How Energy Is Produced - The Renaissance of Renewable Energy - page 120

Power from the Sun: How CSP Works
When I was eight, my father showed me how to singe dry leaves with the lenses of
my grandfather's glasses. Imagining myself as Ra, the Egyptian sun god, I spent the
afternoon subjugating our back garden. Each leaf was a disobedient city rebelling against
my authority. Unfortunately, the summers in Italy's Po Valley are wet as well as hot, and
this put a dampener on my tyrannical ambitions. The concentrated rays from Beppino's
glasses, far from producing the desired inferno of foliage, merely peppered some damp
leaves with brownish holes.
Modern CSP power plants are not terribly different - in terms of the design principle
- from those developed by Mouchot and Pifre. A collection of mirrors focus the sun's
rays onto a target point, where the heat can be converted into electricity. Several different
technologiesareavailable, butthemostcommonisbasedonparabolictroughs.Shapedlike
gutter piping, the trough is designed to automatically follow the path of the sun through
the sky, thus ensuring that it receives maximum direct sunlight throughout the day (see
Figures 4.35 and 4.36 ) . The mirrors that line the trough concentrate sunlight onto receiver
tubes that contain a fluid, such as synthetic oil, that can be heated to high temperatures
(approximately 400°C). As in a solar thermal plant, this oil, known as a thermal transfer
fluidbecauseitallowsheattobetransmitted,ispumpedthroughaseriesofheatexchangers
to produce steam, which in turn drives a turbine to generate electricity.
Figure 4.35. The three main models of CSP plant: linear trough concentrator, parabolic
dish, and solar tower.
Figure 4.36. Linear trough concentrators at the Andasol Power Station (Spain). Source:
Solar Millennium.
The most common alternative designs are solar towers and parabolic dishes (see Figure
4.35 ) . A solar tower is, as the name suggests, a tower with a barrel-like fluid container (the
solar receiver) mounted at its apex. Surrounding the tower in a fanlike array are numerous
large mirrors with sun-tracking motors, known as heliostats (see Figure 4.37 ). Because so
many mirrors concentrate sunlight onto a single point, it produces precisely the effect that
Archimedessetouttoachieve,withtemperaturesexceeding1,000degreesCelsius.Aswith
the parabolic trough, the heat is delivered to a steam turbine.