You may have heard a lot recently about fuel cells. According to many news reports, we may soon be using the new energy-saving technology to generate electrical power for our homes and cars. The technology is extremely interesting to people in all walks of life because it offers a means of making power more efficiently and with less pollution. But how does it do this?
In this edition of HowStuffWorks, we'll take a quick look at each of the existing or emerging fuel-cell technologies. We'll detail how one of the most promising technologies works, and we'll discuss a few potential applications of fuel cells.
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. Combustion engines like the turbine and the gasoline engine burn fuels and use the pressure created by the expansion of the gases to do mechanical work. Batteries store electrical energy by converting it into 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 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.
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.
Proton Exchange Membrane
The proton exchange membrane fuel cell (PEMFC) uses one of the simplest reactions of any fuel cell. First, let's take a look at what's in a PEM fuel cell:
Figure 1. The parts of a PEM fuel cell
In Figure 1 you can see there are four basic elements of 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.
- 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.
Figure 2. Animation of a fuel cell working
Figure 2 shows the pressurized hydrogen gas (H2) entering 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.
|Chemistry of a Fuel Cell
2H2 => 4H+ + 4e-
O2 + 4H+ + 4e- => 2H2O
2H2 + O2 => 2H2O
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.
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.
Problems with Fuel Cells
We learned in the last section that 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. 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. 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.
Efficiency of Fuel Cells
In this section, we will take a look at how fuel cells might improve the efficiency of cars today. Remember that 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. Since all three types of cars have many of the same components (tires, transmissions, etc.), we'll ignore that part of the car and compare efficiencies up to the point where mechanical power is generated. Let's start with the fuel-cell car. (All of these efficiencies are approximations, but they should be close enough to make a rough comparison.)
Fuel-Cell-Powered Electric Car
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. 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.
We still need to convert the electrical energy into mechanical work. This is accomplished by the electric motor and inverter. A reasonable number for the efficiency of the motor/inverter is about 80 percent. So we have 30- to 40-percent efficiency at converting methanol to electricity, and 80-percent efficiency converting electricity to mechanical power. That gives an overall efficiency of about 24 to 32 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. So the overall efficiency of an automotive gas engine is about 20 percent. That is, only about 20 percent of the thermal-energy content of the gasoline is converted into mechanical work.
Battery-Powered Electric Car
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.
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. 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.
Maybe you are surprised by how close these three technologies are. This exercise 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:
This list, of course, goes on and on. In the end, the technology that dominates will be a compromise between efficiency and practicality.
- 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?
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 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.
- 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 1,832 F, 1,000 C). 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 1,112 F (600 C), 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.
Applications of Fuel Cells
As we've discussed, fuel cells could be used in a number of applications. Each proposed use raises its own issues and challenges.
Fuel-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 automibles:
Fuel cells also make sense for portable electronics like laptop computers, 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:
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 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:
Home Power Generation
This 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:
Large Power Generation
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:
For more information on fuels cells and related topics, check out the links on the next page!
Lots More Information!
Related HowStuffWorks Articles
More Great Links
Fuel Cell Kits and Experiments
Fuel Cell Manufacturers