Environmental Engineering Reference
In-Depth Information
higher T, generate large STEP factors, and result in high solar to chemical energy
conversion efficiencies for the splitting of CO 2 to CO and O 2 . As one intermediate
example from Equation 8.4.5, we take the case of an electrolysis efficiency of 80% and
a 34% efficient photovoltaic (
0.272). This will drive STEP solar CO 2
splitting at molten carbonate temperatures (650 C) at a solar conversion efficiency of
35% in the unit activity case, and at 50% when N
η
· η electrolysis =
PV
=
100 (the case of a cell with 1 bar
of CO 2 and
58 mbar CO).
8.4.3 Scaleability of STEP processes
STEP can be used to remove and convert carbon dioxide. As with water splitting,
the electrolysis potential required for CO 2 splitting falls rapidly with increasing tem-
perature (Figure 8.2.1), and we have shown here (Figure 8.2.2) that a photovoltaic,
converting solar to electronic energy at 37% efficiency and 2.7 V, may be used to drive
three CO 2 splitting lithium carbonate electrolysis cells, each operating at 0.9 V, and
each generating a 2 electron CO product. The energy of the CO product is 1.3 V (Equa-
tion 8.2.1), even though generated by electrolysis at only 0.9V due to synergistic use
of solar thermal energy. As seen in Figure 8.2.5, at lower temperature (750 C, rather
than 950 C), carbon, rather than CO, is the preferred product, and this four electron
reduction approaches 100% Faradaic efficiency.
The CO 2 STEP process consists of solar-driven and solar thermal assisted CO 2 elec-
trolysis. Industrial environments provide opportunities to further enhance efficiencies;
for example fossil-fueled burner exhaust provides a source of relatively concentrated,
hot CO 2 . The product carbon may be stored or used, and the higher temperature prod-
uct carbon monoxide can be used to form a myriad of industrially relevant products
including conversion to hydrocarbon fuels with hydrogen (which is generated by STEP
water splitting in Section 8.3.1), such as smaller alkanes, dimethyl ether, or the Fischer
Tropsch generated middle-distillate range fuels of C11-C18 hydrocarbons including
synthetic jet, kerosene and diesel fuels (Andrews and Logan, 2008). Both STEP and
Hy-STEP represent new solar energy conversion processes to produce energetic
molecules. Individual components used in the process are rapidly maturing tech-
nologies including wind electric, (Barbier, 2010) molten carbonate fuel cells (Sun-
macher, 2007), and solar thermal technologies (BrightSource, 2012; AREVA, 2012;
Siemens, 2011; Solar Reserve, 2012; Amonix, 2012; Energy Innovations, 2012;
Pitz-Paul, 2007).
It is of interest whether material resources are sufficient to expand the process to
substantially impact (decrease) atmospheric levels of carbon dioxide. The buildup of
atmospheric CO 2 levels from a 280 to 392 ppm occurring over the industrial revolution
comprises an increase of 1.9
10 11 metric tons) of CO 2 ,(Tans, 2009)
and will take a comparable effort to remove. It would be preferable if this effort results
in useable, rather than sequestered, resources. We calculate below a scaled up STEP
capture process can remove and convert all excess atmospheric CO 2 to carbon.
In STEP, 6 kWh m 2 of sunlight per day, at 500 suns on 1 m 2 of 38% efficient
CPV, will generate 420 kAh at 2.7 V to drive three series-connected molten carbonate
electrolysis cells to CO, or two series-connected molten carbonate electrolysis cells to
form solid carbon. This will capture 7.8
10 16 mole (8.2
×
×
10 3 moles of CO 2 day 1 to form solid carbon
×
2 series cells/4 Faraday mol 1 CO 2 ). The CO 2 consumed per day
is three fold higher to form the carbon monoxide product (based on 3 series cells and
(based on 420 kAh
·
Search WWH ::




Custom Search