446 CHAPTER 18 Renewable Energy ResourcesFlow of electronsBack contact Flow of electronsBoron-enriched
siliconPhosphorus-
enriched siliconLight-absorbing
coatingFront contactCourtesy Solar Design Associates
a. A student seeing the roof of the Intercultural Center of
Georgetown University probably knows that it has arrays of
photovoltaic (PV) cells to collect solar energy. The PV system
supplies about 10 percent of the school’s electricity.b. A scientist looking at those arrays knows that photovoltaic
cells contain silicon and other materials. Sunlight excites
electrons, which are ejected from silicon atoms. Useful electricity
is generated when the ejected electrons flow out of the PV cells
through a wire.WHAT A SCIENTIST SEES
Photovoltaic Cells
PV cells generate electricity with no pollution and
minimal maintenance. They can be used on any scale,
from small portable modules attached to camping lan-
terns to large, multimegawatt power plants, and can
power satellites, uncrewed airplanes, highway signals,
wristwatches, and calculators. The widespread use of PV
cells to generate electricity is currently limited by their
low efficiency at converting solar energy to electricity
and by the amount of land needed to hold the num-
ber of solar panels required for large-scale use. Several
thousand acres of today’s PV panels would be required
to produce the electricity generated by a single large
conventional power plant.
In remote areas not served by electric power plants,
such as the rural areas of developing countries, it can
be more economical to use PV cells for electricity than
to extend powerlines. Photovoltaics generate energy
that can pump water, refrigerate vaccines, grind grain,
charge batteries, and supply rural homes with light-
ing. According to the Institute for Sustainable Power,
more than 1 million households in developing coun-
tries have installed rooftop PV solar cells. A PV panelthe size of two pizza boxes supplies a rural household
with enough electricity for five lights, a radio, and a
television.
Utility companies can purchase PV devices in modu-
lar units, which can become operational in a short pe-
riod, allowing them to increase generating capacity in
small increments. The PV units can provide the addi-
tional energy, for example, to power irrigation pumps on
hot, sunny days.
The cost of manufacturing PV modules has steadily
declined over the past 35 years, from an average factory
price of almost $90 per watt in 1975 to about $4.00 per
watt in 2010. The cost of producing electricity from PVs
has steadily declined from 1970 to the present. As a re-
sult of this progress, in 2010 the cost was about $0.15
per kilowatt-hour (Figure 18.3). Table 18.1 compares
the costs of generating electricity using different energy
sources, including photovoltaics.
Future technological progress may make PVs econom-
ically competitive with electricity produced using conven-
tional energy sources. The production of “thin-film” solar
cells (Figure 18.4), which are much less expensive to