 |
Research at NREL indicates that when certain microalgae encounter a stressful environment, such as nutrient deficiency, they cope by slowing cell division and funneling carbon into storage lipid, or oils, during photosynthesis. The decreased cell division increases the ratio of lipids to other biomass components, and researchers can extract and refine those lipids to produce liquid fuels. In fact, research from the Aquatic Species Program determined that microalgae can produce 10 to 100 times more lipids per acre than soybeans and other oil-seed crops, offering the possibility to significantly reduce our use of fossil fuels.
Learn about algal cultivation and harvesting on this page. See the following pages for information on:
Algal Cultivation
No microalgal biofuels are currently produced at a commercial scale in the United States. Only a small amount of microalgal biomass is used commercially, mainly for the production of high-value, low-volume food supplements and nutraceuticals. Commercial microalgae production facilities use both open- and closed-cultivation systems. Both of these require high-capital input. Closed-cultivation systems, photobioreactors, are significantly more expensive than open-pond systems but have not been engineered to the extent of other reactors in commercial practice. Neither open ponds nor closed photobioreactors are mature technologies. Until large-scale systems exist for a period of time necessary to demonstrate long-term performance, many uncertainties will remain.
Successful microalgal growth for commercial purposes requires the development of strains and conditions for those strains that allow rapid production of biomass with high-lipid content and minimal growth of competing strains. The economics of continuous microalgal propagation can be negatively influenced by the growth of contaminating microalgal species and by the presence of grazers and pathogens. But, unlike terrestrial crops whose failure costs an entire growing cycle, a microalgal pond can be reinoculated and resume production in a matter of days. Microalgae can thrive in a broad range of environmental conditions, but specific strains are more limited by climatic conditions than terrestrial crops. Maximum volumetric productivities can be higher in a closed photobioreactor than in an open pond because the surface-to-volume ratio can be higher. The increased surface area increases the volume of sunlight microalgae can absorb.
Algal Harvesting for Biofuels Production
The amount of biomass that can be harvested from a liter of microalgal culture is still low compared with what can be produced from commercial biomass fermentors because limitations on light availability (because of self-shading) result in low cell densities. Low-cost microalgal harvesting options do not currently exist. Past attempts have taken advantage of spontaneous settling of the microalgal biomass or have enhanced settling through the use of flocculants, a particle-suspension chemical, which could be cost-effective, depending on the amount used. Alternatively, in some cases, the microalgal biomass will float, either because of intrinsic buoyancy or through the use of a dissolved air-floatation process. Nevertheless, further engineering research is required to develop cost-effective microalgal harvesting techniques.
|
|
