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FUEL CELL CONCEPT
A fuel cell is a practical method of achieving a near reversible chemical reaction in which there is a conversion of chemical energy into electrical energy without even the intermediate step of conversion into random molecular energy (i.e. sensible internal energy). Thus whereas the harmonic and thermoelectric forms of direct converter are essentially heat engines and subject to the Carnot limitation on efficiency, the fuel cell is not. It was thought of that if such a device with hydrocarbon fuels or cheap derivatives such as carbon monoxide could be developed it would be a great step forward in the efficient utilization of fuel resources.
The fuel cell as depicted in the fig. is an electrochemical device consisted basically of an electrolyte and two-consumable electrode (often porous plates through which the fuel and oxidant diffuses). The electrodes play a catalytic role in facilitating the ionizing reactions. In simple type of hydrogen oxygen fuel cell the reaction might be written as
Anode H2---- 2H+ +2E
Cathode 2e- +2H+ +1/202 ---- H2O
The open circuit emf can be related to a function of the enthalpy and entropy of the reactants and products (in fact to the change in the Gibbs function (go), and the effected of temperature and pressure of the emf can be determined by chemical thermodynamic analysis. The ideal maximum work for the reaction is not made available in practice because of internal losses. These including inhibiting effects due to absorption of molecules on the surface of the electrodes, uneven concentration of electrolyte when current is flowing, changes in the conductivity of the electrolyte near the electrodes and ohmics heating (12 R loss due to résistance of the electrolyte) depending upon the fuel and oxidant employed, the emf is produced is between 0.5 and1.5 volts and any required output voltage can be obtained by connecting cell in series as in the ordinary electric battery. Current densities of up to 400 amp/m2 have been achieved.
A fuel cell is basically a miniature power plant producing electrical energy through a “cold combustion” reaction between hydrogen and oxygen. Normally, oxygen and hydrogen react in an explosive fashion. In the fuel cell, the two gases are separated by an electrolyte which prevents the hot reaction. An electrochemical process in the electrolyte permits only positively charged hydrogen ions (protons) to pass through, leaving the hydrogen electrons behind. The positive hydrogen ions react with the negative electrons pf oxygen on the other side.
Five different fuel cell technologies are currently competing to serve the market of the future. These employ different types of electrolytes:
THIN MEMBRANE
CAUSTIC POTASH
PHOSPHORIC ACID
CARBONATE
SOLID ZIRCON-OXIDE
Of these membrane fuel clear best suit for mobile applications due to their convenient working temperature range of 20 to 100 degrees Celsius and high performance to weight ratios.
DEVELOPMENT OF FUEL CELL CAR
Daimler’s fuel cell vehicle development project started in 1991 under the guidance of Dr. Hartmurt Weule, then R & D boss of Daimler Benz. In 1993 Vancouver based Ballard Power Systems, an experienced player in the fuel cell arena, joined the research effort.
The first prototype fuel cell car hit the road in 1994. It was a 3.5 tone Mercedes Benz- 180 van powered by a fuel cell system of 50 kW output. It named as New Electric Car (NECAR-1).The fuel cell system occupied so much space that the vehicle could accommodate only driver and one passenger as it cruised along at 90 km/h.The second prototype, NECAR-2, made public in May 1996 was a multipurpose vehicle based on the new V-class Mercedes.
Its 50 kW fuel cell system was made up of only 2 compact stacks and could be tucked away in a suitcase-sized box under rear bench seat, enabling accommodation for 6 occupants. The vehicle could reach a maximum speed of 110km/h and had a range of 250 kms using a fuel load of 280 litters of hydrogen.
A year later came the NEBUS, a zero emission city bus of 12 meters length and 2.5 meters width and curb weight of 14 tones which could accommodate 34 seated and 24 standing passengers. The NEBUS is propelled by a fuel cell system comprising 10 stacks of 25 kW each. Once installed in the bus, the fuel cells supply some190 kW to the vehicle system including power steering pumps, air compressor and door control.
Incidentally, with a cell efficiency of 55% the average energy yield of the fuel cell system is roughly 15% better than that a diesel engine. Enough power is available to drive a fully-laden bus at up to 80 km/h.Seven aluminum tanks mounted on the roof, each containing 150 liters of hydrogen under pressure, totaling 21 kg of the gas, enable NEBUSD to have a range of 250 kms before gas refill is needed.
Having been licensed by the German Technical inspectorate, NEBUS is cruising through the streets of manhiem and Stuttgart just like any other city bus. Fuel cell buses are also operating urban lines services in Chicago and Vancouver.
NECAR -1, NECAR-2 and NEBUS made use of hydrogen stored on board for conversion into electricity in the fuel cells. But hydrogen’s low energy density poses problem for fuel tank d To make matters worse, hydrogen is more difficult to handle than other liquid fuels since it can be transported, filed and stored as liquid at very low temperatures. Moreover, widespread infrastructure for hydrogen refilling is not in place.
Researchers at Daimler Benz came up with a solution to avoid this problem-generate hydrogen gas on board the vehicle itself by reforming a hydro-carbon liquid. This would enable existing refueling, transport and storage structure it be used. Reforming consist of first vaporizing the hydrocarbon liquid then injecting a mixture of the fuel vapor, air and water vapor into a catalytic reformer. At this stage hydrogen and a number of undesirable products such as carbon monoxide as well as well as some nitrogen are produced.
In the presence of water, the hydrogen is partially rid of carbon monoxide and cooled and in a final step called selective oxidation almost all the remaining carbon monoxide is eliminated (carbon monoxide can inhibit the catalytic process in the fuel cell but Daimler-Benz engineers have developed purification method of pushing the concentration of this gas below the level of 2 molecules per million air molecules. Finally the gas mixture, consisting of hydrogen, some carbon dioxide and tiny traces of carbon monoxide, is fed in to the fuel cells. Although gasoline or diesel can be reformed, the problem with this hydrocarbon is that it takes some time for the reformer to reach the highest temperature of around 1000 degrees F.
In practice, this means that the driver has to wait for some time before the vehicle is ready to roll. The researches then homed in on methanol as the ideal. When reforming methanol, temperature between 400 to 575 degrees Fahrenheit suffice to liberate hydrogen. It is cleaner than gasoline and therefore, does not require very complicated purification treatment. Moreover, since it can also be made from bio mass, methanol is a renewable source of energy.
In 1997, the fuel cell project came up with the NECAR-3, a Mercedes-Benz. A class compact car of just 3.57 meters in length, powered by two fuel cell stacks delivering a total of 50 kW of electric power. In a world first, the NECAR-3 generated its own hydrogen on board by reforming methanol. The heat required for vaporization of the methanol and the reformation is provided by a catalytic burner operated with gas from the fuel cells which utilize only around 75% of hydrogen supplied. The water produced as a result of the fuel cell reaction can be reused in the reforming process.
NECAR-3 can travel over 400 Km on 38 liters of methanol. The need for large hydrogen storage tanks is eliminated. With sports utility vehicles representing the most rapidly expanding market segment in the automobile industry, Daimler Chrysler went on to try out a fuel cell powered Jeep Commander using a gasoline reformer system on board to generate the hydrogen supply. It sported two electric motors, one for driving the front axle and one for the rear axle.
An additional electrochemical battery was provided to cold start and run the vehicle during the period required for the reformer to reach its operating temperature and start generating hydrogen. The extra battery system also enabled harnessing the energy normally wasted during braking.
The latest prototype to emerge from the fuel cell initiative is the NECER-4, which uses liquid hydrogen, instead of the compressed hydrogen gas. The fuel is stored in a cryogenic cylinder in the floor of the rear cargo area. Based on the Mercedes Benz A-class compact car, the NECAR-4 was unveiled in WASHINGTON on March 17, 1999
The NECAR-4 represents a new mile-stone in fuel cell technology. Special bipolar plates ensure that both air and gaseous hydrogen flow as freely as possible through the stacks. The platinum catalyst coating is now applied uniformly over the entire surface of the membrane. As a result, the area throughout which the catalyst can react with the hydrogen atoms to produce electrons and protons is spread homogenously across the whole membrane surface. This greatly enhances the fuel cell’s capacity to produce power. It also provides very high energy conversion efficiency. Over average, the new stacks function with an efficiency of between 50 to 80 percent.
The fuel cell system is composed of just 2 stacks, each of which delivers 35 KW of power. The entire fuel cell system fits into a suitcase size unit which conventionally disappears into the sandwich floor of the vehicle. Thus there is ample room for five passengers as well as plenty of cargo space. Improvements have also been made to the electric motor and the transmission. The motor encircles the drive axle. The transmission is integrated snugly with in the right hand side of the motor and connects directly to the right hand front wheel via a drive shaft. The other drive shaft runs through the motor to the left front wheel. The asynchronous electric motor with optimized torque generates up to 55 KW. Using the new European driving cycle method for calculations, the efficiency at wheel comes to as high as 36%. This means more than one-third the chemical energy of the hydrogen fuel is converted to mechanical energy for propelling the car.
