Efficient Use of Photons in a Photobioreactor

Photosynthetic microorganisms have been used to produce various products and by products ranging from waste water treatment, carbon dioxide fixation, pharmaceuticals, food additives, biofuels etc with varying degrees of success. One of the main impediments for large scale production of these products is an efficient scale up of a system which can provide the environment required for these photosynthetic organisms to undergo photosynthesis and grow. The system which provides the required environment for this photosynthetic process is known as a photobioreactor. Up until now most of the production is restricted to large open pond or raceway systems with a limited number of closed systems. Open systems which include open ponds and race ways require large land areas and tend to grow low density cultures. To achieve high density cultures, closed systems should be developed. For such a system to be developed, three main elements of the system (light, photosynthetic microorganism and nutrients) need to be optimized incorporating concepts from multiple disciplines. This dissertation deals only with one main element, light optimization. To understand light optimization, an understanding of the photosynthetic microorganism, the photosynthetic process and how energy is utilized by the photosynthetic microorganisms is explained. It is essential to understand the complexity of the photosynthetic microorganism before attempting to optimize the environment. The dissertation describes light in photon units and evaluates different artificial light sources: white cool fluorescent, incandescent, gro-lux, halogen and light emitting diodes. From this evaluation it can be concluded that light emitting diodes are the best possible light source for converting energy from electricity to useful photons which are defined by the antenna pigments of the photosynthetic microorganism. The photons produced by a light source are limited. To optimize usage of these photons, two methods are proposed (one) development of light guides which would be embedded into the acrylic sheets making up the PBR wall and (two) by mixing the nutrient solution there by moving photosynthetic microorganisms in and out of light regimes. Different light guides are evaluated based on the finish achieved on the light guide ends. Cone shaped light guides are evaluated for their ability to disperse photons. The photobioreactor used in the analysis was a rectangular bubble column reactor. To evaluate hydrodynamics of this reactor the bubble sizes needs to be measured. Bubble images were captured by a high speed camera and a method to measure their size via image processing was devised. For a flow rate of 2 liters/min a bubble size distribution between 1 to 3 mm was identified with 1.6mm for the peak of the distribution (greatest number of bubbles were this size). All the knowledge developed in this dissertation is applied towards design of a system for chlorella species photosynthetic microorganism as an example