Environmental Engineering Reference
In-Depth Information
the one being built by First Solar in China. The market has not supported this claim,
and at present there appears to be no large-scale project of this type under
construction.
7.1.2
A Smaller Scale Concentrator Technology Built on Multijunction Cells
A more modest system, in principle, similar to that described above is marketed by
SolFocus. The individual parabolic glass mirrors are smaller and are packaged in
arrays provided with two-axis tracking of the sun. A conceptual sketch for the
SolFocus devices is shown in Figure 7.8. The literature suggests concentration in
the range of 500
-
650. The dashed curve at the bottom of Figure 7.1 shows that a two-
junction tandem cell at 500 suns has the capability of about 55% ef
ciency, a large
improvement over single cells. SolFocus states the ef
ciency of their multijunction
solar cells is 38%.
The SolFocus concentrating cell design addresses the problem of heating of the
cells PN junctions, by insertion of a nonfocusing optical rod to transmit the multiplied
active spectrum, but not the unutilized and deleterious infrared heat spectrum, to the
tandem junctions. Concentration of light (by about a factor of 500) with individual
parabolic and accompanying secondary mirrors addresses the issue of cost and
availability of elements, for example, Ga, used in the cells, and of the multijunction
fabrication, since the concentration reduces the area of solar cell required. It appears
that water-cooling is not required to operate the tracking panels provided by SolFocus
because of the smaller scale and the use of the glass rod delivering the active spectrum
from the mirrors to the cell.
Figure 7.8 Artist
s impression of
concentrating solar cell including advanced
GaAs-based tandem junctions. SolFocus
markets arrays of these devices provided with
two-axis tracking of the sun. There is a large
change of scale between this approach and the
approach represented by Figure 5.8. SolFocus,
Inc., 510 Logue Avenue, Mountain View,
California, 94043 (2008) http://www.solfocus.
com/en/technology/.
