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ISAT 413 - Module VI: Energy Efficiency in Transportation

Topic VI-1: Fuel Cell Types, Performance and Applications

What is a Fuel Cell?

Proton Exchange Membrane

Problems with Fuel Cells

Efficiency of Fuel Cells

Other Types of Fuel Cells

Applications of Fuel Cells

http://www.howstuffworks.com/fuel-cell.htm

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What is a Fuel Cell?

A fuel cell is an electrochemical energy conversion device that converts hydrogen and oxygen into water, producing electricity and heat in the process. It is very much like a battery that can be recharged while you are drawing power from it. Instead of recharging using electricity, however, a fuel cell uses hydrogen and oxygen.

The fuel cell will compete with many other types of energy conversion devices, including the gas turbine in your city's power plant, the gasoline engine in your car and the battery in your laptop. Batteries store electrical energy by converting it into chemical energy, which can be converted back into electrical energy when needed.

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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 using a different chemistry. Fuel cells are usually classified by the type of electrolyte they use. Some types of fuel cells show promise for use in power generation plants. Others may be useful for small portable applications or for powering cars.

Fuel cells offer a means of making power more efficiently and with less pollution. But how does it do this? Photo at right shows a fuel-cell stack that could power an automobile.

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Proton Exchange Membrane

The proton exchange membrane fuel cell (PEMFC) is one of the most promising technologies. This is the type of fuel cell that will end up powering cars, buses and maybe even your house.

The proton exchange membrane fuel cell (PEMFC) uses one of the simplest reactions of any fuel cell. Figure at right shows four basic elements of a PEMFC.

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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.

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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.

Chemistry of a Fuel CellAnode side:

H2 4H+ + 4e- Cathode side:

O2 + 4H+ + 4e- 2H2O Net reaction:

2H2 + O2 2H2O

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http://www.humboldt.edu/~serc/animation.html

This animation shows the process that goes on inside an individual fuel cell. The red Hs represent hydrogen molecules (H2) from a hydrogen storage tank. The orange H+ represents a hydrogen ion after its electron is removed. The yellow e- represents an electron moving through a circuit to do work (like lighting a light bulb or powering a car). The green Os represent an oxygen molecule (O2) from the air, and the blue drops at the end are for pure water ─ the only byproduct of hydrogen power.

The Proton Exchange Membrane Fuel Cell Animation

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PEMFCs operate at a fairly low temperature (about 176 degrees Fahrenheit, 80 degrees Celsius), which means they warm up quickly and don't require expensive containment structures. 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.

In a single fuel cell produces only about 0.7 volts. To produce power in large amounts, many of these cells are combined into a fuel cell stack. The image at right shows a fuel cell stack composed of 36 cells.

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Problems with Fuel Cells

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.

The hydrogen is not so readily available, however. 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 station. Hydrogen is difficult to store and distribute, so it would be much more convenient if fuel cells could use fuels that are more readily available. This problem is addressed by a device called a reformer.

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A reformer turns hydrocarbon or alcohol fuels into hydrogen, which is then fed to the fuel cell. 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. Many people have natural-gas lines or propane tanks at their house already, so these fuels are the most likely to be used for home fuel cells. Methanol is a liquid fuel that has similar properties to gasoline. It is just as easy to transport and distribute, so methanol may be a likely candidate to power fuel-cell cars.

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Reforming MethanolThe molecular formula for methanol is CH3OH. The goal of the reformer is to remove as much of the hydrogen (H) as possible from this molecule, while minimizing the emission of pollutants such as carbon monoxide (CO). The process starts with the vaporization of liquid methanol and water. Heat produced in the reforming process is used to accomplish this. This mixture of methanol and water vapor is passed through a heated chamber that contains a catalyst. As the methanol molecules hit the catalyst, they split into carbon monoxide (CO) and hydrogen gas (H2): CH3OH CO + 2H2. The water vapor splits into hydrogen gas and oxygen; this oxygen combines with the CO to form CO2. In this way, very little CO is released, as most of it is converted to CO2. H2O + CO CO2 + H2.

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Efficiency of Fuel Cells

Pollution reduction is one of the primary goals of the fuel cell. We will compare a fuel-cell-powered car to a gasoline-engine-powered car and a battery-powered car.

Fuel-Cell-Powered Electric CarIf 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. But, as we learned in the previous section, hydrogen is difficult to store in a car. When we add a reformer to convert methanol to hydrogen, the overall efficiency drops to about 30 to 40 percent, and a reasonable number for the efficiency of the motor/inverter is about 80 percent. Thus,

%Energy Mechanical%ElectricalChemical 288035 %

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Gasoline-Powered CarThe 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. So the overall efficiency of an automotive gas engine is about 27 percent. That is, only about 27 percent of the thermal-energy content of the gasoline is converted into mechanical work.

%Energy Mechanical%ThermalChemical 279030 %

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Battery-Powered Electric Car (1)This type of car has a fairly 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.

%Energy Mechanical%ElectricalChemical 728090 %

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Battery-Powered Electric Car (2)But that is not the whole story. The electricity used to power the car had to be generated somewhere. If it was 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 was 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.

%Energy Mechanical%Chemical

DC to ACfrom%ElectricalChemical

2672

9040

%

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Battery-Powered Electric Car (3)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.

%Energy Mechanical%Chemical

DC to ACfrom%ElectricalPotential

6572

90100

%

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Surprised?Maybe you are surprised by how close these three technologies are. This points out the importance of considering the whole system, not just the car. We could even go a step further and ask what the efficiency of producing gasoline, methanol or coal is. Efficiency is not the only consideration, however. People will not drive a car just because it is the most efficient if it makes them change their behavior. They are concerned about many other issues as well. They want to know:

• Is the car quick and easy to refuel?• Can it travel a good distance before refueling? • Is it as fast as the other cars on the road? • How much pollution does it produce?

This list, of course, goes on and on. In the end, the technology that dominates will be a compromise between efficiency and practicality.

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Well-to-Wheel Efficiency

A report released in March 2003 by MIT's Laboratory for Energy and the Environment concluded, "Even with aggressive research, the hydrogen vehicle will not be better than the diesel hybrid in terms of total energy use and greenhouse gas emission by 2020."

Source: Mechanical Engineering Power (2003)

“Gauging Efficiency, Well to Wheel” By Frank Kreith and R.E. West

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Other Types of Fuel CellsThere 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 program since the 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 cell is unlikely to be commercialized.

Phosphoric-acid fuel cell (PAFC): The phosphoric-acid fuel cell has potential for use in small stationary power-generation systems. It operates at a higher temperature than PEM fuel cells, so it has a longer warm-up time. This makes it unsuitable for use in cars.

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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 temperatures (around 1,832oF, 1,000oC). This high temperature makes reliability a problem, but it also has an advantage: 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 also best suited for large stationary power generators. They operate at 1,112oF (600oC), so they also generate steam that can be used to generate more power. They have a lower operating temperature than the SOFC, which means they don't need such exotic materials. This makes the design a little less expensive.

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Type of Cell Efficiency (%)

Operating Temperature

(oC)

Unit Size (kW)

Proton Exchange Membrane (PEM)

40~50 80 50

Phosphoric Acid 40~50 200 200

Molten Carbonate (NaCO3)

60+ 650 2000

Solid-Oxide Ceramic 60+ 800 100

Alkaline (KOH) 70 60 2~5

Types of Fuel Cells

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Applications of Fuel Cells AutomobilesFuel-cell-powered cars will start to replace gas- and diesel-engine cars in about 2005. A fuel-cell car will be very similar to an electric car but with a fuel cell and reformer instead of batteries. Most likely, you will fill your fuel-cell car up with methanol, but some companies are working on gasoline reformers. Other companies hope to do away with the reformer completely by designing advanced storage devices for hydrogen.

Check out these links for more information about fuel-cell-powered automobiles:

• Fuel Cell Cars • Ballard fuel cells: Transportation Prototypes • Transportation Fuel Cell Pictures

• DaimlerChrysler unveils fuel-cell vehicle

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Portable Power

Fuel cells also make sense for portable electronics like laptop computer, cellular phones or even hearing aids. In these applications, the fuel cell will provide much longer life than a battery would, and you should be able to "recharge" it quickly with a liquid or gaseous fuel.

Check out these links for more information on portable-power fuel cells:

• Tiny Fuel Cell to Power Sensors • New Bicycle Powered by Fuel Cells • Ballard fuel cells: Portable Power

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BusesFuel-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 drive a vehicle. In the first fuel-cell bus, about one-third of the vehicle was filled with fuel cells and fuel-cell equipment. Now the power density has increased to the point that a bus can run on a much smaller fuel cell.

Check out these links for more information on fuel-cell buses:

• Federal Transit Administration: Fuel Cell Transit Bus• MSNBC: Getting on board the fuel-cell bus

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Home Power GenerationThis is a promising application that you may be able to order as soon as 2002. General Electric 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 seven kilowatts of power (which is enough for most houses). A system like this produces electricity and significant amounts of heat, so it is possible that the system could heat your water and help to heat your house without using any additional energy. Check out these links for more information on fuel-cell home power generation:

• Plug Power • General Electric • IdaTech

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Large Power Generation (1)Some fuel-cell technologies have the potential to replace conventional combustion 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 generate electricity directly from hydrogen in the fuel cell, but will also use the heat and water produced in the cell to power steam turbines and generate even more electricity. There are already large portable fuel-cell systems available for providing backup power to hospitals and factories. Check out these links for more information on large fuel-cell power generation:

• Ballard: 250 kW commercial prototype • IdaTech

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Large Power Generation (2)

The International Fuel cells and its sister company, Onsi Corp., of South Windsor, Connecticut fill with their 200-kW phosphoric acid fuel cell power plants near the Central Park in New York City. Photo below shows one employee checks the deionization system; the reformers site to his right. In back, another inspects a second unit before the doors go on.

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Good Luck to You on All Your Finals!

Have a Happy Winter Break! And

Enjoy Your Senior Year!