Efficiency
The electrolysis of water requires a minimum of 286 kJ of electrical energy input to dissociate each mole. Since each mole of water requires two moles of electrons, the specific electrical energy required is 143 kJ/mole (8.9×1023 eV/mole). It follows then that a minimum electrical power input per ampere is implied, namely 1.48 W/ampere. In turn, the minimum electrolytic potential for electrolysis of water of 1.48 V (not 1.23 V). Thus, any current (I) at applied voltage (V) greater than 1.48 V is an overvoltage and results in waste heat which can be estimated as I×(V-1.48).
Water electrolysis does not convert 100% of the electrical energy into the chemical energy of hydrogen. The process requires more extreme potentials than what would be expected based on the cell's total reversible reduction potentials. This excess potential accounts for various forms of overpotential by which the extra energy is eventually lost as heat. For a well designed cell the largest overpotential is the reaction overpotential for the four electron oxidation of water to oxygen at the anode. An effective electrocatalyst to facilitate this reaction has not been developed. Platinum alloys are the default state of the art for this oxidation. Developing a cheap effective electrocatalyst for this reaction would be a great advance (see also). In 2008, a group led by Daniel Nocera announced the development of an electrocatalyst composed of the abundant metal cobalt and phosphate. Other researchers are pursuing carbon-based catalysts.
The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no reaction overpotential by platinum or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode then another large overpotential must be paid.
Efficiency of modern hydrogen generators is measured by Power consumed per volume of mass (kWh/Nm3). The lower this number the higher efficiency is. Efficiency percentage can be calculated by simplified equation:
where (in SI metric units):
| = efficiency in % | |
| = Hydrogen density in normal conditions (0.08988 kg/m3) | |
| E | = Hydrogen specific energy (about 40 kWh/kg) |
| P | = Power consumed per volume of mass (kWh/Nm3) |
Here is a table with market standard efficiency parameters:
| P (kWh/Nm3) | Efficiency (%) |
|---|---|
| 7.3 | 49 |
| 7 | 51 |
| 6.8 | 53 |
| 6.0 | 60 |
| 5.0 | 72 |
| 4.5 | 80 |
According to this equation, 100% efficient hydrogen generator has P about 3.6 kWh/Nm3 (actual = 3.49).
The energy efficiency of water electrolysis varies widely with the numbers cited below on the optimistic side. Some report 50–80%. These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. If one considers simply the electrical energy input to an electrolyser and the enthalpy of combustion of the H2 product (therefore the energy input and the energy output of the system), then efficiency of 95% is achievable (using platinum catalysts and PEM technology, with H2 production occurring at 1.55V, with ideal Faraday efficiency being achieved). This value of 95% correctly refers to the higher heating value (HHV) of H2. Confusingly, the lower heating value of H2 (LHV) is occasionally used to calculate the efficiency for electroysers, and often for fuel cells. However, it is technically correct to always use HHV, as this value represents the total amount of enthalpy available from the H2 product (which is the enthalpy released during the combustion reaction of H2 with O2). This is generally observed correctly with electrolysis calculations, since LHV if used would give lower efficiency values for electrolysis. However, with fuel cells, LHV gives apparently higher efficiencys thus promoting the fuel cell and has crept into use; although LHV can be used to 'approximately' benchmark the reaction at the fuel cell electrodes, it is not the true overall efficiency of the fuel cell itself. This can become confusing and therefore only HHV should ever be used for calculations (since LHV is arbitrarily used to disregard the energy released (lost) during to the cooling and latent heat of vapourisation of the reaction products, which is not used in the basic fuel cell reaction).
On a more general note to consider, the energy lost in generating the electricity for the electrolyser is not included in this figure. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency may be closer to 30–45%, although the inefficiencies of powerplants in turning heat into electrical energy is not usually included in efficiency, so the former measure of 50–80% efficient is probably a more realistic efficiency.
Read more about this topic: Electrolysis Of Water
Famous quotes containing the word efficiency:
“Nothing comes to pass in nature, which can be set down to a flaw therein; for nature is always the same and everywhere one and the same in her efficiency and power of action; that is, nature’s laws and ordinances whereby all things come to pass and change from one form to another, are everywhere and always; so that there should be one and the same method of understanding the nature of all things whatsoever, namely, through nature’s universal laws and rules.”
—Baruch (Benedict)
““Never hug and kiss your children! Mother love may make your children’s infancy unhappy and prevent them from pursuing a career or getting married!” That’s total hogwash, of course. But it shows on extreme example of what state-of-the-art “scientific” parenting was supposed to be in early twentieth-century America. After all, that was the heyday of efficiency experts, time-and-motion studies, and the like.”
—Lawrence Kutner (20th century)
“I’ll take fifty percent efficiency to get one hundred percent loyalty.”
—Samuel Goldwyn (1882–1974)