Environmental Engineering Reference
In-Depth Information
2 F mol
−
1
CO
2
) in lieu of solid carbon. The material resources to decrease atmospheric
carbon dioxide concentrations with STEP carbon capture, appear to be reasonable.
From the daily conversion rate of 7.8
10
3
moles of CO
2
per square meter of CPV,
the capture process, scaled to 700 km
2
of CPV operating for 10 years can remove and
convert all the increase of 1.9
×
10
16
mole of atmospheric CO
2
to solid carbon. A
larger current density at the electrolysis electrodes, will increase the required voltage
and would increase the required area of CPVs. While the STEP product (chemicals,
rather than electricity) is different than contemporary concentrated solar power (CSP)
systems, components including a tracker for effective solar concentration are similar
(although an electrochemical reactor replaces the mechanical turbine). A variety of
CSP installations, which include molten salt heat storage, are being commercialized,
and costs are decreasing. STEP provides higher solar energy conversion efficiencies
than CSP, and secondary losses can be lower (for example, there are no grid-related
transmission losses). Contemporary concentrators, such as based on plastic Fresnel
or flat mirror technologies, are relatively inexpensive, but may become a growing
fraction of cost as concentration increases (Pitz-Paal et al., 2007). A greater degree of
solar concentration, for example 2000 suns, rather than 500 suns, will proportionally
decrease the quantity of required CPV to 175 km
2
, while the concentrator area will
remain the same at 350,000 km
2
, equivalent to 4% of the area of the Sahara desert
(which averages
×
6 kWh m
−
2
∼
of sunlight per day), to remove anthropogenic carbon
dioxide in ten years.
A related resource question is whether there is sufficient lithium carbonate, as an
electrolyte of choice for the STEP carbon capture process, to decrease atmospheric
levels of carbon dioxide. 700 km
2
of CPV plant will generate 5
10
13
A of electrolysis
×
current, and require
2 million metric tonnes of lithium carbonate, as calculated from
a 2 kg/l density of lithium carbonate, and assuming that improved, rather than flat,
morphology electrodes will operate at 5 A/cm
2
(1,000 km
2
) in a cell of 1 mm thick.
Thicker, or lower current density, cells will require proportionally more lithium car-
bonate. Fifty, rather than ten, years to return the atmosphere to pre-industrial carbon
dioxide levels will require proportionally less lithium carbonate. These values are viable
within the current production of lithium carbonate. Lithium carbonate availability as
a global resource has been under recent scrutiny to meet the growing lithium battery
market. It has been estimated that the current global annual production of 0.13 mil-
lion tonnes of LCE (lithium carbonate equivalents) will increase to 0.24 million tonnes
by 2015 (Tahil, 2008). Potassium carbonate is substantially more available, but as
noted in the main portion of the paper can require higher carbon capture electrolysis
potentials than lithium carbonate. An additional modified barium carbonate STEP
electrolyte has been introduced (Licht et al., 2013), and STEP mechanisms continue to
be probed (Cui and Licht, 2013), and the portfolio of new STEP processes and prod-
ucts, such as STEP Cement (Licht, 2012) and STEP Water Treatment (Wang et al.,
2012; Wang et al., 2103), continues to expand.
∼
8.5 CONCLUSIONS
To ameliorate the consequences of rising atmospheric carbon dioxide levels and its
effect on global climate change, there is a drive to replace conventional fossil fuel
driven electrical production by renewable energy driven electrical production. In addi-
tion to the replacement of the fossil fuel economy by a renewable electrical economy,
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