Page 30 - Design of Solar Thermal Power Plants

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION 23

rapidly increase, but also the selective receiver film of the metal receiver
tube surface will deteriorate due to oxidation, which may result in severe
reduction of the receiver tube’s optical efficiency. Under the special
working conditions of high temperature and strong radiation, CSP per-
formance and vacuum life can only be ensured when the materials and
properties of these components satisfy certain requirements:
1. Glazed shield tube. Due to dayenight alternation and temporary
cloud occlusions, alternating stress may be generated at the seal,
which thus requires high hardness and thermal stability as well as
corrosion resistance. Materials that are widely applied at the present
include borosilicate glasses such as Pyrex glass, the expansion
6
coefficient of which is 3.3 10 /K while featuring high hardness,
good optical properties, and acid and alkali corrosion resistance. It
also has the disadvantages of having no corresponding sealing
metal, and its softening temperature approximates 820 C, and thus

the temperature is extremely high during sealing operations.
2. Metal receiver tube. The temperature of the metal receiver tube
under concentration effect will be much higher than 400 C. Thus it

is necessary that it is equipped with high-temperature and corrosion
resistance. To eliminate the influences of axial expansion on the
collector bracket, the expansion coefficient shall be as small as
possible. Due to thermal and gravitational influences, downward
deflectionmayoccur,sotheremust beasufficient distancebetweenthe
exterior wall and the interior wall of the glass tube. Currently, high-
temperature-resistant 316L stainless steel is normally used with an
external diameter of 70 mm, a wall thickness of 3e5.5 mm, a standard
length of 4060 mm, and a mean roughness of less than 0.5 mm.
3. Glass-metal sealing transition piece. A certain sealing alloy is
applied to solve the inconsistency of the expansion coefficients of
the interior metal tube and exterior glass tube. Therefore, both
expansion coefficients shall be as close to each other in value as
possible in order to satisfy matched sealing and easier welding to the
corrugated pipe.
4. Thermal stress buffer section. This buffer is required in order to
compensate the expansions of the metal receiver tube and the
glazed shield tube. Thus it is necessary that is has good flexibility,
excellent tension fatigue strength and life, high-temperature
resistance, and acid and alkali corrosion resistance. The respective
length shall be as short as possible to increase the effective
concentration length of the receiver tube.
5. Getter. A getter is used to absorb the residual gases in the vacuum
interlayer after sealing and the released gases of components
under high-temperature working status to ensure a satisfactory