Pattern light simulation and sensing automation of flat plate photobioreactor for sustainable growth and cultivation of Microalgae

Table of Contents

Abstract

Acknowledgements

List of Figures

List of Tables

List of Appendices

1 Introduction
1.1 Statement of Problem
1.2 Thesis Objectives

2 Literature Review of Microalgae
2.1 What are Microalgae?
2.2 What does Microalgae contain?
2.3 Advantages and Disadvantages of Microalgae Cultivation
2.4 Commercial History of Microalgae
2.5 Production techniques of Microalgae
2.5.1 Open-Systems
2.5.2 Closed-Systems
2.6 Comparison between Open-Systems and Closed-Systems
2.6.1 Economic confrontation
2.7 Net Energy Ratio (NER)
2.8 Commercialization of Microalgae
2.9 Future trends of Microalgae

3 Literature Review of Automation, Control System and Sensor Technology
3.1 Automation
3.2 Historical Background of Automation
3.3 Control System
3.4 Automatic Control System
3.5 Mathematical Modeling of Control Systems
3.5.1 Unit-Step Response of First-Order System
3.5.2 Unit-Ramp Response of First-Order System
3.5.3 Unit-Impulse Response of First-Order System
3.6 On-Off Control Action
3.7 Sensor Technology

4 Materials and Methods
4.1 Variables affecting microalgae cultivation
4.1.1 Temperature
4.1.2 Light
4.1.3 pH
4.2 Flat Plate Photobioreactor at Water Research Center for Latin America & Caribbean at ITESM Monterrey
4.3 Selected Sensors and Components for Monitoring & Control System
4.3.1 PAR-Sensor
4.3.2 pH Sensor
4.3.3 Stainless Steel Temperature Probe
4.3.4 SensorDAQ
4.3.5 Precision Mass Flow Controller
4.3.6 Piping and Instrumentation Diagram (P&ID)
4.4 Development of Monitoring and Control System for Flat Plate Photobioreactor with LabVIEW
4.4.1 Stage 1: Sensor testing with LabVIEW
4.4.2 Stage 2: Control cabinet installation with components and Sensors on PBR
4.4.3 Stage 3: Human Machine Interface (HMI) development for Monitoring and Control System with LabVIEW

5 Tests and Results

6 Conclusion and Recommendations

References

APPENDICES

List of Figures

Figure 1 Elements of Microalgae (Schmid-Straiger U. Algae biorefinery–Concept. National German Workshop on Biorefineries, 15 September 2009, Worms)

Figure 2 Open Pond System (Wang, 2013)

Figure 3 Raceway Pond System (Bahadar & Bilal Khan, 2013)

Figure 4 Schematic for Tubular PBR for outdoor cultivation (Dormido et al., 2014)

Figure 5 Schematic of a horizontal tubular photobioreactor (Bahadar & Bilal Khan, 2013)

Figure 6 Kinds of flat plate PBRs (Chruściak, 2011)

Figure 7 Front and Side view of Vertical FPR (Bahadar & Bilal Khan, 2013)

Figure 8 Basic block diagram (Author)

Figure 9 Open-loop block diagram (Author)

Figure 10 Closed-loop block diagram with error signal (Author)

Figure 11 Block diagram of a First-Order System (Author)

Figure 12 Unit-Step Response of First-Order System [44]

Figure 13 Unit-Ramp Response of First-Order System [44]

Figure 14 Unit-Impulse Response of First-Order System [44]

Figure 15 On-Off Control Action (Author)

Figure 16 Flat Plate Photobioreactor (Author)

Figure 17 Top View of Flat Plate Photobioreactor (Author)

Figure 18 Front View Electrical Structure (Author)

Figure 19 Flat Plate Photobioreactor and Electrical Structure (Author)

Figure 20 PAR Sensor

Figure 21 pH Sensor

Figure 22 Stainless Steel Temperature Probe

Figure 23 Sensor DAQ

Figure 24 Cole-Parmer Precision Mass Flow Controller

Figure 25 Piping and Instrumentation Diagram of Flat Plate Photobioreactor (Author)

Figure 26 Wave graphs of Temperature, PAR and pH Sensors – Test run

Figure 27 Front View of Control Cabinet (Author)

Figure 28 Perspective View of Control Cabinet (Author)

Figure 29 Electrical Drawing of Control Cabinet Wiring (Author)

Figure 30 Connection between SensorDAQ and Relay Board (Author)

Figure 31 Connection between SensorDAQ and Precision mass flow controller (Author)

Figure 32 LabVIEW Block Diagram - Start and Read Data

Figure 33 LabVIEW Block Diagram - Statistical analysis

Figure 34 LabVIEW Block Diagram - Create Path, Folder and Excel File

Figure 35 LabVIEW Block Diagram - Creating Folder and Exporting data into Excel File

Figure 36 LabVIEW Block Diagram - Reset Graphs

Figure 37 LabVIEW Block Diagram - Timestamp on Graphs

Figure 38 LabVIEW Block Diagram - Light Controller

Figure 39 LabVIEW Block Diagram - Flow Controller

Figure 40 LabVIEW Block Diagram - Reset Previous Entry

Figure 41 LabVIEW Block Diagram - Timed Loop

Figure 42 LabVIEW Block Diagram - Feedback Control PAR

Figure 43 Front Panel - LabVIEW Feedback Control PAR

Figure 44 LabVIEW - Front Panel of Monitoring and Control System (1)

Figure 45 Front Panel of Monitoring and Control System (2)

Figure 46 Inner working of Control Cabinet

Figure 47 Light groups switched ON

Figure 48 PAR measurement after activating light groups (1)

Figure 49 PAR measurement after activating light groups (2)

Figure 50 PAR measurement after activating light groups (3)

Figure 51 Testing of Flow Controller

Figure 52 Statistical Analysis of PAR in Monitoring and Control System

Figure 53 Exporting Data to excel file

Figure 54 Temperature - 24 hours test cycle

Figure 55 pH - 24 hours test cycle

Figure 56 PAR - 24 hours test cycle

Figure 57 Flat Plate Photobioreactor at ITESM Monterrey

List of Tables

Table 1 Biochemical composition of some microalgae species (Um & Kim, 2009)

Table 2 Comparison of Open systems and Close systems (Cuellar-Bermudez et al., 2014)

Table 3 Commercial value of microalgae product [35]

Table 4 Ranges of Parameters (Lavens & Sorgeloos 1996)

Table 5 Specifications of PAR Sensor

Table 6 Specifications of pH sensor

Table 7 Specifications of Stainless Steel Temperature Probe

Table 8 Specifications of SensorDAQ

Table 9 Measurements of light combination

List of Appendices

Appendix I: Harvesting techniques of microalgae

Appendix II: Lipid Extraction of Microalgae

Appendix III: Life Cycle Assessment (LCA)

Appendix IV: Block Diagram of Monitoring and Control System

Appendix V: Front Panel of Monitoring and Control System

Appendix VI: Cooling System SP 25°C

Appendix VII: Cooling System SP 30°C

Appendix VIII: Front View of Flat Plate Photobioreactor

Appendix IX: Side View of Flat Plate Photobioreactor (1)

Appendix X: Side View of Flat Plate Photobioreactor (2)

Appendix XI: Back View of Flat Plate Photobioreactor

Appendix XII: Step-Response Simulation in Scilab

Abstract

Nowadays, microalgae are considered as a promising new sustainable feedstock due to its higher photosynthetic activity and growth rates compared to other plants. However, its high energy need in the cultivation process, the prevalence of manual work and the high costs deem the production and commercialization of it difficult. Consequently, the challenge of artificial microalgae production is not only to replicate and enhance the optimum natural growth conditions, but to make it automated and profitable. The purpose of this research was to develop an automated system to monitor and control specific growth conditions in order to improve the algae biomass production process. This research required the planning and installation of a control cabinet on a flat plate photobioreactor, components selection, installation of sensors and software programming in LabVIEW. The result of this research was a system that monitors the basic environmental growth parameters, which are temperature, light and pH. In addition to the monitoring system, a control system for light and CO2 flow was integrated to simulate specific growth conditions of microalgae. Further research is required in order to strengthen the idea of a fully automated flat plate photobioreactor for a more efficient microalgae cultivation. This approach may lead to a technology that can be used as a base model for future applications on more reactors.

Keywords: Microalgae, Photobioreactors, Automation, Control Systems, Sensor Technology, LabVIEW programming

Acknowledgements

I would like to express my gratitude to Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM), Mexico, to the Water Center for Latin America & Caribbean and to the University of Applied Sciences Esslingen, Germany for giving me the chance to accomplish my Bachelor´s thesis and to be a part of an excellent and dynamic research team. My gratitude goes especially to Prof. Federico Guedea and Prof. Roberto Parra for their patience, support and outstanding guidance through the thesis. It was an honor to work with you. Furthermore, I would like to thank Prof. Ben Marx for providing me an opportunity to travel to Mexico and to accomplish my studies. In addition, I wish to thank Sara Paulina Cuéllar-Bermúdez and Leonel Peña Angeles for their support and constant motivation during my staying in Mexico. I would like to express my deep gratitude and respect to all those people behind the screen who guided, inspired and helped me for the completion of my thesis. Furthermore, I want to thank to all the members of the Research Center for giving me the feeling being at home and never alone. This thesis will be add as an asset to my academic profile whereas my staying in Mexico enriched my person.

1 Introduction

Due to concerns on climate change and attempts to promote a sustainable environment by exploring new sources for biofuel production, there is a remarkable increasing interest for the usage of microalgae as a sustainable feedstock in the recent years.

Nowadays biodiesel is made of animal fats and vegetable oils, which affects the worldwide food resources, especially in developing countries. Beneficially, microalgae are microorganisms capable of producing lipids, proteins, carbohydrates and other co-products that can be processed for high value products. Therefore, microalgae are demonstrating a candidacy as a feedstock for commercial and industrial applications due to higher photosynthetic activity and growth rates compared to other plants [1]. In addition, microalgae are promising for promoting sustainability as renewable energy sources, which find their applications in human foods, cosmetics, health, pharmacies, aquaculture feeds and biofertilizers [2].

Microalgae can be grown with different cultivation systems such as open systems or closed systems. However, compared to open systems, mostly close systems like photobioreactors (PBRs) have received high research attention in the last years because of their ability to solve issues like pollution, contamination or space requirements [3]–[5]. Furthermore, PBRs provide high regulation and control of environmental conditions such as temperature, illumination and cultivation process and therefore, cost reductions. Nowadays, flat plate photobioreactors (FPRs) are considered as the most effective cultivation systems because of their high illumination surface, excellent temperature control and easy design features [3]. Owing to these advantages, the interest in automating a flat plate reactor has led to new research approaches. Accordingly, some companies such as “Renewed World Energies” or “Algae Lab Systems” started automating PBRs by developing and providing monitoring and control systems for scaling up microalgae production [6], [7]. Even though photobioreactors provide much more advantages over open cultivation systems, still more work has to be done in order to be economically feasible, productive and sustainable. The reason to introduce an automated system is driven by the aim of precise monitoring and control of the cultivation process of microalgae over a period. Because of the constant improvements in the technology, the system should be easy to upgrade in terms of software or other additional components such as sensors or actuators. All in all, the scientific research and analysis of microalgae can improve significantly with a monitoring and control system.

1.1 Statement of Problem

Microalgae production faces high costs due to the high energy need for their cultivation and harvesting process and this difficult the commercialization of it. In addition, cooling systems are required to control the temperature and flow controllers to regulate the CO2 supply, which at the same time regulates the pH level in the cultivation media during the cultivation process. Therefore, improvements in cultivating and harvesting systems are key for microalgae production scale up by automating the cultivation process. Furthermore, the growth of microalgae is affected by basic environmental conditions such as temperature, pH and light. These parameters need to be evaluated in detail to ensure higher biomass production or to enhance the production of specific metabolites such as fatty acids, lipids, proteins or pigments. For application of the foregoing, the Water Research Center for Latin America & Caribbean at Instituto Tecnológico y de Estudios Superiores de Monterrey, Mexico has built a flat plate photobioreactor to grow and analyze microalgae in certain environmental conditions. Artificial microalgae production is needed in order to achieve the best growth conditions and to avoid contamination of other microorganisms, while the control system is required to simulate specific growth conditions for higher cultivation efficiency.

1.2 Thesis Objectives

The objective of the bachelor’s thesis “Pattern light simulation and sensing automation of flat plate photobioreactor for sustainable growth and cultivation of Microalgae” is to develop a user-friendly monitoring and control system for a flat plate photobioreactor by considering the basic environmental growth conditions of microalgae. In order to achieve that, the installation of a control cabinet and the selection of components are needed in order to carry out the monitoring and control operations. The development and implementation of an automated monitoring and control system for an algae photobioreactor eliminates the need of manual work and measurement processes. Furthermore, the labor expenses to complete the work will decrease enormously. In order to analyze microalgae profoundly, data acquisition is needed. To obtain data, sensors will be set and installed. Data collecting, data management and data experiments will be done efficiently by adjusting and selecting parameters in the system that will be built. With a well installed automated system, future errors or complications in the cultivation process of microalgae would be prevented and easily adjusted.

This whole approach guarantees an operating system that provides a user-friendly usage and an achievement of high efficient work. The computer-based human machine interface (HMI), which is a software application that presents information to the user about the status of a process, will give the ability to control the photobioreactor with the user’s instructions. An interface will be made to displayed information in a graphic format (Graphical User Interface or GUI).

2 Literature Review of Microalgae

2.1 What are Microalgae?

Microalgae are a broad and diversified group of aquatic organisms, which can be found in different environments like thriving water, marine and freshwater, saline conditions or sea water [8]. Also, they are very tinny, small, without roots, stems and leaves, which are making them visible just under a microscope [9]. Microalgae are photosynthetic microorganisms. The main aspects are that microalgae can grow very fast and live in tough conditions because of their simple cell structure [10]. Microalgae convert solar energy into chemical energy in the form of lipids, carbohydrates and proteins by performing photosynthesis [9]. Due to the mentioned simple cell structure of microalgae, they are very efficient energy producers by absorbing sunlight, water and carbon dioxide (CO₂) which in fact represents a higher production of biomass and lipids per hectare than any other superior plant [11]. In addition, the yield per hectare of oil from algae is approximately 200 times higher than from the best performing plant or vegetable oil [12]. Currently, around 200,000 – 800,000 species exist of which around 50,000 species are described [13], but only 30,000 have been studied and analyzed [1].

Cultivation of microalgae is possible by considering different factors like temperature, illumination, pH, CO₂ supply, salt and nutrients in open/closed pond systems or photo-bioreactors (PBRs) [8]. Furthermore, microalgae can be used for commercial and industrial purposes. Commercial usages are for example human food, cosmetics, health and biofuels. Sustainability and renewable energy sources are important aspects in the industry where microalgae find their utilization in pharmaceuticals, aquaculture feed and biofertilizers [2].

2.2 What does Microalgae contain?

Due to the high content of lipids, carbohydrates and nutrients contained in Microalgae, they are becoming more and more an interesting source for biofuels, e.g. biodiesel, bioethanol and biomethane as well as for pharmaceutical and nutraceutical products [11]. They contain many important elements like proteins, carbohydrates, lipids and other high value compounds such as pigments, anti-oxidants, fatty acids, vitamins and enzymes. Figure 1 shows the main compositions of microalgae.

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Figure 1 Elements of Microalgae (Schmid-Straiger U. Algae biorefinery–Concept. National German Workshop on Biorefineries, 15 September 2009, Worms)

Microalgae contain calcium, phosphorus, iron and many other mineral elements. Because of their health benefit they are also known as the “Green Super Food”. Their iron values are higher than in our livers and the calcium content is superior than in cow milk [14]. Therefore, dried microalgae can be used as high-protein feeds for shrimps and fish [12]. Table 1 shows the different kinds of Microalgae and their proportions in percentage (%) related with proteins, carbohydrates and lipids. The strain Spirulina maxima has the most share of protein, which is 60-71%. Spiroyra sp. (33-64%) and Porphyridium cruentum (40-57%) are showing the highest share of carbohydrates. Scenedesmus dimorphus has a lipid share of 16-40% [15].

Table 1 Biochemical composition of some microalgae species (Um & Kim, 2009)

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2.3 Advantages and Disadvantages of Microalgae Cultivation

Microalgae have some advantages and disadvantages when used for commercial and industrial applications, compared to other available feedstocks.

One of the main advantages of microalgae is that their cultivation process can occur almost anywhere. Their cultivation requires 130 times less land compared to rapeseed crops or soybean crops [1], [16]. An estimated need of 2 % of land is required to produce the same amount of biodiesel from oil bearing crops [17]. Furthermore, they don’t need any high quality agricultural land to be cultivated and in contrast to other plants, they can be cultivated in non-arable lands [16]–[18]. This advantages are leading to an easy cultivation process with little or no attention while nutrients can be obtained relatively easily [1]. In addition, microalgae provide faster and more productive growth rates than other agricultural crops or plants and they present short growth cycles and high lipid storages [1], [17]. Another important advantage is that microalgae provide a short harvesting life cycle (see Appendix A for more information about harvesting techniques) since microalgae can be harvested in shorter cycles compared to oilseed crops. Therefore, the harvesting and transportation costs are lower than oilseed crops [17]. Most of the other plants are harvested 1–2 times per year whereas compared with microalgae, the harvesting cycle is approx. 1-10 days depending on the process and the cultivation system [1], [16]. Also, wastewater or seawater that is unsuitable for humans can be used to grow microalgae and to trigger a reduction in clean water utilization as well as in carbon footprints related with water treatment. The benefit of cultivating algae in wastewater is that algae obtain nutrients, which reduce the need of fertilizer [19]. Microalgae are able to reproduce themselves by photosynthesis in order to convert sun energy into chemical energy producing high value co-products like proteins, pigments and carbohydrates including anti-oxidant substances for commercial or pharmaceutical usages [1], [17], [18]. Besides all these advantages, microalgae also carry some disadvantages. The biggest drawback that needs to be considered is the difficult harvesting procedure because of their microscopic size. The cultivation in seawater, as mentioned in the paragraph above, reduces the utilization of clean water, however, it brings also some side effects like affecting walls of bioreactors, pumps and valves, which triggers a reduction in lifetime of the components. Furthermore, it also needs to be considered that biomass with salt needs to go through a filtering process that increases the cost and time aspects [3]. Also, cultivation in closed and controlled systems like photobioreactors are much more efficient than in open pond systems since open ponds are susceptible for contamination [3], [18]. However, more energy is needed, which results in an increase of the costs leading to unprofitability. Further advantages and disadvantages of the different systems will be discussed in the section 2.6. Consequently, the production of biodiesel from algae oil is a relative new technology, the large-scale extraction processes (see Appendix II for more information about Lipid extraction) are complex and still in development stage. In addition, the drying process shows difficulties because the usage of sunlight or artificial light brings a lower yield and also artificial light needs much more energy compared to the output energy from the yield [3].

2.4 Commercial History of Microalgae

Microalgae have a long history that started in the late 1800s. Since then, methods and basic culture concepts were developed as well as techniques and processes for algae cultivation, which were described from 1900 until now. Cultivation of Microalgae is approx. 160 years old while the commercial farming is approx. 80 years old. Ferdinand Julius Cohn (24 January 1828 – 25 June 1898), a German biologist, was the first pioneer that published the first report about algae cultivation. After this breakthrough, in 1871 the plant physiologist Famintzin (17 June 1835 – 8 December 1918) grew for the first time green algae in St. Petersburg, Russia by using solutions [20], [21]. The past 50 years have shown a concentrated focus on Research and Development (R&D) in microalgae and therefore the first large-scale cultivations were established in the early 60s by the Japanese company “Nihon Chlorella Ryoho Kenkyukai FukuokaBr” where the species Chlorella were cultivated [1]. The oil crises in the 70’s led some countries like France, Germany, Japan and USA to launch studies in algae fuel in order to increase the energy security and to support the sustainability of feedstocks. For this reason the interest in using microalgae as a renewable energy source increased rapidly, however, the new technology was expensive and related with high development costs. In the 70s, the first cultivation and harvesting process of the species Spirulina were established in lake Texcoco, Mexico and 1977 the companies Dai Nippon Ink and Chemicals Inc. found Spirulina plants in Thailand. Forty-six large-scale companies in Asia were producing more than 1000kg of the species Chlorella per month, which rose to 2000t in 1996, traded only in Japan. From 1978 – 1996, the U.S. National Renewable Energy Laboratory (NREL), through Aquatic Species Program (ASP), launched R&D in order to find out at which point algae were profitable for biofuel production. In favor of reaching this goal, two 1000m2 open pond systems were launched in Roswell, New Mexico, which helped to detect that a biofuel production by using microalgae was more profitable above a price of 60$ per barrel [1], [15], [22]. Furthermore, it helped to detect that microalgae is a low cost renewable energy source that still needs long-term R&D in order to achieve the highest productivity. Nowadays, microalgae are the most promising and important renewable energy source for sustainable living on earth [23].

2.5 Production techniques of Microalgae

Microalgae have the ability to grow in different environments with different cultivation systems, mainly open systems or closed systems. The following paragraphs will provide information about open pond systems and photobioreactors.

2.5.1 Open-Systems

The oldest, easiest and most common mass cultivation system of microalgae are open-pond systems, e.g. raceway open pond systems [9], [17], [24], [25]. Open pond systems are usually located outdoors and use the direct sunlight. Simple cultivation tanks were used to grow microalgae, which are now replaced by more effective raceways ponds [3]. Cultivation with raceway ponds are achieved with paddle wheels and a water depth of approx. 15-20 cm while samples of microalgae, inorganic nutrients and CO2 are added in the water. The mentioned ingredients and the ability of photosynthetic support a rapid growth of algae [9]. The paddle wheels help to circulate the water, mixing and cycling the algae cell [3], [9], [24]. Raceway ponds are used in Israel, USA, China and in many other countries [25].

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Figure 2 Open Pond System (Wang, 2013)

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Figure 3 Raceway Pond System (Bahadar & Bilal Khan, 2013)

2.5.2 Closed-Systems

The usage of closed systems such as Photobioreactors (PBRs) to grow microalgae received high research attention in the last years because PBRs help to overcome limitations and problems with open pond systems, e.g. pollution and contamination [3]–[5]. Furthermore, PBRs provide high regulation and control of the environmental conditions, e.g. temperature [25] and cultivation process, [5], [26] which triggers high biomass production that turns into a more efficient biofuel production and co-product production than the one in open pond systems [4], [24]. Moreover, it is important to mention that PBRs require less space [5] and water loss by evaporation is considerably low [26]. However, heating and cooling systems are required to control the cultivation; this represents higher capital and energy costs compared to the open pond systems ones. [4], [5], [25]. Generally, photobioreactors are constructed with a thin panel of transparent tubes or plates arranged vertically or horizontally and CO2 cylinders. Tubular PBRs are the most used because of their ability to produce high biomass and to perform short harvesting times [3]. Also, Flat Plate PBRs are used for mass growth of microalgae because of their high photosynthetic efficiency compared to Tubular PBRs [4]. In the following paragraphs, horizontal and vertical Tubular PBRs as well as the Flat Plate PBR will be described.

2.5.2.1 Tubular PBRs

Tubular Photobioreactors are the first developed closed reactors for microalgae cultivation [26] and the most advisable types for outdoor cultivation because of their large illumination surface area [5], [27], [28]. Tubular PBRs are constructed either with glass or plastic tubes, which are arranged straight, coiled, looped, vertically or horizontally in order to maximize sunlight absorption. Additionally, it is equipped with pumps or airlift systems to support the mixing process [27], [28]. On the one hand Tubular PBRs involves advantages like high control of environmental conditions, e.g. temperature, pH, water supply, mixing regime, etc., but on the other hand it needs more energy, which increases the cost aspect negatively [26], [28].

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Figure 4 Schematic for Tubular PBR for outdoor cultivation (Dormido et al., 2014)

2.5.2.2 Horizontal Tubular PBRs

The main usage of Horizontal Tubular PBRs is in photosynthetic bioprocesses for cultivating microalgae by using solar energy for biomass production [29]. These kinds of systems provide high control of cultivation conditions and efficient light utilization enables operation in continuous mode [29] while the chances of contamination are very low [30]. Furthermore, this system can be used indoors or outdoors and provides a large illumination surface area [31]. Horizontal Tubular PBRs are made of straight tubes, thin plates with partition walls, transparent materials, e.g. plastic or glass. Applications of this system are found in cultivation of the species Spriulina and Chlorella. The biggest industrial Horizontal Tubular PBR was built in the year 2000 near to Wolfsburg, Germany where the cultivation of the species Chlorella took place [29].

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Figure 5 Schematic of a horizontal tubular photobioreactor (Bahadar & Bilal Khan, 2013)

2.5.2.3 Vertical Tubular PBRs

Vertical Tubular PBRs such as bubble column or airlift bioreactors are mainly used to cultivate microalgae because of their simple construction and operation [30], [31]. Bubble column or airlift bioreactors are usually made of cylindrical vessels consisting of polyethylene or glass tubes [30], [31]. The airlift reactor circulates the growth medium in a reservoir without moving any parts [3]. Bubble-Column reactor is bubbling air at the bottom of the reservoir in order to support an efficient mixing process. Nowadays, polyethylene bags are used because of their low costs and high transparency [30]. Usage of Vertical Tubular PBRs guarantee higher yields than horizontal tubular PBRs because of their hydrodynamics, low energy consumption and good mixing with low shear stress [27], [31]. In addition, the mentioned system is compact, inexpensive, simple to construct and it is very promising for large-scale cultivation of microalgae [31]. Besides the benefits, the system has drawbacks like small illumination surface area and high air pumping costs [5], [27].

2.5.2.4 Flat Plate Reactor

In the 80s, Flat Plate Reactors (FPRs) were considered expensive because of their high developing costs [30], but nowadays they are considered as an effective cultivation system for microalgae [3]. FPRs received high attention because of their high illumination surface area and optimum usage of sunlight energy [1], [3], [17], [27], [28], [30]. This system is made of transparent materials for the optimum usage of sunlight and narrow panels to achieve high area-to-volume ratios [27], [30]. Usage of FPRs for large scale cultivation of microalgae can be performed outdoor or indoor [1], [17], [27] in order to reach high photosynthetic efficiency [27] and high productivity [30]. On the one hand, it provides excellent temperature control, is easy to clean up and has easy design features. On the other hand, however, it represents high mixing and installation costs [1], [3], [17]. According to publications, it is difficult to scale up, but vertical flat plates with a volume of 1000 – 2000L can be used for long period operations, which prompts a potential for scaling up [1].

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Figure 6 Kinds of flat plate PBRs (Chruściak, 2011)

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Figure 7 Front and Side view of Vertical FPR (Bahadar & Bilal Khan, 2013)

2.6 Comparison between Open-Systems and Closed-Systems

As it is described in the previous chapters, open and closed systems are demonstrating many advantages and disadvantages. The following chapter will contrast both systems with their advantages and disadvantages. Additionally, a confrontation of the economic aspects will be shown.

Before starting the cultivation of microalgae, it is very important to evaluate and select the right cultivation system, which depends on the properties of the species, climatic conditions, cost of land and water availability among others [17] that can be realized with an Life Cycle Assessment (see Appendix III). Open systems like raceway open pond systems are the most commonly used cultivation systems because of their cost effectiveness and simplified operation mode compared to closed systems [17], [24]. The major advantage of raceway open pond systems against closed systems is that the cultivation could be realized in marginal or non-arable land, which makes the cleaning and maintenance process easier, which in fact uses less energy compared to closed systems [3], [17]. However, closed systems present much higher productivity because of, as mentioned before, their ability to control the environmental conditions [4]. In open systems it is common that microalgae are contaminated by other microorganisms whereas the closed systems prevent this issue [3], [4]. Nevertheless, open systems are mainly used to produce high-value products for pharmacy and cosmetics that are more feasible in closed systems [24]. Regarding to several publications, the highest output and the best performance could be achieved by combining open systems with closed systems. Table 2 shows a particularly comparison between microalgae production in raceway open ponds and closed photobioreactors.

Table 2 Comparison of Open systems and Close systems (Cuellar-Bermudez et al., 2014)

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2.6.1 Economic confrontation

Production of microalgae shows important aspects with respect to sustainability, but it is also important to consider the economic aspects for scalable and sustainable processing. Costs of closed systems such as PBRs are much higher than open systems because of their special designs and high energy need. The capital investment of PBRs are approximately 10 times higher than raceway ponds, but in contrast it performs better in terms of productivity [23]. In the publication of [23], it is reported that the production costs of dewatered biomass are around $5.05 to $5.27 US/kg by using tubular PBRs. Furthermore, it is mentioned that the mixing costs for raceway open ponds are around $0.10 US/kg, for tubular PBRs $1.61 US/kg and for FPRs $3.93 US/kg. According to [32], production costs of biomass in open ponds are 4.95 EUR/kg and in FPRs 5.96 EUR/kg, which underlines the fact that PBRs are more expensive than raceway open pond systems. In order to achieve the highest output of the microalgae production, it is essential to decrease the energy input and the costs of PBRs by automating flat plate reactors, using wastewater for the cultivation process and recycling part of the biomass [23].

2.7 Net Energy Ratio (NER)

The Net Energy Ratio is used to demonstrate the relationship between input and output energy in order to show the efficiency of a technology. Production techniques have been compared with the NER to choose the most effective technology in terms of energy usage [3]. A NER higher than one represents a high input energy compared with the output energy, affecting productivity and economical aspects. Equation 1 defines the NER for microalgae production as the sum of energy input in cultivation, drying and oil extraction process divided by the energy content of dry biomass. The NER of all existing PBR systems are higher than one because of their high energy need. However, the flat plate reactor has shown the best performance with respect to the energy usage [3].

Equation 1 – Net Energy Ratio

2.8 Commercialization of Microalgae

Microalgae are showing high potential for commercial applications in the whole world. In recent years microalgae has aroused the interest of many biofuel companies as a possible source for biofuel production since microalgae contain high value of oil and perform a fast cultivation process [2], [33]. Currently, biofuels are made of animal fats and vegetable oils, which effects strongly the world food market [33], mainly in developing countries, were food is limited. Nowadays, over 50 algae biofuel companies exist over the world, but none of them are producing in large-scales because of the high production costs [33]. As mentioned before, microalgae are also used for health, food, supplements, cosmetics, animal feeds, wastewater treatments, biofertilizer and pharmacy [2]. The most commercialized algae species are Chlorella, Spriulina and Dunaliella because of their higher nutrients and protein values. These species are mainly served in tablets, capsules, liquids, pasta, snack food and juice drinks [33]. Algae biomass costs between 5 EUR/kg – 50 EUR/kg and more than 10.000 dry tons of microalgae are produced per year worldwide. As a result, it is predicted that the development of large scale technology and artificial cultivation systems will contribute to the worlds energy need and support the sustainability [34]. Table 3 shows a brief summary of the commercial products and their prices.

Table 3 Commercial value of microalgae product [35]

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2.9 Future trends of Microalgae

The future of the usage and cultivation of microalgae shows a promising perspective. Companies worldwide tend to use more and more these particular microorganisms for commercial purposes such as biofuel production [36]. Microalgae are potential substitute for renewable feedstock for different industries without harming the environment. As microalgae form the basis of the food chain, they have potential applications in disease prevention as most of the diseases are coming due to poor eating habits [35]. In addition, microalgae will be used more in the future in the food industry because of their healthy components. Furthermore, microalgae will be used in cosmetic industries and environmental applications because of their ability to absorb carbon dioxide from the atmosphere [35]. However, as mentioned before, the production costs of these microorganisms are currently very high, so the future orientation will be to develop new systems and techniques to improve the production process of microalgae by analyzing them with special monitoring and control systems and to become more economical and productive.

3 Literature Review of Automation, Control System and Sensor Technology

3.1 Automation

Automation is a word, which originates from the Greek “Auto” (self) and “Matos” (moving) [37], which is a technique to transform processes or to make systems operating automatically. Automated systems provide the possibility to manage and automate hardware and software with different programming languages, such as JAVA, C or C# and at the same time with system-design platforms such as LabVIEW. Systems, which are automated need as input information from the process through hardware such as sensors and it needs actuators to influence the process in the desired way [38]. Automation provides cost effectiveness, competitiveness, flexibility and productiveness within a company or organization. Nowadays, it is essential that a company or organization operates fast, reliable and qualitative to reach the maximum productivity. Automation is a combination of control system and information technology whereby the need of human action is reduced to the minimum. Automation Control System is a system, which is able to control a process with minimal or no human impact and has the ability to initiate, to adjust or to measure the variables in the process and to stop the process to obtain the desired output.

3.2 Historical Background of Automation

The birthplace of automation was basically in the industry and trade, but in 1769, James Watt introduced the steam engine that was an important milestone in the history of automation. For the first time, human or animal work were replaced by a machine [39]. However, the word “automation” was used for the first time in 1936 [40] and therefore the history of automation with special solutions such as mechanical gaming machines, machine control or military applications covers a period of more than 70 years. Nevertheless, this discipline made a significant contribute to the overall progress of humanity. Today, a world without automation is hardly imaginable because without automation we wouldn’t have efficient productions, facilitated work activities, improvements in comfort and security of the people, high safety standards in the automotive industry, great success in the science field, etc. [41].

3.3 Control System

Control system is a system where the components of a process are interconnected together and perform through a configuration the desired output [42]. The traditional control systems are made of sensors, actuators, computers and software [43] and the most common controlled industrial parameters are temperature, pressure and flow [44]. Before designing a control system model and applying it to an actual system, it is essential to set the ranges of the errors in order to avoid any instability. This method is called Robust Control Theory, which is a mathematically complex method. The error signal is needed to ensure that the system will work in the best and desired way. The first step to design a control system is to develop a mathematical model and to define the specifications of the parameters to be controlled precisely [44]. Processes or components which need to be controlled, can be illustrated in a understandable block diagram, which represents the functional activities and interconnecting signal lines to show the output of a system [45]. Figure 8 represents a basic block diagram with an input and an output.

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Figure 8 Basic block diagram (Author)

Control system differs from automation simply because control is the operation of a device that requires interaction from the user, mostly in the form of a single action. Additionally, automation systems may include control systems, but not the other way around. Control systems could be a part of automation systems. There are different kinds of control systems, for example, open-loop systems without feedback and closed-loop systems with feedback [42]. In open-loop control systems, the control action is independent of the system output; that means that the process output is totally independent of the controller output. Open-loop systems are economical and simple in construction, but at the same time, changes in output cannot be done automatically [46]. Fig.9 shows an open-loop system.

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Figure 9 Open-loop block diagram (Author)

Closed-loop systems realize a feedback signal by measuring the actual output and comparing it with the desired output. Basically, a system is closed if the systems are in a cycle. This system provides the ability to adjust the correlation of one process variable to another one by comparing and measuring them. The usage of a precise sensor allows the possibility to assume that the measured output is approximately equal to the actual output of the system. Closed-loop systems are highly accurate because of an error signal, which is the difference between reference input signal and feedback signal, and can be delivered to the controller to reduce the error and to achieve the desired output [46]. Fig. 10 illustrates the block diagram of a closed-loop system with an error signal.

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Figure 10 Closed-loop block diagram with error signal (Author)

3.4 Automatic Control System

A predetermined closed-loop control system is also called an automatic control system, which requires minimum human activity. That means that the process is still in the normal range of the control system. For developing an automatic control system, two process variables are needed:

- Controlled Variable
- Manipulated Variable

Controlled variable is the process variable which is maintained at a specified value or within a specified range. Manipulated variable is the process variable that maintains the controlled variable at the specified value or within the specified range [45].

An automatic control system compares first the output of a closed-loop system with the desired input by sensing it, calculates the difference and finally produces and sends an error signal to the controller that will reduce the difference in order to achieve the desired output. On the one hand, it is essential that the sensor is converting the measured variable into a suitable variable that can be compared with the input variable such as pressure, voltage, etc. On the other hand, the input variable needs to be the same unit as the sensor signal [42].