Energy balance of microalgae biofuels

Microalgae are small organisms that live in the water and use solar energy to grow. Like plants, they can be used to produce biofuels. Since the Second World War there have been repeated attempts to produce biofuels from microalgae. The idea has recently received a boost due to one specific feature of microalgae: unlike other biofuel feedstock, microalgae do not compete with food production for arable land. Biofuel production with microalgae is only sensible when less energy is required to produce the fuel than is stored in the fuel. The ratio of energy demand to energy output, the ‘Net Energy Ratio’ (NER), should be smaller than one. Previous studies have shown that the NER depends significantly on (a) the assumed operation energy, and (b) the expected biomass productivities. Although it is well-known that these two parameters are inherently linked, this dependency has not been considered when calculating the NER. In this dissertation, for the first time biomass productivity is calculated based on operation energy. For this purpose, a correlation between the key parameters to model operation energy and biomass productivity (aeration rate, light intensity and photosynthetic efficiency (PE)) is derived and validated based on a systematic analysis of published experimental data. Based on this correlation, the NER of microalgae biofuels production is calculated. Aerated flat plate photobioreactors are investigated as a method of microalgae cultivation. These have previously been examined as promising systems for outdoor cultivation. As a biofuel, biomethane production is investigated since its production requires the least energy compared to other biofuels. The results of this dissertation show that operation energy and biomass productivities are related non-linearly: to achieve high productivities, disproportionately more energy is required than to achieve low productivities. Consequently, the aim of energy-efficient microalgae cultivation is not to achieve the highest possible biomass yield but to find a good balance between operation energy and biomass yield. Furthermore, due to these interactions, the lowest possible NER is not achieved with the maximum biomass yield. The optimum NER depends on the interaction of all model parameters. The effect of parameter changes on the NER depends also on the aeration rate. The NER calculated in this dissertation for aerated flat plate photobioreactors is around 1.8. This value is achieved at an aeration rate of 0.25 vvm (gas volume gas per liquid volume and minute). This corresponds, when coupled with the further findings and assumptions of this study, to an operation power of 54 W m-3 or 2.2 W m-2 and a biomass productivity of 50 t ha-1 y-1. A NER below one could not be achieved even though expected technological improvement is considered in the calculation. The calculated NER is compared to the NER results in previous studies which were partially below one. The analysis of previous studies showed that there are two main reasons for a NER < 1: one is incomplete system boundaries; the other is that the relation between energy demand and productivity is not considered. With the systematic approach presented in this dissertation, the potential development of microalgae biofuel production can be predicted more reliably. Expected technological development could improve the relation between operation energy and biomass productivities, but it cannot uncouple these parameters. Their correlation is based on the fundamental principles of microalgae growth, which apply to all cultivation systems and all types of algae. The method developed in this thesis can also be applied to quantify the best possible NER for other cultivation systems, based on the relation between operation energy and biomass productivity. The approach to correlating important model parameters based on the underlying scientific mechanisms can be transferred to other systems as well. It can thus also be applied to estimate the potential development of other technologies