, and long-chain hydrocarbons that resemble crude oil. The algal biomass can be converted to biogas through anaerobic fermentation.
Despite this potential, the production capacity for microalgae is presently limited in comparison to land-based energy crops. The current worldwide microalgal manufacturing infrastructure (producing the equivalent of ~5000 tons of dry algal biomass) is devoted to extraction of high-value products such as carotenoids and ω-3 fatty acids used for food and feed ingredients. The total market volume is 1.25 billion, implying an average market price for microalgae of 250/kg dry biomass. As an example for comparison with land-based oleaginous crops, the world production of palm oil is nearly 40 million tons, with a market value of ~0.50 /kg.
Production of microalgae for biofuels needs to take place on a much larger scale at much lower costs. If all transport fuels were to be replaced by biodiesel in Europe, there would be an annual need for nearly 0.4 billion m3. If this biodiesel were to be supplied through microalgae, 9.25 million ha (almost the surface area of Portugal) would be needed to supply the European market, assuming a productivity of 40,000 liters per ha per year. This productivity is based on a 3% solar energy conversion to biomass (theoretical maximum is 9%) and a biomass oil content of 50%, under the solar conditions of Portugal.
A leap in the development of microalgae technology is therefore required; on a practical level, the scale of production needs to increase at least 3 orders of magnitude, with a concomitant decrease in the cost of production by a factor of 10. In the past few years, there has been a rather polarized debate between researchers in the field over technology readiness and the prospects for productivity enhancement, with some parties pressing for scale-up and commercialization now, while others cautiously stress the need for additional research leading to more careful step-by-step development.
—Wijffels and Barbosa
Wijffels and Barbosa suggest that a multidisciplinary approach will be required, with a comprehensive research portfolio covering the whole chain of process development in an integrated and iterative way, including fundamental biology, systems biology, metabolic modeling, strain development, bioprocess engineering, scale-up, biorefineries, integrated production chain, and the whole system design, including logistics.
Commercial production of microalgae is still based on traditional technologies using only a few strains, they note. Unexplored species and genetic engineering to improve known strains offer great potential. They note that the present productivity of penicillin synthesis by fungi is 5,000 times as high as it was 50 years ago due to improvements in microbial fermentation through both technological (reactor design, process control, harvesting, and extraction) and strain improvements.
René H. Wijffels and Maria J. Barbosa (2010) An Outlook on Microalgal Biofuels. Science Vol. 329. no. 5993, pp. 796 - 799 doi: 10.1126/science.1189003