Modelling the impact of temperature on microalgae productivity during outdoor cultivation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering at Massey University, Palmerston North, New Zealand

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Massey University
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Accurate predictions of algal productivity during outdoor cultivation are critically needed to assess the economic feasibility and the environmental impacts of full-scale algal cultivation. The literature shows that current estimations of full-scale productivities are mainly based on experimental data obtained during lab-scale experiments conducted under conditions poorly representative of outdoor conditions. In particular, the effect of temperature variations on algal productivity is often neglected. The main objective of this thesis was to develop a model able to predict algal productivity under the dynamic conditions of temperature and light representative of full-scale cultivation. In a first step, models were developed to predict broth temperature as a function of climatic, operational, and design parameters. The model developed for open ponds could predict temperature at an accuracy of ±2.6oC when assessed against experimental data collected in New Zealand over one year. The temperature model developed for closed photobioreactors was accurate at ±4.3oC when compared to experimental data collected in Singapore and New Zealand over a total of 6 months of cultivation. This second temperature model was then applied at different climatic locations to demonstrate that actively controlling temperature would seriously threaten the economics and sustainability of full-scale cultivation in photobioreactors. To quantify the impact of temperature variations on biomass productivity, a productivity model was developed using Chlorella vulgaris as a representative commercial species. To determine the best methodology, a review of more than 40 models described in the literature revealed that an approach accounting for light gradients combined with an empirical function of temperature for photosynthesis and first-order kinetics for respiration would offer the most pragmatic compromise between accuracy and complexity. The model was parameterized using short-term indoor experiments and subsequently validated using independent benchscale indoor (> 160 days) and pilot-scale outdoor (> 140 days) experiments, showing prediction accuracies of ± 13%. The outdoor data set was obtained from 13 different experiments performed in 4 different reactors operated under various regimes and climatic conditions. The productivity model was found to be accurate enough to significantly refine previous assessments of the economics and the environmental impacts of full-scale algal cultivation. The productivity model was then used in different case studies in order to investigate the impact of location/climate, design (pond depth or reactor diameter), and operation (hydraulic retention time or HRT) on productivity and water demand. Although the qualitative impact of the HRT on process was already known, this application enabled the first quantification of the HRT value on the productivity. Low HRT values around 3 days were found to maximize productivity at most locations investigated but these operating conditions were associated with a large water demand, illustrating a poorly acknowledged trade-off between sustainability and revenues. The model was also used to demonstrate that actively controlling the pond depth can increase the productivity by up to 23% while minimizing the water demand by up to 46%. This thesis therefore revealed that the choice of a location for algal full-scale production must be based on the comparison of optimized systems, contrarily to current assessments assuming the same design and operation at different locations.
Algae culture, Microalgae, Effect of temperature on