Hydrogen Fuel Cells

The fuel cell is an electrochemical device that converts hydrogen and oxygen into electricity and heat. It is similar to a battery that can be recharged while you are drawing power from it. Instead of recharging the device using electricity, a fuel cell uses hydrogen and oxygen.

Since hydrogen is not readily available to consumers, and today is difficult to store and distribute, a device called a reformer is used. A reformer turns hydrocarbon or alcohol fuels into hydrogen, which is then fed to the fuel cell. This process generates heat and produces other gases besides hydrogen, and as a result lowers the efficiency of the fuel cell. Some of the more promising fuels being used with fuel cells are natural gas, propane and methanol.

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, as well as the battery in your laptop computer. A fuel cell provides DC (direct current) voltage, which can be used to power motors, lights or any number of electrical appliances.

Efficiency of Fuel Cells

Let's consider the efficiency of fuel cells as they might relate to cars to better understand the complete system that needs to be considered when determining efficiency.

Fuel Cell-Powered Car

If a 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 have mentioned, hydrogen is difficult to store in a car. When a reformer is added to convert methanol to hydrogen, the overall efficiency drops to about 30 to 40 percent.

At this point, the electrical energy still needs to be converted into mechanical work. This is accomplished with an 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.

Gasoline-Powered Car

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. This results in an overall efficiency for an automotive gas engine of 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 battery-powered electric 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.

However, there is more to consider with a storage-type solution. The electricity stored ion the battery, which is 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 batteries for 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 about 72 percent for the car, 40 percent for the power plant, and 90 percent for charging the car. That gives an overall efficiency of about 26 percent. However, the overall efficiency varies considerably depending on what sort of power plant is used. As an example, if the electricity for a battery-powered car is generated by a hydroelectric plant, then there was no fuel required to generate the electricity. Hence, the overall efficiency of the electric car for this case is about 65 percent.

Applications of Fuel Cells

As we've mentioned, fuel cells could be used in a number of different applications. Each proposed use raises its own issues and challenges that are briefly discussed in the following Sections.

Automobiles

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, the fuel-cell car will utilize methanol, although, some companies are now working on gasoline reformers. And yet, there are other companies that are hoping to do away with the reformers completely by designing advanced storage devices for hydrogen.

Portable Power

Fuel cells also make good 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 typical battery would, and it should be able to be "recharged" quickly, using 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 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 more efficient fuel cells, requiring much smaller real estate to support fuel cell operations.

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 power plant use will generate electricity directly from hydrogen in the fuel cell. In addition, the heat and water produced in the cell will be utilized 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.

Types of Fuel Cell Technology

Of the various types of fuel cells, four are commercially available: phosphoric acid, polymer electrolyte membrane, molten carbonate, and solid oxide.

Phosphoric Acid Fuel Cells

The Phosphoric Acid Fuel Cell is the most mature fuel cell technology in terms of system development and commercialization activities. 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, which results in a longer warm-up time. This makes it unsuitable for use in cars.

Molten Carbonate Fuel Cells

The Molten Carbonate Fuel Cell uses a molten carbonate salt mixture, usually lithium carbonate and potassium carbonate, as its electrolyte. These fuel cells are also well suited for large stationary power generators. They operate at 1,112 F (600 C), so they can also generate steam to be used for the generation of more power. They have a lower operating temperature than the SOFC, which means they don't require the special materials to address the high-temperature reliability concerns. This makes the design a little less expensive.

Proton Exchange Membrane (PEM)

The Proton Exchange Membrane Fuel Cell offers an order of magnitude higher power density than any other fuel cell system, with the exception of the advanced aerospace alkaline fuel cell, which has comparable performance. The proton exchange membrane can operate on reformed hydrocarbon fuels, with pretreatment, and on air. The use of a solid polymer electrolyte eliminates the corrosion and safety concerns associated with liquid electrolyte fuel cells.

Properly designed, a proton exchange membrane fuel cell can be run in reverse, acting as an electrolyzer. This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). A regenerative fuel cell uses water and electrical energy as inputs, electrolyzes the water, and emits hydrogen and oxygen as outputs. These units are currently in the prototype stage, with novel applications such as creating hydrogen during the day with solar electric power, then using the hydrogen fuel at night to power a hybrid solar/hydrogen fuel cell high-altitude unmanned reconnaissance airplane.

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.

Relevant Codes and Standards

There are standards provided by the Institute of Electrical and Electronics Engineers (IEEE) that set requirements for connecting distributed resources with electric power systems. Fuel cells are considered a distributed resource. The IEEE standards that apply to connecting distributed resources are:

Additional information on codes and standards for hydrogen and fuel cells are available below.

Additional Resources

U.S.Department of Energy

Associations
Organizations

Given the large number of hydrogen and fuel cell organizations, performing further research on a particular state is recommended. Resources for certain states are provided below:

Publications
Other Resources