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Fuel Cell Types
Fuel cells are classified by the electrolyte used. The different types of fuel cell developed for various applications are given in Table 7.4. For vehicle application, the temperature of fuel cell operation and start up time are important. The PEM (proton exchange membrane) fuel cell has been accepted presently as best suited for vehicle application as it can be started in about 30 seconds and it operates at acceptably low temperatures. The PEM fuel cell consists of an electrolyte membrane in the form of a thin film of approximately 0.1 mm thickness made of sulfonated fluorocopolymer or an aromatic polymer. A typical automotive fuel cell stack consisting of 640 PEMFC developed 129kW peak power with continuous rating of 102 kW, weighed 100 kg and occupied 58 litres of space
| Table 7.4 |
| Fuel Cell Types and their Characteristics |
| Type |
Electrolyte |
Temperature of operation, ºC |
System Efficiency
% HHV |
Start-up time, hours |
Power range and application |
| Alkaline (AFC) |
KOH (OH-) |
60-120 |
35-55 |
Very short |
< 5kW, military, space |
| Proton Exchange Membrane (PEMFC) |
Polymer Electrolyte (H+) |
20-120 |
32-45 |
< 0.01(30 seconds) |
5 – 250 kW, High power density, automotive |
| PAFC |
Phosphoric Acid (H+) |
160 -220 |
36-45 |
1 -4 |
200 kW, CHP |
| MCFC |
Molten carbonates (CO-3) |
550-650 |
43-55 |
5 -10 |
200 kW - MW, CHP and stand alone |
| Solid oxide (SOFC) |
Solid doped Zr-oxide (O-) |
850-1000 |
43-55 |
5 -10 |
2 kW - MW CHP and stand alone, High efficiency |
Energy Sources for Fuel Cell
The following sources can supply energy to fuel cells
- Hydrogen
- Methanol
- Ethanol
- Hydrocarbon fuels, gasoline and diesel
Hydrogen-oxygen fuel cell provides the highest EMF and power density (W/cm2). Hydrogen either can be directly stored on-board of vehicle or generated by steam- reforming of fuels such as methanol, ethanol and hydrocarbons. The purity of hydrogen is very important for operation and longer life of fuel cell as even small concentrations of carbon monoxide and sulphur are highly detrimental. The products of fuel reforming are to be cleaned to supply hydrogen to the fuel cell. Although in principle, methanol, ethanol, gasoline, diesel and other hydrocarbons can be reformed to supply hydrogen, but so far only methanol reforming on board has been successfully used. Direct methanol fuel cell (DMFC) where methanol is fed directly to the fuel cell for oxidation and generation of electricity, is another option being developed for automotive use. . Electrolysis of water using nuclear energy and the renewable solar, wind, hydro and wave energy is the other route to generate hydrogen. The electrolysis route appears to be a long term solution once the low cost renewable or nuclear power is available. On board storage of hydrogen is another important factor for commercial success of FCV. Hydrogen can be stored in the form of gas, liquid, metal hydrides as hydrogen or in chemically combined form such as methanol and NaBH4 (sodium borohydride). High pressure storage systems of hydrogen at 700 bars have been developed. The different methods of hydrogen storage on board are compared in Table 7.5. So far most FCV prototypes have however, used the high pressure (350 – 700 bar) hydrogen tanks.
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