(322b) A Novel Taylor-Couette Photobioreactor for Energy Efficient Micro Algae Cultivation

Numerous studies have demonstrated that algal growth rate and light utilization efficiency is dramatically impacted by the cycling in the intensity of light exposure as cells move between well-illuminated photic regions near a photobioreactor external surface and the darker regions of the reactor. In particular, when the characteristic frequency of the light/dark cycles is greater than approximately 1 Hz, the "flashing light effect" can boost cell growth rates 50~100% compared to growth rates obtained without light/dark cycling. However, high frequency light/dark cycles are not easy to achieve in large-scale reactors and may be one of the reasons for low yields. In commonly used flat panel bubble column and airlift reactors, the random nature of the agitation provided by the gas bubbles produces a wide spectrum of random light/dark frequencies. In the work presented here, we make use of robust and coherent hydrodynamic structures known as Taylor vortices that arise in the annular region of a Taylor-Couette flow cell (concentric cylinders with the inner cylinder rotating) in order to rapidly and periodically shuttle algal cells between photic and non-photic regions of the reactor. Because the fluid agitation in a Taylor vortex algal reactor is not dependent on bubble-induced turbulence, much lower carbon dioxide flow rates can be used and the carbon capture efficiency is significantly higher than in conventional algal reactors. Here, we present experimental results (using  Chlorella vulgaris ) and analyses based upon computational fluid dynamics simulations to demonstrate that even at relatively low light-dark cycling frequencies, the Taylor vortex algal reactor offers higher biomass yield than can be achieved in corresponding bubble column experiments. Furthermore, we consider important factors for design and scale up of Taylor vortex algal reactors.