In both lab-scale and pilot-scale microalgae cultivation systems, the key factors that need to be considered for the design and operation of microalgae cultivation systems are as follows: (1) how to use appropriate light sources (intensity and wavelength), (2) how to enhance light conversion efficiency, and (3) how to maintain an appropriate microalgae biomass concentra­tion during prolonged operation. In addition, the stability of continuous culture of microalgae is usually poor, because the cell growth and target-product production are sensitive to changes in the environment and the medium composition.

Maintaining a sufficient cell concentration in the continuous microalgae cultivation system is also a challenge. Therefore, many large-scale outdoor microalgae cultivation systems are operated in a semibatch mode, in which a portion of microalgae culture is harvested within a specific cultivation time period and an equal amount of fresh medium is refilled into the cultivation system. In addition, most commercial-scale microalgae cultivation is carried out in open ponds, since solar light energy is directly utilized. Therefore, there are challenges such as contamination by other microorganisms or alien microalgae species, direct exposure to ultraviolet (UV) irradiation, low light intensity or uneven light energy distribution (Kim et al., 1997), day-night cycles, diurnal variation, and requirements for large areas of land (Laws et al., 1986). Moreover, since the intensity of sunlight varies greatly with the seasons, solar spectrum, and operating time, it is very difficult to maintain steady microalgae culture performance in outdoor cultivation.

The limitation of light energy is also one of the most commonly encountered problems in large-scale cultivation when the size of the microalgae cultivation system is increased. In this case, the illumination area per unit volume is often considered as a design criterion. The fac­tors mentioned here greatly limit the light conversion efficiency and productivities of outdoor microalgae cultivation systems. Other factors that may also lower the biomass productivity are consumption of biomass by respiration in the dark zones of the reactor, insufficient mixing of CO2 and nutrients, and the mechanical damage due to the shear stress on the algal cells. Variation in biomass concentration and composition (e. g., carbohydrate or lipid content) may occur when different culture media and operation modes are used.

Despite the fact that good production performances of target products can be achieved using lab-scale microalgae cultivation systems, there are still very few successful commercial-scale processes. This is mainly because of the higher operating costs, unstable light intensity, and lower mixing efficiency when the microalgae are grown outdoors on a large scale. Consequently, appropriate operating configurations with innovative design of microalgae cultivation system are required to achieve commercially viable production of microalgae biomass and target products.

Therefore, highly efficient light sources and good circulation devices are the key to pro­mote microalgae cell growth in the design of commercially feasible microalgae cultivation systems. If the light source has a narrow spectral output that overlaps the photosynthetic ab­sorption spectrum of microalgae, the emission of light at unusable wavelengths would be eliminated, thereby improving the overall energy conversion. Among the available light sources, light-emitting diode (LED) is the only one that meets the foregoing criteria. LEDs are an economic external light source that is energy-saving and small enough to fit into any microalgae cultivation system. They also have a very long life expectancy, and their elec­trical efficiency is so high that heat generation is minimized. LEDs have a half-power band­width of 20-30 nm, which can match photosynthetic needs. On the other hand, circulation is also very important in the outdoor microalgae cultivation system. The benefits include keep­ing microalgae in suspension, decreasing heat generation within the microalgae cultivation system, uniform distribution of the cells and the liquid broth, improving CO2 mass-transfer efficiency, and degassing the O2 produced during photosynthesis. Therefore, the develop­ment of economically successful production of microalgae biomass requires the improvement of both light efficiency and mixing efficiency for microalgae growth at low cost.