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Light is of one the most essential source that affects the autotrophic growth of microalgae. With light, carbon dioxide and water, microalgae can produce its own energy through a process called photosynthesis, where oxygen (O2) is usually the by-product.
This project involves investigating the growth of microalgae, in the presence of the greenhouse gas CO2, using LED technology lighting systems. The objective of this project is to construct an array of LED lighting systems and investigate the potential arrangements of the lighting suitable for cultivating algae (internal lighting for bioreactors and external lighting for growing in small flasks). LED lighting systems have great potential for the growth of algae, as this allows these systems to escape the traditional reactor geometries (tubular reactors) because of their dependency on light. The project will therefore further investigate the potential impact of a bioreactor design and operation and to characterise the heat transfer of such systems to a bioreactor.
Chapter 1 - Introduction to LED and Microalgae
Microalgae are among the fastest growing autotrophs on the earth, which utilize commonly available material for growth. [carotenoid production from microalgae].
In the world of photoautotrophic microalgal cultivation, light is one of the major energy sources for the growth of cells and is one of the most important factors that affects the autotrophic growth of microalgae. Since the photons of light could be absorbed by the microalgal cells as nutrients, the properties of light source, such as wavelength and intensity are definitely critical for the growth of photoautotrophic microalgae [1,2]. This also means that the specific growth rate of algae could be greatly influenced by the light source [3,4].The concept of using LEDs to grow algae has been an ongoing process for many years as LEDs are so efficient. Compared to the conventional tubular discharge lamps and light bulbs, LEDs with the characteristics of narrow band wavelength and low power consumption are considered the optimal light sources for algae growth .
Microalgae are particularly attractive as natural sources of bioactive molecules because algae have the potential to produce structurally complex compounds [6,7]. The growth of microalgae and the composition of microalgal biomass are known to be greatly dependent on the type of light supply (light source and light intensity) , medium composition, such as carbon sources [9-11], and the growth conditions (e.g. pH, temperature, oxygen removal) . Since light supply is known to play a pivotal role in the efficiency of microalgal growth [13, 14], the effect of light sources (i.e. blue and red LEDs) was investigated. [Effect of light supply and carbon source on cell growth and cellular composition of a newly isolated microalga Chlorella vulgaris ESP-31]]
Light-emitting diodes (LED) have been around for many years and are found in all kinds of devices. LED is solid-state semiconductor device that allows electric current to flow through in only one direction and emits light when sufficient current flows through the object. In recent years, LED devices have become widely used in many types of equipments and systems. Their field of application range from popular consumer electronic equipments to LED based photobioreactor.
Researches in new technological advances have led to new LED materials and improved production processes have produced brighter LEDs in colours throughout the visible light spectrum, including white light, with efficiencies greater than incandescent lamps. These brighter, more efficient and colourful LEDs move LED technology into a wider range of lighting applications .
LED was first developed in 1962 by Nick Holonyak Jr., while working at General Electric Company, and was introduced as a practical electronic component which emits visible low-intensity red light . The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators . In 1976, T.P. Pearsall created the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths .
LEDs are, in contrast to incandescent lamps, cold light sources. LEDs are small, lightweight, durable and efficient in terms of longer operating life. LEDs are designed to give of light when electrons pass through them. The amount of light given off is very high compared to the amount of power used. Unlike ordinary incandescent bulbs, LEDs do not have a filament that would burn out and LEDs do not get especially hot. LED technologies are increasingly being used for a wide range of applications such as lightings, displays, indicator lamps, billboards and signs and even in phycological and biological researches.
LED Structure (Design)
The basic structure of a LED consists of the semiconductor compound (commonly referred to as a die), a lead frame on which the die is placed, and the encapsulation epoxy surrounding the assembly (see Figure xx). The LED semiconductor chip is supported in a reflector cup joined into the end of negative electrode (cathode), and in the typical configuration, the top face of the chip is connected with a bonding wire to the positive electrode (anode) . The anode lead is, in general, longer than the cathode lead.
Several junction structure designs require two bonding wires, one to each electrode. In addition to the obvious variation in the radiation wavelength of different LEDs, there are variations in shape, size, and radiation pattern. The typical LED semiconductor chip measures approximately 0.25 mm2, and the epoxy body ranges from 2âˆ’10 mm in diameter. The body of the LED is usually round, but they may be rectangular, square, or triangular .
Figure xx: The typical structure of an LED lamp.
How LED work
Light is generated in the semiconductor chip, a solid crystal material, when current flows across the junction of the different materials . The spectral quality of the emitted light, wavelength and therefore the colour of the light, depends on the composition of the materials used in the semiconductor chip and the operating temperature. The chemical element, gallium, is an essential component of most LEDs, is heavily consumed for LED production.
LEDs are based on conventional diodes and consist of two fused semiconductors, the P-type semiconductor and N-type semiconductor, operated by means of a forward bias current. From figure xx, the grey shaded region on the left is the P-type semiconductor and on the right is the N-type material with white background. The junction region where the two semiconductors are placed in direct contact is known as the P-N junction indicated by the dashed box. In the presence of electric current, the electrons and holes will move and meet in the P-N junction region between the P-type and N-type semiconductor and recombines to generate light energy .
Figure xx: A schematic on how LED light is formed from the P-N junction.
Colours and Materials
LEDs are active emitters of nearly monochromatic light with highly saturated colours. Conventional LEDs are made from a variety of inorganic semiconductor materials. The colour emitted from an LED is determined by the bandgap energy of the semiconductor material used [6.7] and the emitted light is categorised by wavelength and measured in nanometers (nm). Table xx shows the available colours with wavelength ranges of emission and the material used in LEDs in each case .
Wavelength Î» (nm)
Gallium arsenide (GaAs)
Aluminium gallium arsenide (AlGaAs)
610 <Î» <760
AlGaAs, GaAsP, Gallium(III) phosphide (GaP)
Aluminium gallium indium phosphide (AlGaInP)
590 <Î» <610
GaAsP, AlGaInP, GaP
570 <Î» <590
GaAsP, AlGaInP, GaP
500 <Î» <570
Indium gallium nitride (InGaN), Gallium nitride (GaN), GaP, AlGaInP, Aluminium gallium phosphide (AlGaP)
450 <Î» <500
Zinc selenide (ZnSe), InGaN
400 <Î» <450
Diamond, Boron nitride (BN), Aluminium nitride (AlN), AlGaN, AlGaInN
Blue/UV diode with yellow phosphor
Table xx: LED colour variations.
LEDs have a number of advantages over other sources for lighting applications. The following are some of the advantages of LEDs:
Colour: LEDs are available in a range of colours and LEDs emit light of an intended colour without the use of colour filters that traditional lighting methods require. This is more efficient and can lower initial costs.
Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of infrared that can cause damage to sensitive objects or fabrics.
Cycling: LEDs are ideal for use in applications that are subject to cycle between on and off frequently, unlike fluorescent lamps that burn out more quickly when cycled frequently.
Ecologically friendly: LEDs do not contain mercury, unlike fluorescent lamps. LEDs tend to conserve electricity as they are more efficient than others.
Efficiency: LEDs generates more light per watt of consumed power than incandescent bulbs . Conventional incandescent bulbs generates a lot of heat and are eventually lost due to the filament must be heated up during the light production process. Their efficiency is not affected by its shape and size.
Lifetime: LEDs can have a relatively long useful life and are normally very robust due to the lack of mechanical or moving parts. Incandescent bulbs have an expected lifetime of 1k to 5k hours, while good quality LEDs are often quoted of having a lifetime of 50k hours, more than 5 years continuous use. The performance of LEDs eventually degrades over time, and this degradation is strongly affected by factors such as operating current and temperature .
On/Off time: LEDs lights up almost instantly to achieve its full brightness. A traditional red indicator LED will achieve full brightness in microseconds. Size: LEDs can be very small and are easily populated onto printed circuit boards.
Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
Slow failure: LEDs mostly fail by dimming over time, rather than the unexpected burn-out.
Algae are a large and diverse group of simple, typically photosynthetic organisms, ranging from small, unicellular forms to complex multicellular forms. They also vary in sizes from less than a micron in diameter to over ten meters in length. Algae occur in most habitats, ranging from marine and freshwater to desert sands and from hot boiling springs to snow and ice. Algae are found in the fossil record dating back to approximately 3 billion years in the Precambrian . The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens . They exhibit a wide range of reproductive strategies, from simple, asexual cell division to complex forms of sexual reproduction. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in land plants.
Microalgae are unicellular species which live either isolated or in chains. Depending on the species, their sizes range from a few micrometers (Âµm) to a few hundreds of micrometers.
The most important common biochemical characteristic that unites the algae is their ability to split water, producing molecular oxygen (O2) during photosynthesis and concomitantly assimilating CO2. [x]
As with all plants, micro-algae photosynthesize, i.e. they assimilate inorganic carbon for conversion into organic matter. Light is the source of energy which drives this reaction and in this regard intensity, spectral quality and photoperiod need to be considered. Light intensity plays an important role, but the requirements vary greatly with the culture depth and the density of the algal culture: at higher depths and cell concentrations the light intensity must be increased to penetrate through the culture (e.g. 1,000 lux is suitable for erlenmeyer flasks, 5,000-10,000 is required for larger volumes). Light may be natural or supplied by fluorescent tubes. Too high light intensity (e.g. direct sun light, small container close to artificial light) may result in photo-inhibition. Also, overheating due to both natural and artificial illumination should be avoided. Fluorescent tubes emitting either in the blue or the red light spectrum should be preferred as these are the most active portions of the light spectrum for photosynthesis.
Algae normally grow by photosynthesis. (http://www.algae.wur.nl/UK/factsonalgae/growing_algae/).
Prymnesiophyceae - Isochrysis galbana
Eustigmatophyceae - Nannochloropsis oculata
Chlorella sp., a unicellular microalga commonly found in freshwater in Taiwan, contains abundant nutrients, which have been confirmed to be beneficial to human health . The biomass of Chlorella sp. mainly comprises protein (51-58%), carbohydrate (12-17%), and lipid (14-22%) . Mainly due to the high protein content, Chlorella sp. are widely used as health food for human beings and as animal nutritional supplements [1-3].
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[Effect of light supply and carbon source on cell growth and cellular composition of a newly isolated microalga Chlorella vulgaris ESP-31]
Various LEDs with different wavelengths were used to compare the wavelength effects on growing S. platensis in photoautotrophic conditions. From the experimental results, the higher light intensities harvested more biomass. Meanwhile, the largest specific growth rate occurred when using red LED. Blue LEDs yielded the lowest biomass production. The poor performance of blue light in photosynthesis was mainly due to the fact that the absorption bands of chlorophyll were not present in these light wavelengths.
A modified Monod model with a light intensity threshold could properly evaluate the growth kinetics of S. platensis. It was observed that the growth under the threshold displayed a growth pattern similar to that of darkness. According to the model fitting parameters, red light source had the best performance for microalgal growth with the largest maximum specific growth rate and the smallest Monod constant. Furthermore, concerning the economic efficiency of energy to biomass, the red LED also produced the best results with an efficiency of around 70-110 (g Lâˆ’1) $âˆ’1, when the light intensity was higher than 1500_molmâˆ’2 sâˆ’1.
Microalgae can produce various kinds of value-added chemicals along with the oxygen in the photoautotrophic cultivation conditions. The use of narrow bands red LEDs as a photon supplier would be more economical to drive the photosynthesis compared to using a fluorescent lamp or light bulb . Besides, the longer lifetime and less energy consumption would make the LEDs more attractive in the cultivation of algal culture. Although, a preliminary study showed that only the algal cultivation under blueLEDcontained less chlorophyll and phycocyanin as compared to other LEDs sources (data not shown), it is still expected that the applications of various LEDs with different wavelengths might trigger different specific bio-chemical syntheses in the photosynthesis process and are worthy to be further investigated.
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