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Other efficiency measures often are listed on the product brochures of electrolyzer manufacturers, the most frequently used being kilowatt hours (kWh) per normal cubic meter (Nm3) of dry hydrogen produced (kWh/Nm3). The kilowatt-hours per kilogram term also appears frequently in the literature. These measures have more meaning to the customers of electrolysis units who, after all, want hydrogen gas and not heat. Voltage Efficiency of Electrolysis Cells and Stacks A problem exists, however. Using this approach to calculate the maximum thermodynamic effi- ciency of an electrolysis cell operating reversibly produces nonsense numbers that exceed 100%. (HHV) 285.8 kJ/mole ∕ 237.2 kJ/mole = 120.5% (LHV) 241.8 kJ/mole ∕ 228.6 kJ/mole = 105.8% The problem is that it takes both electricity and heat to split water electrochemically and the heat is not being included in the above calculation of the energy input. Although the thermodynamic voltage for splitting water under standard conditions is the same 1.229 volts as the fuel-cell reaction, practical electrolysis cells—like fuel cells—do not operate near this voltage. Whereas the practical fuel cell operates well below 1.23 volts (in the range of 0.750 to 0.900 volts), the practical electrolysis cell operates above this voltage in the range of 1.60 to 2.00 volts. System efficiencies of practical systems calculated using the above approach, although inflated, always are less than 100%; therefore, the problem is not obvious. It is when we attempt to develop a method for calculating individual cell and multiple cell stack efficiency that we see the problem. Splitting a mole of liquid water to produce a mole of hydrogen at 25°C requires 285.8 kJ of energy—237.2 kJ as electricity and 48.6 kJ as heat; there is no way around this fact. In PEM and alkaline electrolysis cells the heat requirement is supplied from the extra heat generated, due to internal resistance as the electric and ionic currents flow through the cell. This heat requirement is directly traceable back to the electricity supplied. In other words, 285.8 kJ—not 237.2 kJ—of electricity is the minimum required to split water in these cells. This translates into a cell voltage of 1.481 volts, not the 1.229 volts used in calculating the theoretical maximum electrical efficiency of a fuel cell. The electrochemical potential (standard potential) corresponding to the HHV is 1.481 V/cell as shown below. This represents the thermoneutral voltage at which hydrogen and oxygen are produced with 100% thermal efficiency (i.e., no waste heat produced from the reaction). This is determined using Faraday’s Law, and dividing the HHV (285,840 J/mole) by the Faraday constant (F = 96,485 coulombs mole-1) and the number of electrons needed to create a molecule of hydrogen (z = 2). EO = Δf Ho zF = 285,840 J mol =1.481 Volts 2∗96,485 C cell mol 6PDF Image | Hydrogen Production: Fundamentals
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