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Energy balance of microalgae biofuels

Weiss, Annika (2016)
Energy balance of microalgae biofuels.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

Abstract

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.

Item Type: Ph.D. Thesis
Erschienen: 2016
Creators: Weiss, Annika
Type of entry: Primary publication
Title: Energy balance of microalgae biofuels
Language: English
Referees: Schebek, Prof. Dr. Liselotte ; Cornel, Prof. Dr. Peter
Date: 2016
Place of Publication: Darmstadt
Refereed: 19 February 2016
URL / URN: http://tuprints.ulb.tu-darmstadt.de/5352
Abstract:

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.

Alternative Abstract:
Alternative abstract Language

Mikroalgen sind im Wasser lebende Mikroorganismen, die mit Hilfe von Sonnenlicht wachsen. Bereits seit dem Zweiten Weltkrieg wird versucht, aus Algen Biotreibstoff herzustellen. Dieser Ansatz wird derzeit wieder verstärkt diskutiert, da Mikroalgen – im Gegensatz zu Landpflanzen – nicht mit Nahrungsmittelproduktion um fruchtbaren Boden konkurrieren. Sinnvoll ist die Gewinnung von Biotreibstoff aus Mikroalgen nur dann, wenn weniger Energie benötigt wird, um den Treibstoff zu produzieren, als im gewonnenen Treibstoff gespeichert ist: Der Quotient dieser beiden Werte (Energieaufwand und Energiegehalt des Treibstoffes), der ‚Net Energy Ratio‘ (NER) muss kleiner eins sein. Bisherige Studien zeigen, dass im Wesentlichen zwei Parameter den NER bestimmen: Kultivierungsenergie und Biomasse-Ertrag. Obwohl diese beiden Parameter offensichtlich voneinander abhängen, wurde diese Abhängigkeit bisher nicht berücksichtigt, um den NER zu berechnen. In dieser Dissertation wird erstmalig der Biomasse-Ertrag abhängig von der Kultivierungsenergie modelliert. Dazu wird eine Korrelation zwischen wichtigen Modellparametern (Begasungsrate, Lichtintensität und photosynthetischer Effizient (PE)) aus Experimentaldaten hergeleitet und anhand weiterer Literatur validiert. Diese Korrelation wird zugrunde gelegt, um den NER der Biotreibstoffproduktion aus Mikroalgen zu berechnen. Als Methode der Algenkultivierung werden begaste flache Photobioreaktoren untersucht. Diese wurden bisher als vielversprechende Systeme für die Freilandkultivierung intensiv erforscht. Als gewonnener Treibstoff wird beispielhaft Biomethan untersucht, da seine Produktion den geringsten Energiebedarf im Vergleich zur Produktion anderer Treibstoffe aufweist. Die Ergebnisse dieser Arbeit zeigen, dass Kultivierungsenergie und Biomasse-Ertrag nichtlinear voneinander abhängen: um hohe Erträge zu erzielen, wird überproportional mehr Energie benötigt, als für niedrige Erträge. Um Mikroalgen möglichst energie-effizient zu kultivieren, sollte daher nicht der höchstmögliche Biomasse-Ertrag angestrebt werden, sondern vielmehr ein ausgewogenes Verhältnis zwischen Energiebedarf und Biomasse-Ertrag. Aus diesem Zusammenhang folgt weiterhin, dass ein niedriger NER nicht mit dem höchstmöglichen Biomasse-Ertrag zu erreichen ist. Der bestmögliche NER hängt von weiteren Modellparametern ab, die sich wechselseitig beeinflussen. Parameteränderungen wirken sich je nach Begasungsrate unterschiedlich stark auf den NER aus. Der in der vorliegenden Arbeit berechnete NER für begaste Photobioreaktoren liegt bei etwa 1,8. Dieser Wert wird bei einer Begasungsrate von 0,25 vvm (Gasvolumen per Flüssigkeitsvolumen und Minute) erreicht. Das entspricht, zusammen mit den weiteren Ergebnissen und Annahmen und dieser Arbeit, einem Leistungseintrag von 54 W m-3 oder 2,2 W m-2 und einem Biomasse-Ertrag von 50 t ha-1 y-1. Ein NER unter eins kann nicht erreicht werden, obwohl zu erwartende Technologieentwicklung in die Berechnung miteinbezogen wurde. Der berechnete NER wird mit anderen Studien verglichen, die teilweise auf deutlich niedrigere NER kommen. Eine Analyse dieser Studien zeigt zwei Ursachen für einen NER < 1: Einerseits sind die Systemgrenzen zum Teil unvollständig, anderseits wird der Zusammenhang zwischen Energiebedarf und Biomasse-Ertrag nicht berücksichtigt. Mit dem hier vorgestellten systematischen Ansatz lassen sich verlässliche Aussagen zum Entwicklungspotential der Biotreibstoffproduktion aus Mikroalgen treffen. Erwartete Fortschritte in der Technologieentwicklung können das Verhältnis von Kultivierungsenergie und Ertrag verbessern. Es ist jedoch nicht möglich, diese beiden Parameter zu entkoppeln, da ihre Abhängigkeit auf den fundamentalen Mechanismen des Algenwachstums basiert. Diese treffen auf alle Algenkultivierungssysteme und alle Arten von Mikroalgen zu. Die Methodik kann angewendet werden, um den Zusammenhang zwischen Kultivierungsenergie und Biomasse-Ertrag auch für andere Mikroalgen-Kultivierungssysteme zu bestimmen und so ihren bestmöglichen NER zu berechnen. Der Ansatz, der die Zusammenhänge wichtiger Modellparameter aufgrund der zugrundeliegenden Mechanismen berücksichtigt, ist systemübergreifend einsetzbar. Er kann daher auch genutzt werden, um das Entwicklungspotential anderer Technologien einzuschätzen.

German
Uncontrolled Keywords: Mikroalgen, Energiebilanz, Bioreaktoren, biotreibstoff, Ökobilanz, Life Cycle Assessment, LCA
Alternative keywords:
Alternative keywordsLanguage
microalgae, energy balance, bioreactors, biofuel, Life Cycle Assessment, LCA, net energy ratioEnglish
URN: urn:nbn:de:tuda-tuprints-53524
Classification DDC: 500 Science and mathematics > 500 Science
500 Science and mathematics > 570 Life sciences, biology
600 Technology, medicine, applied sciences > 620 Engineering and machine engineering
Divisions: 13 Department of Civil and Environmental Engineering Sciences > Institute IWAR > Material Flow Management and Resource Economy
13 Department of Civil and Environmental Engineering Sciences > Institute IWAR
13 Department of Civil and Environmental Engineering Sciences
Date Deposited: 14 Aug 2016 19:55
Last Modified: 14 Aug 2016 19:55
PPN:
Referees: Schebek, Prof. Dr. Liselotte ; Cornel, Prof. Dr. Peter
Refereed / Verteidigung / mdl. Prüfung: 19 February 2016
Alternative keywords:
Alternative keywordsLanguage
microalgae, energy balance, bioreactors, biofuel, Life Cycle Assessment, LCA, net energy ratioEnglish
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