Bioreactors, including photobioreactors using the principles of photosynthesis, are well known in the art. Photobioreactors use photosynthetic organisms such as algae in a liquid medium along with light energy (whether synthetic light or natural sunlight) and carbon dioxide to create chemical energy from light energy while sequestering carbon dioxide. These systems may have open channels such as ponds or closed channels such as cylindrical vessels. Some systems employ tubes of a variety of sizes.
Due to a raised awareness and interest in energy independence and the Earth's climate, there is an increased demand for renewable fuels, decreasing harmful emissions or flue gasses such as those emitted from coal fired power plants, and for achieving these goals in an efficient and effective manner. In addition to the carbon dioxide emitted from electric power plants, which are some of the biggest producers of such gases, other sources of harmful emissions include manufacturing facilities and diesel power generation plants, to note but a few. It is also desirable to remove certain nitrogen oxides and sulfur oxides from flue gasses as well as carbon dioxide. For the pollutant sequestration process to be economically feasible and environmentally responsible, the process should, among other requirements, not consume more energy than it creates, be operable and effective on a substantial production scale, and should not displace crops from farmland or pastures from grassland, to note but a few.
The open channel photobioreactors, such as ponds, have faced difficulties from contamination by hostile species or external pollutants and from the inefficient use of light that illuminates only the top portion of the pond. As a more efficient photobioreactor will have an illumination surface area per unit volume (SN) ratio that is high, shallow ponds are the norm. This, however, greatly increases land space requirements for pond-based photobioreactors. In addition, when these ponds use natural sunlight the process is limited by the available hours of sunlight. Such processing limitations can be important if the photobioreactors are used to process waste gasses from polluting facilities that operate twenty-four hours a day. Further, if these ponds are not insulated from the elements such as seasonal changes in weather, the photosynthetic organisms must be remarkably hardy to withstand changes in temperature, external pollution, and attack from hostile species.
Another approach that has received considerable attention is the closed channel system such as those systems having cylindrical tubes that employ the air lift principle. In general, air lift photobioreactors have photosynthetic material such as algae suspended in a liquid medium into which air or gas is injected into the bottom of the system which then rises through the fluid medium in the cylindrical tube.
Conventional air lift photobioreactors, however, suffer from the lack of flow patterns that can be duplicated, controlled, or even easily defined. By one approach, straight, vertical, concentric tubular containers receive the gases at an inner tube, which creates an annular liquid flow upwards through the inner tube and downwards in a space between itself and another tube. Fluid flow is important to controlling the progression of the photosynthetic stages: light-dependent reactions and light-independent reactions. Difficulty controlling the mixing properties may lead to poor photomodulation, low mass transfer coefficients, and low productivity. In addition, systems having uncontrolled or poorly controlled fluid flow or mixing properties may experience algae pooling or buildup along with damage to the photosynthetic organism that results in poor algal growth.