The fuel cell system is composed of four principal components:
1 - Fuel Processor
2 - Fuel Cell Stack
3 - Electric Power Conversion
4 - Balance of Plant
Most fuel cells operate with hydrogen as the fuel. Hydrogen is not available naturally and must be generated in a reformer from hydrogen-containing compounds such as hydrocarbon fossil fuels, biogas, biomass, and water. A number of components and processes are used for this purpose. A reformer is a vessel within which fuel and other gaseous recycle stream(s) react with steam in the presence of heat. For example:
Fuel reacts with water over a catalyst
Requires heat input
Fuel reacts with air without a catalyst
Typically produces heat
Combination of above (fuel, water, air)
No net heat required or produced
Oxidation states of metals
Alternatively pass fuel and oxidant over parallel beds
Calcium oxide, calcium carbonate for carbon absorption
Fuel Cell Stack
The basic components of the fuel cell stack include the electrodes and electrolyte with additional components required for electrical connections and/or insulation and the flow of fuel and oxidant. These key components include current collectors and separator plates. The current collectors conduct electrons from the anode to the separator plate. The separator plates provide the electrical series connections between cells and physically separate the oxidant flow of one cell from the fuel flow of the adjacent cell. The channels in the current collectors serve as the distribution pathways for the fuel and oxidant. Often, the two current collectors and the separator plate are combined into a single unit called a "bipolar plate."
A fuel cell stack is comprised of an array of individual fuel cells, each one of which is capable of producing about 1 volt. As a result, typical fuel cell designs link together many individual cells to form a "stack" and thereby produce a more useful voltage. A fuel cell stack can be configured with many groups of cells in series and parallel connections to further tailor the voltage, current, and power. The number of individual cells contained within one stack is typically greater than 50 and varies significantly with stack design.
The electrolyte is a non-metallic electrical conductor in which current is carried by the movement of ions. In the case of a proton exchange membrane fuel cell (PEMFC), the electrolyte is a membrane that acts as the separating layer in a fuel cell (ion-exchanger). It is also a film barrier that separates gases in the anode and cathode compartments of the fuel cell.
An electrode is an electric conductor through which an electric current enters or leaves a medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. A fuel cell is comprised of two electrodes:
- Anode - The electrode at which oxidation occurs. For cells that create potential, it is also the electrode towards which the negative ion flows.
- Cathode - The electrode at which reduction occurs.
The catalyst is a chemical substance that increases the rate of a reaction without being consumed. After the reaction, the catalyst can potentially be recovered chemically unchanged from the reaction mixture. A catalyst lowers the activation energy required, allowing the reaction to proceed more quickly or at a lower temperature.
This term is used to describe the conductive material in a fuel cell that collects electrons (on the anode side) or disburses electrons (on the cathode side). The current collectors are microporous (to allow for fluid flow through them), located between the catalyst / electrolyte surfaces and the bipolar plates.
Membrane Electrode Assembly (MEA).
In the case of a PEMFC, the MEA is the structure consisting of an electrolyte (proton-exchange membrane) with surfaces coated with catalyst / carbon / binder layers and sandwiched by two microporous conductive layers (which function as the gas diffusion layers and current collectors).
Bipolar Plates or Separator Plates.
These are conductive plates in a fuel cell stack that act as an anode for one cell and a cathode for the adjacent cell. The plate may be made of metal or a conductive polymer (which may be a carbon-filled composite). The plate usually incorporates flow channels for the fluid feeds and may also contain conduits for heat transfer.
A manifold is the conduit that supplies gas to the fuel cell and can be either external or internal. An external manifold acts as the plumbing of the system which routes the fuel to anode chamber and the oxidant (air) to cathode chamber. Developmental issues include:
- Even distribution of fuel and oxidant
- Exposure to high temperatures and corrosive environments
- Low pressure drop allowable
- Compact space requirements
An internal manifold is a "system with a self-contained reactant delivery system similar to a boxed fuel cell system that would only require connections to the reactant tanks to become operational." Developmental issues include:
- Delivery of fuel and oxidant uniformly to electrode - electrolyte surfaces
- Multiple concepts for integration
- Integrate with separators, bipolar plates
- Integrate with cooling
- Integrate with electrodes
Heat exchangers are used to transfer heat from a fluid (liquid or gas) to another fluid where the two fluids are physically separated, and are needed to cool fuel cells and maintain a consistent operating temperature. They are used primarily in lower-temperature fuel cells (AFC, PEMFC and PAFC) where the typical cooling mechanism is water. Heat exchangers adopted for fuel cells are applied in a variety of configurations:
- Gas - Gas
- Gas - Liquid
Some of the development areas to date are:
Electric Power Conversion
Power Conversion and Electronics.
- Size and weight reduction (transportation markets)
- Heat transfer and size (portable power markets
Fuel cells produce direct current or DC power. While a growing number of "appliances" are powered by DC, such as computers and lighting, most of our buildings and appliances today require alternating current or AC power to operate. Consequently, a conversion device ("inverter") is required to convert DC to AC. Current attention to inverter technology is directed to a reduction in losses, an increase in the reliability, and the cost.
The general characteristics of power conversion devices include:
- Conversion of direct current (DC) to alternating current (AC) when required
- Control of current and/or voltage
- Feedback to control system
- Surge and short-circuit protection
- Connection to and manage energy storage
- Connection to and/or control loads
- Dedicated load
- Motor controller, motors
- Electrical grid
- Other generators
The general requirements of power electronic devices include:
- Over-current protection
- Short circuit protection
- Motor over-speed protection
- Low IGBT junction operating temperatures
- High-efficiency electronics (desire > 97%)
- High system efficiency (desire > 95%, typical ~ 92%)
- Smooth motor controls
- Low parasitic inductance
- Environmentally sealed
- High power quality
- Grid-synchronized power for stationary power generation
- Grid parallel - peak shaving
- Automatic off-grid with stand-alone power generation
- Continuous stand by power
- Uninterruptible power
- Automated black start capability
- Full diagnostic capability
- Control and system communication
- Internal and external control and communication
- Serial (RS-232)
- Large dynamic range