Compact fuel cell that will operate satisfactorily under all driving conditions.

2. OBJECTIVESAfter studying Chapter 15, the reader should be able to:1. Explain how a fuel cell generates electricity.2. Discuss the advantages and disadvantages of fuel cells.3. List the types of fuel cells.4. Explain how ultracapacitors work.5. Discuss alternative energy sources.

3. FUEL-CELL TECHNOLOGYA fuel cell is an electrochemical device in which the chemical energy of hydrogen and oxygen is converted into electrical energy. Fuel cells are being developed to power homes and vehicles while producing low or zero emissions.

4. The chemical reaction in a fuel cell is the opposite of electrolysis. Electrolysis is the process in which electrical current is passed through water in order to break it into its components, hydrogen and oxygen. It is important to note that while hydrogen can be used as a fuel, it is not an energy source. Instead, hydrogen is only an energy carrier, as energy must be expended to generate the hydrogen and store it so it can be used as a fuel.

5. A fuel cell is a hydrogen-powered battery. Hydrogen is an excellent fuel because it has a very high energy density when compared to an equivalent amount of fossil fuel. One pound (lb) of hydrogen has three times the energy content as one pound of gasoline. Hydrogen is the most abundant element on earth, but it does not exist by itself in nature. This is because its natural tendency is to react with oxygen in the atmosphere to form water (H2O).

6. In order to store hydrogen for use as a fuel, processes must be undertaken to separate it from these materials.

7. Benefits of a Fuel CellA fuel cell can be used to move a vehicle by generating electricity to power electric drive motors, as well as powering the remainder of the vehicle’s electrical system. They are powered by hydrogen and oxygen, fuel cells by themselves do not generate carbon emissions such as CO2.A fuel cell is also much more energy-efficient than a typical internal combustion engine. While a vehicle powered by an internal combustion engine (ICE) is anywhere from 15% to 20% efficient, a fuel-cell vehicle can achieve efficiencies upwards of 40%.

8. Compact fuel cell that will operate satisfactorily under all driving conditions.

9. Well-to-Wheel Efficiency.Well-to-wheel efficiency is calculated by multiplying a vehicle’s well-to-tank efficiency by its tank-to-wheel efficiency. The results can be surprising, because while fuel-cell vehicles are very efficient in terms of their tank-to-wheel measurement, they score low on the well-to-tank rating because of the high levels of energy required to produce hydrogen from natural gas. See the accompanying chart.

10. Overall Efficiency (Well-to-Wheel)

11. A fuel-cell vehicle (FCV) uses the fuel cell as its only source of power, whereas a fuel-cell hybrid vehicle (FCHV) would also have an electrical storage device that can be used to power the vehicle. Most new designs of fuel-cell vehicles are now based on a hybrid configuration due to the significant increase in efficiency and driveability.

12. Fuel-Cell ChallengesThere are a number of reasons for this, including:High costLack of refueling infrastructureSafety perceptionInsufficient vehicle rangeLack of durabilityFreeze starting problemsInsufficient power density

13. Fuel-Cell Types The fuel-cell design that is best suited for automotive applications is the Proton Exchange Membrane (PEM).

14. PEM FUEL CELLSThe Proton Exchange Membrane fuel cell is also known as a Polymer Electrolyte Fuel Cell (PEFC). The PEM is a simple design based on a membrane that is coated on both sides with a catalyst such as platinum or palladium. There are two electrodes, one located on each side of the membrane. These are responsible for distributing hydrogen and oxygen over the membrane surface, removing waste heat, and providing a path for electrical current flow. The part of the PEM fuel cell that contains the membrane, catalyst coatings, and electrodes is known as the Membrane Electrode Assembly (MEA).

15. The negative electrode (anode) has hydrogen gas directed to it, while oxygen is sent to the positive electrode (cathode). Hydrogen is sent to the negative electrode as H2 molecules, which break apart into H+ ions (protons) in the presence of the catalyst. The electrons (e-) from the hydrogen atoms are sent through the external circuit, generating electricity that can be utilized to perform work. These same electrons are then sent to the positive electrode where they rejoin the H+ ions that have passed through the membrane and have reacted with oxygen in the presence of the catalyst. This creates H2O and waste heat, which are the only emissions from a PEM fuel cell.

17. Fuel-Cell StacksA single fuel cell by itself is not particularly useful, as it will generate less than 1 volt of electrical potential. It is more common for hundreds of fuel cells to be built together in a fuel- cell stack. The fuel cells are placed end-to-end in the stack much like slices in a loaf of bread. Automotive fuel-cell stacks contain upwards of 400 cells in their construction.

18. Direct Methanol Fuel CellsHigh-pressure cylinders are one method of storing hydrogen onboard a vehicle for use in a fuel cell. This is a simple and lightweight storage method, but often does not provide sufficient vehicle driving range. Another approach has been to fuel a modified PEM fuel cell with liquid methanol instead of hydrogen gas.

19. Methanol has a higher energy density than gaseous hydrogen because it exists in a liquid state at normal temperatures, and is easier to handle since no compressors or other high-pressure equipment is needed. This means that a fuel-cell vehicle can be refueled with a liquid instead of high-pressure gas, which makes the refueling process simpler and produces a greater vehicle driving range.

20. FUEL-CELL VEHICLE SYSTEMSHumidifiersWater management inside a PEM fuel cell is critical. Too much water can prevent oxygen from making contact with the positive electrode; too little water can allow the electrolyte to dry out and lower its conductivity. The role of the humidifier is to achieve a balance where it is providing sufficient moisture to the fuel cell by recycling water that is evaporating at the cathode. The humidifier is located in the air line leading to the cathode of the fuel-cell stack.

22. Fuel-Cell Cooling SystemsOne of the major challenges for engineers in this regard is the fact that the heat generated by the fuel cell is classified as low-grade heat. This means that there is only a small difference between the temperature of the coolant and that of the ambient air. Heat transfers very slowly under these conditions, so heat exchangers with a much larger surface area must be utilized.

23. In some cases, heat exchangers may be placed in other areas of the vehicle when available space at the front of the engine compartment is insufficient. An auxiliary heat exchanger is located underneath the vehicle to increase the cooling system heat-rejection capacity.

24. Air Supply PumpsAir must be supplied to the fuel-cell stack at the proper pressure and flow rate to enable proper performance under all driving conditions.

25. Fuel-Cell Hybrid VehiclesHybridization tends to increase efficiency in vehicles with conventional drive trains, as energy that was once lost during braking and otherwise normal operation is instead stored for later use in a high-voltage battery or ultracapacitor.

26. Secondary Batteries. In most FCHV designs, a high-voltage nickel-metal hydride (NiMH) battery pack is used as a secondary battery.

27. Ultracapacitors. An alternative to storing electrical energy in batteries is to use ultracapacitors. A capacitor is best known as an electrical device that will block DC current, but allow AC to pass. However, a capacitor can also be used to store electrical energy, and it is able to do this without a chemical reaction.

28. Ultracapacitors are built very different from conventional capacitors. Ultracapacitor cells are based on double-layer technology, in which two activated-carbon electrodes are immersed in an organic electrolyte. The electrodes have a very large surface area and are separated by a membrane that allows ions to migrate but prevents the electrodes from touching. Ultracapacitors can charge and discharge quickly and efficiently, making them especially suited for electric assist applications in fuel-cell hybrid vehicles.

29. Ultracapacitors that are used in fuel-cell hybrid vehicles are made up of multiple cylindrical cells connected in parallel.

30. Fuel-Cell Traction Motors. The electric traction motors used in fuel-cell hybrid vehicles are very similar to those being used in current hybrid electric vehicles. The typical drive motor is based on an AC synchronous design, which is sometimes referred to as a DC brushless motor. This design is very reliable as it does not use a commutator or brushes, but instead has a three-phase stator and a permanent magnet rotor.

31. Some fuel cell hybrid vehicles use a single electric drive motor and a transaxle to direct power to the vehicle’s wheels. It is also possible to use wheel motors to drive individual wheels.

32. Transaxles. Fuel-cell hybrid vehicles are effectively pure electric vehicles in that their drive train is electrically driven.Fuel-cell hybrid vehicles use electric drive motors that require only a simple reduction in their final drive and a differential to send power to the drive wheels. No gear shifting is required and mechanisms such as torque converters and clutches are done away with completely. The transaxles used in fuel cell hybrid vehicles are extremely simple with few moving parts, making them extremely durable, quiet, and reliable.

34. Power Control Units. The drive train of a fuel-cell hybrid vehicle is controlled by a power control unit (PCU), which controls fuel-cell output and directs the flow of electricity between the various components.

35. Power to and from the secondary battery is directed through the power control unit, which is also responsible for maintaining the battery pack’s state of charge and for controlling and directing the output of the fuel-cell stack.

36. Hydrogen StorageModern drivers have grown accustomed to having a minimum of 300 miles between refueling stops, a goal that is extremely difficult to achieve when fueling the vehicle with hydrogen. Hydrogen has a very high energy content on a pound-for-pound basis, but its energy density is less than that of conventional liquid fuels. This is because gaseous hydrogen, even at high pressure, has a very low physical density (mass per unit volume).

37. A number of methods of hydrogen storage are being considered for use in fuel-cell hybrid vehicles. These include high-pressure compressed gas, liquefied hydrogen, and solid storage in metal hydrides.

38. High-Pressure Compressed Gas. It is common for a pressure of 5,000 psi (350 bar) to be used, but technology is available to store hydrogen at up to 10,000 psi (700 bar).

39. In order to refuel the compressed hydrogen storage tanks, a special high-pressure fitting is installed in place of the filler neck used for conventional vehicles.

40. There is also a special electrical connector that is used to enable communication between the vehicle and the filling station during the refueling process.

41. Liquid Hydrogen. Hydrogen can be liquefied in an effort to increase its energy density, but this requires that it be stored in cryogenic tanks at -423°F (-253°C).One liter of liquid hydrogen only has one-fourth the energy content of 1 liter of gasoline.

43. Solid Storage of Hydrogen. One method discovered to store hydrogen in solid form is as a metal hydride, similar to how a nickel-metal hydride (NiMH) battery works.A metal hydride is formed when gaseous hydrogen molecules disassociate into individual hydrogen atoms and bond with the metal atoms in the storage tank. This process uses powdered metallic alloys capable of rapidly absorbing hydrogen to make this occur.

44. Hydrogen Fuel = No CarbonDuring combustion, the first element that is burned is the hydrogen. If combustion is not complete, carbon monoxide is formed, plus leaving some unburned carbon to accumulate in the combustion chamber.

45. HYDRAULIC HYBRID STORAGE SYSTEMFord Motor Co. is experimenting with a system it calls Hydraulic Power Assist (HPA). This system converts kinetic energy to hydraulic pressure, and then uses that pressure to help accelerate the vehicle. It is currently being tested on a four-wheel-drive (4WD) Lincoln Navigator with a 4.0-L V-8 engine in place of the standard 5.4-L engine.

46. While the concept is simple, the system itself is very complicated. Additional components include: Pulse suppressors Filters An electric circulator pump for cooling the main pump/motor

47. HCCIHomogeneous-Charge Compression Ignition (HCCI) is a combustion process. HCCI is the combustion of a very lean gasoline air-fuel mixture without the use of a spark ignition. It is a low-temperature, chemically controlled (flameless) combustion process.

48. While the challenges of HCCI are difficult, the advantages include having a gasoline engine being able to deliver 80% of diesel efficiency (a 20% increase in fuel economy) for 50% of the cost. A diesel engine using HCCI can deliver gasoline-like emissions.

49. PLUG-IN HYBRID ELECTRIC VEHICLESA plug-in hybrid electrical vehicle (PHEV) is a hybrid electric vehicle that is designed to be plugged into an electrical outlet at night to charge the batteries. By charging the batteries in the vehicle, it can operate using electric power alone (stealth mode) for a longer time thereby reducing the use of the internal combustion engine (ICE).

50. FUTURE FOR ELECTRIC VEHICLESThe future of electric vehicles depends on many factors including:1. The legislative and environmental incentives to overcome the cost and research efforts to bring a usable electric vehicle to the market.2. The cost of alternative energy.3. Advancement in battery technology that would allow the use of lighter-weight and high-energy batteries.