The mobilization of large quantities of fossil fuels in the course of global industrialization has led to an increase in atmospheric CO2 concentrations in recent decades and, as a consequence, to climate change that is becoming apparent worldwide. Accordingly, a reduction of CO2 emissions is one of the most important ecological priorities for the coming decades. In this context, great hopes are pinned on a switch from fossil to renewable energy sources, since the latter can be produced and used in a climate-neutral manner. However, after first-generation renewable energy feedstocks have come under criticism due to significant land-use conflicts, intensive research is currently being conducted into more resource-efficient feedstocks. In this regard, microalgae and macroalgae appear particularly promising, as they are highly productive compared to terrestrial plants and can be grown on inhospitable land or land unsuitable for food production. In order to gain further insights into the properties of macroalgae as an energy source, a seawater-independent marine macroalgae plant for energy production is being designed at BU Weimar.
Aiming at the development of a sustainable strategy for a resource-efficient production of renewables, a macroalgae cultivation plant is to be introduced and implemented on an agricultural biogas plant. By thriving on waste materials as nutrient and carbon supply, macroalgae reduce the overall environmental impact of the biogas plant and increase its economic potential.
Core elements of the project are:
the investigation of optimal system parameters for macroalgae cultivation in closed systems,
the development of a seawater-independent, land-based macroalgae cultivation plant,
the design of a cultivation process, sustainable both in an energetic and financial perspective,
the implementation of a user-friendly cultivation system.
Eventually, the overall sustainability of the concept is to be assessed using balancing methods focussing on environmental and financial issues.
Starting with an investigation on the methane production potentials, lab-scale experiments using different algae to inoculum ratios demonstrated that macroalgae are not detrimental to an anaerobic process. Instead, they reached promising results lying between 139 and 518 NL CH4/kg oTR. The red algae Palmaria palmata achieved the highest methane production, even lying within the vicinity of the yields achievable with maize (518 NL compared to 570 NL CH4/kg oTR).
Based on literature data and lab-scale experiments on growth enhancing parameters, a prototypal cultivation plant has been built with a capacity of 3.6 m3. The fully automated plant is equipped with sensors continuously measuring water pressure and flow to monitor hydraulics in the recirculation system. Feedback control triggers the flue gas (e.g. carbon dioxide) supply to the reactors using the actual pH-values of the cultivation medium. Consecutive cultivation experiments with selected macroalgae species demonstrated the overall functionality of the cultivation system. However, acid-forming flue gas components made the pH-value of the cultivation medium drop to algae damaging levels within days. The flue gas supply thereby had little effect on the carbon limitation of the algae and thus cultivation experiments had to be conducted without a supplementary carbon supply. The achieved daily growth rates with Ulva spp. (1 - 7%) remained behind the expected growth rates documented in literature - suggesting that unknown factors are limiting growth. In order to increase the productivity of the cultivation plant, further research needs to focus on optimizing the nutrient and light supply.
Balancing the overall concept of cultivating macroalgae as a renewable energy source, demonstrated that it is neither sustainable in an energetic, nor in a financial way. However, considering current market prices, selling it as a product, for instance as food, would result in a plus of 32€ per kg of cultivated macroalgae.