Biodiesel is a fuel similar to diesel that is obtained from oil rich plants such as reapeseed, soy, palm oil, sunflower or used cooking oils or phototrophic microorganisms. It has the advantage of producing lesser amounts of green house gases than fossil fuels. The main reason for the success of biodiesel is because it can be used without modification to engines and distribution systems.
Biodiesel has some of the same problems as bioethanol. It can start a competition for land with other agricultural crops, causing decrease in food supply or increase in food prices. Hence, the focus is on microalgae and cyanobacteria to produce biodiesel. Since, the microbes can grow in saline environments, they are not as much a threat to food crop cultivation. These methods for production of biodiesel from microbes are still experimental and slow. Developments in bioreactor design and genetic modification of the microbes may make these methods more viable in the future.
Growing the microbes required for biodiesel production requires photobioreactors as the microbes get their energy by photosynthesis. The photobioreactors can be mainly classified into open and closed systems. Open systems are lakes and natural ponds which can be used to grow the microorganisms. Closed systems are tubular or flat panel shaped bioreactors. The tubular bioreactors can be horizontal or vertical. Closed bioreactors have the advantage of not being contaminated and can be easily controlled. Open bioreactors have cost benefits.
The design of the bioreactor is driven by various factors such as light considerations, gas exchange, nutrient availability, product recovery and contamination. Proper mixing is required to ensure time for both dark and light reactions to occur. Cooling is required to remove the heat due to high irradiation. Too much light is observed by the cells at the surface of the culture and lost as heat, this is known as the shading problem. Genetic engineering changes to the cells to have smaller photosynthetic antenna seems to reduce this problem considerably.
Idea: The shading problem can be overcome by having cells of two different types in the reactor. The first type of cells do the light reaction and are positioned at the surface of the culture. Second type of cells do the dark reaction below the surface. The two cell types interact and exchange the products of their respective reactions through the medium.