Review of water electrolysis
technologies and design of renewable
hydrogen production systems
Progress of Master’s Thesis
Joonas Koponen, Antti Kosonen, & Jero Ahola
Objectives of the work
•
Describe the fundamentals of water electrolysis
•
Review state-of-the-art technologies
•
Review how hydrogen production has been integrated into renewable power
generating systems
•
Design of electrolytic hydrogen production laboratory setup for LUT
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Why hydrogen?
•
Most abundant element in the universe
•
High energy density on a mass basis
• Hydrogen’s HHV is 39.4 kWh/kg
• Gasoline’s HHV is 13.1 kWh/kg
•
Versatility as an energy carrier
• Hydrogen-to-Chemical, Hydrogen-to-Liquid fuel, Hydrogen-to-Gas, Hydrogen-to-Electricity…
• Hydrogen must be produced
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Present state of hydrogen production
•
Majority (48 %) of hydrogen comes from reforming of natural gas and refinery gas
•
Only 4 % of global hydrogen production from water electrolysis
•
Electricity accounts for 70 – 90 % of the cost of a kilogram of hydrogen in electrolytic
hydrogen production
•
Source: Bertuccioli, A. Chan, D. Hart, F. Lehner, B. Madden, and E. Standen, “Study on development of water
electrolysis in the EU,” Final report in fuel cells and hydrogen joint undertaking, Feb. 2014.
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Fundamentals of water electrolysis
•
The principle of water electrolysis is to pass a direct current between two electrodes
immersed in an electrolyte
• Hydrogen is formed at the cathode, oxygen at the anode
• H20 (l)
H2(g) + ½O2(g)
• First performed by W. Nicholson and A. Carlisle in 1800
•
Production of hydrogen is directly proportional to the current
• Laws of electrolysis by M. Faraday in 1833-1834
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Thermodynamics of water electrolysis
•
Electrical and thermal energy are converted into chemical energy
•
Energy required to decompose water is the enthalpy of formation of water
•
Only the free energy of this reaction has to be supplied as electrical energy
•
Remainder of required energy is thermal energy
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Thermodynamics of water electrolysis
•
Idealised operating conditions for water electrolysis:
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Thermodynamics of water electrolysis
•
Illustrative cell efficiency and H2 production rate as a function of cell voltage:
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Thermodynamics of water electrolysis
•
The reversible cell voltage is a
thermodynamic state function
• Decreases with temperature
• Increases with pressure
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Thermodynamics of water electrolysis
•
Vapour pressure of water:
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Electrochemistry
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Electrolyser efficiency
•
Different definitions of water electrolysis efficiencies
• HHV of hydrogen is 39.4 kWh/kg
• LHV of hydrogen is 33.3 kWh/kg (excludes heat required for water vaporization)
• Cell-level, stack-level, and system-level efficiencies
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Electrolyser efficiency
•
Simulated alkaline electrolyser cell efficiency (HHV) with a current density 0.2 A/cm 2
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Overview of electrolyser technologies
Monopolar
Bipolar
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Alkaline electrolysers
•
Most developed water electrolysis technology
•
Accounts for the majority of the installed water electrolysis capacity worldwide
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Alkaline electrolysers
•
Electrolyte typically 20 – 40 wt% aqueous solution of KOH
•
Non-noble catalysts, Ni-based electrodes
•
Relatively low cost materials, suitable for large-scale production
•
Limitations in dynamic operation
• Diaphragm limits minimum partial load (gas cross-diffusion)
• Liquid electrolyte limits ramping rate (inertia of the liquid)
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Proton exchange membrane electrolysers
•
Electrolyte is a gas-tight, thin polymeric
membrane which has H+ conducting ability
• Presence of side chain ending with HSO3
• Decreasing water content decreases
conductivity
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Proton exchange membrane electrolysers
•
The solid polymer electrolyte creates corrosive low pH condition
•
Use of scarce, expensive materials
• Noble catalysts, titanium-based current collectors and separator plates
•
Responds more quickly to fluctuations in input power
•
Compact system design
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Comparison of alkaline and PEM technologies
Alkaline
PEM
Current density
0.2 - 0.4 A/cm2
1.0 - 2.0 A/cm2
Start-up time from
Cell area
< 4 m2
< 0.3 m2
cold to minimum
10 - 30 bar
load
< 200 bar
H2 purity
(1)
Hydrogen output
pressure
Operating
temperature
Min. load
Ramp-up from
minimum load to full
load
•
•
0.05 - 30 bar
Alkaline
(1)
System efficiency
60 - 80 °C
50 - 80 °C
20 - 40 %
5 - 10 %
System size range
0.13 - 10 %(full
10 - 100 %(full
Lifetime stack
load)/second
load)/second
(HHV)
(2)
20 min - several
hours
PEM
5 - 15 min
99.5 - 99.9998 %
99.9 - 99.9999 %
68 - 77 %
62 - 77 %
0.25 - 760 Nm3/h
0.01 - 240 Nm3/h
1.8 - 5300 kW
0.2 - 1150 kW
60 000 - 90 000 h
20 000 - 90 000 h
< 20 000 h
(3)
Values from L. Bertuccioli, A. Chan, D. Hart, F. Lehner, B. Madden, and E. Standen, “Study on development of water electrolysis in the EU,” Final
report in fuel cells and hydrogen joint undertaking, Feb. 2014.
Except
• (1) M. Lehner, R. Tichler, H. Steinmüller, and M. Koppe, Power-to-gas: technology and business models, New York: Springer International
Publishing, 2014.
• (2) B. Decourt, B. Lajoie, R. Debarre, and O. Soupa, “The hydrogen-based energy conversion FactBook,” The SBC Energy Institute, Feb. 2014.
• (3) M. Carmo, D. Fritz, J. Mergen, and D. Stolten, “A comprehensive review on PEM water electrolysis,” Int. J. Hydrogen Energy, vol. 38, no. 12,
pp. 4901–4934, Apr. 2013.
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Solid oxide electrolyte electrolysers
•
High operating temperatures, typically 700 – 1000 °C
• Steam electrolysis
•
System efficiencies typically over 90 %
•
Fast degradation of cell components due to high operating temperatures
•
Still in the R&D stage
•
SOEC systems can be operated in reverse mode (fuel cell)
•
SOEC devices can be used to reduce CO2 to CO
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Overview of electrolyser plants
Publication: J. Koponen et al., Specific energy consumption of alkaline and PEM electrolysers in atmospheric and
pressurized conditions, to be published in Proc. 17th European Conf. on Power Electronics and Applications, 2015.
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NEO-CARBON ENERGY project is one of the Tekes’ strategic research openings.
The project is carried out in cooperation between VTT, Lappeenranta University of Technology and
University of Turku / Futures Research Centre.