Page 29 - Design of Solar Thermal Power Plants

22 1. INTRODUCTION

FIGURE 1.11 Structural diagram of receiver tube of parabolic trough solar collector.

consists of a backboard and adhesive and reflective materials. The back-
board functions to create a parabolic surface, and it can be made from
steel plate, aluminum plate, float glass, and fiberglass. Reflection mate-
rials can be thin glass mirror, metal film, or firm composite materials.
Adhesive materials can be PVB, neutral organic silicone, etc., in which the
aluminum reflector has a high reflection rate created with an aluminum
plate through the use of surface finishing and oxidation protection. A
silver-coated polymer mirror is a reflection surface with a high reflection
rate that is created by coating with silver on one side of the high-
transmittance, strong weather-resistance polymer film; it is equipped
with multiple layers of protective film that are attached to the bottom of
the curved surface to create a curved mirror.
As shown in Fig. 1.11, the receiver tube of the parabolic trough solar
collector is a core component of the parabolic trough collector and is
typically about 4 m long. The interior tube is a commercial-type metal
receiver tube with an external diameter of 70 mm, whereas the exterior
tube is a glazed shield tube with an external diameter that falls in the
range of 115e125 mm. Due to the metal receiver tube and glazed shield
tube having different coefficients of expansion and thermal intensities
during operation, high-temperature-resistant glass and metal sealing
pieces are required as transition pieces to ensure an airtight connection. In
addition, a metal corrugated pipe is used as the thermal stress buffer
section to relieve the longitudinal thermal expansion difference between
the metal receiver tube and the glazed shield tube. To ensure degree of
vacuum degree in the vacuum interlayers of the receiver tube, a getter
must be mounted between the metal receiver tube and the glazed shield
tube. Furthermore, with any focusing solar irradiation, the seal un-
dertakes great thermal stress that may easily invalidate the sealing of
glass and metal. Therefore, thin-walled materials with good reflection
performance are required as a solar shade to block radiation while
reflecting it to the metal receiver tube.
Both the thermal properties and life of the parabolic trough receiver
tube are determined by the vacuum degree of the vacuum interlayer. If the
vacuum environment is damaged, not only will the respective heat losses