Energy efficient hydrogen production by water electrolysis
Koponen, Joonas (2020-02-28)
Väitöskirja
28.02.2020
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Sähkötekniikka
Kaikki oikeudet pidätetään.
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In reference to IEEE copyrighted material which is used with permission in this thesis, the IEEE does not endorse any of Lappeenranta-Lahti University of Technology LUT's products or services. Internal or personal use of this material is permitted. If interested in reprinting/republishing IEEE copyrighted material for advertising or promotional purposes or for creating new collective works for resale or redistribution, please go to http://www.ieee.org/publications_ standards/publications/rights/rights_ link.html to learn how to obtain a License from RightsLink.
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-491-3
https://urn.fi/URN:ISBN:978-952-335-491-3
Tiivistelmä
Hydrogen is the most abundant element in the universe. On Earth, it not only forms water molecules, but also commonly combines with carbon into hydrocarbons that power the current world economy. The use of these natural hydrocarbons for energy depletes the resources of our planet, increases the concentration of carbon dioxide in the atmosphere, and accelerates the acidification of oceans. Globally, the whole energy system must be driven towards a net zero-emission level and beyond through re- and decarbonization and emission-free power generation. Hydrogen would remain a key element in the world economy and electricity could become the main form of energy through this global energy transition.
Water electrolyzers decompose water into hydrogen and oxygen gas with the aid of electric current. Part of the electrical energy supplied to the water electrolysis process is carried as the chemical energy of hydrogen. Water electrolysis is feasible for large-scale production and can transfer emission-free electrical energy to the production of carbonneutral fuels, raw materials, and chemicals in the hydrogen required to form the constituent synthetic compounds.
AC to DC power conversion, rectification, is required to execute controlled and energy efficient power consumption in electrolyzer systems connected to the main AC electricity grid. Firstly, water electrolyzers are current-controlled DC loads whose optimal and safe operating conditions depend especially on temperature, pressure, and current density. Secondly, the rectification is responsible for the quality of the DC power supplied to the water electrolyzer. Industrial water electrolyzers are characterized by high DC currents and low DC voltages, and therefore, the rectifiers employed have typically been based on thyristors and diodes. The natural commutation of the thyristors introduces notable harmonics to the supplied DC current and DC voltage causing additional heat losses and imposing a constant dynamic operation on the electrolytic cells.
This work focuses on the factors affecting the specific energy consumption of commercial water electrolysis technologies from the viewpoint of power electronics and power supply. The aim is to improve understanding of how the commercial water electrolysis processes can be optimized, integrated into renewable power production systems, and operated in Power-to-X systems. The results highlight the water electrolyzer as the main energy consumer in Power-to-X systems. The dynamic operation capabilities of a water electrolyzer may have to be artificially limited under fluctuating renewable power to ensure energy efficient operation over time. Operation at elevated pressures, which may be required by other processes in a Power-to-X system, may further limit the safe control range and energy efficiency of a water electrolyzer by decreasing the Faraday efficiency.
The improvement in DC power quality, controllability, and connectivity to the main AC electricity grid can be achieved by using transistor-based rectifiers. The results imply that improving the power quality can have a significant impact on the specific energy consumption and controllability of water electrolyzer systems. With improved power quality, the specific energy consumption of a conventional industrial alkaline water electrolyzer stack may be improved by up to 14%. Understanding of the energy efficient operation and control of water electrolyzers, together with the impact that power quality can have, can notably decrease the end price of hydrogen and carbon-neutral end products required in the global energy transition.
Water electrolyzers decompose water into hydrogen and oxygen gas with the aid of electric current. Part of the electrical energy supplied to the water electrolysis process is carried as the chemical energy of hydrogen. Water electrolysis is feasible for large-scale production and can transfer emission-free electrical energy to the production of carbonneutral fuels, raw materials, and chemicals in the hydrogen required to form the constituent synthetic compounds.
AC to DC power conversion, rectification, is required to execute controlled and energy efficient power consumption in electrolyzer systems connected to the main AC electricity grid. Firstly, water electrolyzers are current-controlled DC loads whose optimal and safe operating conditions depend especially on temperature, pressure, and current density. Secondly, the rectification is responsible for the quality of the DC power supplied to the water electrolyzer. Industrial water electrolyzers are characterized by high DC currents and low DC voltages, and therefore, the rectifiers employed have typically been based on thyristors and diodes. The natural commutation of the thyristors introduces notable harmonics to the supplied DC current and DC voltage causing additional heat losses and imposing a constant dynamic operation on the electrolytic cells.
This work focuses on the factors affecting the specific energy consumption of commercial water electrolysis technologies from the viewpoint of power electronics and power supply. The aim is to improve understanding of how the commercial water electrolysis processes can be optimized, integrated into renewable power production systems, and operated in Power-to-X systems. The results highlight the water electrolyzer as the main energy consumer in Power-to-X systems. The dynamic operation capabilities of a water electrolyzer may have to be artificially limited under fluctuating renewable power to ensure energy efficient operation over time. Operation at elevated pressures, which may be required by other processes in a Power-to-X system, may further limit the safe control range and energy efficiency of a water electrolyzer by decreasing the Faraday efficiency.
The improvement in DC power quality, controllability, and connectivity to the main AC electricity grid can be achieved by using transistor-based rectifiers. The results imply that improving the power quality can have a significant impact on the specific energy consumption and controllability of water electrolyzer systems. With improved power quality, the specific energy consumption of a conventional industrial alkaline water electrolyzer stack may be improved by up to 14%. Understanding of the energy efficient operation and control of water electrolyzers, together with the impact that power quality can have, can notably decrease the end price of hydrogen and carbon-neutral end products required in the global energy transition.
Kokoelmat
- Väitöskirjat [1070]