Page 122 - Design of Solar Thermal Power Plants

108 2. THE SOLAR RESOURCE AND METEOROLOGICAL PARAMETERS

FIGURE 2.19 Constitution of the pyranometer.

a sealed glazed shield (quartz glazed shield), a level, and electronic
and mechanical accessories (refer to Fig. 2.19).
The sensor consists of a sensing surface and thermopile. The
blackened sensing surface is normally circular or square. The
thermopile is made of constantan or plated-copper constantan,
whereas the black-and-white sensing surfaceismadeofblack-and-
white metal sheets that measure the lower-level thermopile
thermoelectromotive force by utilizing the different absorption
rates of black-and-white metal sheets and converting them into
2
irradiance. The sensitivity of the instrument is 7e14 mVm /W with
response time 60 s (99% response). The annual stability is
superior to 5%. As for cosine response indices, solar altitudes of 10
and 30 degrees result in cosine response errors that are less than
10% and 5% separately. The all-black glazed shield is made of
hemispherical double-layered quartz glass, which not only is
windproof but also can penetrate short-wavelength radiation
within a range of 0.29e3.05 mm with a constant transparency of
nearly 0.9. A double-layered shield is used to prevent the outer
shield from being influenced by infrared radiation and to eliminate
measurement errors.
Accessories include the machine body, dryer, white baffle, base,
level, and wiring terminal. In addition, a metal cover protects the
glazed shield (also called the protective shield). The dryer is filled
with the drying agent (silica gel) and connected with the glazed
shield to keep dried air inside the shield. The while baffle is used to
prevent solar radiation from heating the lower section of the
machine body as well as prevent the sensing surface from being
influenced by radiation below the horizontal surface of the
instrument. The base is designed with fixing screw holes for
instrument installation and three screws for adjusting the sensing
surface on the horizontal level.