Environmental Engineering Reference
In-Depth Information
consists of passing this CO
2
through a heat exchange with the outgoing products, (3)
tertiary heat is applied through concentrated, split solar thermal energy (Figure 8.2.2).
An upper limit to the energy required to maintain a constant system temperature
is given in the case in which neither solar IR, excess solar visible, nor heat exchange
from the environment or products would be applied to the system. When an 0.90 V
electrolysis occurs, an additional 0.56 V, over E
tn
1.46V , is required to maintain
a constant system temperature. Hence, in the case of three electrolyses in series, as
in Figure 8.2.2, an additional 3
=
1.68 V will maintain constant tempera-
ture. This is less than the 63% of the solar energy (equivalent to 4.6 V) not used in
generating the 2.7 V of maximum power point voltage of electronic charge from the
CPV in this experiment. Heating requirements are even less, when the reactant activ-
ity is maintained at a level that is higher than the product activity. For example, this
is accomplished when products are continuously removed to ensure that the partial
pressure of the products is lower than that of the CO
2
. This lowers the total heat
required for temperature neutrality to below that of the unit activity thermoneutral
potential 1.46 V.
The STEP effective solar energy conversion efficiency,
×
0.56 V
=
η
STEP
, is constrained by both
photovoltaic and thermal boost conversion efficiencies,
η
PV
and
η
thermal-boost
(Licht
et al., 2011a). Here, the CPV sustains a conversion efficiency of
37.0%. In the
system, passage of electrolysis current requires an additional, combined (ohmic, &
anodic
η
PV
=
cathodic over-) potential above the thermodynamic potential. However,
mobility and kinetics improve at higher temperature to decrease this overpotential.
The generated CO contains an increase in oxidation potential compared to carbon
dioxide at room temperature (E
CO2
/
CO
(25
◦
C)
+
1/2 O
2
in
Figure 8.2.1), an increase of 0.43 V compared to the 0.90 V used to generate the
CO. The electrolysis efficiency compares the stored potential to the applied poten-
tial,
=
1.33 V for CO
2
→
CO
+
E
◦
electrolysis
(25
◦
C)/V
electrolysis
(T) (Licht et al., 2010a). Given a stable
temperature electrolysis environment, the experimental STEP solar to CO carbon cap-
ture and conversion efficiency is the product of this relative gain in energy and the
electronic solar efficiency:
η
thermal-boost
=
η
= η
· η
=
37
.
0%
·
(1
.
33 V
/
0
.
90 V)
=
54
.
7%
(8.2.8)
STEP
PV
thermal-boost
Ohmic and overpotential losses are already included in the measured electroly-
sis potential. This 54.7% STEP solar conversion efficiency is an upper limit of the
present experiment, and as with the Hy-STEP mode, improvements are expected in
electrocatalysis and CPV efficiency. Additional losses will occur when beamsplitter and
secondary concentrator optics losses, and thermal systems matching are incorporated,
but serves to demonstrate the synergy of this solar/photo/electrochemical/thermal
process, leads to energy efficiency higher than that for solar generated electricity, (King
et al., 2007; Green et al., 2011) or for photochemical, (Miller et al., 2008) photoelec-
trochemical, (Licht, 2002; Barton et al., 2008) solar thermal, (Woolerton et al., 2010)
or other CO
2
reduction processes (Benson et al., 2009).
The CPV does not need, nor function with, sunlight of energy less than that of the
0.67 eV bandgap of the multi-junction Ge bottom layer. From our previous calcula-
tions, this thermal energy comprises 10% of AM1.5 insolation, which will be further
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