Page 276 - Design of Solar Thermal Power Plants

258 4. DESIGN OF THE CONCENTRATION SYSTEM

field controller as well as offering a variety of information to the
heliostat controller, such as meteorological conditions and the
working status of the receiver, thermal storage unit, and steam
turbine. The heliostat field controller determines the working status
of each heliostat based on such information. All instructions for any
actions of the heliostat will be sent by the heliostat field controller,
which means that every heliostat only receives action instructions
from the solar field controller.

The control of the heliostat field can also be not connected to the host
computer of the power plant; instead, it receives direct signals from the
temperature sensor and meteorological conditions from the receiver to
control the actions of the heliostat.

4.4.2 Correction of Heliostat Tracking Errors

Heliostat tracking precision is a key index for the solar thermal tower
power generation system. The heliostat’s current position can be
calculated in accordance with the astronomical formula of the Sun
while achieving very high calculation precision. Nevertheless, in the
case of manufacturing, installing, and operating a heliostat, it is
inevitable that various kinds of errors will be encountered. For
example, the azimuth rotation axis of the heliostat shall be perpendic-
ular to the horizontal plane, and the elevation rotation axis shall be
parallel to the horizontal plane; yet during the manufacture and
installation process, absolute perpendicularity and parallelism cannot
be achieved. Furthermore, the higher the requirement for precision, the
higher the respective cost. Because of various influencing factors,
heliostat tracking precision is normally low. Although it does not
deviate greatly from the target center, it is unable to satisfy the request
for power generation. Therefore, alternative deviation calibration
methods must be used to improve tracking precision. If deviation
corrections cannot be performed in time, deviation of the concentrated
solar spot from the target point may result and lead to burnup of the
support structure of the heat-absorbing tower.
Equipment used in heliostat tracking deviation correction includes
a single-heliostat control system, a CCD (charge-coupled device) image-
acquisition camera, an image processing and analyzing system, a full
heliostat field control programmable logic controller, and a host monitoring
system for the entire heliostat field (refer to Fig. 4.21).
It applies fully closed-loop inspection and correction of tracking
deviation, recording of historical deviation correction curve, interpolation
calculation, successive approximation, and the like, which features good
deviation correction effects and satisfactory adaptability.