Losses in Fuel Cells
- The theoretical e.m.f. generated is given by the Equation as, E = △G/n.F. It is also called as open circuit voltage or no load voltage. However, as the fuel cell is subjected to load i.e. current is supplied, the voltage of the cell drops considerably with increase in load. The voltage- current characteristics of a fuel cell is represented in Figure A
- The electrical energy generated by the fuel cell depends on the free Gibbs energy, △G and not on the heat energy for overall cell reaction.
- The maximum efficiency of a H2-O2 fuel cell as calculated in is about 83% at 25°C and 1 atmospheric pressure while the actual efficiency is in the range of about 50 to 60 percent.
- The actual value of c.m.f. generated at about 180°C is 1.12 volts.
- This loss of voltage at no load is called activation loss (E – Vb) as shown in Figure A by curve (a – b). These losses are associated with the activity of cell i.e. its ability to dissociate and derive the chemical reaction at low temperature.
- The voltage of the cell further drops as the current is drawn from the cell when subjected to load as shown by the curve (b – c). These losses represent the ohmic or resistance losses as a result of electrical resistance of the cell to the current . These loss (Vc-Vb) are called resistance losses.
- At moderate currents the voltage Vc, after activation and resistance losses of a (H2-O2) fuel cell is in the range of 0.7 to 0.8 volts.
- Beyond the point ‘c’ the c.m.f. of cell drops suddenly due to mass transport processes in the cell. These losses occur when the cell’s ability reduces to maintain adequate concentrations of H2 and O2 in it due to high current demand.
- For the reasons explained above, a fuel cell is normally operated in the region of the curve (bc) of Figure A It can be further observed that voltage drop across the cell increases with the increase in temperature. Therefore in practice, a fuel cell is operated at higher end of its temperature range.
- All the above losses combined together produce heat in the fuel cell. Thus, it becomes necessary to cool the fuel cell continuously to remove the heat generated so that the cell works efficiently.