Environmental Engineering Reference
In-Depth Information
One salt source for the STEP generation of magnesium and chlorine from MgCl
2
are via chlorides extracted from salt water, with the added advantage of the generation
of less saline water as a secondary product. In the absence of effective heat exchanger,
concentrator photovoltaics heat up to over 100
◦
C, which decreases cell performance.
Heat exchange with the (non-illuminated side of) concentrator photovoltaics can
vaporize seawater for desalinization and simultaneously prevent overheating of the
CPV. The simple concentrator STEP mode (coupling super-bandgap electronic charge
with solar thermal heat) is applicable when sunlight is sufficient to both generate
electronic current for electrolysis and sustain the electrolysis temperature. In cases
requiring both the separation of salts from aqueous solution followed by molten elec-
trolysis of the salts, a single source of concentrated sunlight can be insufficient, to both
drive water desalinization and to also heat and drive electrolysis of the molten salts.
Figure 8.3.8 includes a schematic representation of a Hybrid-Solar Thermal Electro-
chemical Production process with separate (i) solar thermal and (ii) photovoltaic field
to drive both desalinization and the endothermic carbon dioxide-free electrolysis of
the separated salts, or water splitting, to useful products. As illustrated, the separate
thermal and electronic sources may each be driven by insolation, or alternatively, can
be (i) solar thermal and (ii) (not illustrated) wind, water, nuclear or geothermal driven
electronic transfer.
8.4 STEP CONSTRAINTS
8.4.1 STEP limiting equations
As illustrated on the left side of Scheme 8.2.2, the ideal
STEP
electrolysis potential
incorporates not only the enthalpy needed to heat the reactants to T
STEP
from T
ambient
,
but also the heat recovered via heat exchange of the products with the inflowing
reactant. In this derivation it is convenient to describe this combined heat in units
of voltage via the conversion factor nF:
Q
T
≡
H
i
(R
i
,T
STEP
)
−
H
i
(R
i
,T
ambient
)
−
H
i
(C
i
,T
STEP
)
+
H
i
(C
i
,T
ambient
);
i
i
i
i
E
Q
(V)
=−
Q
T
(J
/
mol)
/
nF
(8.4.1)
The energy for the process, incorporates E
T
,E
Q
, and the non-unit activities, via
inclusion of Equation 8.4.1 into Equation 8.2.4, and is termed the
STEP
potential,
E
STEP
:
x
y
G
◦
(T)
a(R
i
)
ri
/
a(P
i
)
pi
)]
/
nF;
E
STEP
(T, a)
=
[
−
−
Q
T
−
RT
·
ln(
i
=
1
i
=
1
E
STEP
(a
E
T
+
=
1)
=
E
Q
(8.4.2)
In a pragmatic electrolysis system, product(s) can be be drawn off at activities that are
less than that of the reactant(s). This leads to large activity effects in Equation 8.4.2
at higher temperature, (Licht, 2009; Licht et al., 2010a; Licht and Wang, 2010;
Licht et al., 2011b; Licht et al., 2011a; Licht, 2002; Licht, 2003; Licht et al., 2003;
Licht, 2005) as the RT/nF potential slope increases with T (e.g. increasing 3-fold from
0.0592 V/n at 25
◦
C to 0.183 V/n at 650
◦
C).
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