High temperature electrolysis
High temperature electrolysis ( HTE or steam electrolysis ) is an electrolysis process for the production of hydrogen (H 2 ) from water at high temperatures between 100 ° C and 850 ° C. Depending on the electrolyzer , carbon and other compounds can also be broken down.
Efficiency
HTE is more economical than conventional water electrolysis at room temperature because some of the energy is supplied in the form of heat, which is cheaper than electrical energy and the electrolysis reaction is more efficient at higher temperatures. No electrical supply is required from 2500 ° C, since water breaks down into hydrogen and oxygen in the thermolysis . Such temperatures are impractical in HTE; HTE systems work between 100 ° C and 850 ° C. The increase in efficiency of the HTE can best be identified by assuming that the electrical energy comes from a heat engine and taking into account the amount of thermal energy that is required to produce 1 kg of hydrogen (141.86 MJ ), both in the HTE process as well as for generating electrical energy. At 100 ° C, 350 MJ of thermal energy are required (41% efficiency). At 850 ° C, 225 MJ are required (64% efficiency). As of 2018, the achieved efficiency based on the upper calorific value (see below) is a median of 82%, with a maximum of around 91%.
materials
The choice of materials for the electrodes and the electrolyte in a solid oxide electrolytic cell is important. One option that is being investigated for the process uses yttria-stabilized zirconia (YSZ) electrolytes, nickel- cermet vapor / hydrogen electrodes, and mixed oxides of lanthanum, strontium, and cobalt oxygen electrodes.
Economic potential
HTE is of interest as a more efficient way to produce hydrogen, RE gas ( synthesis gas ), carbon-neutral fuel and energy storage. For economic operation, however, in addition to the availability of cheap, CO 2 -neutral energy sources, a reduction in capital costs is necessary, which is currently (as of 2018) around 2500 € / kWel and thus significantly above those of alkaline electrolysis with 1000 € / kWel can be specified. However, considerable savings potential is expected and, according to industry surveys, 750 € / kWel should be achieved in 2030 (as of 2018), in 2050 even 153 € / kWel (adjusted for inflation, based on 2017). The problem with the use of fluctuating current is the narrower operating range compared to alkaline or PEM electrolyzers, as well as the unavoidable material stresses during load changes due to the high thermal stresses. In addition, the high temperature required for operation can lead to long heating phases, so that the activation time from standstill is relatively long. All in all, PEM electrolysers seem to be better suited for the operation of strongly fluctuating electricity, while the higher efficiency of the HTE promises cheaper production in continuous operation.
Possible cheap sources of heat for HTE are all non-chemical, including nuclear reactors , concentrating solar thermal collectors , and geothermal sources. HTE has been demonstrated in a laboratory at 108 kJ (electrical) per gram of hydrogen, but not on a commercial scale. The first generation 5 commercial nuclear reactors are expected around 2030.
Electrolysis and thermodynamics
In electrolysis, the amount of electrical energy supplied, the free enthalpy , also known as Gibbs energy, corresponds to the reaction plus the losses in the system. The losses can - theoretically - be close to zero, i.e. the maximum thermodynamic efficiency of an electrochemical process, which would be 100%. In practice the efficiency of the electrical work divided by the released Gibbs energy is given.
In most cases, such as water electrolysis at room temperature, the electrical input is greater than the change in enthalpy of the reaction, so that part of the energy is released in waste heat . In the case of the electrolysis of steam from hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the environment and the calorific value of the hydrogen produced is higher than the electrical supply. In this case, it can be said that the efficiency in relation to the electrical energy input is greater than 100%. The maximum theoretical efficiency of the fuel cell is the inverse of the electrolysis at the same temperature. It is therefore not possible, by combining the two processes, to get back more energy than has flowed into the process, which would represent a perpetual motion machine .
Mars ISRU
It has been proposed to generate oxygen from atmospheric carbon dioxide of Mars using zirconia based high temperature electrolysis with solid oxide electrolyzer cells .
Web links
Individual evidence
- ^ Highly Efficient high temperature electrolysis . In: J. Mater. Chem. . 18, 2008, pp. 2331-2340. doi : 10.1039 / b718822f .
- ↑ SPS Badwal: Hydrogen production via solid electrolytic routes . In: WIREs Energy and Environment . 2, No. 5, 2012, pp. 473-487. doi : 10.1002 / wene.50 .
- ↑ Technical status and flexibility of the power-to-gas process (PDF) Energy and Resource Management, Technical University of Berlin. August 2018. Accessed July 9, 2020.
- ↑ Kazuya Yamada, Shinichi Makino, Kiyoshi Ono, Kentaro Matsunaga, Masato Yoshino, Takashi Ogawa, Shigeo Kasai, Seiji Fujiwara, Hiroyuki Yamauchi: High Temperature Electrolysis for Hydrogen Production Using Solid Oxide Electrolyte Tubular Cells Assembly Unit. AICHE Annual Meeting, San Francisco, CA, November 2006 ( abstract ).
- ↑ Technical status and flexibility of the power-to-gas process (PDF) Energy and Resource Management, Technical University of Berlin. August 2018. Accessed July 9, 2020.
- ↑ Steam heat: researchers gear up for full-scale hydrogen plant . In: Science Daily . September 19, 2008.
- ↑ Nuclear hydrogen R&D plan (PDF) US Dept. of Energy . March 2004. Retrieved May 9, 2008.
- ^ Mike Wall: Oxygen-Generating Mars Rover to Bring Colonization Closer . In: Space.com , August 1, 2014. Retrieved November 5, 2014.