The special design of flat plate photobioreactors with small distances between their translucent rectangular covers allows for cultivation with small layer thick­ness in response to the limited light path length (Figs. 4 and 6). Flat plate reactors can be compared with bubble columns with regard to aeration and mixing.

Movement of gas bubbles through the reactor induces mixing mainly along the vertical axis. The main difference is given by the short thickness of flat plate reactors which generally amounts only a few centimeters (e. g., [42]). If biomass concentrations during operation, absorbance and incident light intensities can be reasonably estimated during the planning process, adjustment to the light path length can widely prevent the occurrence of dark volumes. The relatively simple geometry facilitates scale-up tremendously, e. g., when several reactor modules are placed in north-south oriented “fences” (Fig. 4).

Instead of using glass plates, in some applications plastic bags are fixed in a metallic frame (Green Wall Panel [45], see also Fig. 4). The replacement of glass by much cheaper, transparent, disposable plastic bags is particularly interesting for commercial application. In this case, the reaction vessel can be exchanged when fouling or contamination makes a further utilization unfavorable.

The company Solix Biofuels (Fort Collins, CO, USA), for example, cultivates algae in submerged flat plastic bags. In principle, the concept can be traced back to the basic design of flat plate reactors. The fundamental setup of the third generation reactor concept (3G) is depicted in Fig. 7. Major advantage of the submerged reac­tion compartments is the fact that additional temperature control is not necessary in the system because the surrounding water acts as temperature buffer. Moreover, construction costs for the reactor are reduced as there is no need for a special scaf­fold supporting the flat reaction compartments. The demonstration facility (south­western Colorado) utilizes wastewater from coal-bed methane production. The innovative gas sparger system is integrated in the seam of the plastic bags and dis­tributes CO2 enriched gas of a nearby amine plant. Therewith, the concept aims at an integrated environmental-friendly biomass production connected with CO2 captur­ing. According to the information given by the official website Solix produces 5,000-8,000 gallons of algal oil per acre, per year (circa 42-67 t/ha/a, assuming an

oil density of 900 kg/m3) [43]. A fourth generation reactor is currently under devel­opment. Investigations on the replacement of the spargers by an integrated mem­brane aeration system are undertaken by the company [7, 47, 48].

Another commercially applied advancement of classic flat plate reactors was realized by Proviron (Hemiksem, Belgium) (Fig. 8). Their major focus was set on development of an efficient low-cost reactor suitable for large-scale outdoor appli­cations. Their approach comprises the incorporation of flat growth compartments (less than 1 cm thick) within water-filled plastic bags without any rigid structure. The major part of the setup is represented by water-filled chambers that are sepa­rated from reaction compartments. Water diffuses the impinging solar radiation, which should result in an equalized light distribution within the water-filled chamber. At the same time, temperature is regulated without any additional energy input. Moreover, the water-filled chambers themselves constitute the scaffold of the reac­tor. In the future, the low auxiliary energy demand of 20 kW/ha should be further reduced with control strategies that aim at adaption of aeration to light. According to the company’s outlook investment cost is expected to drop from currently 200,000€/ha to 100,000€/ha [31].

A straightforward approach to improve flat plate reactor productivity and enforce beneficial light/dark cycles was implemented in the flat-panel-airlift reactor [14] , a concept that was further improved for large-scale outdoor applications by the company Subitec (Fig. 9) . This reactor works according to the airlift principle. Compressed air is injected in the riser which induces an upward flow of liquid. Specific flow regimes are induced by the elaborate arrangement of interconnected chambers that are separated by baffles alternatively located at the front — and back­side of the reactor. Therewith radial mixing is substantially improved and cells circu­late between darker and more illuminated regions of the reactor. The specific design with indentations (acting as baffles) provides additional surfaces for light capturing and certainly contributes to light availability within the culture. After reaching the top of the flat plate reactor, the liquid volume descends in a downcomer with small diameter, so that the culture circulates repeatedly through the compartments.

According to information given by the company, the energy input ranges in between 100 and 200 W/m3. Total energy consumed in the process referred to bio­mass produced is specified to be below 20 MJ/kg dry mass and thus below the aver­age energy content of algae biomass (see above). Efforts are undertaken to further decrease this value [35].

To sum up, the flat-panel-airlift reactor concept integrates three beneficial char­acteristics: a short light path length, efficient mixing, and utilization of the intermit­tent light effect.