Type of DG: Fuel Cells
Fuel Cells produce power electrochemically like a battery rather than like a conventional generating system that converts fuel to heat, then to rotating shaft-power, and finally to electricity. Unlike a battery, which generates power from internally stored chemicals, Fuel Cells produce power when hydrogen fuel is delivered to the positive pole (anode) of the cell and oxygen in air is delivered to the negative pole (cathode). The hydrogen fuel can come from a variety of sources, but the most economic is steam reformation of natural gas (a thermally-activated chemical process that strips the hydrogen from both the fuel and the steam). The diagram below provides an overview of the structure and flow of reactants in a typical fuel cell.
Sample Fuel Cell Structure
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Example Fuel Cell Stacks |
A PEM Fuel Cell System with Fuel Reformation Included |
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Tubular SOFC Design Utilized in Long-Lifetime Systems |
Large (1MW) MCFC System |
The electrochemical reaction which then takes place in the Fuel Cell produces
heat, which itself can be recovered if the device is to be used in combined
heat and power mode. Another typical product of this electrolytic reaction
is pure water, which can also be utilized at the installation site.
A Fuel Cell normally consists of three major sections (See Figure Below):
- Fuel Processing System: Converts the raw fuel feed into a hydrogen rich gas. This is usually accomplished through the use of a reformer. If hydrogen is directly fed, this section is not needed.
- Cell Stack Assembly: Each cell consists of an electrolyte and two electrodes. Each cell, and the Assembly as a whole, converts the gas into electrical energy which is produced as direct or continuous current (DC)
- Power Conditioning System: Converts unregulated DC output from the cell stack into a regulated AC supply. This is usually accomplished through the use of an inverter.
Fuel Cell System Schematic
Several different liquid and solid media can be used to create the fuel cell’s electro-chemical reactions – alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and proton exchange membrane fuel cell (PEM). Each of these media comprises a distinct fuel cell technology with its own performance characteristics and development schedule.
Fuel Cell Type |
Development Status |
Efficiency |
AFC |
Huge role on space missions. Lately on electrolyzers, |
60-70% |
PAFC |
Early Commercial Market. 200 kW Units Delivered to over 120 customers. |
35-40% |
SOFC |
Field Testing and Demonstration |
Up to 60% |
MCFC |
Early Commercial Market. 6 Commercial
250 kW Units Delivered. |
Up to 60% |
PEM |
Early Development and Testing. |
30-40% |
Direct electrochemical reactions are generally more efficient than using fuel to drive a heat engine to produce electricity. It is for this reason that Fuel Cell efficiencies are typically higher than efficiencies for combustion systems. Fuel Cells are also inherently quiet and extremely clean running. Typically they generate only 1 percent of the emissions produced by reciprocating engines. Like a battery, fuel cells produce direct current (DC) that must be converted through an inverter to obtain 60 Hz alternating current (AC). These power electronics components can be integrated with other components as part of a power quality control strategy for sensitive customers.
Because of current high costs, Fuel Cells are best suited to environmentally sensitive areas and customers with power quality concerns. Some fuel cell technology is modular and capable of application in small commercial and even residential markets; other technology utilizes high temperatures in larger sized systems that would be well suited to industrial and energy intensive commercial cogeneration applications.
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