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Renewable Energy: Scaling Deployment in the United States Chapter j 5 93

Solar Photovoltaics
Solar photovoltaic (PV) technologies harness the photoelectric effect, using
sunlight to excite electrons in a semiconductor and generate direct current
electricity. Two main categories of PV technologies that currently exist are
silicon crystal and thin film. Today, silicon is the most commonly used
semiconductor in the PV industry, on account of both the relatively high ef-
ficiencies it can achieve as well as its relative abundance as a raw material,
although the process of refining silicon to a sufficient level of purity is
expensive. Thin-film technologies, such as those made from cadmium telluride
or copper indium gallium selenide, promise to address the high materials costs
of silicon PV, combining low-cost materials (such as glass or plastic) with
thinly spread semiconductors. Although thin-film technologies have limited
efficiencies, they have the potential to provide solar power at a lower cost per
watt than silicon crystals.
In the United States, PV originally had a niche application in the space
program, providing power to shuttles and orbiting satellites. With improved
efficiencies (from laboratory-best figures of 15% in the early 1980s to better
than 40% today (New world record, 2014)) and decreasing production costs
(by about a factor 100 since the 1950s), PV came to be an economical power
source of a variety of Earth-bound applications, including off-grid living and
remote communication devices. Prior to 2005, the majority of cumulative US
PV installations were dedicated to off-grid applications, even though utility-
scale PV arrays started to be deployed in the early 1980s. Since 2005,
however, most of the added PV capacity has been grid-tied, with the sizeable
majority taking the form of utility-scale developments. [In 2016, more
than 70% of the added solar capacity came from systems larger than 2 MW
(Utility-Scale Solar, 2015).] The cost of installing solar panels has fallen
dramatically in the last decade, at about $1.50 per Watt for a utility-scale
system today from a cost of about $4.50 per Watt in 2009 (U.S. Solar
Market Insight, 2016). Electricity is produced by current solar PV systems at a
cost of about 5e7 cents per kWh, which is competitive with many other
sources of power.

Solar Thermal
Solar thermal technologies harness sunlight to produce thermal energy. This
heat is then used directly or to generate electricity.
In contrast with solar PV, electricity generation by solar thermal tech-
nology is almost exclusively the province of large, utility-scale generators.
Concentrated solar power (CSP) systems use mirrors or lenses to focus solar
energy, heating a working fluid (such as wateroroil) thatdrivesan
electricity-generating turbine. Based on how they collect solar energy, there
are four mainstream CSP designsdtrough, linear Fresnel reflector, tower,