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How does it work?
A fuel cell generates electrical power by continuously converting the chemical energy of a fuel into electrical
energy by way of an electrochemical reaction. The fuel cell itself has no moving parts, making it a quiet
and reliable source of power. Fuel cells typically utilize hydrogen as the fuel, and oxygen (usually from air)
as the oxidant in the electrochemical reaction. The reaction results in electricity, by-product water, and
by-product heat.
Because the fuel is converted directly to electricity, a fuel cell can operate at much higher efficiencies than
internal combustion engines, extracting more electricity from the same amount of fuel.
When hydrogen gas is introduced into the system, the catalyst surface of the membrane splits hydrogen gas
molecules into protons and electrons. The protons pass through the membrane to react with oxygen in the
air (forming water). The electrons, which cannot pass through the membrane, must travel around it, thus
creating the source of DC electricity.
Individual fuel cells can be then combined into a fuel cell "stack". The number of fuel cells in the stack
determines the total voltage, and the surface area of each cell determines the total current. Multiplying the
voltage by the current yields the total electrical power generated.
The Proton Exchange Membrane Fuel Cell (PEMFC) Example
- The PEMFC stack is the heart of the fuel cell system. It is made up of a membrane electrode assembly
(MEA) sandwiched between two gas diffusion layers (GDLs) with bipolar plates on each side.
- The reformate arriving from the CO cleanup system feeds the fuel side of the fuel cell. The hydrogen in the
fuel passes through the GDL, which typically serves three functions within a PEM fuel cell:
- Diffuse the reactant gases across the surface of the membrane
- Manage the water around the membrane
- Provide a highly conductive path between the membrane and bipolar plates
- The MEA ionizes the hydrogen, passes the hydrogen ions, and combines the ions with oxygen to form water.
MEA catalysts are typically made of precious metals such as platinum. The improvement in platinum application to
the MEA is one major reason for decreasing the costs of a fuel cell.
- The fuel processor portion of a fuel cell system has two operating components.
- The fuel reformer processes a hydrocarbon fuel, such as natural gas, into a hydrogen-rich gas known as reformate.
Reformate contains heavy concentrations of CO so a CO cleanup system is applied to reduce the CO concentrations
to acceptable levels (under 50 ppm).
- The carbon monoxide (CO) cleanup unit.
- The last step is the power conditioner. The power conditioner first converts the low-voltage direct current (DC) produced by the PEMFC to a
high-voltage alternating current (AC).
- Batteries are used to ensure that power surges from such things as air conditioner start-ups can be handled.
- Batteries also meet any extended peak period of demand, which is higher than stack peak output.
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