With regard to the aforementioned restrictions, an increase in PCE of more than a factor two cannot be expected. Process costs need to be drastically reduced by lower investment cost and lower demand for auxiliary energy to target a price range of less than 5 US$/kg biomass.

Aeration with membranes can be one major improvement for future reactor developments [13, 17]. The interfacial area for CO2 input and O2 removal from the culture in case of membrane gas exchange is defined by the surface area of the membrane itself and no longer by the surface of gas bubbles. This concept could drastically reduce the overall input of auxiliary energy since there is no longer the need to generate bubbles, and energy loss when bubbles fuse with the top headspace gas phase is avoided. However, gentle agitation will still be required for gas disper­sion. A proof of concept is missing in photo-biotechnology but implementation of membrane systems are considered by Solix Biofuels [26,47].

If beneficial light/dark cycles cannot be attained with low inputs of auxiliary energy, laminar flow patterns can be accepted if diffusion paths from gas membranes to all volume elements are small enough to supply all cells with CO2 solely by diffusion. In this case, additional power input for dispersion could be spared and the overall energy balance tremendously improved. However, this approach requires large surface areas for membranes and a drastic reduction of diffusion path length and thus simultaneously light path length. Such a short light path length could, by contrast, allow for high cell concentrations which is favorable for DSP. This approach also lacks a proof of concept.

Future development will show if membrane gassing will be established on a large scale.

With regard to the current high demand of auxiliary energy for photobioreactors, reduction of the height of reactors could significantly contribute to increased energy efficiency. “Low ceiling” concepts aim at reducing the hydrostatic pressure and therewith the energy demand for aeration systems. Additional improvements in control engineering can certainly facilitate further energy saving in the future. Requirements of carbon dioxide and therewith removal of oxygen is dependent on cell concentration and on light availability in the culture. Therewith, carbon dioxide supply and energy input for mixing should be adjusted to photosynthetic activity, photon flux-density respectively, and cell concentration. Proviron, for example, claims that the auxiliary energy input can be halved in the future by adapting aera­tion to light availability [31].

Infrared radiation is not photosynthetically convertible into chemical energy but contributes to heating of algae culture. Infrared reflecting materials or coatings could additionally improve the overall energy balance by reducing the energy required for cooling. Transparent materials with selective transmittance are avail­able and were developed for the installation in buildings, cars, and greenhouses [19,38,50].

A different approach to the difficulty of light capturing and distribution aims at harvesting the light in a module that is spatially separated from the reactor itself. The Green Solar Collector [52] harvests light by moving lenses whose orientation is guided by a computer that calculates position and altitude of the sun in order to capture maximum amount of photons. The light is then focused and transported via plastic light guides where light is totally reflected. A change in refraction index releases the photons in the microalgae suspension. It is suggested that light redis­tributing plates are integrated in small distances from each other in airlift-photobio­reactors. According to the authors biomass concentrations of up to 20 g/L could be maintained in such a setting with good light distribution and induction of beneficial high frequency light/dark cycles [20] .

Spatially separated light harvesting and reactor modules can have beneficial advantages because parameters like temperature can be controlled more easily. Furthermore, influences of unfavorable weather conditions, such as hail, will be confined only to the light collector. That will positively influence maintenance costs.

Spatial separation of light harvest and cultivation differs from all other concepts presented here, but the basic principles of other reactor designs can also be retrieved here. In order to optimize light utilization and productivity, light path length is also limited in this reactor, as the reactor contains flat panel compartments with short light path lengths. Moreover, turbulent flow patterns, induced by aeration, ensure rapid circulation of microalgae between dark zones and illuminated volume ele­ments in order to benefit from the intermittent light effect [20] .

3 Conclusion

Many efforts in the field of photo-biotechnology have not brought out the “perfect” photobioreactor, yet. All basic concepts show specific advantages and disadvan­tages that finally lead to the development of more sophisticated reactors. These should be characterized by outstanding light utilization and mass transfer, yet be operated with minimum energy input. Moreover, diverse algae strains show different behavior in terms of light saturation, robustness towards shear stress, and other cul­ture conditions. Therefore, benefits and drawbacks of different reactor concepts should be taken into account in terms of producing biomass or energy rich products for the energy market. At the same time, adjustment of the particular system to the specific algae strain will be unavoidable. Nevertheless, high productivities and photoconversion efficiencies give rise to high expectations in this field of research.