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SVS College of Engineering
Coimbatore- 641109.
PAPER ON,
“ELECTRICITY FROM ALGAE”
PRESENTED BY
S.MONIKA, C.KIRUTHIKA,
III-YEAR (EEE), III-YEAR (EEE),
CONTACT NO: 9500328313. CONTACT NO:
9894579425.
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“ELECTRICITY FROM ALGAE”
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Abstract — the algae will be cultivated in
laboratories and put in plastic cylinders
where water, carbon dioxide, and sunshine
can trigger photosynthesis. The resultingbiomass will be treated further to produce a
fuel to turn turbines. The carbon dioxide
produced in the process will be fed back to
the algae, resulting in zero emissions from the
plant.
I NTRODUCTION
High oil prices, global warming, and emphasisin renewable technology are attracting new
interest in a potentially rich source of biofuels:algae. Currently, no alternative technologyseems to be able to entirely replace our vastenergy demands. Yet, we need new technologiesto provide us the energy we need. One of thesetechnologies which are receiving more and moreattention is bioenergy derived from algae.
Microalgae can contain up to 40% oil, they cangrow in wastewater and in live places where noagriculture is possible. They are able to grow
very fast and they even capture large amounts
carbon dioxide while doing so. These facts look very promising for use of algae in bioenergy.
CHARACTERISTICS OF ALGAE
The algae are an ancient group of aquatic plants.
There are thought to be about 23,000 species of algae. There are three features which distinguishthe algae from other plants, namely their body
plan and reproductive system. There is nospecialisation of the algal body into root, stem,leaves with vascular tissue. The photosynthetic
portion of the alga is a thallus while theattachment portion comprises hair-like rhizoids.For most algae, sperm and eggs fuse in the openwater and the zygote develops into a new plantwithout any protection. For other plant groups
the zygote develops into an embryo within the protection of the parent plant. The gametes are produced within a single cell. There is no jacketof sterile cells protecting the gametes.
Being aquatic, algae are marine, freshwater andterrestrial. Terrestrial algae are effectivelysurviving in an aquatic environment on land.Soil algae survive in a film of soil water. Algaeare primary producers, i.e. they are the start of the food chain. One third of all the carbon fixed
on this planet is achieved by algae, largely in theoceans!
Algae are largely classified on the basis ofcolor (photosynthetic pigments), storage material,flagella, and cell wall. The skeletons of a groupof algae, the diatoms, are glass-like and thismaterial is put to a variety of uses, such asabrasives , reflective road signs, swimming poolfilters.
In general there are two types of algae: themacro algae and microalgae. Macro algae arealso known as seaweed and can become verylarge, up to like 60 meter in length. There arealso microalgae, which are far smaller thanmacro algae and are practically small
photosynthetic factories. Both types of algaegrow in marine and fresh waters and can befound over the whole earth.
METHODS OF ALGAE GROWTH
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To make algae a viable source for biomass theyhave to be produced in large quantities. Thereare some important factors that determine thegrowth rate of algae
• Algae type: different types of algae havedifferent growth rates
• Medium/nutrients: composition of the water
• Light: light is needed for the photosynthesis process
• pH: algae need a pH between 7 and 9 to have anoptimum growth rate
• Aeration: the algae need to have contact with air (CO2)
• Mixing: mixing prevents sedimentation of algaeand makes sure all cells are equally exposed tolight
• Temperature: there is an ideal temperature for algae to growIdeal circumstances to grow algae can becreated in a laboratory. However, the costs tocreate these circumstances are very high and itis very difficult to scale up the laboratoryenvironment efficiently.At the moment there aretwo distinct methods that are being researchedfor growing algae.
RACEWAY POND
The 'raceway pond' is a large open water raceway track where algae and nutrients are
pumped around by a motorized paddle. Carbondioxide also has to be added to the pond. Thealgae culture will grow continuously and part of the algae will be removed during the growing
process. The biggest advantage of these open
ponds is their simplicity, yielding low production costs and low operating costs.However, not all algae species can be grown inthese ponds, due to contamination of other algaeand bacteria. Also the process conditions, liketemperature and the amount of light, are hard or impossible to control.
Fig. 1 Sketch of Raceway pond
Fig. 2 Raceway pond.
ANAEROBIC DIGESTION
Anaerobic digestion is a biological process that produces a gas principally composed of methane(CH4) and carbon dioxide (CO2) otherwiseknown as biogas. These gases are producedfrom organic wastes such as livestock manure,food processing waste, etc. Anaerobic processescould either occur naturally or in a controlledenvironment such as a biogas plant.
METHANE FROM ANAEROBIC
DIGESTORS
Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4). It is the major component of the "natural" gas used in manyhomes for cooking and heating. It is odorless,colorless, and yields about 1,000 BritishThermal Units (Btu) [252 kilocalories (kcal)] of
heat energy per cubic foot (0.028 cubic meters)when burned. Natural gas is a fossil fuel that
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was created eons ago by the anaerobicdecomposition of organic materials. It is oftenfound in association with oil and coal.
The same types of anaerobic bacteria that produced natural gas also produce methane
today. Anaerobic bacteria are some of the oldestforms of life on earth. They evolved before the photosynthesis of green plants released largequantities of oxygen into the atmosphere.Anaerobic bacteria break down or "digest"organic material in the absence of oxygen and
produce "biogas" as a waste product. (Aerobicdecomposition, or composting, requires largeamounts of oxygen and produces heat.)Anaerobic decomposition occurs naturally inswamps, water-logged soils and rice fields, deep
bodies of water, and in the digestive systems of termites and large animals. Anaerobic processescan be managed in a "digester" (an airtight tank)or a covered lagoon (a pond used to storemanure) for waste treatment. The primary
benefits of anaerobic digestion are nutrientrecycling, waste treatment, and odor control.Except in very large systems, biogas productionis a highly useful but secondary benefit.
Biogas produced in anaerobic digesters consists
of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such ashydrogen, carbon monoxide, nitrogen, oxygen,and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feedmaterial and management of the process. When
burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heatenergy per percentage of methane composition.For example, biogas composed of 65% methaneyields 650 Btu per cubic foot (5,857 kcal/cubicmeters).
DIGESTION PROCESS
Anaerobic decomposition is a complex process.It occurs in three basic stages as the result of theactivity of a variety of microorganisms. Initially,a group of microorganisms converts organicmaterial to a form that a second group of organisms utilizes to form organic acids.
Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete thedecomposition process.
A variety of factors affect the rate of digestionand biogas production. The most important istemperature. Anaerobic bacteria communitiescan endure temperatures ranging from belowfreezing to above 135° Fahrenheit (F) (57.2°Centigrade [C]), but they thrive best at
temperatures of about 98°F (36.7°C)(mesophilic) and 130°F (54.4°C) (thermophilic).Bacteria activity, and thus biogas production,falls off significantly between about 103° and125°F (39.4° and 51.7°C) and gradually from95° to 32°F (35° to 0°C).
In the thermophilic range, decomposition and biogas production occur more rapidly than in themesophilic range. However, the process ishighly sensitive to disturbances such as changes
in feed materials or temperature. While allanaerobic digesters reduce the viability of weedseeds and disease-producing (pathogenic)organisms, the higher temperatures of
thermophilic digestion result in more complete
destruction. Although digesters operated in themesophilic range must be larger (toaccommodate a longer period of decompositionwithin the tank [residence time]), the process isless sensitive to upset or change in operatingregimen.
To optimize the digestion process, the digester must be kept at a consistent temperature, asrapid changes will upset bacterial activity. Inmost areas of the United States, digestionvessels require some level of insulation and/or heating. Some installations circulate the coolantfrom their biogas-powered engines in or aroundthe digester to keep it warm, while others burn
part of the biogas to heat the digester. In a
properly designed system, heating generallyresults in an increase in biogas productionduring colder periods. The trade-offs inmaintaining optimum digester temperatures tomaximize gas production while minimizingexpenses are somewhat complex. Studies ondigesters in the north-central areas of thecountry indicate that maximum net biogas
production can occur in digesters maintained attemperatures as low as 72°F (22.2°C).
Other factors affect the rate and amount of biogas output. These include pH, water/solidsratio, carbon/nitrogen ratio, mixing of the
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digesting material, the particle size of thematerial being digested, and retention time. Pre-sizing and mixing of the feed material for auniform consistency allows the bacteria to work more quickly. The pH is self-regulating in mostcases. Bicarbonate of soda can be added to
maintain a consistent pH, for example when toomuch "green" or material high in nitrogencontent is added. It may be necessary to addwater to the feed material if it is too dry, or if the nitrogen content is very high. Acarbon/nitrogen ratio of 20/1 to 30/1 is best.Occasional mixing or agitation of the digestingmaterial can aid the digestion process.Antibiotics in livestock feed have been knownto kill the anaerobic bacteria in digesters.Complete digestion, and retention times,
depends on all of the above factors
As long as proper conditions are present,anaerobic bacteria will continuously produce
biogas. Minor fluctuations may occur thatreflect the loading routine. Biogas can be usedfor heating, cooking, and to operate an internalcombustion engine for mechanical and electric
power. For engine applications, it may beadvisable to scrub out hydrogen sulfide (ahighly corrosive and toxic gas). Very large-scale
systems/producers may be able to sell the gas tonatural gas companies, but this may requirescrubbing out the carbon dioxide.
The material drawn from the digester is calledsludge, or effluent. This is transferred tomicrobial fuel cell.
MICROBIAL FUEL CELL
A microbial fuel cell (MFC) or biological fuelcell is a bio-electrochemical system that drives acurrent by mimicking bacterial interactionsfound in nature. A microbial fuel cell is a devicethat converts chemical energy to electricalenergy by the catalytic reaction of microorganisms.
DESCRIPTION
A typical microbial fuel cell consists of anode and cathode compartments separated by a cation
(positively charged ion) specific membrane. Inthe anode compartment, fuel is oxidized bymicroorganisms, generating electrons and
protons. Electrons are transferred to the cathodecompartment through an external electriccircuit, and the protons are transferred to the
cathode compartment through the membrane.Electrons and protons are consumed in thecathode compartment, combining with oxygento form water. In general, there are two types of microbial fuel cells: mediator and mediator-lessmicrobial fuel cells.Bacteria in mediator-lessMFCs typically have electrochemically activeredox enzymes such as cytochromes on their outer membrane that can transfer electrons toexternal materials.
GENERATING ELECTRICITY
When micro-organisms consume a substratesuch as sugar in aerobic conditions they producecarbon dioxide and water . However whenoxygen is not present they produce carbondioxide, protons and electrons as described
below
C12H22O11 + 13H2O --> 12CO2 + 48H+ + 48e-
Microbial fuel cells use inorganic mediators totap into the electron transport chain of cells andsteal the electrons that are produced. Themediator crosses the outer cell lipid membranesand plasma wall; it then begins to liberateelectrons from the electron transport chain thatwould normally be taken up by oxygen or other intermediates. The now-reduced mediator exitsthe cell laden with electrons that it shuttles to anelectrode where it deposits them; this electrode
becomes the electro-generic anode (negatively
charged electrode). The release of the electronsmeans that the mediator returns to its originaloxidized state ready to repeat the process. It isimportant to note that this can only happenunder anaerobic conditions, if oxygen is presentthen it will collect all the electrons as it has agreater electronegativity than the mediator.
In a microbial fuel cell operation, the anode isthe terminal electron acceptor recognized by
bacteria in the anodic chamber. Therefore, themicrobial activity is strongly dependent on theredox potential of the anode.
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This is the principle behind generating a flow of electrons from most micro-organisms. Theorganisms capable of producing an electriccurrent are termed Exoelectrogens. In order toturn this into a usable supply of electricity this
process has to be accommodated in a fuel cell so
that it creates a complete circuit, not just shuttleselectrons to a single point. In the secondchamber of the MFC is another solution andelectrode. This electrode, called the cathode is
positively charged and is the equivalent of theoxygen sink at the end of the electron transportchain, only now it is external to the biologicalcell. The solution is an oxidizing agent that
picks up the electrons at the cathode. As withthe electron chain in the yeast cell, this could bea number of molecules such as oxygen. However,
this is not particularly practical as it would require largevolumes of circulating gas. A more convenientoption is to use a solution of a solid oxidizingagent.
Connecting the two electrodes is a wire (or other electrically conductive path which may includesome electrically powered device such as a light
bulb) and completing the circuit and connectingthe two chambers is a salt bridge or ion-exchange membrane. This last feature allows the
protons produced, to pass from the anodechamber to the cathode chamber.
The reduced mediator carries electrons from thecell to the electrode. Here the mediator isoxidized as it deposits the electrons. These thenflow across the wire to the second electrode,which acts as an electron sink. From here they
pass to an oxidizing material.
Microbial fuel cell
ADVANTAGES
One of the biggest advantages of using algae asa source for methane is the fact that the total
process is neutral with regard to carbon dioxide.
With an increasing focus on environmentalfriendliness this could point out to be the mostimportant advantage. Also the fact that algaecan use carbon dioxide and other flue gasses togrow makes them a very interesting subject of research. Furthermore, algae are a sustainablesource of energy. Furthermore all conditions for algae to grow fast are present in abundance:feedstock in the form of carbon dioxide andwater and energy source in the form of sunlight.
A second advantage is the fact that algae arehighly efficient converters of solar energy to
biomass, compared to crops or trees. It is possible for algae to use almost 10% of theincoming sunlight for the photosynthesis
process. This makes it possible to get a high biomass output per square meter per day.
A third advantage is that some algae grow inwater and not on land. If grown in traditionallakes, or even salty waters, they don't compete
with other sources for biomass.
DISADVANTAGES
The biggest disadvantage at this moment is thefact that it is still hard to grow mass cultures of algae for a competitive price. To earn maximumgrowth per square meter a photo bioreactor isnecessary. These photo bioreactors however areextremely expensive compared to the traditionalopen pond systems.
The disadvantage of the open pool systemshowever is the fact that it is hard to control thegrowth process of the algae. Only a few algaespecies can be grown in open pool systems atthis moment, because there are only a fewspecies that are not very sensitive tocontamination. These species are often not themost efficient converters of sunlight and carbondioxide to biomass.
Besides the fact that growing algae is noteconomically viable at the moment, theconversion is not optimal as well. Because algae
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are wet they cannot be gasified without drying,which consumes a lot of energy and thereforelowers the overall efficiency. Supercritical water gasification is a solution for this problem
because wet biomass can be a feedstock for this process. However, this technology is still very
immature and not applied on a large scale.
One of the problems with current methods for producing biodiesel from algae oil is the processing cost, and the New York researcherssay their innovative process is at least 40
percent cheaper than that of others now beingused. Supply will not be a problem: There is alimitless amount of algae growing in oceans,lakes, and rivers, throughout the world.
GREEN FUTURE OF VENICE
The city of Venice hopes to get at least 50 per cent of electricity from renewable sources by theyear 2011. It plans to use algae to generateelectricity. Venice, known as the City of Bridges, plans to end its reliance on fossil fuelsin the near future by primarily using biofuels.
As a first step the city officials have invested €200 million ($264 million) for a biofuels plant.They will use two types of algae,
Sargassummuticum and Undariapinnafitida.They will cultivate them in laboratories, whichwill then be used to generate electricity in a new40 MW power plant. This plant will provide upto 50 per cent of the city’s electricity needs.
CONCLUSION
At this moment it is commercially not viable to produce methane from algae. Cheap algae can be produced by open pool systems, but theyneed a too large surface to replace fossil fuels.Bioreactors need about ten times less surface to
produce the algae, but the costs for these
systems are far higher. A lot of technological progression has to be made in order to make power generation from algae feasible. Initialfocus of research should be on growing masscultures of algae efficiently. By geneticengineering more productive species and speciesthat are more suitable for open pond systemsshould be created.
A big question at the moment is how to usealgae as efficient as possible as a source of
energy. There are many paths to take, but at themoment major research is focused on the
production of biodiesel. The process of making biodiesel from biomass is well known andalready used on a large scale. Another path,which is easy to follow, is the fermentation of
biomass to ethanol. This process is also appliedon a large scale. If there are not any major
breakthroughs in the conversion of algae tomethane (supercritical water gasification for example) methane from algae is not expected to
be the source of energy for the future. However,with increasing demands of gas and moresubsidies on biogas (subsidies on biodiesel arevery high globally) it is possible that moreresearch will be done on the production of methane from algae.
R EFERENCES
[1] http://www.digitaljournal.com/article/
[2] http://www.digitaljournal.com/article/
[3] http://www.alternative-energy-news.info/engineers-tap-algae-cells-for-electricity/
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[4] http://www.wired.com/science/discoveries/news/
[5] http://algaetobioenergy.wordpress.com/2008/04/06/a-small-introduction-on-bioenergy-and-algae/
[6] http://biomassmagazine.com/articles/3153/rwe-plans-to-generate-electricity-from-algal-diesel
[7] http://freeenergynews.com/Directory/Ethanol/
[8] http://cogeneration.net/anaerobic-digester/
[9] www.ecofriend.org.
[10] www.bio-pro.de.
