Page 123 - Design of Solar Thermal Power Plants

2.9 MEASURING INSTRUMENT 109

2. Installation and maintenance of the instrument first involves firmly
mounting the solar radiation meter on a specific stand. The stand is a
piece of metal pipe or wooden post fixed on the top with a metal
sheet or wooden plate slightly larger than the base of the global
radiation meter. The stand is more than 1.50 m from the ground to
avoid the influences of ground radiation. The lower section of the
stand shall be firmly buried in the ground and prevent sinking or
deforming for a long period even while under severe shock
vibration (such as a heavy wind), and shall not change the
horizontality of the instrument.
During installation, the white baffle of the global radiation meter
shall be dismantled and the global radiation meter shall be mounted
on the stand while keeping the wiring terminal of the instrument
facing northward. The instrument shall be fixed on the stand with
three screws (rustless materials are best); if metal plates are used for
the stand, three holes shall be drilled in advance and the instrument
shall be fixed by bolts. Then the three screws on the base can be
adjusted by utilizing the attached leveling instrument to keep the
sensing surface of the global radiation meter horizontal before
finally mounting the white baffle.
After the instrument is installed, the wiring terminal and recorder
shall be connected by wires (please pay attention to the positive and
negative electrodes during connection). A wiring terminal normally
has three lead-out lines, with one piece connected to the machine
body and the shielding layer of the cables to avoid interference and
inductive lightning strikes.
3. The instrument is used for observing global radiation. Before
sunrise, the metal cover shall be opened so that the radiation
meter starts to function, and the recorder automatically displays
instantaneous values and cumulative total of global radiation. After
sunset, observation shall be stopped and the instrument shall be
covered. In cases of no rainfall or when no possible damage can
occur to the instrument during the night, the global radiation meter
and the regular meter may remain uncovered.
Opening and closing of the metal cover shall be handled with special
care. As the quartz glazed shield is expensive, heavy, and fragile, the
metal cover shall be closed with slight force without touching the glazed
shield. In winter, water drips and other coagulations on the glazed shield
and its surrounding areas shall be wiped out before closing the metal
cover to avoid freezing. Once the metal cover becomes frozen it becomes
difficult to remove, so it is suggested that frozen parts are melted using a
blower or by adopting other methods before removing the metal cover;
the operator shall be careful and strive to not cause any damage to the
glazed shield.