Page 274 - Design of Solar Thermal Power Plants

256 4. DESIGN OF THE CONCENTRATION SYSTEM

temperature control when no power is generated. Electric
auxiliary heating of the transmission pipeline is not reliable, as
salts are easily frozen in partial areas. When applying this
method, heat tracing of all narrow parts should be fully
completed, mainly including the valve, flange, sensor interface,
elbow, and pumping hole. Good insulation measures shall be
taken on the aperture surface of a molten-salt receiver.
For an external cylinder receiver (Fig. 4.13), the respective
antifreezing energy consumption of molten salt is comparatively
great. However, due to geometrical features, a larger concentration
ratio can be obtained for an external receiver than using a cavity
receiver.
b. For water/steam receivers, only when temperature of the
receiver’s heat absorber exceeds zero can water be added to make
it start working. Heat absorber preheating can be conducted by
integrating solar concentration using a small number of mirrors
and absorber surface temperature measurements.
c. For air receivers, before heliostats are put into operation, the fan
or compressor should first be turned on and the heliostat
aimpoints adjusted according to the surface temperature of the
heat absorber or the air temperature at the receiver outlet.

4.4 CONTROL DESIGN OF THE HELIOSTAT FIELD OF
A SOLAR TOWER POWER PLANT

4.4.1 Technical Conditions for the Heliostat Field
Control System
1. Control requirements of the heliostat field shall be determined
according to solar irradiation conditions, wind speed, ambient
temperature, and characteristics of the heat-transfer working fluid
while meeting the following requirements:
a. Division of the heliostat field. The solar-concentrating field
consists of many heliostats. The receiver shall be free of major
thermal shock during starting and stopping stages, which in a
heliostat field can be divided into several sections. During
startup, solar energy is gradually concentrated into the receiver
by sections at different time intervals; the corresponding
operation time point and time interval of each section of heliostats
are given by instructions from the solar field master controller.
During operation control of a heliostat, energy input to the
receiver shall be estimated, and the position of the projected solar
beam inside the receiver shall be determined.