Page 132 - A Comprehensive Guide to Solar Energy Systems

Chapter 7 • Concentrating Solar Thermal Power 131

7.2 Parabolic-Trough Collectors

The deployment of parabolic-trough collectors (PTC) has experienced a surge since 2000,
with about 4 GW e of installed power worldwide at the end of 2016. This section gives an
overview of the state-of-the-art of this technology, describing the main components,
working fluids, and commercial applications of PTC. Further information on the design,
energy balance, operation, and maintenance of PTC can be found in Ref. [9].

7.2.1 Main Components
Fig. 7.3 shows a typical PTC with its three main components: (1) the parabolic-trough con-
centrator, (2) the receiver pipe, and (3) the supporting structure providing the required ri-
gidity and mechanical stiffness. The parabolic-trough concentrator is used to concentrate
the direct solar radiation onto a receiver pipe placed at its focal line. This concentrated
solar radiation increases the enthalpy of a working fluid as it circulates inside the receiver
tube from the collector inlet to its outlet. The maximum temperature of the working fluid
in current commercial applications is 395°C. However, one of the current research and
development (R + d) topics is the increase of this temperature up to 500°C or even 550°C
to increase the overall plant efficiency.
Back-silvered thick-glass reflectors are the most widely used type of reflector for para-
bolic-trough concentrators. This type of reflector is similar to conventional mirrors apart
from the curvature and the type of glass used. As standard glass has some iron content
that reduces its transmissivity for solar radiation, borosilicate glass (glass with much low-
er iron content than standard glass) is commonly used for PTC, and for glass components
used in CST systems in general. Although there are other types of reflectors available
(e.g., polymeric-film reflectors, aluminum-sheet reflectors, and thin-glass reflectors),

FIGURE 7.3 A Typical parabolic-trough collector (PTC).