Environmental Engineering Reference
In-Depth Information
Figure 10.2.1
Solar water splitting based on PEC cells, PV cells or combined arrangements systems.
The red line represents the “holy grail'' of the PEC system.
alone devices - semiconductor-liquid junction (SCLJ) photoelectrochemical cell (Krol
et al., 2008; Grimes et al., 2008; Aruchamy et al., 1982; Minggu et al., 2010); and
iii
) by thermochemical cycles (Coelho et al, 2010). The third technology will not be
considered in this chapter.
10.2.1.1 Composed devices
Up to today, no semiconductor photoelectrode is able to efficiently perform alone
water-splitting and thus an extra bias must be supplied. The most developed technol-
ogy is the PV device/electrolyzer arrangement where the photovoltaic cells are silicon
based, achieving maximum efficiencies of 15%, and efficiencies of electrolyzers is often
around 75%
1
. For instance, combining commercial 12% efficiency PV modules with a
water electrolysis unit operating with an energy conversion efficiency of approximately
65% (output voltage of 1.9 V) results in a solar-to-hydrogen efficiency of about 7.8%
(Kruse et al., 2002; Bansal et al., 1999; Bilgen, 2001). Only by combining optimized PV
technologies it is possible to achieve higher solar-to-hydrogen conversion efficiencies
(Green et al., 2008; Conibeer and Richards, 2007).
Even if biasing an electrolyzer with a separate set of solar cells is very attractive
from an efficiency point of view, the fact of involving two separate devices complicates
the system and increases the cost (Krol et al., 2008), besides being more energy dissipa-
tive. Furthermore, in a system PV
electrolyzer at least four silicon PV cells connected
in series are required to generate the desired voltage for water-splitting, which under
+
1
The efficiency of an electrolyzer is defined as
η
Z
=
E
◦
/
V
, where
E
◦
is the thermodynamic cell
potential (1.23 V for water electrolysis) and
V
is the voltage applied to the cell under operating
conditions.
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