298
Water for Energy and Fuel Production
been operating with 1.6 V per cell. There are several indirect methods to improve
electrolytic cell performance, and these are described by Bockris et al. [11]. In the
recent years, hydrogen is also created by coal slurry electrolysis. Recent advances
can also reduce the potential for the electrolysis to as low as 0.5 V.
If the electricity is obtained with a heat engine, Carnot efficiency limitation applies.
Thus, if the electricity is obtained from coal, the normal efficiency is about 39% and
the remaining 61% is lost as heat. This loss of thermal energy makes the electricity
generated by wind or hydroelectric energy more efficient. The processes are often
considered in combination with a nuclear or solar heat source. A high-temperature
electrolysis (HTE) process may be favorable when high-temperature heat is available
as waste heat from other processes. The use of such waste heat makes the overall
process cost efficient.
11.2.1 AlkAline eleCTrolySiS
Alkaline electrolyzers use an aqueous KOH solution (caustic) as an electrolyte that
usually circulates through the electrolytic cells [1–11]. Alkaline electrolyzers are
suited for stationary applications and are available at operating pressures up to
25 bar. Alkaline electrolysis is a mature technology allowing unmanned remote
operation with significant operating experience in industrial applications. The following reactions take place inside the alkaline electrolytic cell:
Electrolyte: 4H → 4
+
+ 4
−
2 O
H
OH
(11.3)
Cathode: 4H
+
+ 4e
−
→ 2H 2
(11.4)
Anode: 4OH
−
→ O 2 + 2H 2 O + 4e
−
(11.5)
Sum: 2H 2 O → O 2 + 2H 2
(11.6)
Commercial electrolyzers usually consist of a number of electrolytic cells arranged
in a cell stack. The major research challenges for the future are the design and manufacturing of electrolyzer equipment at lower costs with higher energy efficiency and
large turndown ratios.
11.2.2 hTe ProCeSS
HTE is more efficient economically than traditional room-temperature electrolysis
because some of the energy is supplied by heat that is cheaper than electricity and the
electrolysis reactions are more efficient at higher temperatures. While at 2500°C, thermal
energy alone can dissociate water molecules, generally HTE systems operate between
100°C and 850°C [12,14,16–19] (Laguna-Barcero et al., 2012, pers. comm.). The efficiency of HTE process can be easily estimated by assuming that the heat required comes
from heat engines and heat energy required for 1 kg of hydrogen (350 MJ) at 100°C
gives the efficiency of 41%. Similar calculation at 850°C gives the efficiency of 64%.
The process requires a careful use of materials for electrodes and electrolyte.
For a solid oxide electrolyzer cell (SOEC), numerous materials for electrodes and
Water for Energy and Fuel Production
been operating with 1.6 V per cell. There are several indirect methods to improve
electrolytic cell performance, and these are described by Bockris et al. [11]. In the
recent years, hydrogen is also created by coal slurry electrolysis. Recent advances
can also reduce the potential for the electrolysis to as low as 0.5 V.
If the electricity is obtained with a heat engine, Carnot efficiency limitation applies.
Thus, if the electricity is obtained from coal, the normal efficiency is about 39% and
the remaining 61% is lost as heat. This loss of thermal energy makes the electricity
generated by wind or hydroelectric energy more efficient. The processes are often
considered in combination with a nuclear or solar heat source. A high-temperature
electrolysis (HTE) process may be favorable when high-temperature heat is available
as waste heat from other processes. The use of such waste heat makes the overall
process cost efficient.
11.2.1 AlkAline eleCTrolySiS
Alkaline electrolyzers use an aqueous KOH solution (caustic) as an electrolyte that
usually circulates through the electrolytic cells [1–11]. Alkaline electrolyzers are
suited for stationary applications and are available at operating pressures up to
25 bar. Alkaline electrolysis is a mature technology allowing unmanned remote
operation with significant operating experience in industrial applications. The following reactions take place inside the alkaline electrolytic cell:
Electrolyte: 4H → 4
+
+ 4
−
2 O
H
OH
(11.3)
Cathode: 4H
+
+ 4e
−
→ 2H 2
(11.4)
Anode: 4OH
−
→ O 2 + 2H 2 O + 4e
−
(11.5)
Sum: 2H 2 O → O 2 + 2H 2
(11.6)
Commercial electrolyzers usually consist of a number of electrolytic cells arranged
in a cell stack. The major research challenges for the future are the design and manufacturing of electrolyzer equipment at lower costs with higher energy efficiency and
large turndown ratios.
11.2.2 hTe ProCeSS
HTE is more efficient economically than traditional room-temperature electrolysis
because some of the energy is supplied by heat that is cheaper than electricity and the
electrolysis reactions are more efficient at higher temperatures. While at 2500°C, thermal
energy alone can dissociate water molecules, generally HTE systems operate between
100°C and 850°C [12,14,16–19] (Laguna-Barcero et al., 2012, pers. comm.). The efficiency of HTE process can be easily estimated by assuming that the heat required comes
from heat engines and heat energy required for 1 kg of hydrogen (350 MJ) at 100°C
gives the efficiency of 41%. Similar calculation at 850°C gives the efficiency of 64%.
The process requires a careful use of materials for electrodes and electrolyte.
For a solid oxide electrolyzer cell (SOEC), numerous materials for electrodes and