(440g) Design of Raceway Ponds for the Production of Algae Biomass

Global warming and scarcity in fossil fuels have driven an interest towards producing biodiesel from algal biomass. However, substantial challenges are encountered in deriving fuels from algae. Primary amongst them is the relatively higher costs of cultivation. Although there are several cultivation techniques that are commercially viable, this presentation focuses on designing optimum outdoor cultivation technology that considers the effect of both sunlight and temperature on algae growth. Raceway ponds are closed loop, oval shaped recirculation channels with paddle wheel that enables circulation and prevent sedimentation. In this work, raceway ponds are designed based on a given geographical location and algae species. Culture properties such as biomass productivity, growth rate, and concentration, and physical properties such as mass flow rate, average velocity, and pond temperature are estimated for a whole year depending on the dynamic behavior of zenith angle, diurnal pattern of solar irradiance, and temperature fluctuations at the location. The designs are modeled as nonlinear programming formulations with about 4100 variables and 3800 constraints. The objective is to meet a specific biodiesel production demand at a minimum net present sink over 10 years. Net present sink includes both equipment and operating costs. Operating costs involve the cost of electricity used for pumping algae biomass. Constraints connect culture properties and raceway pond geometry through experimentally validated growth model [1], temperature model [2], and irradiance model [3]. Maximum specific growth rate of algae biomass is constrained by temperature of the raceway pond, which in turn is constrained by geometry of the raceway pond and surrounding air temperature. Air temperature is calculated based on parameters such as maximum and minimum temperatures of the day along with sunrise time. Biomass concentration is constrained by the available irradiance at a location and geometry of the raceway pond. Solar irradiance at a location is calculated based on the zenith angle. Biomass productivity is calculated from growth rate and biomass concentration. In order to analyze the influence of the type of algae strain and geographical location on these culture properties and geometry of the raceway ponds, case studies with two algae species (P. Tricornutum and I. Galbana) and four locations (Tulsa, USA; Hyderabad, India; Cape-Town, South Africa; and Rio-de-Janeiro, Brazil) are investigated. The resulting eight Nonlinear Programming (NLP) models: Case 1 – P. Tricornutum, Tulsa, USA, and Raceway pond design, Case 2 – P. Tricornutum, Hyderabad, India, and Raceway pond design, Case 3 – P. Tricornutum, Cape-Town, South Africa, and Raceway pond design, Case 4 – P. Tricornutum, Rio-de-Janeiro, Brazil, and Raceway pond design, Case 5 – I. Galbana, Tulsa, USA, and Raceway pond design, Case 6 – I. Galbana, Hyderabad, India, and Raceway pond design, Case 7 – I. Galbana, Cape-Town, South Africa, and Raceway pond design, Case 8 – I. Galbana, Rio-de-Janeiro, Brazil, and Raceway pond design, are solved using GAMS 24.2/CONOPT solver. The problems are initialized thousand times using Latin Hypercube Sampling. Preliminary results show that with the model parameters used and a demand of 1 ton/year of biodiesel, Cases 1 through 8 require areas of about 29.99 ha, 25.36 ha, 35.50 ha, 21.92 ha, 31.14 ha, 26.20 ha, 37.03 ha, and 22.89 ha respectively to meet the demand. These results are comparable to the designs presented in [4]. To analyze the impact of algae species over the raceway pond geometry, notice that Cases 1 through 4, where P. Tricornutum species is cultured, have lower area than Cases 5 through 8, where I. Galbana species are cultured. P. Tricornutum species have higher lipid content than I. Galbana species and hence requires lower volume to meet the demand. In order to analyze the impact of location on the geometry of raceway pond, notice that Cases 4 and 8, where algae is cultured at Rio-de-Janeiro, Brazil, occupy lower area than other locations. Rio-de-Janeiro, Brazil has higher temperatures and algae growth is favorable at these high temperatures. In this work, a novel mathematical model is developed to estimate the best combination of algae species, geographical location, and raceway pond geometry by combining experimentally validated temperature, irradiance, and algae growth models with optimization. Benefits of such a method include realistic design of raceway ponds that produce the desired amount of biodiesel cost efficiently.

REFERENCES
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