Thermal modelling of parabolic trough collectors - Solar Energy Sciences and Engineering Applications - page 172

Figure 6.1.2 Photographs of parabolic trough collectors (left picture Industrial Solar Technology
collector, right picture Eurotrough collector).
annual output. Therefore, the choice of orientation should depend on the application
and whether more energy is needed during summer or winter (Kalogirou 2004, 2009).
Parabolic trough collector technology is the most advanced of the solar thermal
technologies because of considerable experience gained so far and the development of a
small commercial industry to produce and market these systems. Parabolic trough col-
lectors are built in modules that are supported from the ground by simple pedestals at
specific intervals or at either end if the collector is short. Photographs of two parabolic
trough collectors are shown in Figure 6.1.2.
Parabolic trough collectors are the most mature solar technology to generate heat
at temperatures up to 400 ◦ C for solar thermal electricity generation or process heat
applications. This is due to the application of this type of system in the Southern
California power plants, which have a total installed capacity of 354 MWe (Kearney
and Price 1992) and are known as Solar Electric Generating Systems (SEGS). SEGS I
is 14 MWe, SEGS II-VII are 30 MWe each and SEGS VIII and IX are 80 MWe each.
Recently, new systems have been installed in Spain and the USA, and new plants are
under development in many Middle East countries.
New developments in the field of parabolic trough collectors are focussed on cost
reduction and improvements of the technology. In one such development, the collector
is washed automatically during the night, thus reducing drastically the maintenance
cost, as this is the most used maintenance process (Kalogirou 2009).
A linear receiver is used in a parabolic trough which is a metallic tube placed along
the focal line of the parabola surrounded by a glass cover envelope (see Figure 6.1.1
detail). The size of the tube, and therefore the concentration ratio, is determined by
the size of the reflected sun image and the manufacturing tolerances of the trough. The
surface of the metal receiver is usually plated with selective coating that has a high
absorptance for solar irradiation but a low emittance for thermal radiation.
The purpose of the glass cover tube placed around the receiver tube is to reduce
the convective heat loss from the receiver, so as to decrease the heat loss coefficient.
A disadvantage resulting from the use of the glass cover tube is that the reflected light
from the concentrator must first pass through the glass to reach the receiver, and in
doing so a transmittance loss is added of about 0.9, when the glass is clean. The glass
envelope usually has an anti-reflective coating to improve transmissivity. Particularly