Page 245 - Design of Solar Thermal Power Plants

228 4. DESIGN OF THE CONCENTRATION SYSTEM

inputeoutput relationship and (2) according to the annual mean inpute
output relationship.
In the first mode, it is feasible to determine the area of the heliostat field
before receiver design so that the corresponding rated input power is
equivalent to the peak value output of the heliostat field; alternatively, it is
feasible to determine receiver input power before heliostat field design so
that the peak value of heliostat field output is equivalent to the rated
receiver input.
In the second mode, receiver input power and solar-concentrating field
output power have an equivalent annual average. The problem with this
method is that when concentrating-field output exceeds the annual mean
value, partial concentrators must be shut down so that receiver input does
not become supersaturated. Therefore the annual mean calculation
method is usually not applied. For parabolic trough and Fresnel collector
systems, receiver power depends on the Nusselt number of the fluid
inside the heat-absorbing tube. It is required that the heat-transfer system
is designed to adapt to the concentrator’s maximum output power.
While performing power matching, energy inside the receiver must be
distributed on a reasonable basis, especially for a “water/superheated
steam” type receiver with a phase-change process; the concentrating field
shall feature high optical precision and good flexibility, and be capable of
projecting solar radiation onto different positions inside the receiver at
different times; otherwise, dry-heating can easily occur on the super-
heated section of the heat absorber, especially during receiver startup.
Because complex processes are involved, the control design techniques
corresponding to this method are especially difficult.

4.1.6 Influences of Dust Accumulation on Mirror
Dust accumulation on the mirror may result in a sharp decline in
mirror reflectivity. Attenuation of reflectivity is related to both time and
location. Fig. 4.2 shows the record of dust accumulation’s influence on
mirror reflectivity during the period from August 28, 2011, to May 24,
2012. During the test period, no manual cleaning were engaged; mirrors
were cleaned only by rainwater.
According to Fig. 4.2, the degree of dust accumulation on the mirror is
relatedtothe angleinwhich themirrorisplaced; thereflectivity
attenuations of mirrors placed perpendicularly to the ground and
those with their reflective surface facing downward are comparatively
slow; due to the cleaning effect of rainwater, the reflectivity attenuation
of a mirror placed perpendicularly to the ground is slower than it is
for one with its reflective surface facing downward. According to
Fig. 4.2, after being washed by rainwater, all mirror reflectivities
increased significantly; the reflectivity of mirrors placed facing the