Solar integration takes a page from the semi wafer CSP playbook
major drawback to this approach is that arranged in series
the paste spreads wider with thickness, and feeding into a
!"#$%&'()*#+,'#!',--.-&/01,2123#
which makes the photon absorption rectifier, where it
area less and the problem is further is converted into
compounded because high metal struc- AC power. Dirt or
tures induce shadowing on the cell. shadows generated
by trees or clouds
As solar cell performance increases, series
create localized high
resistance becomes a larger issue. With
resistances, or even
increased current output, the power drop
failing cells unbal-
across the resistances become more signifi-
ance the panel since
cant (i
2
R).
43#5)-0&#
the cells are not all
The integrated circuit industry has per-
43#
at a uniform bias.
formed a large body of work on metallurgi-
5()0&#
The low bias cells
cal contacts and systems. Plating is currently
become the bottle-
being implemented in solar cells to reduce
neck in the chain—
resistance by electrolytically depositing on
power becomes lost
top of the paste. Figure 10 shows plated
at this point in the
Figure 10. Silver over silver paste plating.
silver on top of the paste. Note the high
load rather than at
porosity of the paste and the high density of
an external power
plating. Cu and other systems are also being
monitored, a new level of performance for
generator. Losses can approach 50%. Cur-
evaluated for over-paste metal. Although this
solar can be achieved.
rently, bypass diodes are mounted into the
plating reduces the resistance, it does not ad-
panel to circumvent the mismatched cells,
dress the issue of wide metal traces. Plating
Standards
but the drawback to this approach is that the
as well as metal-organic printing are being
One underrated area of the packaging in-
power that is being outputted from the low
pursued as replacements for the paste
9
.
dustry is the development of comprehensive
performing cells is lost.
standards. Standards allow precise design
In terms of manufacturing, solar cells
System integration
and great reliability. The solar industry must
must be held to a tight standard of perfor-
Traditional solar power systems are based
follow certain standards due to the fact that
mance to avoid building these mismatches
on modules connected to balance of system
it is part of a power system (e.g., UL1703).
into the system. Out of spec cells that are
electronics. However, alternative approaches
However, not all standards are mandatory,
functional but do not meet spec are binned
based on integrating more functions at the
and there are limited certifications required.
out. To solve these issues, electronics are be-
cell level are emerging and the opportunity
In the United States, there is only one
ing integrated at the cell level. This reduces
exists to gain performance by distributing
certification lab (Arizona State University
the required binning, and much of the lost
functionality in innovative modes. With
Photovoltaic Testing Laboratory). ASU-PTL
power due to environmental effects can be
WLP and CSP, the semi industry has recog-
offers a certification service per IEC 61215,
reclaimed
10
. Many of these system electron-
nized the efficacy of merging functions into
IEC 61646, and IEEE 1262 standards and
ics can also be self-powered, since power
a common substrate. The solar industry is
PV safety testing services per UL 1703, and
consumption can be in the mW range.
taking a cue from the IC business and work-
will soon offer IEC 61730 standards.
The next step for such an approach
ing with form factor and integration to im-
would be to build intelligence into the
prove performance. Adding concentrators to
What semi can learn from solar
system. Current power systems have manage-
the cell offers many advantages. The increase
Metallization is a key bottleneck to grid par-
ment chips and in-system tracking. This level
in power is not linear since the increase in
ity. The metal system affects recombination,
of functionality does not exist in solar pan-
power is tracked by the increase in short
series resistance, module interconnect and,
els. If a panel fails, it is likely to be scrapped.
circuit current, so they cancel. However, the
potentially, heat sinking. In addition to the
The EVA encapsulation, the series ribbons
open circuit voltage increases by ln(E) and
technical requirements, the metal process
soldering and the fragile nature of solar cells
concentrators have been shown to increase
must be supported by tools that can meet
(170-200 µm thick multicrystalinity) make
energy by orders of magnitude.
the throughput and operating cost goals.
reclaim a low yield proposition. However, by
Figure 11 shows the two standard
The current silver paste system is
building modularity as well as intelligence
approaches to concentrating solar power—
straightforward to implement but has limita-
into the system where individual cells can be
either by using lenses to focus energy or
tions. The rising cost of silver, linewidth con
by employing reflectors. One advantage of
using concentrators is that less silicon can !
be used, which drives down the cost. Instead
of full wafers, slats, squares or spheres can
be utilized. The drawbacks to this approach
are assembly cost and heat dissipation (the
increase in power from concentrators can be
cancelled by the increase in temperature of
the silicon).
Another opportunity to improve power
output through system design involves cell
mismatches. A solar panel has the cells
Figure 11. Concentrator configurations. Light can be focused through lenses or reflectors.
10 – Global Solar Technology – March/April 2009
www.globalsolartechnology.com
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