Innovation of photobioreactor for microalgae by using fluid dynamics and proposal of its usage for CO2 fixation
Sato T.1, Usui K.1, Nakatsuka N. 1, Sakurai S. 1, Tsuchiya Y. 1, Kondo Y.2, Hirabayashi S.3
1University of Tokyo, Tokyo, Japan
2Yamaha Motor Co. Ltd., Iwata, Japan
3Micro Gaia Inc., Maui, USA
It has been said that the fixation of carbon dioxide by using microalgae is one of promising methods to reduce the so-called greenhouse effects. For this purpose, it is necessary to innovate high-performance photobioreactors for mass-production use. However, it seems not-so-easy to attain the productivity accomplished by laboratory-scale reactors. This is partly due to the thickness of reactor chambers and high concentration of algae, which are simply for accommodating necessary volume of cultivates in a reasonably economical mass-production system, but prevent the algae from receiving sufficient sunlight and the cultivate medium from mixing nutrients and dissolved gasses equally.
We focused on the Bio-dome developed by one of the authors (SH), which is a discrete-type reactor for mass production and was adopted for commercial use recently. Firstly, the flow in a Bio-dome was elucidated by using flow-visualization techniques. Then, the shape of a photobioreactor was analyzed. We classified its functions into three, i.e. sunlight-reception, global mixing, and local mixing. While the first and third were mathematically modeled, the second was numerically simulated by using two-phase CFD (computational fluid dynamics). Here we were interested in the third function of photobioreactors; the local mixing, in other words, vortices mixing algae in the thickness direction of a reactor chamber. Our mathematical model for the dark-light cycle for photosynthesis can predict the increase of productivity by intensifying these vortices, although the model needs further investigation to acquire sufficient accuracy.
Next, we tried to innovate novel forms of photobioreactors. It is important for mass production that the system of reactors must be simple. Our design policy is that steady flow should provide algae with equal opportunity for sunlight, nutrients, and dissolved gases. Apparatus for unsteady flow, which is regarded as effective to reduce unfairness among algae in terms of sunlight-reception, sometimes cause small troubles. However, this is serious, if small, for mass production using numbers of discrete reactors. To select better shapes of reactor chamber, the CFD code was used to evaluate the global mixing performance. Here we introduce two parameters; deviations of random numbers initially set in each cell of computational mesh and of light intensity added in each cell at every computational time-step depending on the distance from the reactor surface. Another kind of selection was done in terms of the sunlight-reception capacity. Two types, which had survived through the above selections, were manufactured and tested by using Chlorococum littorale for the 12 mostly clear winter days in Iwata. The winner of this last selection performed 0.15g/L/day (6.8g/m2/day) at the alga-concentration of 1-2.5g/L, that is 57% more productivity than the Bio-dome.
Lastly, we consider proposing a reasonably economical CO2 fixation system by using high-performance photobioreactors (0.5g/L/day, 24g/m2/day) and Botryococcus braunii. The system is supposed to be constructed in the Hobuqi Desert in China, using water from the Huang He River and exhausts from a coal power plant in Paotou City. Our calculation shows that the total CO2 fixation cost is about 100yen($0.8)/kg-CO2. The reduced CO2 is the one supposed to be exhausted from current Chinese power generators burning coal, which is replaced by Botryococcus braunii as biomass fuel. Here, the exhaust gas from the coal power plant is directly lead to the photobioreactors, so that the used Botryococcus braunii should be genetically manipulated to endure sulfuric acid and nitrogen oxides.
1. Terry, K.L. (1986) Photosynthesis in modulated light: quantitative dependence of photosynthetic enhancement on flashing rate, Biotech. Bioeng. 28:988-995.
2. Jones, R.W. (1999) Mass transfer in a bubble column photobioreactor, Biotech. Bioeng. 76:1345-1351.