The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 247

The STEP factor, A STEP is the extent of improvement in carrying out a solar driven
electrolysis process at T STEP , rather than at T ambient . For example, when applying the
same solar energy, to electronically drive the electrochemical splitting of a molecule
which requires only two thirds the electrolysis potential at a higher temperature, then
A STEP (2/3) − 1

= = 1.5. In general, the factor is given by:

A STEP = E STEP (T ambient ,a) / E STEP (T STEP ,a); e . g . T ambient = 298K (8.4.3)

The STEP solar efficiency,

STEP , is constrained by both photovoltaic and elec-
trolysis conversion efficiencies,

η

electrolysis , and the STEP factor. In the
operational process, passage of electrolysis current requires an additional, com-
bined (anodic and cathodic) overpotential above the thermodynamic potential;
that is V redox

η

PV and

η

= +

(1 z)E redox , Mobility and kinetics improve at higher tempera-

ture and ξ (T > T ambient ) < ξ (T ambient ,) (Light, 1987; Sunmacher, 2007). Hence, a

lower limit of η STEP (V T ) is given by η STEP-ideal (E T ). At T ambient ,A STEP = 1, yielding

η STEP (T ambient ) η STEP is additionally limited by entropy and black

body constraints on maximum solar energy conversion efficiency. Consideration of
a black body source emitted at the sun's surface temperature and collected at ambient
earth temperature, limits solar conversion to 0.933 when radiative losses are consid-
ered, (Solanki and Beaucarne, 2007) which is further limited to

= η PV · η electrolysis .

0.868
when the entropy limits of perfect energy conversion are included (Luque and Marti
2003). These constraints on

η PV < η limit =

η STEP-ideal and the maximum value of solar conversion, are
imposed to yield the solar chemical conversion efficiency, η STEP :

η STEP-ideal (T, a) = η PV · η electrolysis · A STEP (T, a)

STEP (T, a) = η

η · η electrolysis (T ambient ,a) · A STEP (T, a); ( η

STEP < 0 . 868)

(8.4.4)

PV

As calculated from Equation 8.2.3 and the thermochemical component data (Chase,
1998) and as presented in Figure 8.2.1, the electrochemical driving force for a variety
of chemicals of widespread use by society, including aluminium, iron, magnesium and
chlorine, significantly decreases with increasing temperature.

8.4.2 Predicted STEP efficiencies for solar splitting of CO 2
The global community is increasingly aware of the climate consequences of elevated
greenhouse gases. A solution to rising carbon dioxide levels is needed, yet carbon
dioxide is a highly stable, noncombustible molecule, and its thermodynamic stability
makes its activation energy demanding and challenging. The most challenging stage
in converting CO 2 to useful products and fuels is the initial activation of CO 2, for
which energy is required. It is obvious that using traditional fossil fuels as the energy
source would completely defeat the goal of mitigating greenhouse gases. A preferred
route is to recycle and reuse the CO 2 and provide a useful carbon resource. We limit
the non-unit activity examples of CO 2 mitigation in Equation 8.3.1 to the case when
CO and O 2 are present as electrolysis products, which yields a O2 = 0.5a CO , and upon

substitution into Equation 8.4.2:

E STEP (T)

E STEP (T, a) = − (RT / 2F) · ln(N);

= √ 2a CO 2 a − 3 / 2 E ◦ (25 ◦ C) = 1 . 333 V; N (8.4.5)

CO