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Innovation of photobioreactor for microalgae by using fluid dynamics and proposal of its usage forCO2fixation

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, Japan2

Yamaha Motor Co. Ltd., Iwata, Japan3

Micro 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-calledgreenhouse 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 thethickness of reactor chambers and high concentration of algae, which are simply for accommodating necessary volume ofcultivates in a reasonably economical mass-production system, but prevent the algae from receiving sufficient sunlight and thecultivate 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 andwas 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 usingtwo-phase CFD (computational fluid dynamics). Here we were interested in the third function of photobioreactors; the localmixing, in other words, vortices mixing algae in the thickness direction of a reactor chamber. Our mathematical model for thedark-light cycle for photosynthesis can predict the increase of productivity by intensifying these vortices, although the modelneeds 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 dissolvedgases. 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 discretereactors. To select better shapes of reactor chamber, the CFD code was used to evaluate the global mixing performance. Here weintroduce 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 selectionwas done in terms of the sunlight-reception capacity. Two types, which had survived through the above selections, weremanufactured and tested by using Chlorococum littoralefor 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 theBio-dome. Lastly, we consider proposing a reasonably economical CO2 fixation system by using high-performance photobioreactors

(0.5g/L/day, 24g/m2/day) andBotryococcus 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 totalCO2 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 byBotryococcus braunii as biomass fuel. Here, the exhaust gas from the coalpower plant is directly lead to the photobioreactors, so that the usedBotryococcus braunii should be genetically manipulated toendure sulfuric acid and nitrogen oxides.

References: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.

er page: Photo from web page http://lemons.k.u-tokyo.ac.jp/SATO/research/Ghgt/reactor.ht

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Chlorococum Littorale

Photo-Bioreactors

Computer Simulations

er page: Photo from web page http://lemons.k.u-tokyo.ac.jp/SATO/research/Ghgt/reactor.ht

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