Environmental Engineering Reference
In-Depth Information
Scheme 8.2.3
Comparison of solar energy utilization in STEP and Hy-STEP implementations of
the solar thermal electrochemical production of energetic molecules. Modified with
permission from Licht 2011.
temperature, E
◦
, can comprise a significant fraction of the thermoneutral potential.
The first STEP mode, energetically represented next to the room temperature process
in the scheme, separates sunlight into thermal and visible radiation. The solar visible
generates electronic charge which drives electrolysis charge transfer. The solar thermal
component heats the electrolysis and decreases both the E
◦
at this higher T, and the
overpotential. The second mode, termed Hy-STEP (on the right) from “hybrid-STEP'',
does not separate sunlight, and instead directs all sunlight to heating the electrolysis,
generating the highest T and smallest E, while the electrical energy for electrolysis is
generated by a separate source (such as by photovoltaic, solar thermal electric, wind
turbine, hydro, nuclear or fossil fuel generated electronic charge). As shown on the
right side, high relative concentrations of the electrolysis reactant (such as CO
2
or iron
oxide will further decrease the electrolysis potential).
8.2.2 STEP solar to chemical energy conversion efficiency
The Hy-STEP mode is being studied outdoors with wind or solar CPV generated elec-
tricity to drive E
electrolysis
. The STEP mode is experimentally more complex and is
presently studied indoors under solar simulator illumination. Determination of the
efficiency of Hy-STEP with solar electric is straightforward in the domain in which
E
electrolysis
<
E
thermoneutral
and the coulombic efficiency is high. Solar thermal energy is
collected at an efficiency of
η
thermal
to decrease the energy from E
thermoneutral
to E
electrolysis
,
and then electrolysis is driven at a solar electric energy efficiency of
η
solar-electric
:
η
Hy-STEPsolar
=
(
(E
thermoneutral
-E
electrolysis
)
+ η
solar-electric
·
η
thermal
·
E
electrolysis
)
/
E
thermoneutral
(8.2.6)
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