Fuel cells are often compared to both internal combustion engines (ICEs) and batteries and, in fact, they share some characteristics with each. All three types of devices are used to transform one type of energy into another. A diesel engine turns chemical energy contained in diesel fuel into heat energy through combustion with oxygen from the air, and then turns that heat energy into mechanical energy, turning the vehicle’s wheels through a transmission and drive shaft.
On the other hand, a battery is a galvanic cell; it uses reactions between chemicals stored inside to turn chemical energy directly into electricity, which can then be used to power a number of devices, including an electric motor to produce mechanical energy.
Like a diesel engine, a fuel cell requires fuel (hydrogen) and oxygen (air). However, like a battery, it is capable of directly producing electricity.
A fuel cell is also a galvanic cell; the hydrogen and oxygen do not combust inside the device. Instead, the hydrogen and oxygen react electrochemically and produce electricity. See Table 3 for a comparison of the major differences between fuel cells and ICEs and batteries.
As with a battery, the electricity produced by a fuel cell can be used to power any number of devices. In the case of a vehicle, it is most often used to power an electric motor to move the vehicle down the road. A fuel cell vehicle is, therefore, an electric vehicle, powered by an electric motor.
Fuel cells have been around for a long time and have been used in the United States space program since the 1960s (College of the Desert, 2001c). It has only been in the last few years, however, that they have been developed for use in conventional vehicles.
There are a number of different fuel cell technologies that employ different chemical reactions to combine hydrogen and oxygen to produce electricity. The most common technology used in vehicles is called a Proton Exchange Membrane (PEM) fuel cell. See Figure 1, which shows the layout and operation of a PEM fuel cell. Also see Appendix A for a more in-depth description of the construction of a PEM fuel cell and the chemical reactions that take place in the cell.
The maximum voltage that one PEM “cell” can produce is 1.2 VDC, but the actual voltage depends on how much current is being drawn from the cell. The cell can put out the greatest amount of power at between 0.5 and 0.6 volts, so that is where they are generally designed to operate. To create a device powerful enough to power a large vehicle, up to 1,200 cells are connected in series, to produce a peak power of 100 kW or more at between 300 and 600 VDC (nominal). Physically the individual PEM cells are usually stacked together, separated by a cooling plate between each set of cells. These cooling plates circulate a mixture of water and ethylene glycol to remove excess heat created during operation of the cells. These cooling plates are part of a cooling system that is similar in both design and function to the cooling systems used with diesel engines. A collection of individual fuel cells used to create a practical device is usually referred to as a fuel cell “stack” (see Figure 2).
The fuel cell stack must be supported by a number of auxiliary systems that together make up the “fuel cell engine.” In addition to a cooling system, the fuel cell engine needs a fuel system, an air system, and a water management system. See Figure 3 for a generic schematic of a PEM fuel cell engine. In a PEM fuel cell, engine hydrogen and air are saturated with water and fed into the fuel cell stack. Inside each PEM cell, the hydrogen and air react with each other across a thin plastic-like film, called a proton exchange membrane, but they never mix. Electricity is produced by each cell, and water and a small amount of heat are the only by-products. Excess water not needed to humidify the gases is exhausted, with air, out the tailpipe.
PEM fuel cells generally operate at relatively low temperatures (140 to 180 ºF) and pressures (from 0 to 15 psig). The exact layout and details of a fuel cell engine and its subsystems will depend on the specifics of the design and its specified operating parameters. Packaging and layout of the fuel cell engine in the vehicle can also vary significantly. See Figure 4 for a photo of a fuel cell engine and electric drive motor that was installed in a transit bus.
PEM fuel cell engines fueled by hydrogen produce virtually none of the volatile organic hydrocarbon or nitrogen oxide tailpipe emissions that come from combustion of fuel in gasoline and diesel engines, and which together produce ground-level ozone, or “smog” in the atmosphere in the presence of sunlight. They also produce virtually none of the harmful particulate emissions produced by diesel engines and zero carbon dioxide emissions. Carbon dioxide is a major by-product of fuel combustion in diesel and gasoline engines. As a so-called “greenhouse gas,” carbon dioxide is a contributor to global warming.
In addition to reduced exhaust emissions, the potential benefits of using hydrogen fuel cells to power commercial vehicles include lower total energy use due to improved efficiency of the fuel cell compared to an internal combustion engine. The actual “wells-to-wheels” efficiency of a fuel cell vehicle will depend on how the system is designed, as well as how the hydrogen fuel is produced. Many fuel cell vehicles are designed with a hybrid propulsion system that incorporates a large battery to supplement the fuel cell. The battery provides power during acceleration, allowing the fuel cell to be smaller. It is also used to capture energy that is normally wasted in braking, which can later be re-used, increasing net efficiency, especially in stop-and-go city driving. See Figure 5 for a comparison of “wells-to-wheels” fuel use (liters per mile)2 for vehicles with different types of power sources. This figure is illustrative only and does not include all potential combinations of fuel and propulsion technology.
One liter per mile is equivalent to 0.26 gal/mile.
Usually pure hydrogen is used to fuel a PEM fuel cell engine. While a mixture of hydrogen and carbon dioxide can be used, other “contaminants” must be kept to a minimum in the fuel supplied to the cells, especially carbon monoxide (CO) and sulfur. Both CO and sulfur can reduce the activity of the platinum catalysts used in the PEM cells, reducing the amount of power that the cells can produce (EG&G, 2004).
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