Page 275 - Design of Solar Thermal Power Plants

4.4 CONTROL DESIGN OF THE HELIOSTAT FIELD 257

b. Concentrated solar irradiation control. The heliostat field
controller determines the quantity of heliostats to be input into
the receiver according to external meteorological conditions,
heat-transfer loop emergency alarm, and the like, which can also
be determined on the basis of instructions from the power plant’s
main controller.
c. Constitution of the heliostat field control system. The controller
consists of hardware and software to control the actions of
heliostats as well as the BCS (beam characterization system) for
inspecting the tracking precision of heliostats. Input signals of the
controller include wind speed, temperature, heat absorber
temperature, drum pressure, receiver inlet flow, and fluid
temperature at the receiver outlet.
2. Control structure of the heliostat field. The heliostat has two rotation
axes. The commonly used transmission equipment includes the gear
transmission, linear actuator, and hydraulic transmission. Each
heliostat is equipped with a local controller for controlling actions of
the rotation axis; the heliostat field controller controls rotation of the
heliostat through the local controller:
a. The heliostat local controller either calculates the heliostat’s
rotational position corresponding to each time point by using the
astronomical formula of the Sun or receives heliostat tracking
position instructions distributed from the host controller, which has
more powerful computational capabilities; heliostats themselves
can be only equipped with an emergency response function.
b. The heliostat field controller is connected to the solar
concentration field wind speed sensor and the receiver safety
alarm device in order to provide heliostat orientation control in
emergencies. The heliostat field controller can also be connected
to the power plant’s main controller.
3. Heliostat grounding can be divided into two sections, lightning
protection grounding and control electrical appliance grounding,
both of which shall be executed in accordance with respective
national standards. Heliostat field grounding shall be considered
together with grounding of the entire power plant; the grounding
electrodes of all power plant electrical equipment shall be connected
in an equipotential manner. Grounding materials shall be selected
while fully considering the chemical constituents and nature of soil
to ensure that the designed service life of the power plant can be
achieved. The grounding resistance shall be designed while
considering winter soil freezing and summer meltdown.
4. The logical relationship between the heliostat field controller and
power plant control. The power plant’s control host receives
heliostat output energy information and the like from the heliostat