A User's Manual for DELSOL3 - prod.sandia.gov - Sandia National ...

J1.D. Heliostats
Either rectangular (IROUND=O) or circular (IROUND=l) mirror shapes
(Figure II-ll(A) and (B)) can be accommodated by the code. The overall dimen-
sions of the heliostat, WM and HM, enclose the mirrored surface, the edge sup-
ports, and cutouts or slots, if any. The fraction of the area defined by WM and
HM which actually reflects sunlight is specified by the parameter DENSMR. The
user has the option of generating more accurate images from canted heliostats by
specifying the size and location of the cant panels (ICPANL=l, namelist HSTAT)
rather than letting DELSOL specify based on NCANTX, NCANTY, HM, and
WM. This option is highly recommended in small systems. The width and height
of the reflective surface of each cant panel are WPANL and HPANL meters, re-
spectively (Figure 11-ll(C)). The center of the Ith cant panel is displaced from
the pivot point (center of the heliostat) by HXCANT(1) meters parallel to the
horizontal edge of the heliostat and HYCANT(1) meters parallel to the vertical
edge of the heliostat.
The reflectivity, given by the value of RMIRL, represents the time averaged
value and the value just after washing. RMIRL should also include transmission
losses due to any enclosure surrounding the heliostat. (See also Section
111. G-2.)
The heliostats are assumed to have altitude-azimuth drive systems pivoted at
the center of the mirrored surface. Therefore, the geometrical center of a heliostat
is also the center of rotation of the mirrored surface.
11.D-I. Heliostat Error Sources-The performance of heliostats is degraded
by several error sources. Care must be taken with the input of these terms be-
cause different reports often use different descriptions for the same errors. Specif-
ically, a distinction must be made between an error source (e.g., backlash in the
azimuthal motor drive) and the effect of the error source (i.e., the magnitude of
the displacement and/or distortion of the heliostat image on the receivers). The
latter, the effect of the error source, depends on the geometry between the sun,
heliostat, and receiver. Thus, a heliostat with a constant error source will pro-
vide variable effects on the image at different times of the year for the same field
position, or at different field positions for the same time of the year, due to the
changing relative positions of the sun and receiver.
Consider the example of the effect of a constant backlash error in the az-
imuthal drive in otherwise perfect heliostats. At noon on any given day of the
year, a heliostat located due north of the tower will produce a larger displacement
of the image on the receiver than its counterpart at the same distance due south.
In fact, as heliostats in the south field approach a horizontal orientation (Le., mir-
ror normal ii vertical), errors in the azimuthal drive produce no displacement in
the image on the receiver.
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