Page 133 - A Comprehensive Guide to Solar Energy Systems

132 A COmPREHEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

back-silvered thick-glass reflectors are the preferred ones for CST plants due to their bet-
ter solar reflectance, good durability, and affordable cost. Sandwich-glass reflectors (i.e.,
reflectors made of two pieces of glass with a silver layer deposited in between) have good
optical properties, but their higher cost limits their use.
The receiver pipe of a PTC is composed of several receiver tubes connected in series
and placed at the focal line of the parabolic-trough concentrator. The place where two
adjacent tubes are connected is used to install the brackets and metallic supports required
to hold the receiver tubes at the focal line. These metallic supports can be seen in Fig. 7.3.
Receiver tubes of PTC used in CSTP plants are composed of two pipes: (1) an inner steel
pipe inside which the working fluid circulates, and (2) an outer glass envelope surround-
ing the inner steel tube with an annular gap in between. Both ends of the cylindrical glass
envelope are connected to the inner steel tube by means of stainless-steel flexible bellows,
which compensates for the different thermal expansion of the steel and glass tubes. One
end of the flexible bellows is welded to the steel tube, while the other end is connected
to the glass envelope using a glass-to-metal special weld. A hermetic annular gap is thus
created between the steel tube and the glass envelope. The air can be extracted from this
−2
annulus creating a high-vacuum condition (P < 10 Pa), thus avoiding thermal losses
by convection and increasing the efficiency of the system. This efficiency is further in-
creased by depositing a selective coating on the outer surface of the steel tube. Such se-
lective coating has a high solar absorptance (≥0.94) and a low emittance (≤0.1 at 400°C)
in the infrared region. Additionally, both sides of the glass envelope are provided with an
antireflective coating that increases the solar transmittance to 0.96. These receiver tubes
are sophisticated from a technical standpoint to efficiently convert the concentrated solar
radiation reflected by the parabolic-trough concentrator into heat.
Another key element of a PTC is the sun tracking system, which keeps the parabolic
concentrator correctly aimed at the sun during operation to reflect the concentrated solar
radiation toward the receiver tube. All concentrating solar systems must be provided with
a sun tracking system to ensure that the incoming direct solar radiation is impinging on
the concentrator with the correct incidence angle. The sun tracking system of a PTC must
have a high accuracy (equal or better than 0.1 degree) to ensure that most of the reflected
radiation goes to the focal line where the receiver tube is located. An error of 0.5 degree
would be enough to lose a significant fraction (>25%) of the concentrated radiation.

7.2.2 Working Fluids for PTC
The working fluid currently used in CSTP plants with PTC is a thermal oil composed of an
eutectic mixture of two very stable compounds: (1) biphenyl (C 12 H 10 ) and (2) diphenyl ox-
ide (C 12 H 10 O). This thermal oil has a maximum working temperature of 398°C and is stable
with time so long as this maximum temperature is not surpassed and a suitable treatment
system (the so-called “oil ullage system”) is installed in the plant. under these conditions,
this thermal oil can be used for more than 30 years. Another advantage of this thermal oil
is its low vapor pressure (1.06 MPa at 398°C), which reduces the pressure required in the