448 CHAPTER 18 Renewable Energy Resources
SunlightPipe with
heated oilSolar
collector
Pasquale Sorrentino/Science Source Imagesa. A solar thermal plant in California uses troughs to
focus sunlight on a fluid-filled tube, as shown in b.
The heated oil is pumped to a water tank, where
it generates steam used to produce electricity. For
simplicity, arrows show sunlight converging on
several points; sunlight actually converges on the
pipe throughout its length.a bsunlight and concentrate it using mirrors or lenses to
heat a fluid to high temperatures.
In one such system, computer-guided trough-shaped
mirrors track the sun for optimum efficiency, center
sunlight on nearby oil-filled pipes, and heat the oil to
390°C (735°F) (Figure 18.5). The hot oil is circulated
to a water storage system and used to boil water into su-
perheated steam, which turns a turbine to generate elec-
tricity. The plant produces about 370 MW of electricity.
Worldwide, concentrated solar thermal plants produce
over 1 gigawatt (GW). In 2011, an additional 17 GW of
capacity was planned or under construction.
Solar thermal systems often have a backup—usually
natural gas—to generate electricity at night and dur-
ing cloudy days when solar power isn’t operating. The
world’s largest solar thermal system of this type currently
operates in the Mojave Desert in southern California.
Solar thermal energy systems are inherently more
effi cient than other solar technologies because they con-
centrate the sun’s energy. With improved engineering,
manufacturing, and construction methods, solar ther-
mal energy is becoming cost- competitive with fossil fuels
(see Table 18.1). In addition, the environmental benefits
of solar thermal plants are significant: These plants don’t
produce air pollution or contribute to acid rain or global
climate change.Solar-Generated Hydrogen
Increasingly, people think of hydrogen as the fuel of
the future, as it is abundant as well as easily produced.
Electricity generated by any energy source can split water
into the gases oxygen and hydrogen. Consequently, pro-
ducing hydrogen results in the same environmental and
security problems associated with the underlying energy
source, including those discussed in Chapter 17.
Hydrogen is a clean fuel; it produces water and heat
as it burns, but it produces no sulfur oxides, carbon mon-
oxide, hydrocarbon particulates, or CO 2 emissions. It
does produce some nitrogen oxides, though in amounts
fairly easy to control. Hydrogen has the potential to pro-
vide energy for transportation (in the form of hydrogen-
powered automobiles) as well as for heating buildings
and producing electricity.
It may seem wasteful to use electricity generated from
solar energy to make hydrogen that will then be used to
generate electricity. However, the electricity generated
by existing photovoltaic cells must be used immediately,->ÀÊÌ iÀ>ÊiiVÌÀVÊ}iiÀ>ÌÊUÊ}ÕÀiÊ£n°xÊ