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Fuel Cells: fuelling cars to the future
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1. NEED FOR FUEL CELLS After a century of constant improvements, the internal combustion engine still only converts on average about 16 percent of the energy in gasoline to turn the car’s wheels. All heat engines have efficiencies limited by the Carnot Cycle. The theoretical thermodynamic derivation of the Carnot Cycle shows that even under ideal conditions, a heat engine, used to power a vehicle or generator, cannot convert all the heat energy supplied to it into mechanical energy. Some of the heat energy is rejected. In an internal combustion engine, the engine accepts heat from a source at a high temperature (T1), converts part of the energy into mechanical work and rejects the remainder to a heat sink at a low temperature (T2). The greater the temperatures difference between source and sink, the greater the efficiency: Maximum Efficiency = (T1 – T2) / T1 Where the temperatures T1 and T2 are given in degrees Kelvin Fuel cell vehicles, not limited by the Carnot Cycle, are expected to achieve energy efficiencies of 40 to 45 percent and very possibly higher. Thus Fuel cell vehicles have proven to be much more efficient than similar internal combustion vehicles Secondly, the conventional ICE vehicle, however refined they may be, emit harmful exhaust, whose effects is not evident today will be bourn by our children in the future. Given the significant improvement in energy efficiency, fuel cell vehicles offer substantial reductions in greenhouse gas emissions, and higher mileage too. Thirdly, the conventional fuel resources are limited on this planet. Today or tomorrow they will be exhausted bringing the entire world to a standstill. So it is imperative that we look for other avenues to power our modern transportation systems. Fuel cells provide an excellent alternative to today’s gasoline. Since it uses simple elements to produce energy which are abundantly available, there is no question of depletion these resources. All the above reasons emphasis the importance of fuel cells in today’s world. In the further sections of the paper, we will trying explain the basic working of fuel cells, their important types along with advantages and drawbacks and further scope of development. 2.INTRODUCTION In principle, a fuel cell operates like a battery. Unlike a battery, a fuel cell does not run down or require recharging. It will produce energy in the form of electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat. Refer Figure 1. This type of a fuel cell is called Proton Exchange Membrane (PEM) fuel cell. This is the most basic fuel cell based on its principle other fuel cell are made. A basic overview of the component and the working of a PEM fuel cell is given below: 2.1 Parts of a fuel cell: (Refer Fig.2) • The anode, the negative post of the fuel cell, has several jobs. It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. It has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst. • The cathode, the positive post of the fuel cell, has channels etched into it that distribute the oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. • The electrolyte is the proton exchange membrane. This specially treated material, which looks something like ordinary kitchen plastic wrap, only conducts positively charged ions. The membrane blocks electrons. • The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM. 2.2 Working of a PEM fuel cell: (Refer Fig no.3) Pressurized hydrogen gas (H2) enters the fuel cell on the anode side. This gas is forced through the catalyst by the pressure. When an H2 molecule comes in contact with the platinum on the catalyst, it splits into two H+ ions and two electrons (e-). The electrons are conducted through the anode, where they make their way through the external circuit (doing useful work such as turning a motor) and return to the cathode side of the fuel cell. Meanwhile, on the cathode side of the fuel cell, oxygen gas (O2) is being forced through the catalyst, where it forms two oxygen atoms. Each of these atoms has a strong negative charge. This negative charge attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two of the electrons from the external circuit to form a water molecule (H2O). This reaction in a single fuel cell produces only about 0.7 volts. To get this voltage up to a reasonable level, many separate fuel cells must be combined to form a fuel-cell stack.
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