The straight, looped or coiled transparent glass or plastic tubes of tubular reactors are usually arranged in horizontal or vertical arrays [18, 28]. Liquid flow is induced by pumping of the liquid volume or by airlift circulators [24].

Scale-up by increasing the tube diameter is limited with respect to light penetra­tion depth into the culture. However, light is focused in radial direction to the center of the tube. This effect partially compensates for exponential decrease in light inten­sity due to absorption which is described by Lambert-Beer’s law. Diameters in the range of 3-6 cm are common [18, 24]. The occurrence of dark volumes in the center of the reactor does not necessarily lower productivity. Induction of favorable light/ dark cycles can be achieved by high liquid velocities or installation of static mixers and thus increase volumetric productivity [27] . Molina et al. suggested identical light/dark cycle frequencies on different scales as a suitable scale-up criterion for achieving similar productivities [24] .

Since scale-up potential with regard to tube diameter is restricted, tube lengths are increased and modular units are multiplied, e. g., mounted on vertical scaffolds

(Fig. 10).

Gas exchange is crucial for this type of reactor. Aeration and oxygen removal usually take place in specific gassing and degassing compartments, whereas gassing

at several points along a tubular track is a conceivable option. The flow regime within the tubes can be regarded as plug flow regime with minimal backward and forward mixing. Therefore, considerable spatial gradients of oxygen and CO2 along tubular axis occur and gain importance with increasing lengths of tubes. Limited availability of carbon dioxide limits cell growth at some point. One should also keep in mind that pH gradients are concurrent with CO2 gradients on account of the car­bonic acid equilibrium [39]. Oxygen removal is another important aspect since oxy­gen supersaturation inhibits growth or even causes oxygen-induced cell damage [3]. Therefore, dimensions of the degassing section, length of tubes, liquid flow rates, and mass transfer must be reasonably adjusted in order to avoid detrimental oxygen concentrations and CO2 limitation as well.

Tubular reactors can attain high productivities, e. g., 1.4 g/L/day in a helical reac­tor [18] or 1.9 g/L/day in an airlift-driven tubular reactor with flat arrangement of solar collecting tubes [25] . However, power supply for tubular reactors is usually much higher compared to the aforementioned alternative design concepts. Power supply in magnitudes ranging from 800 to 3,200 W/m3 [18, 42] is unfavorable for production of biodiesel and hydrogen. Therefore, tubular reactors should rather be used for cultivation of high value products, e. g., for the pharmaceutical market.

Most prominent commercial application of tubular photobioreactors is probably the biggest indoor tubular reactor, set up by the company Bisantech in Klotze, Germany (Fig. 10).

About 500 km of tubes with a total volume of circa 700 m3 are located in a green­house where tubes are arranged in vertically oriented scaffolds to attain maximum areal productivity.

The tubes are not all interconnected because gradients resulting from such an arrangement would necessarily lead to inhibiting accumulation of oxygen and simultaneously to carbon dioxide limitation. Before being recirculated through the tubular system, the culture suspension intermittently enters degassers. In the facil­ity, temperature is actively regulated to adjust to optimal culture conditions [30]. The facility’s output targets the nourishment market sector with production costs of circa 15€/kg [approximately 20 US$/kg] biomass. Productivities of 100 t/ha are attained under mixotrophic growth conditions (Prof. Steinberg, personal communication).