New SAT Reading Practice Test 23

This passage is adapted from Kevin Bullis, “What Tech Is Next for the Solar Industry?” ©2013 by MIT Technology Review.

Solar panel installations continue to grow quickly,
but the solar panel manufacturing industry is in the
doldrums because supply far exceeds demand. The
poor market may be slowing innovation, but
5 advances continue; judging by the mood this week at
the IEEE Photovoltaics Specialists Conference in
Tampa, Florida, people in the industry remain
optimistic about its long-term prospects.
The technology that’s surprised almost everyone
10 is conventional crystalline silicon. A few years ago,
silicon solar panels cost $4 per watt, and
Martin Green, professor at the University of
New South Wales and one of the leading silicon solar
panel researchers, declared that they’d never go
15 below $1 a watt. “Now it’s down to something like
50 cents a watt, and there’s talk of hitting 36 cents per
watt,” he says.
The U.S. Department of Energy has set a goal of
reaching less than $1 a watt—not just for the solar
20 panels, but for complete, installed systems—by 2020.
Green thinks the solar industry will hit that target
even sooner than that. If so, that would bring the
direct cost of solar power to six cents per
kilowatt-hour, which is cheaper than the average cost
25 expected for power from new natural gas power
plants.
All parts of the silicon solar panel industry have
been looking for ways to cut costs and improve the
power output of solar panels, and that’s led to steady
30 cost reductions. Green points to something as
mundane as the pastes used to screen-print some of
the features on solar panels. Green’s lab built a solar
cell in the 1990s that set a record efficiency for silicon
solar cells—a record that stands to this day. To
35 achieve that record, he had to use expensive
lithography techniques to make fine wires for
collecting current from the solar cell. But gradual
improvements have made it possible to use screen
printing to produce ever-finer lines. Recent research
40 suggests that screen-printing techniques can produce
lines as thin as 30 micrometers—about the width of
the lines Green used for his record solar cells, but at
costs far lower than his lithography techniques.
Meanwhile, researchers at the National Renewable
45 Energy Laboratory have made flexible solar cells on a
new type of glass from Corning called Willow Glass,
which is thin and can be rolled up. The type of solar
cell they made is the only current challenger to
silicon in terms of large-scale production—thin-film
50 cadmium telluride. Flexible solar cells could lower
the cost of installing solar cells, making solar power
cheaper.
One of Green’s former students and colleagues,
Jianhua Zhao, cofounder of solar panel manufacturer
55 China Sunergy, announced this week that he is
building a pilot manufacturing line for a two-sided
solar cell that can absorb light from both the front
and back. The basic idea, which isn’t new, is that
during some parts of the day, sunlight falls on the
60 land between rows of solar panels in a solar power
plant. That light reflects onto the back of the panels
and could be harvested to increase the power output.
This works particularly well when the solar panels
are built on sand, which is highly reflective. Where a
65 one-sided solar panel might generate 340 watts, a
two-sided one might generate up to 400 watts. He
expects the panels to generate 10 to 20 percent more
electricity over the course of a year.
Even longer-term, Green is betting on silicon,
70 aiming to take advantage of the huge reductions in
cost already seen with the technology. He hopes to
greatly increase the efficiency of silicon solar panels
by combining silicon with one or two other
semiconductors, each selected to efficiently convert a
75 part of the solar spectrum that silicon doesn’t convert
efficiently. Adding one semiconductor could boost
efficiencies from the 20 to 25 percent range to
around 40 percent. Adding another could make
efficiencies as high as 50 percent feasible, which
80 would cut in half the number of solar panels needed
for a given installation. The challenge is to produce
good connections between these semiconductors,
something made challenging by the arrangement of
silicon atoms in crystalline silicon.