Control systems development for the optimization of outdoor microalgae raceway cultivation systems

Oseikhuemen E. Isiramen

The cultivation of microalgae has numerous benefits to clean energy production, wastewater treatment, carbon dioxide fixation, production of pharmaceuticals, energy generation, etc. As a result, substantial monetary and human resources have been committed to microalgal studies. Currently, mass cultivation of microalgae is mainly achieved with the use of raceway ponds due to their economic superiority over closed photobioreactors. Raceway ponds used for microalgae cultivation are generally limited by light, temperature, pH, dissolved oxygen, turbidity, depth, location, mixing approach etc. Therefore, it is important to continually investigate optimum conditions that would provide for enhanced microalgae productivity. From relevant literature, most of the cultivation optimisation approaches being investigated or proposed for enhanced microalgae productivity are design focused. That is, most of the work is directed towards making design adjustments to the pond (e.g., depth change, replacement of traditional paddlewheels with pumps for mixing, etc.). Apart from the standard regulation of pH in raceway ponds using feedback on/off controllers, there is little to no focus on research concerning the regulation of some of the key environmental factors (e.g., temperature) that greatly influence raceway productivity. Additionally, the use of more advanced/improved control systems for pH control and where possible multivariable (pH and temperature) control for microalgae cultivation is an area that is lacking in the literature. The conclusion from the detailed review study presented in Chapter 2 was that the application of appropriate control strategies in the enhancement of multiple industrial processes, bioprocesses, and photobioreactor systems, for the cultivation of microalgae in outdoor open ponds would yield appreciable outcomes. To this end, a series of research studies was developed to address some of the neglected control related areas that greatly influence microalgae productivity. Specifically, the two main areas of focus were pH and temperature control. Details of the major aims, objectives, and relevant research questions of this thesis are presented in Chapter 1. Following the review study, the potential benefit of using a more improved control strategy for regulating the pH levels in raceway ponds was investigated (Chapter 3). To do this, an innovative proportional-integral (PI) + dead-zone control strategy was developed. This was done to address the pH control issue and to overcome the negative environmental impact and challenges associated with the currently employed on/off pH stat control schemes used for microalgae cultivation. Scenedesmus sp. grown in anaerobically digested abattoir effluent (ADAE) in two outdoor paddlewheel-driven raceway ponds was used to determine the effectiveness of the improved control strategy with respect to the on/off control strategy. The results obtained from the study showed that the PI + dead-zone control strategy significantly improved the regulation of the pH levels in the pond with respect to the on/off control strategy. The total integral squared error (ISE), integral absolute error (IAE), integral timed square error (ITSE), and integral timed absolute error (ITAE) controller performance indicators were reduced by a noteworthy 94.94%, 73.87%, 95.9%, and 73.74% respectively with regards to the on/off control scheme. Also, the advanced control strategy reduced CO2 usage by a remarkable 59.21% (highest recorded in related literature), resulting in a reduction in CO2 associated costs and most importantly, a reduction in the amount of greenhouse gas lost to the atmosphere. Furthermore, the advanced control scheme did not have an adverse effect on biomass productivity. In Chapter 4, details of the study involving the control of the other environmental factor (temperature) that greatly influences the cultivation of microalgae in raceway ponds are presented. The aim of the temperature control study was to determine if it would be possible to develop a temperature control system in conjunction with pH control (multivariable control) that would improve the biomass productivity of raceway ponds in colder periods of the year while keeping the energy requirements minimal. An experiment was conducted outdoor using eight raceway ponds. Seven of the ponds were fitted with various temperature control regimes ranging from all-day (24 h) to daytime-only temperature control and at different temperature levels ranging from 15°C to 25°C. One of the ponds was left with no temperature regulation to make it possible to evaluate the benefits of the various control schemes with a traditional raceway pond. From the results obtained, an approximate 60% increase in biomass productivity was observed for ponds operated at a minimum temperature of 15°C coupled with a 24 h (T15P24) and 12.5 h (T15P12.5) control period in relation to the uncontrolled temperature pond (UTP). This means that by using lower temperature set-points coupled with windows of control rather than 24hrs control, the energy requirement of the heating system would be minimal. In other words, significant productivity improvements were observed for the systems with lower energy demands. In general, the ponds regulated at a minimum of 15°C used 60% less energy for heating, were more efficient in nutrient removal and showed significantly higher biomass productivity with respect to the uncontrolled/higher temperature regulated ponds. This study also involved the cultivation of Scenedesmus sp. in ADAE. Following the temperature control study was an economic analysis carried out to determine the possibility of implementing temperature regulation on a large scale for raceway cultivation systems used for wastewater treatment (Chapter 5). The economic study followed a step-by-step procedure to; a) determine suitable animal farming locations within Australia where temperature regulation might be feasible, b) estimate the various heat load of the different locations considered, c) estimate the expected microalgae productivity/treatment capacity based on the considered locations, d) determine the best heating approach/scenario for an economical temperature-regulated treatment system, and e) perform financial analysis to compare the various thermal regulated scenarios to the traditional non-thermal regulated wastewater treatment ponds. The result obtained showed that for a location with already existing biogas facilities, the heat energy obtained from the methane combustion in the plant is more than sufficient to provide the necessary energy required for regulating the temperature of the wastewater treatment ponds. With the temperature-regulated system, the estimates in the study showed that the required cultivation area would be reduced by approximately 21%, the overall unit cost of microalgae production would reduce by approximately 18%, and the annual biomass produced would increase by approximately 26%. This translates to a much more reliable/efficient wastewater treatment facility where the energy from the waste (methane) is used to improve the treatment of the waste produced by the anaerobic digestion process. Additionally, the steps followed in the study were outlined such that it can be replicated for any other given location where wastewater treatment via microalgae cultivation is necessary. The major outcomes of this thesis are summarised in the final Chapter (Chapter 6). After a review of the literature and identification of neglected areas in the field of study, multiple studies were conducted to provide valuable inputs to those neglected areas. The pH analysis provided a much better control system for regulating pH levels in algal raceway ponds, the temperature control analysis provided valuable insights on how temperature regulation of raceway ponds can be achieved with low energy demands, and the economic analysis showed that there are suitable locations/scenarios where multivariable control (pH and temperature) can be deployed for enhanced commercial microalgae cultivation.

Control systems development for the optimization of outdoor microalgae raceway cultivation systems

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