Recent Changes - Search:




edit SideBar

Water Splitting Processes

Low-Temperature Electrolysis

The oldest and world wide well established technology of water electrolysis is the alkaline electrolysis. Approx. 20 billion Nm3 of H2 are being generated actually as a byproduct of the chlorine production. Electrical energy requirement is in the order of 4 to 4.5 kWh/Nm3 H2. Capacities of electrolyzer units are ranging between 20-5000 Nm3/h. The largest integrated installation is currently in Assuan, Egypt, with a production capacity of 33,000 Nm3/h. First alkaline electrolyzers for hydrogen production were developed by Norsk Hydro in Norway, where cheap electricity from hydro power could form the basis for this process. Electrolysis has become a mature technology at both large (125 MW) and small scale (1 kW). Today’s units are available in sizes up to about 2 MW(e) corresponding to ~ 470 Nm3/h of H2 production with multiple units being combined to larger capacities. They typically have an availability of > 98 % and an energy consumption of 4.1 kW/Nm3 operating at about atmospheric pressure (NorskHydro:2002). Additional components like purification of water and products, rectifier and reprocessing of alkaline solution are necessary. Pressurized systems operating at 3 MPa help to save compression energy. Plant operation is simple, highly flexible and appropriate for off-peak electricity use.

The more advanced method is solid polymer electrolyte membrane (PEM) water electrolysis which can be operated at higher pressures and at higher current densities due to volume reduction compared to cells with a liquid electrolyte. Typical operation temperatures are 200-400°C. This membrane electrolysis is simpler in its design and promises a longer lifetime and a higher efficiency. The requirement of electricity will be reduced to values below 4 kWh/Nm3 of H2. High-pressure systems are established in the smaller power range with pressures of 3 MPa achieved, small-scale units (8-260 Nm3/h) exhibit somewhat lower efficiencies . Main disadvantage is the still high cost of membrane manufacture.

High-Temperature Electrolysis

Another principal variant of electrolysis considered promising for the future is the high temperature electrolysis (HTE). An operation at temperatures between 800 and 1000°C offers the advantage of a smaller specific electricity requirement compared to conventional electrolysis. This process is also known as reverted electrolysis. High temperature electrolysis work has been undertaken in Germany (DoenitzW:1982), Japan and in the US (OBrienJE:2005).

Thermochemical (Hybrid) Cycles

Thermochemical cycles can be used to split water through a series of thermally driven chemical reactions where the net result is the production of hydrogen and oxygen at much lower temperatures than direct thermal decomposition. All supporting chemical substances are regenerated and recycled, and remain – ideally – completely in the system.

The only input is water and high temperature heat. Numerous instances of such cycles have been proposed in the past and checked against features such as reaction kinetics, thermodynamics, separation of substances, material stability, processing scheme, and cost analysis. Thermochemical cycles are being investigated mainly with respect to primary heat input from solar or nuclear power. Some of the most promising cycles include those based on the sulfur family, which all have in common the thermal decomposition of sulfuric acid at high temperatures. One cycle considered with a high priority is the sulfur-iodine (S-I) process which was originally developed by the US company General Atomics and later modified and successfully demonstrated by JAEA in Japan, see also ch. 1.3.5, in a closed cycle in continuous operation over one week. The facility consisted of more than 10 process units primarily made of glass and quartz with a hydrogen production rate achieved of 30 Nl/h. The next step which started in 2005 is the design and construction of a pilot plant with a production rate of 30 Nm3/h of H2. The theoretical limit of efficiency for the total process is assessed to be 51% assuming ideal reversible chemical reactions. A best estimate was found to be around 33-36% (GoldsteinS:2005), but it is hoped that 40-50% be achievable. The decomposition of H2 SO4 and HI were found to cause severe corrosion problems.


D{\"o}nitz W. and Schmidberger R. (1982) Concepts and design for scaling up high temperature water vapour electrolysis. International Journal of Hydrogen Energy, 4:321-330.(BibTeX)
Goldstein S., Borgard J.M. and Vitart X. (2005) Upperbound and best estimate of the efficiency of the IS cycle. International Journal of Hydrogen Energy, 30:619-626.(BibTeX)
Norsk Hydro (2002) Hydro Electrolysers..(BibTeX)
O'Brien J.E., Herring J.S., Lessing P.A. and Stoots C.M. (2005) High-temperature electrolysis for hydrogen production from nuclear energy. 11th Int. Topical Meeting on Nuclear Reactor Thermal Hydraulics, 2--6 October, Avignon, France.(BibTeX)

<< Hydrocarbon Splitting Processes | Content | Liquid Hydrogen Production in the World >>

Edit - History - Print - Recent Changes - Search
Page last modified on March 01, 2009, at 10:02 PM