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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Date
2014
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Massey University
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Abstract
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.
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Keywords
Algae culture, Microalgae, Effect of temperature on