The production of biofuels from microalgae requires currently more energy than is stored in the fuel. Can future developments overcome this hurdle? Annika Weiss’ analysis of the correlation between energy input and output provides new evidence.

Microalgae are discussed as feedstock for biofuels. In contrary to land plants, they do not compete with food production for fertile soil. However, the energy demand to produce biofuels from algae is currently higher than the energy stored in the fuel – the quotient of these values, the so called net energy ratio (NER) is above one, but should be smaller than one for an effective fuel production. Previous studies come to different conclusions whether this will be possible in the future.

The NER depends strongly on (i) the biomass yield and (ii) the energy to cultivate (mix and gas) algae.

Annika Weiss analyses in her dissertation how the yield depends on the cultivation energy. Based on that, she calculates the NER with the method of Live Cycle Assessment (LCA). She focused her analysis on aerated flat plate photobioreactors as promising systems for outdoor cultivation and biomethane as biofuel since its production requires comparably low energy.

Results show: the more biomass is produced, the more energy is required in relation to the biomass. In other words: the energy-efficiency of microalgae cultivation sinks with increasing yield. A low NER can consequently not be achieved with maximum biomass yields. Due to this correlation a low NER can – in contrary to the prevalent assumption – not be achieved with maximum biomass yields. In the investigated case, a NER below one is not achieved despite assumed technology development and a good relation between energy input and yield. Consequently, energy production with microalgae is not feasible.

Annika Weiss analyzed in her dissertation seven other LCA studies which partially predict a net energy output (a NER below one). She found two reasons for this: either not all energy-relevant processes are considered in the calculation and/or the relation between operation energy and yield is neglected.

Bibliographic reference:

Weiss, A.
Energy balance of microalgae biofuels (Dissertation). Darmstadt: Technische Universität Darmstadt 2016
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