The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 227

At lower temperature, small values of E electrolysis can occur at higher reactant and lower
product activities, as described in Equation 8.2.4. In the present configuration sunlight
is concentrated at 75% solar to thermal efficiency, heating the electrolysis to 950 ◦ C,
which decreases the high current density CO 2 splitting potential to 0.9 V, and the elec-
trolysis charge is provided by CPV at 37% solar to electric efficiency. The solar to
chemical energy conversion efficiency is in accordance with Equation 8.2.6:

η Hy-STEPsolar = (75% · (1 . 46 V − 0 . 90 V) + 37% · 0 . 90 V) / 1 . 46 V = 52% (8.2.7)

A relatively high concentration of reactants lowers the voltage of electrolysis via the
Nernst term in Equation 8.2.4. With appropriate choice of high temperature elec-
trolyte, this effect can be dramatic, for example both in STEP iron and in comparing
the benefits of the molten carbonate to solid oxide (gas phase) reactants for STEP
CO 2 electrolytic reduction, sequestration and fuel formation. Fe(III) (as found in the
common iron ore, hematite) is nearly insoluble in sodium carbonate, while it is soluble
to over 10 m (molal) in lithium carbonate, (Licht et al., 2011b; Licht 2011) and as
discussed in Section 8.2.3, molten carbonate electrolyzer provides 10 3 to 10 6 times
higher concentration of reactant at the cathode surface than a solid oxide electrolyzer.
In practice, for STEP iron or carbon capture, we simultaneously drive lithium
carbonate electrolysis cells together in series, at the CPV maximum power point (Fig-
ure 8.2.2). Specifically, a Spectrolab CDO-100-C1MJ concentrator solar cell is used
to generate 2.7 V at maximum power point, with solar to electrical energy efficiencies
of 37% under 500 suns illumination. As seen in Figure 8.2.2, at maximum power, the
0.99 cm 2 cell generates 1.3 A at 100 suns, and when masked to 0.2 cm 2 area generates
1.4 A at 500 suns. Electrolysis electrode surface areas were chosen to match the solar
cell generated power. At 950 ◦ C at 0.9 V, the electrolysis cells generate carbon monoxide
at 1.3 to 1.5 A (the electrolysis current stability is shown at the bottom of Figure 8.2.2).
In accord with Equation 8.2.6 and Scheme 8.2.3, Hy-STEP efficiency improves
with temperature increase to decrease overpotential and E electrolysis , and with increase
in the relative reactant activity. Higher solar efficiencies will be expected, both with
more effective carbonate electrocatalysts (as morphologies with higher effective surface
area and lower overpotential) are developed, and as also as PV efficiencies increase.
Increases in solar to electric (both PV, CPV and solar thermal-electric) efficiencies
continue to be reported, and will improve Equation 8.2.7 efficiency. For example,
multijunction CPV have been reported improved to

η =

40.7% (King et al., 2007;

PV

Green et al., 2011).
Engineering refinements will improve some aspects, and decrease other aspects,
of the system efficiency. Preheating the CO 2 , by circulating it as a coolant under the
CPV (as we currently do in the indoor STEP experiment, but not outdoor, Hy-STEP
experiments) will improve the system efficiency. In the present configuration outgoing
CO and O 2 gases at the cathode and anode heat the incoming CO 2 . Isolation of the
electrolysis products will require heat exchangers with accompanying radiative heat
losses, and for electrolyses in which there are side reactions or product recombination
losses, η Hy-STEPsolar will decrease proportional to the decrease in coulombic efficiency.

At present, wind turbine generated electricity is more cost effective than solar-electric,
and we have demonstrated a Hy-STEP process with wind-electric, for CO 2 free pro-
duction of iron (delineated in Section 8.3.3). Addition of long-term (overnight) molten