Energy and Human Ambitions on a Finite Planet, 2021a

13 Solar <strong>Energy</strong> 220
front of the reflector carrying a fluid (usually oil) that can be heated
to a high temperature by the absorbed sunlight. The hot oil pipes can
then be run through water to boil it <strong>and</strong> make steam, thereafter driving
a traditional steam power plant. Such ST arrangements are sometimes
called concentrated solar power (CSP). Another common variant—called
a “power tower”—is shown in Figure 13.25, in which an array of steerable
flat mirrors on the ground direct sunlight to the top of a central tower to
make steam.
As for efficiency, solar thermal is at face value similar to PV: 15–20%
is fairly typical. Broken down, roughly 50–75% of the available energy
successfully transfers to the fluid, <strong>and</strong> then the heat engine delivers
about 25–30% efficiency. But these numbers only apply if we count just
the area of the reflective collector. Because they have to track the sun, <strong>and</strong>
self-shadowing is to be avoided, only a small amount of the l<strong>and</strong> area is
occupied by the reflectors. Characterization of real facilities indicates
that only 3% of the solar energy hitting the patch of l<strong>and</strong> corresponding
to the power plant is exported in the form of electrical energy.
But efficiency is not everything. 3% of a gigantic resource like solar
energy input can still be tremendously large. It translates to over 6 W/m 2
for a st<strong>and</strong>ard insolation of 200 W/m 2 , which is about thirty-times better
than wind, per l<strong>and</strong> area. While a field of PV panels outperform an ST
installation by a factor of 5–6, the technologically simpler solar thermal
designs can be more cost effective than PV. Reflectors <strong>and</strong> oil pipes
are low-tech cheap devices, compared to photovoltaic material. The
production cost for solar thermal is estimated to be about $0.06/kWh,
which is lower than the typical retail cost of electricity, but still a factor
of two higher than fossil fuel electricity production costs.
Figure 13.24: Parabolic trough-based ST
plant, in which part of the power generation
facility is seen in the background.
Reflectors must be spaced out to prevent
self-shadowing. From U.S. DoE.
Figure 13.25: One of three “power towers”
of the Ivanpah facility in California. By
Craig Dietrich.
One disadvantage of solar thermal is that concentration only works
when the sun itself is visible in the sky: no obscuring clouds. One way to
think of it is: if you can’t see your shadow, solar concentration will not
work. Meanwhile, PV panels will still produce a meaningful amount of
daytime electricity from the bright sky <strong>and</strong> clouds even if the sun itself
is not “out.”
Balancing this disadvantage is the fact that solar thermal has some
built-in storage capacity, in that the heated oil can be “banked” for some
hours 92 <strong>and</strong> continue to produce electricity even during the passage of
a cloud or for a few hours into the evening. In this sense, it can better
match the peak of electrical dem<strong>and</strong> (early evening: Figure 13.15) than
can PV, which goes to zero once the sun sets.
92: . . . thus the “optional storage” block in
Figure 13.23
As seen in Figure 13.13, the desert southwest is the best place in the U.S.
for solar thermal electricity generation. It makes sense that deserts would
be good spots, since effective concentration requires no interference from
clouds. Incidentally, transmitting electricity over intermediate distances
(across regions) is fairly efficient: typically better than 90% for distances
shorter than ∼1,000 km.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.