Dynamic simulation of microalgae growth and microclimate in large-scale closed photobioreactors

Microalgae are considered as the most promising feedstock for biofuel production of the future. The most efficient way to produce vast amounts of algal biomass is the use of closed tubular photobioreactors (PBR). In contrast to open pond systems, closed PBRs provide a controlled environment with a high surface-to-volume ratio, regulated inputs and outputs, and contamination free growth medium. However, for the commercial large-scale (multiple hectares) use of PBRs, economics are still critical. One major concern is the heating or cooling requirement of the systems since the best algae growth rates are obtained at about 25-30°C, depending on the specific strain. A dynamic mathematical model was developed to simulate heating and cooling energy requirements of a horizontal tubular PBR for any desired location. Operating the model using hourly weather data as input, heating and cooling loads can be calculated early in the planning stage of a project. Furthermore, the model makes it possible to compare the operation inside a greenhouse to the outdoor operation, and consequently provides fundamental information for an economic feasibility study. Simulation of different PBR row spaces, tube diameters, lengths and distances between the tubes provides essential information for energy studies as well as efficient use of space. The best configuration for a specific location can be evaluated easily. Preliminary results proved the importance of a simulation model and the impact of a sophisticated heat management on economics. Energy savings due to an optimized heat management system will eventually increase proficiency of the systems, and support a new sustainable industry and fuel source to grow further. The model was exemplary tested for a virtual 100,000-L photobioreactor located in San Luis Obispo, California, USA. Average algae productivity rates of 23 and 67% for outdoor and indoor PBR operations, respectively, were obtained before providing supplemental cooling or heating for optimal thermal environmental conditions.