Environmental Engineering Reference
In-Depth Information
Figure 8.2.2 Left: STEP carbon capture in which three molten carbonate electrolysis in series are driven
by a concentrator photovoltaic. Sunlight is split into two spectral regions; visible drives the
CPV and thermal heats the electrolysis cell. In Hy-STEP (not shown) sunlight is not split and
the full spectrum heats the electrolysis cell, and electronic charge is generated separately
by solar, wind, or other source. Right: The maximum power point photovoltage of one
Spectrolab CPV is sufficient to drive three in series carbon dioxide splitting 950 C molten
Li 2 CO 3 electrolysis cells. Top: Photocurrent at 500 suns (masked (0.20 cm 2 ) Spectrolab
CDO-100 CPV, or electrolysis current, versus voltage; electrolysis current is shown of
one, two or three series 950 CLi 2 CO 3 electrolysis cells with 200 cm 2 Ni electrodes.
Three in series electrolysis cells provide a power match at the 2.7V maximum power
point of the CPV at 950 C; similarly (not shown), two 750 CLi 2 CO 3 electrolysis cells in
series provide a power match at 2.7V to the CPV. Bottom: Stable carbon capture (with
200 cm 2 “aged'' Ni electrodes at 750 C; fresh electrodes (not shown) exhibit an initial
fluctuation as carbon forms at the cathode and Ni oxide layer forms on the anode. The
rate of solid carbon deposition gradually increases as the cathode surface area slowly
increases in time. Modified with permission from Licht et al. 2010a.
salt-insulated storage will permit continuous operation of the STEP process. Both STEP
implementations provide a basis for practical, high solar efficiencies.
Components for STEP CO 2 capture and conversion to solid carbon are represented
on the left side of Figure 8.2.2, and are detailed in Licht et al., 2010a; Licht et al.,
2010; Licht et al., 2011b; Licht et al., 2010b; Licht et al., 2011a;. A 2.7 V CPV
photopotential drives three in series electrolyses at 950 C. Fundamental details of the
heat balance are provided in Licht et al., 2010a. The CPV has an experimental solar
efficiency of 37%, and the 63% of insolation not converted to electricity comprises a
significant heat source. The challenge is to direct a substantial fraction of this heat to
the electrolysis. An example of this challenge is in the first stage of heating, in which
higher temperatures increases CO 2 preheat, but diminishes the CPV power. Heating of
the reactant CO 2 is a three tier process in the current configuration: the preheating of
room temperature CO 2 consists of either (1a) flow-through a heat exchange fixed to
the back of the concentrator solar cell and/or (1b) preheating to simulate CO 2 extracted
from an available heat source such as a hot smoke (flue) stack, (2) secondary heating
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