The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 230

diminished by the solar thermal absorption efficiency and heat exchange to the elec-
trolysis efficiency, (Licht, 2003) and under 0.5 MW m − 2 of incident sunlight (500 suns
illumination), yields 50 kW m − 2 , which may be split off as thermal energy towards

heating the electrolysis cell without decreasing the CPV electronic power. The CPV,
while efficient, utilizes less than half of the super-bandgap (h ν> 0.67 eV) sunlight. A
portion of this >

∼

250 kW m − 2 available energy, is extracted through heat exchange at
the backside of the CPV. Another useful source for consideration as supplemental heat
is industrial exhaust. The temperature of industrial flue stacks varies widely, with fossil
fuel source and application, and ranges up to 650 ◦ C for an open circuit gas turbine.
The efficiency of thermal energy transfer will limit use of this available heat.
A lower limit to the STEP efficiency is determined when no heat is recovered,
either from the CPV or remaining solar IR, and when heat is not recovered via heat
exchange from the electrolysis products, and when an external heat source is used to
maintain a constant electrolysis temperature. In this case, the difference between the
electrolysis potential and the thermoneutral potential represents the enthalpy required
to keep the system from cooling. In this case, our 0.9 V electrolysis occurs at an effi-
ciency of (0.90 V/1.46 V)

∼

34%. While the STEP energy analysis, detailed in
Section 8.4.2 for example for CO 2 to CO splitting, is more complex than that of the
Hy-STEP mode, more solar thermal energy is available including a PV's unused or
waste heat to drive the process and to improve the solar to chemical energy conversion
efficiency. We determine the STEP solar efficiency over the range from inclusion of no
solar thermal heat (based on the enthalpy, rather than free energy, of reaction) to the
case where the solar thermal heat is sufficient to sustain the reaction (based on the free
energy of reaction). This determines the efficiency range, as chemical flow out to the
solar flow in (as measured by the increase in chemical energy of the products compared
to the reactants), from 34% to over 50%.

· 54.7% =

8.2.3 Identification of STEP consistent endothermic processes
The electrochemical driving force for a variety of chemicals of widespread usewill be
shown to significantly decrease with increasing temperature. As calculated and sum-
marized in the top left of Figure 8.2.1, the electrochemical driving force for electrolysis
of either carbon dioxide or water, significantly decreases with increasing temperature.
The ability to remove CO 2 from exhaust stacks or atmospheric sources, provides a
response to linked environmental impacts, including global warming due to anthro-
pogenic CO 2 emission.From the known thermochemical data for CO 2 , CO and O 2 ,
and in accord with Equation 8.2.1, CO 2 splitting can be described by:

CO 2 (g) → CO (g) + 1 / 2O 2 (g);

E CO 2 -split (G CO + 0 . 5G ◦ O 2 − G CO 2 ) / 2F; E ◦ (25 ◦ C) = = 1 . 333 V (8.2.9)

As an example of the solar energy efficiency gains, this progress report focuses on CO 2
splitting potentials, and provides examples of other useful STEP processes. As seen
in Figure 8.2.1, CO 2 splitting potentials decrease more rapidly with temperature than
those for water splitting, signifying that the STEP process may be readily applied to
CO 2 electrolysis. Efficient, renewable, non-fossil fuel energy-rich carbon sources are