Page 134 - Design of Solar Thermal Power Plants

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS 121

point M i (x i , y i , z i ), and s i is the absorptivity (absorption ratio) of the
receiving surface. In Fig. 3.1, the grid element of the mirror surface at the
lower corner marked with dotted lines is invalid, possibly due to shading
and blocking by neighboring heliostats or the solar tower.
Fig. 3.1 schematically shows the BRT method’s ray trace principle on
the mirror surface plane and the target plane for simulating concentrated
solar flux density at target surface point T j (u j , v j , w j ), but the equation
shown in Fig. 3.1 is simply the general expression for the BRT method,
.
with mirror surface normal n i changing with mirror point positions and
.
the target surface normal p changing with target surface positions.
j
Since the shading-and-blocking computation is the most complicated,
time-consuming, and important optical computing section for the
concentrated solar flux map on the receiving-surface for a field of helio-
stats, we also developed a new shading-and-blocking method to compute
the shading-and-blocking factors (efficiencies) of all heliostats given a
time or sun position in the sky. The shading-and-blocking mirror region of
a heliostat by neighboring heliostats and solar tower shading on the he-
liostat mirror are all computed using a flat-ground-plane platform; i.e.,
the mirror contours and the envelope cylinder of the tower are projected
onto the horizontal ground plane along the sunbeam incident direction or
in the directions of the reflection. If the shading projection of a sampled
mirror point of the current heliostat is inside the shade cast by a neigh-
boring heliostat or in the shade cast by the tower, that mirror point should
be shaded from the incident sun beam.
Fig. 3.2 illustrates the new model for identifying the invalid shading-
and-blocking mirror region of a heliostat by neighboring heliostats and
also the solar tower. The shading-and-blocking mirror region of a heliostat
by neighboring heliostats and also the solar tower shading on the heliostat
mirror are all computed on a flat-ground-plane platform; i.e., projecting
the mirror contours and envelope cylinder of the tower onto the hori-
zontal ground plane along the sunbeam incident direction or along
reflection directions. In Fig. 3.2, the shading-and-blocking projections on
the horizontal ground plane of the heliostat and solar tower contours are
drawn with dashed lines. Fig. 3.2A shows the orientations of all normal-
working heliostats around the circle cylinder solar tower marked in red
given a solar vector, whereas the current heliostat, and the incident and
reflected solar rays by the heliostat surface center, are marked with green
lines. In this imaginary case, the field ground slopes from north to south.
Fig. 3.2B shows the shading projections for all heliostat contours
(including the current heliostat marked in green) and the solar tower
cylinder contour together with heliostat plan positions. Fig. 3.2C shows
the reflection-blocking projections for all heliostat contours (including the
current heliostat marked in green) together with the heliostat plan posi-
tions. Fig. 3.2D is the partial enlarged drawing of Fig. 3.2B, in which the