Light impinges on the reactor surface, is absorbed by cells, scattered and reflected and thus light intensities necessarily decrease with increasing distance from the surface. Consequently, light cannot be provided with equal intensities for all cells

reaction

Fig. 2 Interdependency ofbiochemical reaction, light transfer, and fluid dynamics in a photobio­process [37]

within the reactor at the same time. Furthermore, the incident light intensity is sub­ject to daily and seasonal changes, as well as weather influences. Even increasing cell concentrations strongly alter light distribution in the time course of a single cultivation due to absorption, scattering, and mutual shading.

All photobioreactor concepts apply the same common design principle of a lim­ited light path length (Table 1). Light gradients in the reactor are inevitable. Nevertheless, a plate or tubular thickness that significantly exceeds the light path length, leads to an increased dark volume. This generally impairs the overall pro­ductivity because microalgae shift to respiratory metabolism when photosynthesis is stopped. The significance of respiratory losses can be deduced from respiratory maintenance metabolism during night hours. Respiration can cause a biomass loss of up to 25% of biomass produced during the day [11, 23].

Many attempts to simulate growth of algae cultures assume an exponential decline of light intensity with increasing distance from irradiated reactor surface [44]. High cell densities tremendously limit the light path length. Exemplary mea­surements show that at a cell concentration of 10 g/L (Arthrospira platensis), about 95% of incident light (I0 = 1,925 mE/m2/s) is absorbed or scattered along the first 2 mm of the light path [46].

However, high cell concentrations are desirable because downstream processing (DSP) usually requires high energy input, e. g., for centrifugation or spray drying. High cell concentrations increase efficiency of DSP by reducing energy input and cost per biomass yielded. Cell densities of about 5 g/L were reached with Chlorella in semicontinuous cultivation experiments in airlift-photobioreactors [12] and max­imal dry weight concentrations of Phaeodactylum tricornutum cultures under out­door conditions of around 7-8 g/L were attained in a Flat-Panel-Airlift reactor (Subitec, Germany) [35].

Light intensity on reactor surface l0 / (|jEm*2S‘1)

Fig-3 Effect of light intensities on growth kinetics of P purpureum in turbidostat cultivation mode under homogenous light conditions (filled circle growth rates resulting from continuous illumina­tion; filled square growth rates when cells are exposed to light/dark cycles)

Provided that the light path length exceeds the plate thickness in flat plate reac­tors, exponential growth of the culture can generally be achieved. Transmitted light is not necessarily “lost” but can be captured by other compartments of the facility, e. g., when parallel reactors are arranged in fence-like structures (Fig. 4). Otherwise, all photons will be absorbed thus leading to linear growth on condition that no sub­strates become limiting.

The tremendous implication on scale-up is that the rule of geometric similarity on different scales cannot be applied to photobioreactors. Instead, one dimension is more or less fixed. Scale-up is limited to the remaining dimensions.

The assumption that exposure of microalgae cultures to high irradiances neces­sarily increases productivity would be misleading.

A look at Fig. 3 reveals that growth rates show a linear increase with light inten­sities only in a very narrow range. As shown here for the model organism, Porphyridium purpureum cultures are light limited when light intensities impinging on the reactor surface range up to ca. 100 mE/m2/s. Higher intensities have almost no advantageous effect on growth kinetics since dark reactions in the CO2 fixing Calvin-Benson cycle become kinetically limiting and therefore the availability of NADP+ and ADP for any further conversion of the H+ gradient across the thylakoid membrane to NADPH and ATP is restricted. Excess light that is harvested by the algae is dissipated as heat or fluorescence by pigments [51]. Cells are said to be light saturated. Growth rates do not increase linearly when light intensities are raised but rather stay constant over a wide range. Such inefficient utilization of light will necessarily result in low PCE values.

Further increase in light intensities can even damage proteins involved in the photosynthetic apparatus and inhibit cell growth. This phenomenon is called

light inhibition. It is characterized by reduced growth rates when light intensities are further increased [2].

The reaction of microalgae to various light intensities is affected by adaptation processes over the day cycle and also dependent on the specific strain. The latter should already be taken account of in screening programs for isolation of new strains.

An optimized operating point for photobioreactors lies in a range where light is limiting and saturation is avoided (as indicated by the vertical, dashed lines in Fig. 3). These conditions fundamentally determine photobioreactor geometry.

A high surface to aperture area (or ground area) is attributable to the fact that high midday summer light intensities need to be avoided and thus light is spread over a larger surface area. Otherwise, cultures would be exposed to the high light intensities that lead to saturation or even inhibition. This generally applied concept is referred to as “light dilution.” Furthermore, limited light penetration depth confines reactor geometry in one dimension. Consequently, reactors are flat or con­sist of tubes with small diameters. Finally, a high surface to volume ratio is attribut­able to the other two demands on geometry.

The Green Wall Panel reactor is one example for light dilution in practice [45]. Vertical reactor compartments of flat panels in fence like arrangements collect light rays at large angles and therewith achieve a dilution effect (Fig. 4). In particular, high areal productivities and efficient light utilization can be obtained since the modules can be placed in relatively short distances (e. g., 1 m high modules, 0.9 m distance [36]) and even light-averted surfaces collect reflected and diffuse light [46].

Table 1 gives an overview of different reactor concepts and their corresponding surface to aperture area ratios.

Although individual reactor and scale-up concepts significantly vary, it becomes clear that all reactors provide high surface areas to collect the incident light (surface:volume ratios of 30-86 are shown) and small depths (3-7 cm) account for limited light path length. Moreover, all reactors dilute light that is collected from a certain area at least by a factor 3.6 (surface:aperture area ratios: 3.61-5.45) in order to avoid excess light intensities.

Midday light intensities of about 1,000 pE/m2/s are not uncommon in Europe in the summer season (2,000 pE/m2/s in equatorial regions) [ 10, 30]. Therewith, a surface to aperture area ratio of 10 or even more would be reasonable if the specific algal strain reacted similarly to high photon flux densities like P. purpureum, as depicted in Fig. 2.

Adjustments for every individual facility to the requirements and characteristics of the specific algae as well as the location, latitude, and climatic conditions need to be considered.