Fuel Cells

FUEL CELLS BY RAVI BHASKAR MECH- B ROLL NO.-80

CONTENTS • • • • • • • •

INTRODUCTION PROTON EXCHANGE MEMBRANE CHEMISTRY OF FUEL CELLS PROBLEMS WITH FUEL CELLS EFFICIENCY OF FUEL CELLS TYPES OF FUEL CELLS APPLICATIONS CURRENT SCENARIOS

INTRODUCTION A fuel cell is an electrochemical energy conversion device. This device converts hydrogen and oxygen into water, producing electricity and heat in the process. It is like a battery that can be recharged while you are deriving power from it. In fuel cell hydrogen and oxygen are used for recharging. The fuel cell will compete with many other types of energy conversion devices such as gas turbine, petrol or diesel engine etc. combustion engines like the turbine and the gasoline engine burn fuels and use the pressure created by the expansion of gases to do mechanical work. Batteries store electrical energy by converting it to chemical energy, which can be converted back into electrical energy when needed. A fuel cell provides a DC( direct current) voltage that can be used to power motors, lights or any number of electrical appliances. There are several different types of fuel cells, each having a different chemistry.

PROTON EXCHANGE MEMBRANE The proton exchange membrane fuel cell( PEMFC) is one of the most promising technologies. PEMFC uses one of the simplest reactions of any fuel cell. DIAGRAM: -

H-H

Proton exchange membrane e e

H+

O Oxygen from air

H+ exhaust

H2O

CONSTRUCTION •

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There are four basic elements in a PEMFC: 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. 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 like an 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.

WORKING The pressurized hydrogen gas (H2) entering the fuel cell on the anode side. The 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. The electrons are conducted through the anode, where they make way through the external circuit and return to the cathode side of the furl 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 have a strong negative charge. This negative charge attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two electrons form the external circuit to form a water molecule (H2O). This reaction in an a single fuel cell produces only about 0.7V. To get this voltage up to a reasonable level, many separate fuel cells must be combined to form a fuel cell stock. PEMFCs operate at fairly low temperatures of about 80 degree C. which means they warm up quickly and do not require expensive containment structure. Constant improvements in the engineering and materials used in these cells have increased the power density to a level where a device about the size of a small piece of luggage can power a car.

CHEMISTRY OF FUEL CELLS REACTIONS: ANODE SIDE: 2H2 4H+ + 4eCATHODE SIDE: O2 + 4H+ +4e2H2O NET REACTION: 2H2 + O2 2H2O

PROBLEMS WITH FUEL CELL A fuel cell uses oxygen and hydrogen to produce electricity. The oxygen required for a fuel cell comes from the air. In fact, in the PEM fuel cell, ordinary air is pumped into the cathode. Hydrogen has some limitations that make it impractical for use in most applications. For instance, you don't have a hydrogen pipeline coming to your house, and you can’t pull up to a hydrogen pump at your local gas. Hydrogen is difficult to store and distribute, so it would be much more convenient if fuel cells could use fuel that are more readily available. A device called REFORMER addresses this problem. A reformer turns hydrocarbon or alcohol fuels into hydrogen which is then fed to the fuel cells. Unfortunately, reformers are not perfect. They generate heat and produce other gases besides hydrogen. They use various devices to try to clean up the hydrogen, but even so, the hydrogen that comes out of them is not pure, and this lowers the efficiency of the fuel cell. Some of the more promising fuels are natural gas, propane and methanol. These fuels are more likely to be used for home fuel cells. Methanol is a liquid fuel that has similar properties to gasoline. It is easier to transport and distribute, so methanol may be a likely candidate to power fuel cell cars.

EFFICIENCY OF FUEL CELLS In this section, it will be seen how fuel cells might improve the efficiency of cars today. It is important to remember that pollution reduction is one of the primary goals of the fuel cell. A fuel cell powered car is compared to a gasoline engine powered car and a battery powered car. Since all the cars have many of the same components, those parts of the car will be ignored and the efficiency compared up to the point where mechanical power is generated.

Fuel cell powered electric car: If the fuel cell is powered with pure hydrogen, it has the potential to be up to 80% efficient. That is, it converts 80% of the energy content of the hydrogen into electric energy. But hydrogen is difficult to store in a car. When we add reformer to convert methanol to hydrogen, the overall efficiency drops to about 30 to 40%. The electric energy should be converted into mechanical work. The electric inverter and motor accomplish this. A reasonable number for the efficiency of the motor/inverter is about 80%. We have 30 to40% efficiency at converting methanol to electricity and 80% efficiency converting electricity to mechanical power. This gives an overall efficiency of about 24 to 32%.

Gasoline powered car: The efficiency of gasoline powered car is surprisingly low. All the heat that comes out as exhaust or goes into the radiator is wasted energy. The engine also uses a lot of energy turning various pumps, fans and generators that keep it going. So the overall efficiency of an automotive gas engine is about 20%. That is, only 20% of the thermal energy content of the gasoline is converted into mechanical work.

Battery powered electric car: This type of car has fairly high efficiency. The battery is about 90% efficient and that of electric motor/inverter is about 80%. This gives an overall efficiency of about 72%. The electricity used to power the car has to be generated somewhere. It is generated at a power plant that uses combustion process, then only about 40% of the fuel required by the power plant is converted into electricity. The process of charging the car requires the conversion of alternating current power to direct current power. The process has an efficiency of about 90%.

So, by looking at the whole cycle, the efficiency of an electric car is 72%, 40% for the power plant and 90% for charging the car. That gives and overall efficiency of 26%. 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 power plant, then it is basically free( no fuel is burned), and the efficiency of the electric car is about 65%. This points out the importance of considering the whole system, not just the car. Efficiency is not the only consideration however. People will not drive a car just because it is efficient. Other issues are important such as: Is the car quick and easy to refuel? How much pollution does it produce? Can it travel a good distance before refueling? In the end the technology that dominates will be a compromise between efficiency and practicality.

OTHER TYPES OF FUEL CELLS There are several other types of fuel cell technologies being developed for possible commercial uses : -

• ALKALINE FUEL CELL(AFC) : This is one of the oldest designs. It has been used in the U.S. space programs since 1960s. The AFC is very susceptible to contamination, so it requires pure hydrogen and oxygen. It is also very expensive, so this type of fuel cells are unlikely to be commercialized.

• PHOSPHORIC ACID FUEL CELL(PAFC) : The PAFC cells have potential for use in small stationary power generation systems. It operates at higher temperatures than PEM fuel cells, so it has longer warm up time. This makes it unsuitable for use in cars.

• SOLID OXIDE FUEL CELL(SOFC) : These fuel cells are best suited for large scale stationary power generators that could provide electricity for factories or towns. This type of fuel cell operates at very high temperature of about 1000 degree C. This high temperature makes reliability a problem, it has also an advantage that is the steam produced by the fuel cell can be channeled into turbines to generate more electricity. This improves the overall efficiency of the system.

• MOLTEN CARBONATE FUEL CELL(MCFC) : These fuel cells are best suited for large stationary power generators. They operate at 600 degree C, so they also generate steam that can be used to generate more power. They have lower operating temperature than SOFC, which means they don’t need exotic materials. This makes the design less expensive.

APPLICATIONS OF FUEL CELLS As it is specified, fuel calls can be used in a number of applications such as : -

• AUTOMOBILES Fuel cell powered cars will replace gas and diesel engine cars in near future. A fuel cell car will be very similar to an electric car but with a fuel cell and reformer instead of batteries. Most likely a fuel cell car will be filled up with methanol., but some companies are working on gasoline reforms. Other companies have to do away with the reforms completely by designing advanced storage devices for hydrogen. General Motors (GM) are the leading automaker right now that funds related research and implementation of fuel cell automobiles.

• PORTABLE POWER Fuel cells also make sense for portable electronics like laptops, cell phones and hearing aids. In these applications fuel cell will provide much longer life than a battery would, and it can be recharged quickly with a liquid or gaseous fuel.

• BUSES Fuel cell powered buses are already running in several cities. The bus was one of the first applications of the fuel cell because initially, fuel cells needed to be quite large to produce enough power to run a vehicle. In the first fuel cell bus, about one third of the bus was filled with fuel cell and fuel cell equipment. Now the power density has increased to the point that a bus can run on a much smaller fuel cell.

• HOME POWER GENERATION This is a promising application that a person may be able to order or use in near future. General Electric (GE) is going to offer a fuel cell generator system made by plug power. This system will use a natural gas or propane reformer and produce up to 7KW of power. A system like this produces electricity and significant amount of heat, so it is possible that the system could heat your water and help to heat your house without using additional energy.

• LARGE POWER GENERATION Some fuel cell technologies have the potential to replace conventional power plants. Large fuel cells will be able to generate electricity more efficiently than today’s power plants. The fuel cell technologies being developed for these power plants will develop electricity directly from hydrogen in the fuel cell, but will also use the heat and water generated in the cell to power steam turbines and generate even more electricity. They are already large portable fuel cell systems available for providing back up power to hospitals and factories.

CURRENT SCENARIO IN DEVELOPMENT OF FUEL CELLS • FUTURE BICYCLE : Manhattan Scientifics Inc. has developed a fuel cell powered mountain bike that uses hydrogen and air as fuel and emits only water vapor as waste product. According to the developers, the hydro cycle has a top range of 70100Km along a flat surface and can attain a top speed of 30Kmph. Because a fuel cell stack powers its electric motor, the hydro motor is extremely quiet and does not needs to be recharged like existing electric bicycles, it only needs to be refueled. This would come as an advantage to electric bike riders who wait hours to recharge their battery powered bicycles. However, the hydro cycle will come with inconvenience of its own, as there is no system of hydrogen refueling stations in place.

• BALLARD FUEL CELL : Ballard delivers fuel cell modules and engines to automotive customers who are developing, building and testing fuel cell vehicle prototype for demonstration and fleet applications. Ballard’s customers are some of the largest car and bus manufacturers in the world.