FUEL CELL-DRIVEN AUTOMOBILES
SK ANSARI, SEEMA ANSARI
July 26 - Aug 1, 2010
In future, fuel cells will play an important role. Fuel cell systems are more environmentally friendly than conventional systems and their size can be adapted to the market requirements, ranging from fuel cells used to drive cars to those for electricity generation in the power plants.
Fuel cells directly convert chemical into electrical energy ñ without noise emission or open combustion. They use hydrogen or hydrogen containing gases as the fuel and pure oxygen or air as oxidant. In contrast to the detonation oxy-hydrogen reaction, which releases thermal energy by way of explosion, the electrochemical reaction in a fuel cell releases energy in the form of electrical current and only minor amounts as heat.
Growing environmental awareness in the late 20th Century and the large increase in environmental pollution due to existing petroleum fuel vehicles and efforts to improve air quality led to the introduction of the clean air act. The law demanded a set proportion of emission-free automobiles (zero emission vehicles) to be in operation from 2004 onward. This put the automobile industry under strong pressure to develop clean driving techniques. Various prototypes of fuel cell vehicles have been developed since the mid-1990s.
FUEL CELLS AND ITS APPLICATIONS
Fuel cells have several highly attractive characteristics. The efficiency of a fuel cell can be higher than in conventional energy conversion processes and the performance remains good even at partial loads. The only waste product is oxidized fuel. Normally, the fuel is hydrogen and consequently, the product is water. Carbon dioxide may be present if a hydrocarbon fuel is used. Fuel reformation adds the benefit of fuel flexibility, but multi-fuel reformer technology is still in early development stage.
FUEL CELL EFFICIENCY
Pollution reduction is one of the primary goals of the fuel cell. By comparing a fuel-cell-powered car to a gasoline-engine-powered car and a battery-powered car, you can see how fuel cells might improve the efficiency of cars today.
Since all cars have many of the same components (tyres, transmissions, et cetera), we'll ignore that part of the car and compare efficiencies up to the point where mechanical power is generated.
If the fuel cell is powered with pure hydrogen, it has the potential to be up to 80-percent efficient. That is, it converts 80 percent of the energy content of the hydrogen into electrical energy. However, we still need to convert the electrical energy into mechanical work. The electric motor and inverter accomplish this. A reasonable number for the efficiency of the motor/inverter is about 80 percent. So we have 80-percent efficiency in generating electricity, and 80-percent efficiency converting it to mechanical power. That gives an overall efficiency of about 64 percent.
A battery-powered electric car has a high efficiency. The battery is about 90-percent efficient (most batteries generate some heat, or require heating), and the electric motor/inverter is about 80-percent efficient. This gives an overall efficiency of about 72 percent.
But that is not the whole story. The electricity used to power the car has to be generated somewhere. If it is generated at a power plant that used a combustion process (rather than nuclear, hydroelectric, solar or wind), then only about 40 percent of the fuel required by the power plant is converted into electricity. The process of charging the car requires the conversion of alternating current (AC) power to direct current (DC) power. This process has an efficiency of about 90 percent.
So, if we look at the whole cycle, the efficiency of an electric car is 72 percent for the car, 40 percent for the power plant and 90 percent for charging the car. That gives an overall efficiency of 26 percent. The overall efficiency varies considerably depending on what sort of power plant is used. If the electricity for the car is generated by a hydroelectric plant for instance, then it is basically free (we didn't burn any fuel to generate it), and the efficiency of the electric car is about 65 percent.
The efficiency of a gasoline-powered car is surprisingly low. All of the heat that comes out as exhaust or goes into the radiator is wasted energy. The engine also uses a lot of energy turning the various pumps, fans, and generators that keep it going.
STORAGE AND OTHER CONSIDERATIONS
Three hundred miles is a conventional driving range (the distance you can drive in a car with a full tank of gas). In order to create a comparable result with a fuel cell vehicle, researchers must overcome hydrogen storage considerations, vehicle weight and volume, cost, and safety.
POLYMER ELECTROLYTE MEMBRANE FUEL CELLS
PEM fuel cells are an attractive option for a wide range of applications, including vehicle power sources, distributed power and heat production, and even portable and mobile systems, for example consumer electronics. At this moment, automotive industry is the largest investor in the PEM fuel cell development. Many manufacturers have announced their commitment to introduce commercial fuel cell cars.
New applications are emerging in the field of portable power generation, where fuel cell systems may offer benefits compared to primary and rechargeable batteries in portable electronics. Major drawbacks of batteries are limited capacity and slow recharging. With a suitable hydrogen storage method, fuel cell systems are quicker to recharge and will achieve higher power and energy densities.
A Polymer Electrolyte Membranes fuel cell is an electrochemical device that produces electricity silently, without combustion. Hydrogen fuel, which can be obtained from natural gas or methanol, and oxygen from the air are electrochemically combined in the fuel cell to produce electricity. Heat and pure water vapor are the only by-products.
Canada's Ballard Power System Inc is currently the leader in fuel cell technology. Not only produced many fuel cell buses servicing in Europe, it also won Mercedes-Benz and Ford's contract to develop fuel cell power system for their passenger cars. Mercedes invested $450M in Ballard and planned to produce 100,000 fuel cell cars in 2005. It seems that fuel cell technology will be no longer a research topic like other alternative fuel.
MERCEDES NECAR 4
The NECAR 4 is the fourth experimental fuel cell car Mercedes-Benz. Unlike all its predecessors, it is practicality like any conventional cars because it is based on the A-class. Only the high cost of the fuel cell power unit - some £19,000 versus piston engine's £1,200 - prevents it from going into production.
It is nonsense to say NECAR 4 performs as good as ordinary cars, although it is already the most advanced fuel cell car ever appeared. The 75 hp fuel cell stack plus all the accessories like electric motor and high-pressure fuel tank put some 410 kg over the slowest petrol A class, yet the A140 output 7 more horsepower than the NECAR 4. Although, the electric motor has constant torque at any RPM, it still fails to compensate the weight penalty - considering it weighs as heavy as a well-specified E320. Top speed barely reaches 90 mph, or 15 mph lower than A140.
More questions about the fuel supply should be raised. Fuel cells can drink hydrogen as well as methanol. The former is the more favorable as it generates only pure water during the reaction, hence no air pollution at all to our cities. However, the highly explosive liquid hydrogen should be stored in a strong, high-pressure tank cooling at minus 230∫C, thus arouses concerns about safety. In particular, collision from behind will hit right on the fuel tank.
STOP-START ENGINE HYBRIDS
A stop-start hybrid is the simplest kind, but this minimal technology may become the most common within a few years. It composes simply of an energy storage device-like a battery-and a beefed-up starter-motor that can also act as a generator.
Stop-start systems are also called idle-stop because it puts an end to burning fuel and emitting pollutants when a conventional car would be idling.
In practice, the car's engine control unit shuts off the engine when the car slows down or comes to a stop. As soon as the driver puts in the clutch, moves the shift lever, or accelerates, the battery powers the starter motor, which quickly switches on the engine.
Stop-stop systems are the lowest-cost hybrid alternative, but if fitted to large numbers of cars they could substantially reduce fuel consumption and air pollution from idling vehicles, especially in crowded city centers. BMW has announced plans to put start-stop systems in all of its vehicles, without using the word "hybrid". Other carmakers, including Citroen, Mazda, and Mercedes-Benz are enthusiastically embracing the technology for its ability to boost efficiency at a relatively low cost.
BATTERY TYPE ENERGY DENSITY (WH/KG) SPECIFIC POWER (W/KG) RECHARGE TIME (HOURS) LIFE (NO. OF CHARGE) ENERGY EFFICIENCY Lead-Acid 30 130 8 400 65% Ni-MH 80 250 <6 600 90% Lithium-ion 100 300 <3 1200 .
Most EVs use traditional DC brush motors. Two motors, one drives the right front wheels and another drives the left one, provides the power for the whole car. DC motors are cheap, but cannot provide sufficient power that a really fast EV needs. Therefore, GM EV1 adopts a complicated 3-phase AC motor, which is supplied by an inverter which transforms DC supply to AC. Since the motor is induction motor, it has no friction that a DC brush generates; therefore, it could be a lot more powerful.
All EVs do not need a transmission. The flat torque characteristic of electric motor eliminates the need for gearing. Reverse gear is also saved because it can be simulated by reversing the polarity of the motor input.
Another special feature of EV appears during braking. Physical principles tell us that while rotating a motor by external force, the motor will become a generator. EVs make good use of this principle to recharge its batteries during braking.
Owing to the low working temperature of PEMFCs neither hydrocarbons nor alcohols can be used directly as fuel. They must be transformed into the hydrogen-containing gas. The processes of transforming a raw material with chemically bonded hydrogen into a hydrogen-containing gas mixture (reforming), and subsequent purification of this gas from unwanted components (gas purification), are known collectively as fuel processing.
Several strategies have been developed to produce a viable PEMFC that can function in spite of residual CO in the feed gas: some of the simplest measures include the operation of the fuel cell under pressure or the mixing of a small amount of air with the hydrogen obtained by reforming. The amount of air added must be metered carefully so that it is clearly below the lower explosion limit of a hydrogen-air mixture. Oxygen in the air oxidizes the CO adsorbed at the surface of the catalyst to CO2, which is subsequently desorbed. This procedure is referred to in the literature as an air bleed technique. It has the disadvantage that the added oxygen also reacts with hydrogen to form water, which reduces the efficiency of the fuel cell.
At present, a widespread supply of hydrogen is not possible due to inadequate infrastructure. In addition, problems in the handling and storage of hydrogen have not yet been solved satisfactorily. For example, the transport of compressed hydrogen by road consumes more energy than the hydrogen itself provides. Therefore, decentralised generation of hydrogen is an appropriate alternative. This can be either a decentralised production of stockpiles of hydrogen at, for example, fueling stations, or the transformation of available energy carriers in to a hydrogen-rich gas (reforming) directly before its use in a fuel cell.
While PEMFC systems have become lighter and smaller as improvements are made, they still are too large and heavy for use in standard vehicles.
There are also safety concerns related to fuel cell use. Engineers will have to design safe, reliable hydrogen delivery systems.