Flat-Panel Airlift Photobioreactor

The most important process parameter in the mass cultivation of microalgae in photobioreactors is the light intensity, which has an impact on every algal cell in the photobioreactor volume. This determines the biomass productivity and thus the growth rate and cell concentration of the algae in the reactor. To achieve high cell concentrations, the light availability for every individual cell in the photobioreactor has to be increased.

Light supply to all cells due to low layer thickness

Fraunhofer IGB has developed and patented a novel reactor platform (Patent number WO 00926833.5; EP 1326959). Airlift-driven intermixing combined with static mixers offers efficient distribution of light with a low energy input for intermixing and low shear forces taking effect on the algal cells.

Precise flow control using static mixers

Due to the static mixers, uprising gas bubbles induce definite vortices in the interconnected reactor compartments. In these definite vortices algal cells are transported at short intervals to the reactor surface to intercept high light intensities and then transported back to the dark. Sufficient CO₂ and O₂ mass transfer for unlimited growth is ensured by the combination of the airlift-driven principle and static mixers.

The flat panel airlift (FPA) reactor is well-suited for small-scale and large-scale production of microalgae. The reactor itself is inexpensively made from two deep-drawn plastic sheets including static mixers, manufactured by twin-sheet technology. In a scale-up process the volume of the FPA reactor was increased from 5 liters lab scale to 30 liters and finally to 180 liters. Today, the volume comprises 125 liters. The scale-up step to a pilot plant consists of linking several reactor modules (each 125 liters).

To design an outdoor process which is independent of light and temperature, the Fraunhofer IGB developed an automation concept with an easily accessible measuring technique. The automation concept was achieved – in line with the current industry standard – with the aid of a programmable logic controller (SIMATIC S7-1200, Siemens).

Both reactor temperature and pH are controlled. Control of pH is achieved by control of CO₂ concentration in the supply air: the higher the CO₂ concentration in the supply air, the more becomes dissolved as carbon dioxide in the culture medium. This lowers the pH value. This is counteracted by the ammonium dissolved in the medium: the higher the ammonium concentration, the higher the pH value in the culture medium. If in such a system the pH value is constantly regulated by means of the carbon dioxide concentration in the supply air, this allows conclusions to be drawn about the ammonium concentration in the reactor. This correlation was used to determine the consumption of nutrients in the reactor. On the basis of these calculations, we were able to successfully control feeding cycles and exclude nutrient and carbon dioxide limitation.

When setting up the control software, it was ensured that it was very user and operator-friendly. The overall process is visualized on a display screen and all online data continuously recorded. The control software is constructed in a modular way and can therefore be implemented easily in new production facilities.

Advantages of automation system

• Continuous process monitoring
• Automated feeding and harvesting cycles possible

By estimating the amount of ammonium in the culture via CO₂ concentration in the supplied air:

• Allows constant nutrient supply
• Allows consistent nutrient concentration in the culture due to low feeding amounts
• Feeding of nutrients depends on nutrient consumption and is independent of weather conditions and therefore suitable for outdoor production
• Growth limitations by culture medium components are detectable (via decreasing ammonium consumption rates)
• Monitoring of growth is possible if correlation factor of nutrient demand per gram biomass is known
  
  

Photobioreactors based on the flat-panel principle offer the significant advantage of distributing a particularly large amount of light to all algal cells due to their high surface-to-volume ratio. Effective mixing of the reactor volume enhances this effect through light integration, which – compared to other established systems and depending on the algal strain being cultivated – leads to the highest biomass and product yields.

Limitations of outdoor cultivation

Outdoor algae cultivation, which has been the predominant method to date and is characterized by highly fluctuating light and temperature conditions, yields only low space/time yields despite the use of the most advanced flat-plate reactor systems. Compounding the challenges for widespread adoption of these systems is an insufficient level of technological maturity. In particular, for rapid, modular scaling and commercialization of the technology, investment and operating costs must be significantly reduced.

Higher productivity and year-round operation through artificial lighting

Artificial lighting in photobioreactors is becoming an increasingly popular alternative for supplying algae with photons. Since the LED industry has made a huge leap in efficiency in recent years and there has been a rapid decline in the price of small SMD chips in mass production, this increasingly enables year-round operation of industrial algae plants regardless of weather conditions, while maintaining consistent product quality. Electricity costs account for approximately 80 to 90 percent of operating costs, while system cooling now plays only a minor role. Energy consumption currently amounts to approximately 150 kWh/kg of algal biomass for single-stage processes in industrial applications. Initial laboratory results show improved energy consumption of approximately 80 to 100 kWh/kg of algal biomass with a further significant increase in productivity compared to the volatile sunlight in outdoor systems.

Through further developments and optimization of the photobioreactor system in various projects, we have succeeded in further reducing the energy consumption per kilogram of algal biomass. As a result, algae cultivation using artificial lighting results in 500 times better land utilization compared to the land area required for agricultural production. This advantage comes at the cost of high electricity consumption of approximately 70–100 kWh per kilogram of algae biomass.

Compact and modular photobioreactor using a stack design

The goal of further concepts and investigations was to create a cost-effective, modular, scalable reactor platform that can be advantageously deployed in the process industry depending on the algae strain, market, and product. Aspects such as low “downtime” due to quick cleanability and flexibility in handling such systems had to be taken into account during further development, as well as good mixing and optimal temperature control during operation.

A key element here was a novel, compact stack configuration of the photobioreactors into a modular reactor system. This is characterized by flat LED lighting and the interconnection of the individual reactor chambers to form a single total volume.

When comparing the yields in these systems with agricultural production, the extremely compact design allows the same amount of biomass to be produced in just a few square meters as was previously produced on an entire hectare using sunlight. If electricity from renewable energy sources is used for such a system – for example, by integrating it with photovoltaics – economically viable operation is already evident today, depending on the product.