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Fuel cells

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Fuel cells

  1. 1. Presentation on Fuel Cells Prepared By : Mahida Hiren R. Mechanical Engg. Dept.
  2. 2. Outlines:  Introduction  Design Principle & operation of Fuel Cell.  Types of Fuel Cells.  Conversion Efficiency of Fuel Cell.  Application of Fuel Cell.
  3. 3. Introduction  It is an electrochemical device which convert hydrogen and oxygen into water producing electricity and heat in the process.  It is much like a battery that can be recharged while you are drawing power from it.  It provides a DC voltage that can be used to power motors, lights and any number of electrical appliances.
  4. 4. Continue……  Fuel cells differ from conventional cells in the respect that active material (fuel & oxygen) are not contained within the cell but are supplied from outside.  Pure or fairly pure hydrogen gas would be preferred fuel for fuel cell.  Alternatively impure hydrogen obtained from hydrocarbon fuels, such as natural gas , methane, LPG & liquid petroleum products can be used in fuel cell as a fuel.  Efforts are going on to develop cells that can use carbon monoxide as the fuel; if they are successful, it should be possible to utilize coal as the primary energy source.  Main uses of fuel cells are in power production, automobile vehicles and in special military use.
  5. 5. First demonstrated by Welsh scientist Sir William Robert Grove in February 1839.
  6. 6. The Invention of the Fuel Cell  Sir William Grove invented the first fuel cell in 1839. Grove knew that water could be split into hydrogen and oxygen by sending an electric current through it (a process called electrolysis). He hypothesized that by reversing the procedure you could produce electricity and water.  He created a primitive fuel cell and called it a gas voltaic battery. After experimenting with his new invention, Grove proved his hypothesis. Fifty years later, scientists Ludwig Mond and Charles Langer coined the term fuel cell while attempting to build a practical model to produce electricity
  7. 7. What is a fuel cell?  A fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity.  A fuel cell is a device that generates electricity by a chemical reaction. Every fuel cell has two electrodes, one positive and one negative, called cathode and anode respectively. The reactions that produce electricity take place at the electrodes.
  8. 8. Why we need fuel cell?  Due to energy crisis all over the world.  Due to the issue of global warming.  Due to the unavailability of different renewable sources at each and every place due to geographic condition.  Fuel cell provides an alternate efficient non polluting power source that produces no noise and has no moving parts.  It is expected that by 2050 the global energy demand is going to rise by 2 to 3 times.  This calls for optimization of generation of energy through well- known sources, preferably renewable energy for commercial exploitation.
  9. 9. Classification Of Fuel Cells: Classification of fuel cells is very difficult as several operational variable exists.  Based on the temperature range in which they operate: low temperature(25-100 C), medium temperature (100-500) , high temperature(500-1000) & very high temperature(above 1000)  According to the type of electrolyte : aqueous, non aqueous, molten or solid.  According to the physical state of the fuel: Gas(hydrogen, lower hydrocarbons ), Liquid(alcohols, hydrazine, higher hydrocarbons), Solid(Metals)  Primary fuel cell: Reactants are passed through the cell only once & the products of the reaction being discarded. (H 2 – O 2 fuel cell )  Secondary fuel cell: Reactants are passed through the cell many times because they are regenerated by different methods.( Nitric oxide – chlorine fuel cell)
  10. 10. Types of fuel cell As per the fuel used the fuel calls are classified as follows.  Hydrogen  Fossil Fuel  Hydrocarbon fuel.  Alcohol fuel.  Hydrazine fuel.
  11. 11. Design Principle & Operation of Fuel Cell:  Chemistry of fuel cell At anode:- 2H2  4H + 4e At Cathode:- O2 + 4H + 4e  2H2O NET REACTION 2H2 + O2  2H2O
  12. 12. Hydrogen Oxygen Cell:  40% KOH solution as electrolyte (Ion exchange membrane).  The membrane is non permeable to the reactant gases, hydrogen and oxygen, which thus prevents them from coming into contact.  The membrane is however , permeable to hydrogen ions which are the current carriers in the electrolyte.  The desired properties of an ideal ion exchange membrane electrolyte are:  High ionic conductivity.  Zero electronic conductivity  Low permeability of fuel and oxidant  Low degree of electro-osmosis.  High resistance to dehydration.  High resistance to its oxidation or hydrolysis and,  Mechanical stability
  13. 13. Continue……  Considerable amount of research has been carried out in a search for the ideal membrane.  Interpolymers of polyflurocarbon and poltstyerene sulfonic acids have been found to be quite satisfactory.  In order that electrolyte resistance be low as possible, a thin sheet of this material (0.076 cm thickness) is used as the electrolyte.  An advantageous feature of this electrolyte is that it retains only a limited quantity of water and rejects excess water produced in fuel cell.  This cell operates at about 40-60 ̊C. The thermodynamic reversible potentials for the reaction is 1.23 volts at 25˚C.
  14. 14. Fossil Fuel Cells:  The most interesting fuel cells for the near future are modified hydrogen- oxygen cells, in which a gaseous or liquid hydrocarbon is the source of hydrogen.
  15. 15. Continue…..  Coal may be serve as the primary energy source for fuel cells. Cells based on fossil fuels have three main components.  The fuel processor which converts the fossil fuel into a hydrogen rich gas.  The power section consisting of the actual fuel cell (or combination of cells), and  The inverter for changing the direct current generated by the fuel cell into alternating current to be transmitted to user.
  16. 16. Continue……  In phosphoric cell utilizes a concentrated aqueous solution of phosphoric acid as the electrolyte.  The primary fuel is light hydrocarbon, such natural gas or nephtha.  The operating temperature is 150 to 200˚ C and the discharge voltage is 0.7 to 0.8 volt.  Each cell unit is only a few millimeters thick so that a large number can be stacked in a package of reasonable size to produce the desired voltage and power.
  17. 17. Molten Carbonate Cells:  These high temperature fuel cells offer the prospect for use with variety of fossil fuels, including coal.  A special feature of these cells is that during operation they can oxidize CO to CO2 as well as hydrogen to water.
  18. 18. Continue…..  Hence gaseous mixture of H2 and CO, which are relatively inexpensive to manufacture, can be used in the cell, the presence of CO2 would have only a minor effect.  The electrolyte in the high temperature fuel cells under development is a molten mixture of alkali metal (lithium, sodium, potassium) carbonates at a temperature of 600-700 ˚C.  This is retained in an inert porous matrix sandwiched between two porous nickel electrodes.  The mixture of H2 and CO is supplied to negative electrodes and oxygen to the positive electrodes.  The discharge e.m.f of the cell is above 0.8 volt.
  19. 19. Continue…….  An important aspect of the molten carbonate fuel cell is that the discharged gases, consisting of the steam and CO2 products and nitrogen from the air, are at a temperature exceeding 540˚ C.  The hot gases could be used to provide industrial process heat, to operate a gas turbine or to produce steam in a waste heat boiler to drive a steam turbine which produce electric power.
  20. 20. Solid Oxide Electrolyte Cells:  Certain solid, ceramic oxides are able to connect electricity at high temperatures and can serve as electrolyte for fuel cells.  A possible electrolyte is zirconium dioxide containing a small amount of another oxide to stabilize the crystal structure; this material is able to conduct oxygen ions at high temperature.  The electrode material might be porous nickel and the operating temperature in the range 600-1000˚ C .  Electrochemical catalyst would not be required.  Other energy sources or fuels, that can be conveniently stored and transported in liquid form such as methanol, ammonia and hydrazine have been proposed for fuel cell.
  21. 21. Methanol as a fuel:  Methanol can be catalytically reformed with steam at about 200˚C to yield a mixture of hydrogen (75 volume percent) and carbon dioxide.  This gas can be supplied to the negative electrode of a fuel cell with air at the positive electrode.  The cell with aqueous phosphoric acid solution as the electrolyte, might be similar to those already described.
  22. 22. Ammonia (NH3) as a fuel:  In this the ammonia gas obtained from the stored liquid is decomposed catalytically into hydrogen and nitrogen.  Part of the hydrogen is burned in air to provide the heat required for the decomposition.  Other hydrogen is supplied to the negative electrode of the hydrogen cell in which potassium hydroxide is used as electrolyte.  The nitrogen produced due to decomposition is an inert gas and plays no roll in the cell.  The advantage of ammonia as fuel is that it can be stored in the liquid form.  The disadvantage include higher vapour pressure and cell operate satisfactory at low temperature.
  23. 23. Hydrazine(N2H4) as a fuel:  A compact fuel cell for mobile application possibly for vehicle propulsion utilizes the liquid hydrazine as a fuel and hydrogen peroxide or air as the energy source.  Hydrazine is injected as required into the aqueous potassium hydroxide potassium hydroxide electrolyte to provide the active material at the negative electrode.  The oxygen is either supplied by decomposition or from air.  The electrode is made up of nickel (negative) or silver (positive) as the electrochemical catalyst.  The overall cell reaction is the oxidation of hydrazine to water and nitrogen, but the discharged e.m.f is similar to that of the hydrogen oxygen cell.
  24. 24. Continue…….  Basic drawback of hydrogen oxygen cell is that hydrogen and oxygen are gases, so there is storage difficulty.  Liquid hydrazine is highly reactive/ toxic a well as costly.  Power output is more (1.56V)as compared to hydrogen oxygen (1.23 V).  In Aluminum oxygen cell , the aluminum forms the negative electrode and the oxygen is the positive electrode.  The electrolyte is an aqueous solution of sodium hydroxide.
  25. 25. Regenerative Systems:  A regenerative fuel cell is one in which the fuel cell product (water) is recovered into the reactants (hydrogen and oxygen) by one of the several possible methods – thermal, chemical, photochemical, electrical and radio chemical.  There are two stages in a regenerative fuel cell:  Conversion of fuel cell reactants into products while producing electrical energy, and  Reconversion of fuel cell products into reactants.
  26. 26. Conversion efficiency of Fuel Cell:  The electrical energy generated by fuel cell depends on what is called the “free” energy rather than on heat energy of the overall cell reaction.  The free energy of formation of 1 mole (18 grams) of liquid water from hydrogen and oxygen gases at atmospheric pressure is 56.67 Kcal or 237 KJ at 25˚C while the heat energy of the reaction under the same conditions is 68.26 Kcal (286 KJ)  Thus theoretical efficiency of the conversion of the heat energy into mechanical energy in a hydrogen oxygen fuel cell is (56.67/68.26)*100 = 83 %  Efficiencies as high as 70% have been observed, but the practical cells using pure fuel have conversion efficiency of 50-60%.  The efficiency is lower when we use air as the source of oxygen and when hydrogen is derived from hydrocarbon sources.  However this value is higher than the efficiency that is obtain by using these sources in the conventional plants.
  27. 27. Continue…..  The discharged voltages observed in actual cells are always below the theoretical value, the difference increasing with increasing strength of the current drawn from the cell.  For the hydrogen oxygen cell at 25˚C with the gases at atmospheric pressure , the ideal e.m.f is 1.23 Volts & for the moderate currents at which fuel cells normally operate the e.m.f is 0.7 to 0.8 volts.  This deviation from the theoretical e.m.f accounts for the conversion efficiency of a fuel cell being below the ideal maximum value.  The departure of a fuel cell from ideal behavior arises from several factors one is the Inherent slowness of the electrode reactions. This is dominant at low current drains.  It can be reduced by an effective electrochemical catalyst and by increasing the operating temperature.
  28. 28. Continue…….  At larger currents there is an additional contribution from the electrical resistance of the electrolyte.  A low resistance (i.e. high conductivity) electrolyte is therefore desirable.  In ideal hydrogen oxygen cell, (100-83) = 17 % of the chemical reaction energy would be liberated as heat & this proportion increases in an actual cell.  In order to avoid this temperature rise, heat is removed from the fuel cell during operation.  In some cell design it is proposed to utilize the heat to provide space heating of a building and to supply hot water.  The heat released in high temperature cells might be used for industrial purposes (i.e. process heat) or to generate steam.
  29. 29. Polarization in Fuel Cells:  The difference between the theoretical voltage and the actual voltage is known as the polarization. This is also called as overvoltage.  The effect of polarization is to reduce the efficiency of the cell from the theoretical maximum.  Significant drop in voltage and hence energy loss takes place as the current density is increased.
  30. 30. Continue…..  There are mainly three types of polarization:  Activation Polarization.(Chemical polarization)  Resistance or ohmic Polarization and  Concentration Polarization.
  31. 31. Activation Polarization:  This is related to the activation energy barrier for the electron transfer process at the electrode.  In fuel cells electrons are liberated and reaction is chemisorption reaction.  At low current densities significant number of electrons are not emitted, which results in such a potential loss.  This process requires that certain minimum activation energy supplied so that sufficient number of electrons are emitted, this energy is supplied by the output of the cell.  This loss is known as activation or chemical polarization.  This polarization may be reduced by using better electrode catalysts, increasing surface area, and by raising operating temperature.
  32. 32. Resistance Polarization:  The voltage drop in linearly related to the current flow according to the ohm’s law.  The internal resistance is composed of the electrode resistance, the bulk electrolyte resistance and interface contact resistance between electrode and electrolyte.  The loss due to resistance polarization is significant when current density is quite large.  The reduction in the internal resistance is the main design criteria for low resistance polarization losses.  The electrolyte resistance can be decreased usually by using more concentrated electrolyte by closer spacing of electrodes and by increased temperature.
  33. 33. Continue…..  Loss due to resistance polarization can be reduced by:  Selecting proper shape of the electrode to have minimum contact between electrode and electrolyte.  Reducing the gap between electrodes.  By using concentrated electrolyte.
  34. 34. Concentrated Polarization:  This type of polarization tends to limit the current drawn.  It is generally divided into two categories  Electrolyte side polarization: This is due to the slow diffusion in the electrolyte causing a change in concentration at the electrodes.  This effect can be minimized by increasing the electrolyte concentration or by stirring or circulating the electrolyte.  Gas side polarization: This is caused from slow diffusion of reactants through the porous electrode to the reaction site or of slow diffusion of products away from the reaction site.  The loss in voltage due to gas side polarization is reduced by using electrodes of smaller pore size and by increasing temperature.  We have observed that all the three loses in a fuel cell are decreased by increasing temperature , due to this a given cell is usually operated in practice at the higher end of its temperature range.
  35. 35. PEM
  36. 36. Requirement for fuel cells  Hydrocarbons  Oxygen  Water  Hydrogen  Carbon dioxide  Carbon monoxide  And hardware elements are  Fuel processor or reformer  Fuel cell  Convertor
  37. 37. Alkali Fuel Cell
  38. 38. Advantages & Disadvantages:  Fuel cell system are environmentally.  High conversion efficiency .  Extremely low emission.  Noise less operations so readily accepted in residential areas.  Has no moving parts.  Availability to use at any location. So less transmission & distribution losses.  No requirements of cooling tower as conventional plants.  Less space require as compared to conventional plants. The main disadvantages of fuel cells are their high initial costs and low service life.
  39. 39. Application: The application of the fuel cell may be discussed in the following areas.  Domestic Power  Central Power Station.  Automotive Vehicles.  Special Application
  40. 40. Some useful points regarding application:  The e.m.f. or voltage of a fuel cell depends to some extend on the discharge current strength. The average voltage per cell is 0.75 volt.  By joining a number of cells in series and parallel can provide any reasonable voltage and current.  Types of current that are generated by fuel cell.  If the fuel cells of reasonably low cost and long life can be produced , a major use might be by electrical utilities for load leveling  A long term possibility is a central station power plant in which coal is gasified and the gas is used to generate electricity directly by means of fuel cells.  Such an installation is expected to have a higher efficiency for fuel utilization than a conventional steam- electrical plant.
  41. 41. Continue……  Portable generating sets seem to be a favorable field for fuel cells.  Here already fuel cells appear to be competitive as compared with conventional sources. Low temperature fuel cells have a favorable position for operating times of 3000 to 4000 hours per year, using methanol as a fuel.  As we know that demand for power is variable. when the demand is less than the rated output, the excess would be used to generate hydrogen by electrolysis of water.  At the times when the load is greater than the rated power, the hydrogen would be used in fuel cells to satisfy the additional demands.  By using fuel cell at the site of power, the transmission and distribution cost would be reduced.  For new load centers different fuel might be utilized more economically in fuel cells located near the new load centers.
  42. 42. Continue……  Some fuel cells have been proposed for remote or rural areas or unattended location , for mobile and emergency power sources, and for vehicle propulsion.  The aluminum- air cell is of special interest for electrical vehicle propulsion because of the high specific energy that is possible. The weight of these batteries are same as conventional batteries.  More than this methanol –air and hydrogen – oxygen cell are also used for vehicle propulsion.  Many of the fuel cells currently under development are for special application where convenience is of importance, cost is secondary.  For these application hydrogen is the superior fuel from the view point of the reactivity and availability of invariant electrolyte, although it is relatively costly.  It seems likely that hydrogen- oxygen and hydrogen carbon- oxygen cells will be used to an increasing extent in special military & space application.
  43. 43. Applications  Fuel cells powered cars will start to replace gas and diesel engine cars in about 2055.  Fuel cell powered buses are already running in several cities..  This promising application will one day even power our houses.
  44. 44. Applications  Fuel cells also make sense for portable power like laptop computers and cellular phones.
  45. 45.  Telecommunications - With the use of computers, the Internet, and communication networks steadily increasing, there comes a need for more reliable power than is available on the current electrical grid, and fuel cells have proven to be up to 99.999% (five nines) reliable. Fuel cells can replace batteries to provide power for 1kW to 5kW telecom sites without noise or emissions, and are durable, providing power in sites that are either hard to access or are subject to inclement weather. Such systems would be used to provide primary or backup power for telecom switch nodes, cell towers, and other electronic systems that would benefit from on-site, direct DC power supply.
  46. 46. Nokia mobile with fuel cell battery
  47. 47. Wheel chair powered by fuel cell
  48. 48. Toshiba mp3 with a fuel cell
  49. 49. Major organizations working in the field:  Ministry of NCES  IITs  CSIR labs  BHEL  GAIL  BARC  MIT
  50. 50. Disadvantages  Initial cost of installation is higher.  Comparative cost of energy storage of fuel cells is around twice that of conventional sources of energy.  Energy produced by one fuel cell is around 0.7 volts.
  51. 51. Recent development:  LLC, Latham, NY has successfully developed 50 KW power plant.  Fuel cell of capacity 1.5KW is powering houses in Australia.  GAIL is actively involved in establishing fuel infrastructure for fuel cell vehicles in India.  CECRI, Karaikudi has developed and tested MCFC stack.
  52. 52. Conclusion  The above discussion prove the existence of huge market for fuel cells.  The commercial power units are technically feasible .  Research and development should be aimed at reducing cost and increasing life.  Future cities can be planned on fuel cell systems for their power and energy requirements.
  53. 53. Thank you
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