Open-pond culture systems and enclosed bioreactor facilities have been used commer­cially in the recently evolved alga biomass biotechnology, but bioreactor design in algal hy­drogen production is still in the research and development stage. Because biohydrogen metabolism is primarily the domain of microalgae, diatoms, or cyanobacteria, the design of a photobioreactor depends on microbiological processes associated with bacteria and microalgae (Show et al., 2008; 2011; 2012). Although these photoheterotrophic bacteria differ in photochemical efficiency, absorption coefficient, and size, the light regime, including light and dark cycles, is assumed to be much more determining than biological factors (Akkerman et al., 2003). Hence the productivity of a photobioreactor is light-dependent, and a large surface-to-volume ratio is a prerequisite for a productive photobioreactor for optimal light exposure of the algae. Provisions for thermal control and monitoring of factors, including flow rates, pH, and dissolved oxygen, sulfur, and hydrogen are essential. Technical develop­ment is now moving toward devising gas-tight systems, engineered microalgae culturing, and computer-controlled systems for monitoring and automatic nutrient delivery and culture dilution.

Photobioreactors have been designed to achieve an economical, rapid multiplication and high algal biomass density (>1012 cells per cubic meter of culture) (Evens et al., 2000). Various photobioreactor designs, including flat plate, tubular, pond, and pool type, have been inves­tigated (Akkerman et al., 2003). The photobioreactor process, whether of batch or continuous flow, should be designed for optimal light exposure to the algae. Sufficient light supply is vital for adequate biomass growth in achieving high-density culture and for photosynthetic generation of hydrogen. Light conversion efficiencies are low (limited to 10% theoretically) and tend to decrease at higher light intensities because of the light saturation effect (Akkerman et al., 2003). The reason for this inefficiency is that, at high solar intensities, the rate of photon absorption by the chlorophyll antenna of the upper layers of algal biomass far exceeds the rate at which photosynthesis can consume. This phenomenon is attributable to the fact that algae have an intrinsic tendency to accumulate a large assembly of photon­absorbing chlorophyll antenna molecules as a survival strategy. The overabsorption of light by the chlorophyll antenna results in loss of excess photon as heat or other rays. Moreover, cells at the upper layers of the algal mass are subject to severe photoinhibition of photosyn­thesis due to the high rate of photon absorption (Baroli and Melis, 1996; Melis, 1999).

Research is underway in improving further algal photosynthetic capacity using an molec­ular engineering approach, whereas algal strains have been manipulated to increase hydro­gen production (Hankamer et al., 2007; Beer et al., 2009). It has been reported that a truncated chlorophyll antenna size of the photosystems in the chloroplast of the microalgae could alle­viate the optical shortcomings associated with a fully pigmented chlorophyll antenna (Melis et al., 1999; Neidhardt et al., 1998). The work on the truncated chlorophyll antenna size in maximizing solar conversion efficiencies is delineated in Section 9.5. Mutant algae with less chlorophyll were cultured and are able to distribute more sunlight to deeper layers in the algal biomass for large-scale applications (Hankamer et al., 2007). In this manner, sunlight is made available for more algal cells to generate hydrogen. Hence, for efficient photoproduction of hydrogen, it is critical to dilute the light and distribute it over the entire reactor volume and to mix the culture at high rates so that cells are exposed to the light for only a short time.

Algal photobioreactors can be designed to regulate light inputs to the algal culture to im­prove its photon conversion efficiency. A substantial increase in light utilization efficiency of up to 15% has been reported (Tetali et al., 2007; Laurinavichene et al., 2008). Conversion ef­ficiency between 10% and 13% is feasible using engineered microbial culture to better utilize the solar energy (Turner et al., 2008). However, improvements must be made to the solar con­version efficiency of the algae for commercial purposes. Critical issues such as the optical shortcomings associated with the chlorophyll antenna size and the light saturation of photo­synthesis must be addressed under mass culture conditions (Melis et al., 1999). Technological advancement addressing these issues is discussed in Section 9.5.

Appropriate configuration of the bioreactor needs to be established for the most effective use of light and surface area. Biomass mixing is hence significant to ensure uniform disper­sion of nutrients and light illumination in the culture as well as to prevent agglomeration and sedimentation of algal biomass (Melis, 2002). Modular design of experimental systems should be allowed for possible scale-up. Such commercial scale should achieve sustainable gas out­put and high hydrogen yields with compact configuration. Trapping and withdrawal of hy­drogen gas in the system are also important design considerations for photobioreactors. Given the current advancement in photobiohydrogen production, technical and economic strategies for cycling the microalgae between sulfur deprivation and supply must be devel­oped (Laurinavichene et al., 2008).

Various types of photobioreactors had been investigated in a study by Janssen (2002). Small-scale flat panel reactors consisting of a rectangular transparent box were mixed with gas introduced via a perforated tube at the bottom of the reactor. To create a high degree of turbulence, 3 to 4 liters of air per liter of reactor volume per minute must be provided. The panels were illuminated from one side by direct sunlight, and the panels are placed vertically or inclined toward the sun. Light/dark cycles were short in flat panel reactors, and this is probably the key factor leading to high photochemical efficiency. A disadvantage of flat panel reactors systems is that the power consumption of aeration (or mixing with another gas) is high, although mixing is always necessary in any reactor. The large-scale flat plate reactor consists of a rectangular airlift photobioreactor with a large number of light — redistributing plates fixed a few centimeters from each other. Mixing was provided by air injected between adjacent plates, and the culture liquid rises in between.

Tubular photobioreactors consist of long transparent tubes with diameters ranging from 3 to 6 cm and lengths ranging from 10 to 100 m (Janssen, 2002). The culture liquid is pumped through these tubes by means of mechanical or airlift pumps. The tubes can be positioned in many different ways: in a horizontal plane as straight tubes with a small or large number of U-bends; vertical, coiled as a cylinder or a cone; in a vertical plane, positioned in a fence-like structure using U-bends or connected by manifolds; or horizontal or inclined, parallel tubes connected by manifolds. In addition, horizontal tubes can be placed on different reflective surfaces with a certain distance between the tubes. Although tubular reactor design is rather diverse, the predominant effect of the specific designs on the light regime is a difference in the photon flux density incident on the reactor surface. The shape of the light gradient in the tubes is similar in most designs. Also with respect to liquid mixing, the circumstances in most de­signs are similar. The length of the tubes is limited because of accumulation of gas, though this might not be so important for nitrogenase-based processes, since they may be less inhibited by hydrogen. The way to scale up is to connect a number of tubes via manifolds. Flat panel reactors normally show a high photochemical efficiency or biomass yield on light energy, while biomass density is also high. Tubular bioreactors in theory should show better efficiencies because of the shorter average light/dark cycles.

Although much of the research has been focused on single-stage photobioreactor systems, multistage bioreactors entailing three or even four bioreactors in biohydrogen production have also been examined (see Figure 9.2) (U. S. DOE, 2007; Wang et al., 2011; Show et al., 2011). Sunlight is first filtered through first-stage direct photolysis, in which visible light is utilized by blue-green algae, and the unfiltered infrared ray is used by photosynthetic mi­crobes in the second-stage photofermentative reactor. The effluent from the second-stage photofermentation, together with the biomass feedstock, is fed into a third-stage dark fermen­tation reactor, where the microorganisms convert the substrate into hydrogen and organic acids. As the effluent is enriched with organic acids, a supply of external organic acids for the photofermentative process can be eliminated. The fourth stage involves the use of a microbial electrolysis cell to convert the organic acids generated from the dark fermentation into hydrogen in a light-independent process. This stage thus can be operated during the night or in low-light conditions.

The increasing attention on hythane has led to research interest in hydrogen production by dark fermentation of biomass in hybrid or multistage bioreactors. Hythane, a mixture of hydrogen and methane, is a highly efficient and ultraclean-burning alternative fuel that is probably the most promising biogas for industrial applications (Cavinato et al., 2009). How­ever, there are issues to be addressed before multistage bioreactors can be put to practical applications. Integration of multiple biochemical conversion processes poses significant chal­lenges for multistage reactor engineering, system design, process control, operation, and maintenance. Major challenges with the simultaneous production of hydrogen and oxygen from photolytic hydrogen production include respiration-to-photosynthetic-capacity ratio, co-culture balance, and concentration and processing of cell biomass (Holladay et al., 2009).