Lipid-rich algae biomass as regenerative energy source

A high lipid content is a prerequisite for the economic use and efficient processing of the algae biomass. Furthermore, the fatty acid spectrum should have a high proportion of saturated and monounsaturated fatty acids, as polyunsaturated fatty acids reduce the storage stability of the algae oil. The accumulation of fatty acids in the form of triglycerides can be induced by nitrogen limitation of the microalgae culture. The triglycerides are stored as storage molecules inside the cell (see picture). In laboratory experiments at the Fraunhofer IGB we were able to achieve lipid contents of up to 70 percent [w/w] under permanent artificial lighting.

For the production of algae biomass for energetic use, it is necessary to transfer this process to the field using sunlight. The challenge here is to establish a process that is stable even under varying conditions in the field and generates biomass with a high lipid content. This is because the given day-night rhythm and changing weather conditions result in fluctuating process conditions.

In order to characterize the lipid production process with the microalga Chlorella vulgaris, we operated a pilot plant with five south-facing 30-liter flat plate airlift reactors in the field in 2010 and 2011. A two-stage batch process was established. In an initial growth phase of four to seven days, biomass was produced with an optimal nutrient supply. This was followed by the lipid production phase, in which the algae cells enriched lipids by limiting nitrogen and phosphate. The focus of the study was to maximise the lipid content of the algae and to establish a quantitative relationship between the relative light availability and biomass concentration. Relative light availability describes the ratio of light input to biomass concentration in the reactor and is expressed in Einstein (1 mol photons) per gram dry matter and day.

We have succeeded in establishing a stable process under field conditions that allows the production of algae biomass with a very high lipid content. With Chlorella vulgaris we were able to achieve a maximum lipid productivity of 0.3 g fatty acids / (L*d) in the field. It has been shown that the flat plate airlift reactor used here, which was developed at Fraunhofer IGB a few years ago, is optimally suited for this process. It is known from the literature, for example, that a maximum lipid productivity of only 0.2 g fatty acids / (L*d) was achieved during cultivation in photobioreactors, as developed and operated in Italy [1]. We determined the influence of the relative light availability on the lipid content of the biomass by parallel operation of flat plate airlift reactors with different biomass concentrations in the field. We could show that high lipid contents of more than 45 percent [w / w] are achieved at low bio-dry mass concentrations and thus high relative light availability.

As a result of these results, the high lipid content of the biomass can be specifically adjusted by process control in the case of outdoor production. A defined, consistent quality of the biomass with a high lipid content is an optimal basis for the development of a purification process for obtaining biodiesel from algae.

[1] Rodolfi, L. et al. (2009) Biotechnology and Bioengineering 102(1): 100-12

Promotion

We would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the project "EtaMax - More Biogas from Low-Lignocellulose Waste and Microalgae Residues by Combined Bio-/Hydrothermal Gasification", funding code 03SF0350A.

Project partners

• Daimler AG
• EnBW Baden-Württemberg AG; FairEnergie GmbH
• Fraunhofer Institute for Process Engineering and Packaging IVV
• Karlsruhe Institute of Technology (KIT)
• Netzsch Mohnopumpen GmbH
• Paul Scherrer Institute PSI
• City of Stuttgart
• Stulz Water and Process Technology GmbH